Our first attempts to compile a API guideline from scratch, made little sense when there are good open standards available in all sizes. We forked the [Zalando RESTful API guidelines](https://github.com/zalando/restful-api-guidelines) and adapted it to fit our needs.

Since this is not based on our use case exactly, this document is meant to be a living document which can be improved overtime to fit our needs.

Our architecture is centered around decoupled microservices that provide functionality via RESTful APIs with a JSON payload. Small engineering teams own, deploy and operate these microservices. Our APIs most purely express what our systems do, and are therefore highly valuable business assets. Designing high-quality, long-lasting APIs has become even more critical for our business.

With this in mind, we’ve adopted "API First" as one of our key engineering principles. Microservices development begins with API definition outside the code and ideally involves ample peer-review feedback to achieve high-quality APIs. API First encompasses a set of quality-related standards and fosters a peer review culture including a lightweight review procedure. We encourage our teams to follow them to ensure that our APIs:

Ideally, all APIs will look like the same author created them.

The requirement level keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" used in this document (case insensitive) are to be interpreted as described in RFC 2119.

The purpose of our "RESTful API guidelines" is to define standards to successfully establish "consistent API look and feel" quality. Teams are responsible to fulfill these guidelines during API development and are encouraged to contribute to guideline evolution via pull requests.

These guidelines will, to some extent, remain work in progress as our work evolves, but teams can confidently follow and trust them.

In case guidelines are changing, following rules apply:

Furthermore you should keep in mind that once an API becomes public externally available, it has to be re-reviewed and changed according to current guidelines - for sake of overall consistency.

Comparing SOA web service interfacing style of SOAP vs. REST, the former tend to be centered around operations that are usually use-case specific and specialized. In contrast, REST is centered around business (data) entities exposed as resources that are identified via URIs and can be manipulated via standardized CRUD-like methods using different representations, and hypermedia. RESTful APIs tend to be less use-case specific and come with less rigid client / server coupling and are more suitable for an ecosystem of (core) services providing a platform of APIs to build diverse new business services. We apply the RESTful web service principles to all kind of application (micro-) service components, independently from whether they provide functionality via the internet or intranet.

An important principle for API design and usage is Postel’s Law, aka The Robustness Principle (see also RFC 1122):

Readings: Here are some recommended reads on the RESTful API design style and service architecture:

As part of our API strategy, we encourage product and platform thinking. This should be expressed in the delivery of APIs as parts of a coherent platform.

Platform products provide their functionality via (public) APIs; hence, the design of our APIs should be based on the API as a Product principle:

Embracing 'API as a Product' facilitates a service ecosystem, which can be evolved more easily and used to experiment quickly with new business ideas by recombining core capabilities. It makes the difference between agile, innovative product service business built on a platform of APIs and ordinary enterprise integration business where APIs are provided as "appendix" of existing products to support system integration and optimised for local server-side realization.

Understand the concrete use cases of your customers and carefully check the trade-offs of your API design variants with a product mindset. Avoid short-term implementation optimizations at the expense of unnecessary client side obligations, and have a high attention on API quality and client developer experience.

API as a Product is closely related to our API First principle (see next chapter) which is more focused on how we engineer high quality APIs.

In a nutshell API First requires two aspects:

By defining APIs outside the code, we want to facilitate early review feedback and also a development discipline that focus service interface design on…​

Moreover, API definitions with standardized specification format also facilitate…​

Elements of API First are also this API Guidelines and a standardized API review process as to get early review feedback from peers and client developers. Peer review is important for us to get high quality APIs, to enable architectural and design alignment and to supported development of client applications decoupled from service provider engineering life cycle.

It is important to learn, that API First is not in conflict with the agile development principles that we love. Service applications should evolve incrementally — and so its APIs. Of course, our API specification will and should evolve iteratively in different cycles; however, each starting with draft status and early team and peer review feedback. API may change and profit from implementation concerns and automated testing feedback. API evolution during development life cycle may include breaking changes for not yet productive features and as long as we have aligned the changes with the clients. Hence, API First does not mean that you must have 100% domain and requirement understanding and can never produce code before you have defined the complete API and get it confirmed by peer review.

On the other hand, API First obviously is in conflict with the bad practice of publishing API definition and asking for peer review after the service integration or even the service productive operation has started. It is crucial to request and get early feedback — as early as possible, but not before the API changes are comprehensive with focus to the next evolution step and have a certain quality (including API Guideline compliance), already confirmed via team internal reviews.

You must follow the API First Principle, more specifically:

We use the OpenAPI specification as standard to define API specification files. API designers are required to provide the API specification using a single self-contained YAML file to improve readability. We encourage to use OpenAPI 3.0 version, but still support OpenAPI 2.0 (a.k.a. Swagger 2).

The API specification files should be subject to version control using a source code management system - best together with the implementing sources.

You must / should publish the component external / internal API specification with the deployment of the implementing service.

Hint: A good way to explore OpenAPI 3.0/2.0 is to navigate through the OpenAPI specification mind map and use our Swagger Plugin for IntelliJ IDEA to create your first API. To explore and validate/evaluate existing APIs the Swagger Editor may be a good starting point.

Hint: We do not yet provide guidelines for GraphQL. We focus on resource oriented HTTP/REST API style (and related tooling and infrastructure support) for general purpose peer-to-peer microservice communication. Here, we think that GraphQL has no major benefits, but a couple of downsides compared to REST. However, GraphQL can provide a lot of value for specific target domain problems, especially backends for frontends (BFF) and mobile clients.

In addition to the API Specification, it is good practice to provide an API user manual to improve client developer experience, especially of engineers that are less experienced in using this API. A helpful API user manual typically describes the following API aspects:

The user manual must be published online, e.g. via our documentation hosting platform service, GH pages, or specific team web servers. Please do not forget to include a link to the API user manual into the API specification using the #/externalDocs/url property.

Normally, API specification files must be self-contained, i.e. files should not contain references to local or remote content, e.g. ../fragment.yaml#/element or $ref: 'https://github.com/zalando/zally/blob/master/server/src/main/resources/api/zally-api.yaml#/schemas/LintingRequest'. The reason is, that the content referred to is in general not durable and not immutable. As a consequence, the semantic of an API may change in unexpected ways.

However, you may use remote references to resources accessible by the following service URLs:

As we control these URLs, we ensure that their content is durable and immutable. This allows to define API specifications by using fragments published via this sources, as suggested in MUST specify success and error responses.

API specifications must contain the following OpenAPI meta information to allow for API management:

Following OpenAPI extension properties must be provided in addition:

OpenAPI allows to specify the API specification version in #/info/version. To share a common semantic of version information we expect API designers to comply to Semantic Versioning 2.0 rules 1 to 8 and 11 restricted to the format <MAJOR>.<MINOR>.<PATCH> for versions as follows:

Additional Notes:

Example:

Each API specification must be provisioned with a globally unique and immutable API identifier. The API identifier is defined in the info-block of the OpenAPI specification and must conform to the following definition:

API specifications will evolve and any aspect of an OpenAPI specification may change. We require API identifiers because we want to support API clients and providers with API lifecycle management features, like change trackability and history or automated backward compatibility checks. The immutable API identifier allows the identification of all API specification versions of an API evolution. By using API semantic version information or API publishing date as order criteria you get the version or publication history as a sequence of API specifications.

Note: While it is nice to use human readable API identifiers based on self-managed URNs, it is recommend to stick to UUIDs to relief API designers from any urge of changing the API identifier while evolving the API. Example:

Each API must be classified with respect to the intended target audience supposed to consume the API, to facilitate differentiated standards on APIs for discoverability, changeability, quality of design and documentation, as well as permission granting. We differentiate the following API audience groups with clear organisational and legal boundaries:

Note: a smaller audience group is intentionally included in the wider group and thus does not need to be declared additionally.

The API audience is provided as API meta information in the info-block of the OpenAPI specification and must conform to the following specification:

Note: Exactly one audience per API specification is allowed. For this reason a smaller audience group is intentionally included in the wider group and thus does not need to be declared additionally. If parts of your API have a different target audience, we recommend to split API specifications along the target audience — even if this creates redundancies (rationale (internal link)).

Example:

For details and more information on audience groups see the API Audience narrative (internal link).

Functional naming is a powerful, yet easy way to align global resources as host, permission, and event names within an the application landscape. It helps to preserve uniqueness of names while giving readers meaningful context information about the addressed component. Besides, the most important aspect is, that it allows to keep APIs stable in the case of technical and organizational changes.

A unique functional-name is assigned to each functional component serving an API. It is built of the domain name of the functional group the component is belonging to and a unique a short identifier for the functional component itself:

Depending on the API audience, you must/should/may follow the functional naming schema for hostnames and event names (and also permission names, in future) as follows:

Please see the following rules for detailed functional naming patterns: * MUST follow naming convention for hostnames * [213]

Internal Guideance: You must use the simple functional name registry (internal link) to register your functional name before using it. The registry is a centralized infrastructure service to ensure uniqueness of your functional names (and available domains — including subdomains) and to support hostname DNS resolution.
Hint: Due to lexicalic restrictions of DNS names there is no specific separator to split a functional name into (sub) domain and component; this knowledge is only managed in the registry.

Hostnames in APIs must, respectively should conform to the functional naming depending on the audience as follows (see MUST/SHOULD use functional naming schema for details and <functional-name> definition):

Hint: The following convention (e.g. used by legacy STUPS infrastructure) is deprecated and only allowed for hostnames of component-internal APIs:

Exception: There are legacy hostnames used for APIs with external-partner audience which may not follow this rule due to backward compatibility constraints. The API Linter maintains a whitelist for this exceptions (including e.g. api.merchants.zalando.com and api-sandbox.merchants.zalando.com).

Every API endpoint must be protected and armed with authentication and authorization. As part of the API definition you must specify how you protect your API using either the http typed bearer or oauth2 typed security schemes defined in the OpenAPI Authentication Specification.

The majority of our APIs (especially the company internal APIs) are protected using JWT tokens provided by the platform IAM token service. In these situations you should use the http typed Bearer Authentication security scheme — it is based on OAuth2.0 RFC 6750 defining the standard header Auhorization: Bearer <token>. The following code snippet shows how to define the bearer security scheme.

The bearer security schema can than be applied to all API endpoints, e.g. requiring the token to have api-repository.read scope for permission as follows (see also MUST define and assign permissions (scopes)):

In other, more specific situations e.g. with customer and partner facing APIs you may use other OAuth 2.0 authorization flows as defined by RFC 6749. Please consult the OpenAPI OAuth 2.0 Authentication section for details on how to define oauth2 typed security schemes correctly.

Note: Do not use OpenAPI oauth2 typed security scheme flows (e.g. implicit) if your service does not fully support it and implements a simple bearer token scheme, because it exposes authentication server address details and may make use of redirection.

APIs must define permissions to protect their resources. Thus, at least one permission must be assigned to each API endpoint.

The naming schema for permissions corresponds to the naming schema for hostnames and event type names. Please refer to MUST follow naming convention for permissions (scopes) for designing permission names and see the following examples.

Note: APIs should stick to component specific permissions without resource extension to avoid the complexity of too many fine grained permissions. For the majority of use cases, restricting access for specific API endpoints using read or write is sufficient.

The defined permissions are than assigned to each API endpoint based on the security schema (see example in previous section) by specifying the security requirement as follows:

In some cases a whole API or selected API endpoints may not require speciifc permissions, e.g. if information is public or protected by object level authorization. To make this explicit you should assign the uid pseudo permission, that is always available as OAuth2 default scope.

Hint: Following a minimal a minimal API specification approach, the Authorization-header does not need to be defined on each API endpoint, since it is required and so to say implicitly defined via the security section.

As long as the functional naming is not yet supported by our permission registry, permission names in APIs must conform to the following naming pattern:

This pattern is compatible with the previous definition.

Open API (based on JSON Schema Validation vocabulary) defines formats from ISO and IETF standards for date/time, integers/numbers and binary data. You must use these formats, whenever applicable:

Note: Formats bigint and decimal have been added to the OpenAPI defined formats — see also MUST define a format for number and integer types and MUST use standard formats for date and time properties below.

We add further OpenAPI formats that are useful especially in an e-commerce environment e.g. language code, country code, and currency based other ISO and IETF standards. You must use these formats, whenever applicable:

Remark: Please note that this list of standard data formats is not exhaustive and everyone is encouraged to propose additions.

In MUST use standard data formats we added bigint and decimal to the OpenAPI defined formats. As an implication, you must always provide one of the formats int32, int64, bigint or float, double, decimal when you define an API property of JSON type number or integer.

By this we prevent clients from guessing the precision incorrectly, and thereby changing the value unintentionally. The precision must be translated by clients and servers into the most specific language types; in Java, for instance, the number type with decimal format will translate into BigDecimal and integer type with int32 format will translate to int or Integer Java types.

As a specific case of MUST use standard data formats, you must use the string typed formats date, date-time, time, duration, or period for the definition of date and time properties. The formats are based on the standard RFC 3339 internet profile -- a subset of ISO 8601

Exception: For passing date/time information via standard protocol headers, HTTP RFC 7231 requires to follow the date and time specification used by the Internet Message Format RFC 5322.

As defined by the standard, time zone offset may be used, however, we recommend to only use times based on UTC without local offsets. For example 2015-05-28T14:07:17Z rather than 2015-05-28T14:07:17+00:00. From experience we have learned that zone offsets are not easy to understand and often not correctly handled. Note also that zone offsets are different from local times which may include daylight saving time. When it comes to storage, all dates should be consistently stored in UTC without a zone offset. Localization should be done locally by the services that provide user interfaces, if required.

Hint: We discourage using numerical timestamps. It typically creates issues with precision, e.g. whether to represent a timestamp as 1460062925, 1460062925000 or 1460062925.000. Date strings, though more verbose and requiring more effort to parse, avoid this ambiguity.

Schema based JSON properties that are by design durations and intervals could be strings formatted as defined by ISO 8601 (Appendix A of RFC 3339 contains a grammar for durations).

As a specific case of MUST use standard data formats you must use the following standard formats:

In some situations the API supports serving different representations of a specific resource (at the same URL) e.g. JSON, PDF, TEXT, or HTML representations for an invoice resource. You should use content negotiation to support clients specifying via the standard HTTP headers Accept, Accept-Language, Accept-Encoding which representation is best suited for their use case, for example, which language of a document, representation / content format, or content encoding. You SHOULD use standard media types like application/json or application/pdf for defining the content format in the Accept header.

Generating IDs can be a scaling problem in high frequency and near real time use cases. UUIDs solve this problem, as they can be generated without collisions in a distributed, non-coordinated way and without additional server round trips.

However, they also come with some disadvantages:

UUIDs should be avoided when not needed for large scale id generation. Instead, for instance, server side support with id generation can be preferred (POST on id resource, followed by idempotent PUT on entity resource). Usage of UUIDs is especially discouraged as primary keys of master and configuration data, like brand-ids or attribute-ids which have low id volume but widespread steering functionality.

Please be aware that sequential, strictly monotonically increasing numeric identifiers may reveal critical, confidential business information, like order volume, to non-privileged clients.

In any case, we should always use string rather than number type for identifiers. This gives us more flexibility to evolve the identifier naming scheme. Accordingly, if used as identifiers, UUIDs should not be qualified using a format property.

Hint: Usually, random UUID is used - see UUID version 4 in RFC 4122. Though UUID version 1 also contains leading timestamps it is not reflected by its lexicographic sorting. This deficit is addressed by ULID (Universally Unique Lexicographically Sortable Identifier). You may favour ULID instead of UUID, for instance, for pagination use cases ordered along creation time.

In most cases, all resources provided by a service are part of the public API, and therefore should be made available under the root "/" base path.

If the service should also support non-public, internal APIs — for specific operational support functions, for example — we encourage you to maintain two different API specifications and provide API audience. For both APIs, you should not use /api as base path.

We see API’s base path as a part of deployment variant configuration. Therefore, this information has to be declared in the server object.

Usually, a collection of resource instances is provided (at least the API should be ready here). The special case of a resource singleton must be modeled as a collection with cardinality 1 including definition of maxItems = minItems = 1 for the returned array structure to make the cardinality constraint explicit.

Exception: the pseudo identifier self used to specify a resource endpoint where the resource identifier is provided by authorization information (see MUST identify resources and sub-resources via path segments).

To simplify encoding of resource IDs in URLs they must match the regex [a-zA-Z0-9:._\-/]*. Resource IDs only consist of ASCII strings using letters, numbers, underscore, minus, colon, period, and - on rare occasions - slash.

Note: slashes are only allowed to build and signal resource identifiers consisting of compound keys.

Note: to prevent ambiguities of unnormalized paths resource identifiers must never be empty. Consequently, support of empty strings for path parameters is forbidden.

Path segments are restricted to ASCII kebab-case strings matching regex ^[a-z][a-z\-0-9]*$. The first character must be a lower case letter, and subsequent characters can be a letter, or a dash(-), or a number.

Example:

Hint: kebab-case applies to concrete path segments and not necessarily the names of path parameters.

You must not specify paths with duplicate or trailing slashes, e.g. /customers//addresses or /customers/. As a consequence, you must also not specify or use path variables with empty string values.

Reasoning: Non standard paths have no clear semantics. As a result, behavior for non standard paths varies between different HTTP infrastructure components and libraries. This may leads to ambiguous and unexpected results during request handling and monitoring.

We recommend to implement services robust against clients not following this rule. All services should normalize request paths before processing by removing duplicate and trailing slashes. Hence, the following requests should refer to the same resource:

Note: path normalization is not supported by all framework out-of-the-box. Services are required to support at least the normalized path while rejecting all alternatives paths, if failing to deliver the same resource.

The API describes resources, so the only place where actions should appear is in the HTTP methods. In URLs, use only nouns. Instead of thinking of actions (verbs), it’s often helpful to think about putting a message in a letter box: e.g., instead of having the verb cancel in the url, think of sending a message to cancel an order to the cancellations letter box on the server side.

REST is all about your resources, so consider the domain entities that take part in web service interaction, and aim to model your API around these using the standard HTTP methods as operation indicators. For instance, if an application has to lock articles explicitly so that only one user may edit them, create an article lock with PUT or POST instead of using a lock action.

Request:

The added benefit is that you already have a service for browsing and filtering article locks.

As a rule of thumb resources should be defined to cover 90% of all its client’s use cases. A useful resource should contain as much information as necessary, but as little as possible. A great way to support the last 10% is to allow clients to specify their needs for more/less information by supporting filtering and embedding.

API resources represent elements of the application’s domain model. Using domain-specific nomenclature for resource names helps developers to understand the functionality and basic semantics of your resources. It also reduces the need for further documentation outside the API definition. For example, "sales-order-items" is superior to "order-items" in that it clearly indicates which business object it represents. Along these lines, "items" is too general.

An API should contain the complete business processes containing all resources representing the process. This enables clients to understand the business process, foster a consistent design of the business process, allow for synergies from description and implementation perspective, and eliminates implicit invisible dependencies between APIs.

In addition, it prevents services from being designed as thin wrappers around databases, which normally tends to shift business logic to the clients.

Some API resources may contain or reference sub-resources. Embedded sub-resources, which are not top-level resources, are parts of a higher-level resource and cannot be used outside of its scope. Sub-resources should be referenced by their name and identifier in the path segments as follows:

In order to improve the consumer experience, you should aim for intuitively understandable URLs, where each sub-path is a valid reference to a resource or a set of resources. For instance, if /partners/{partner-id}/addresses/{address-id} is valid, then, in principle, also /partners/{partner-id}/addresses, /partners/{partner-id} and /partners must be valid. Examples of concrete url paths:

Note: resource identifiers may be build of multiple other resource identifiers (see MAY expose compound keys as resource identifiers).

Exception: In some situations the resource identifier is not passed as a path segment but via the authorization information, e.g. an authorization token or session cookie. Here, it is reasonable to use self as pseudo-identifier path segment. For instance, you may define /employees/self or /employees/self/personal-details as resource paths —  and may additionally define endpoints that support identifier passing in the resource path, like define /employees/{empl-id} or /employees/{empl-id}/personal-details.

If a resource is best identified by a compound key consisting of multiple other resource identifiers, it is allowed to reuse the compound key in its natural form containing slashes instead of technical resource identifier in the resource path without violating the above rule MUST identify resources and sub-resources via path segments as follows:

Example paths:

Warning: Exposing a compound key as described above limits ability to evolve the structure of the resource identifier as it is no longer opaque.

To compensate for this drawback, APIs must apply a compound key abstraction consistently in all requests and responses parameters and attributes allowing consumers to treat these as technical resource identifier replacement. The use of independent compound key components must be limited to search and creation requests, as follows:

Where id-1 is representing the opaque provision of the compound key sku-1/sid-1 as technical resource identifier.

Remark: A compound key component may itself be used as another resource identifier providing another resource endpoint, e.g /article-size-advices/{sku}.

If a sub-resource is only accessible via its parent resource and may not exist without parent resource, consider using a nested URL structure, for instance:

However, if the resource can be accessed directly via its unique id, then the API should expose it as a top-level resource. For example, customer has a collection for sales orders; however, sales orders have globally unique id and some services may choose to access the orders directly, for instance:

To keep maintenance and service evolution manageable, we should follow "functional segmentation" and "separation of concern" design principles and do not mix different business functionalities in same API definition. In practice this means that the number of resource types exposed via an API should be limited. In this context a resource type is defined as a set of highly related resources such as a collection, its members and any direct sub-resources.

For example, the resources below would be counted as three resource types, one for customers, one for the addresses, and one for the customers' related addresses:

Note that:

Given this definition, our experience is that well defined APIs involve no more than 4 to 8 resource types. There may be exceptions with more complex business domains that require more resources, but you should first check if you can split them into separate subdomains with distinct APIs.

Nevertheless one API should hold all necessary resources to model complete business processes helping clients to understand these flows.

There are main resources (with root url paths) and sub-resources (or nested resources with non-root urls paths). Use sub-resources if their life cycle is (loosely) coupled to the main resource, i.e. the main resource works as collection resource of the subresource entities. You should use ⇐ 3 sub-resource (nesting) levels — more levels increase API complexity and url path length. (Remember, some popular web browsers do not support URLs of more than 2000 characters.)

See also MUST property names must be snake_case (and never camelCase).

If you provide query support for searching, sorting, filtering, and paginating, you must stick to the following naming conventions:

Use JSON (RFC 7159) to represent structured (resource) data passed with HTTP requests and responses as body payload. The JSON payload must use a JSON object as top-level data structure (if possible) to allow for future extension. This also applies to collection resources, where you ad-hoc would use an array — see also MUST always return JSON objects as top-level data structures.

Additionally, the JSON payload must comply to the more restrictive Internet JSON (RFC 7493), particularly

As a consequence, a JSON payload must

Non-JSON media types may be supported, if you stick to a business object specific standard format for the payload data, for instance, image data format (JPG, PNG, GIF), document format (PDF, DOC, ODF, PPT), or archive format (TAR, ZIP).

Generic structured data interchange formats other than JSON (e.g. XML, CSV) may be provided, but only additionally to JSON as default format using content negotiation, for specific use cases where clients may not interpret the payload structure.

You should use standard media types (defined in media type registry of Internet Assigned Numbers Authority (IANA)) as content-type (or accept) header information. More specifically, for JSON payload you should use the standard media type application/json (or application/problem+json for MUST support problem JSON).

You should avoid using custom media types like application/vendor.article+json. Custom media types beginning with x bring no advantage compared to the standard media type for JSON, and make automated processing more difficult.

Exception: Custom media type should be only used in situations where you need to provide API endpoint versioning (with content negotiation) due to incompatible changes.

Names of arrays should be pluralized to indicate that they contain multiple values. This implies in turn that object names should be singular.

Property names are restricted to ASCII snake_case strings matching regex ^[a-z_][a-z_0-9]*$. The first character must be a lower case letter, or an underscore, and subsequent characters can be a letter, an underscore, or a number.

Examples:

Rationale: No established industry standard exists, but many popular Internet companies prefer snake_case: e.g. GitHub, Stack Exchange, Twitter. Others, like Google and Amazon, use both - but not only camelCase. It’s essential to establish a consistent look and feel such that JSON looks as if it came from the same hand.

Enumerations should be represented as string typed OpenAPI definitions of request parameters or model properties. Enum values (using enum or x-extensible-enum) need to consistently use the upper-snake case format, e.g. VALUE or YET_ANOTHER_VALUE. This approach allows to clearly distinguish values from properties or other elements.

Exception: This rule does not apply for case sensitive values sourced from outside API definition scope, e.g. for language codes from ISO 639-1, or when declaring possible values for a rule 137 [sort parameter].

Dates and date-time properties should end with _at to distinguish them from boolean properties which otherwise would have very similar or even identical names:

Note: created and modified were mentioned in an earlier version of the guideline and are therefore still accepted for APIs that predate this rule.

A "map" here is a mapping from string keys to some other type. In JSON this is represented as an object, the key-value pairs being represented by property names and property values. In OpenAPI schema (as well as in JSON schema) they should be represented using additionalProperties with a schema defining the value type. Such an object should normally have no other defined properties.

The map keys don’t count as property names in the sense of rule 118, and can follow whatever format is natural for their domain. Please document this in the description of the map object’s schema.

Here is an example for such a map definition (the translations property):

An actual JSON object described by this might then look like this:

Schema based JSON properties that are by design booleans must not be presented as nulls. A boolean is essentially a closed enumeration of two values, true and false. If the content has a meaningful null value, strongly prefer to replace the boolean with enumeration of named values or statuses - for example accepted_terms_and_conditions with true or false can be replaced with terms_and_conditions with values yes, no and unknown.

OpenAPI 3.x allows to mark properties as required and as nullable to specify whether properties may be absent ({}) or null ({"example":null}). If a property is defined to be not required and nullable (see 2nd row in Table below), this rule demands that both cases must be handled in the exact same manner by specification.

The following table shows all combinations and whether the examples are valid:

While API designers and implementers may be tempted to assign different semantics to both cases, we explicitly decide against that option, because we think that any gain in expressiveness is far outweighed by the risk of clients not understanding and implementing the subtle differences incorrectly.

As an example, an API that provides the ability for different users to coordinate on a time schedule, e.g. a meeting, may have a resource for options in which every user has to make a choice. The difference between undecided and decided against any of the options could be modeled as absent and null respectively. It would be safer to express the null case with a dedicated Null object, e.g. {} compared to {"id":"42"}.

Moreover, many major libraries have somewhere between little to no support for a null/absent pattern (see Gson, Moshi, Jackson, JSON-B). Especially strongly-typed languages suffer from this since a new composite type is required to express the third state. Nullable Option/Optional/Maybe types could be used but having nullable references of these types completely contradicts their purpose.

The only exception to this rule is JSON Merge Patch RFC 7396) which uses null to explicitly indicate property deletion while absent properties are ignored, i.e. not modified.

Empty array values can unambiguously be represented as the empty list, [].

You must use common field names and semantics whenever applicable. Common fields are idiomatic, create consistency across APIs and support common understanding for API consumers.

We define the following common field names:

Further common fields are defined in SHOULD name date/time properties with _at suffix. The following guidelines define standard objects and fields:

Example JSON schema:

Address structures play a role in different business and use-case contexts, including country variances. All attributes that relate to address information must follow the naming and semantics defined below.

Grouping and cardinality of fields in specific data types may vary based on the specific use case (e.g. combining addressee and address fields into a single type when modeling an address label vs distinct addressee and address types when modeling users and their addresses).

Use the following common money structure:

APIs are encouraged to include a reference to the global schema for Money.

Please note that APIs have to treat Money as a closed data type, i.e. it’s not meant to be used in an inheritance hierarchy. That means the following usage is not allowed:

Be compliant with the standardized HTTP method semantics (see HTTP/1 RFC-7230 and RFC-7230 updates from 2014) summarized as follows:

Request methods in RESTful services can be…​

Note: The above definitions, of intended (side) effect allows the server to provide additional state changing behavior as logging, accounting, pre- fetching, etc. However, these actual effects and state changes, must not be intended by the operation so that it can be held accountable.

Method implementations must fulfill the following basic properties according to RFC 7231:

Note: MUST document cachable GET, HEAD, and POST endpoints.

In many cases it is helpful or even necessary to design POST and PATCH idempotent for clients to expose conflicts and prevent resource duplicate (a.k.a. zombie resources) or lost updates, e.g. if same resources may be created or changed in parallel or multiple times. To design an idempotent API endpoint owners should consider to apply one of the following three patterns.

Note: While conditional key and secondary key are focused on handling concurrent requests, the idempotency key is focused on providing the exact same responses, which is even a stronger requirement than the idempotency defined above. It can be combined with the two other patterns.

To decide, which pattern is suitable for your use case, please consult the following table showing the major properties of each pattern:

Note: The patterns applicable to PATCH can be applied in the same way to PUT and DELETE providing the same properties.

If you mainly aim to support safe retries, we suggest to apply conditional key and secondary key pattern before the Idempotency Key pattern.

The most important pattern to design POST idempotent for creation is to introduce a resource specific secondary key provided in the request body, to eliminate the problem of duplicate (a.k.a zombie) resources.

The secondary key is stored permanently in the resource as alternate key or combined key (if consisting of multiple properties) guarded by a uniqueness constraint enforced server-side, that is visible when reading the resource. The best and often naturally existing candidate is a unique foreign key, that points to another resource having one-on-one relationship with the newly created resource, e.g. a parent process identifier.

A good example here for a secondary key is the shopping cart ID in an order resource.

Note: When using the secondary key pattern without Idempotency-Key all subsequent retries should fail with status code 409 (conflict). We suggest to avoid 200 here unless you make sure, that the delivered resource is the original one implementing a well defined behavior. Using 204 without content would be a similar well defined option.

Header and query parameters allow to provide a collection of values, either by providing a comma-separated list of values or by repeating the parameter multiple times with different values as follows:

As OpenAPI does not support both schemas at once, an API specification must explicitly define the collection format to guide consumers as follows:

When choosing the collection format, take into account the tool support, the escaping of special characters and the maximal URL length.

We prefer the use of query parameters to describe resource-specific query languages for the majority of APIs because it’s native to HTTP, easy to extend and has excellent implementation support in HTTP clients and web frameworks.

Query parameters should have the following aspects specified:

How query parameters are named and used is up to individual API designers. The following examples should serve as ideas:

We don’t advocate for or against certain names because in the end APIs should be free to choose the terminology that fits their domain the best.

Minimalistic query languages based on query parameters are suitable for simple use cases with a small set of available filters that are combined in one way and one way only (e.g. and semantics). Simple query languages are generally preferred over complex ones.

Some APIs will have a need for sophisticated and more complex query languages. Dominant examples are APIs around search (incl. faceting) and product catalogs.

Aspects that set those APIs apart from the rest include but are not limited to:

APIs that qualify for a specific, complex query language are encouraged to use nested JSON data structures and define them using OpenAPI directly. The provides the following benefits:

JSON-specific rules and most certainly needs to make use of the GET-with-body pattern.

Sometimes certain collection resources or queries will not list all the possible elements they have, but only those for which the current client is authorized to access.

Implicit filtering could be done on:

In such cases, the fact that implicit filtering is applied must be documented in the API specification’s endpoint description. Consider caching aspects when implicit filtering is provided. Example:

If an employee of the company Foo accesses one of our business-to-business service and performs a GET /business-partners, it must, for legal reasons, not display any other business partner that is not owned or contractually managed by her/his company. It should never see that we are doing business also with company Bar.

Response as seen from a consumer working at FOO:

Response as seen from a consumer working at BAR:

The API Specification should then specify something like this:

APIs should define the functional, business view and abstract from implementation aspects. Success and error responses are a vital part to define how an API is used correctly.

Therefore, you must define all success and service specific error responses in your API specification. Both are part of the interface definition and provide important information for service clients to handle standard as well as exceptional situations.

Hint: In most cases it is not useful to document all technical errors, especially if they are not under control of the service provider. Thus unless a response code conveys application-specific functional semantics or is used in a none standard way that requires additional explanation, multiple error response specifications can be combined using the following pattern (see also MUST only use durable and immutable remote references):

API designers should also think about a troubleshooting board as part of the associated online API documentation. It provides information and handling guidance on application-specific errors and is referenced via links from the API specification. This can reduce service support tasks and contribute to service client and provider performance.

You must only use official HTTP status codes consistently with their intended semantics. Official HTTP status codes are defined via RFC standards and registered in the IANA Status Code Registry. Main RFC standards are RFC7231 and RFC 6585 (and there are upcoming new ones, e.g. draft legally-restricted-status). An overview on the official error codes provides Wikipedia: HTTP status codes (which also lists some unofficial status codes, e.g. defined by popular web servers like Nginx, that we do not suggest to use).

The most commonly used codes are best understood and listed below as subset of the official HTTP status codes and consistent with their semantics in the RFCs. We avoid less commonly used codes that easily create misconceptions due to less familar semantics and API specific interpretations.

Important: As long as your HTTP status code usage is well covered by the semantic defined here, you should not describe it to avoid an overload with common sense information and the risk of inconsistent definitions. Only if the HTTP status code is not in the list below or its usage requires additional information aside the well defined semantic, the API specification must provide a clear description of the HTTP status code in the response.

You must use the most specific HTTP status code when returning information about your request processing status or error situations.

Some APIs are required to provide either batch or bulk requests using POST for performance reasons, i.e. for communication and processing efficiency. In this case services may be in need to signal multiple response codes for each part of an batch or bulk request. As HTTP does not provide proper guidance for handling batch/bulk requests and responses, we herewith define the following approach:

Note: These rules apply even in the case that processing of all individual parts fail or each part is executed asynchronously!

The rules are intended to allow clients to act on batch and bulk responses in a consistent way by inspecting the individual results. We explicitly reject the option to apply 200 for a completely successful batch as proposed in Nakadi’s POST /event-types/{name}/events as short cut without inspecting the result, as we want to avoid risks and expect clients to handle partial batch failures anyway.

The bulk or batch response may look as follows:

Note: while a batch defines a collection of requests triggering independent processes, a bulk defines a collection of independent resources created or updated together in one request. With respect to response processing this distinction normally does not matter.

APIs that wish to manage the request rate of clients must use the 429 (Too Many Requests) response code, if the client exceeded the request rate (see RFC 6585). Such responses must also contain header information providing further details to the client. There are two approaches a service can take for header information:

The X-RateLimit headers are:

The reason to allow both approaches is that APIs can have different needs. Retry-After is often sufficient for general load handling and request throttling scenarios and notably, does not strictly require the concept of a calling entity such as a tenant or named account. In turn this allows resource owners to minimise the amount of state they have to carry with respect to client requests. The 'X-RateLimit' headers are suitable for scenarios where clients are associated with pre-existing account or tenancy structures. 'X-RateLimit' headers are generally returned on every request and not just on a 429, which implies the service implementing the API is carrying sufficient state to track the number of requests made within a given window for each named entity.

RFC 7807 defines a Problem JSON object using the media type application/problem+json to provide an extensible human and machine readable failure information beyond the HTTP response status code to transports the failure kind (type / title) and the failure cause and location (instant / detail). To make best use of this additional failure information, every endpoints must be capable of returning a Problem JSON on client usage errors (4xx status codes) as well as server side processing errors (5xx status codes).

Note: Clients must be robust and not rely on a Problem JSON object being returned, since (a) failure responses may be created by infrastructure components not aware of this guideline or (b) service may be unable to comply with this guidelines in certain error situations.

Hint: The media type application/problem+json is often not implemented as a subset of application/json by libraries and services! Thus clients need to include application/problem+json in the Accept-Header to trigger delivery of the extended failure information.

The OpenAPI schema definition of the Problem JSON object can be found on GitHub. You can reference it by using:

You may define custom problem types as extensions of the Problem JSON object if your API needs to return specific, additional, and more detailed error information.

Note: Problem type and instance identifiers in our APIs are not meant to be resolved. RFC 7807 encourages that problem types are URI references that point to human-readable documentation, but we deliberately decided against that, as all important parts of the API must be documented using OpenAPI anyway. In addition, URLs tend to be fragile and not very stable over longer periods because of organizational and documentation changes and descriptions might easily get out of sync.

In order to stay compatible with RFC 7807 we proposed to use relative URI references usually defined by absolute-path [ '?' query ] [ '#' fragment ] as simplified identifiers in type and instance fields:

Hint: The use of absolute URIs is not forbidden but strongly discouraged. If you use absolute URIs, please reference problem-1.0.0.yaml#/Problem instead.

Stack traces contain implementation details that are not part of an API, and on which clients should never rely. Moreover, stack traces can leak sensitive information that partners and third parties are not allowed to receive and may disclose insights about vulnerabilities to attackers.

Use this list and explicitly mention its support in your OpenAPI definition.

This convention is followed by (most of) the standard headers e.g. as defined in RFC 2616 and RFC 4229. Examples:

Note, HTTP standard defines headers as case-insensitive (RFC 7230, p.22). However, for sake of readability and consistency you should follow the convention when using standard or proprietary headers. Exceptions are common abbreviations like ID.

Content or entity headers are headers with a Content- prefix. They describe the content of the body of the message and they can be used in both, HTTP requests and responses. Commonly used content headers include but are not limited to:

As the correct usage of Content-Location response header (see below) with respect to caching and its method specific semantics is difficult, we discourage the use of Content-Location. In most cases it is sufficient to inform clients about the resource location in create or re-direct responses by using the Location header while avoiding the Content-Location specific ambiguities and complexities.

More details in RFC 7231 7.1.2 Location, 3.1.4.2 Content-Location

Content-Location is an optional response header that can be used in successful write operations (PUT, POST, or PATCH) or read operations (GET, HEAD) to guide caching and signal a receiver the actual location of the resource transmitted in the response body. This allows clients to identify the resource and to update their local copy when receiving a response with this header.

The Content-Location header can be used to support the following use cases:

Note: When using the Content-Location header, the Content-Type header has to be set as well. For example:

The Prefer header defined in RFC 7240 allows clients to request processing behaviors from servers. It pre-defines a number of preferences and is extensible, to allow others to be defined. Support for the Prefer header is entirely optional and at the discretion of API designers, but as an existing Internet Standard, is recommended over defining proprietary "X-" headers for processing directives.

The Prefer header can defined like this in an API definition:

Note: Please copy only the behaviors into your Prefer header specification that are supported by your API endpoint. If necessary, specify different Prefer headers for each supported use case.

Supporting APIs may return the Preference-Applied header also defined in RFC 7240 to indicate whether a preference has been applied.

When creating or updating resources it may be necessary to expose conflicts and to prevent the 'lost update' or 'initially created' problem. Following RFC 7232 "HTTP: Conditional Requests" this can be best accomplished by supporting the ETag header together with the If-Match or If-None-Match conditional header. The contents of an ETag: <entity-tag> header is either (a) a hash of the response body, (b) a hash of the last modified field of the entity, or (c) a version number or identifier of the entity version.

To expose conflicts between concurrent update operations via PUT, POST, or PATCH, the If-Match: <entity-tag> header can be used to force the server to check whether the version of the updated entity is conforming to the requested <entity-tag>. If no matching entity is found, the operation is supposed a to respond with status code 412 - precondition failed.

Beside other use cases, If-None-Match: * can be used in a similar way to expose conflicts in resource creation. If any matching entity is found, the operation is supposed a to respond with status code 412 - precondition failed.

The ETag, If-Match, and If-None-Match headers can be defined as follows in the API definition:

Please see Optimistic locking in RESTful APIs for a detailed discussion and options.

When creating or updating resources it can be helpful or necessary to ensure a strong idempotent behavior comprising same responses, to prevent duplicate execution in case of retries after timeout and network outages. Generally, this can be achieved by sending a client specific unique request key – that is not part of the resource – via Idempotency-Key header.

The unique request key is stored temporarily, e.g. for 24 hours, together with the response and the request hash (optionally) of the first request in a key cache, regardless of whether it succeeded or failed. The service can now look up the unique request key in the key cache and serve the response from the key cache, instead of re-executing the request, to ensure idempotent behavior. Optionally, it can check the request hash for consistency before serving the response. If the key is not in the key store, the request is executed as usual and the response is stored in the key cache.

This allows clients to safely retry requests after timeouts, network outages, etc. while receive the same response multiple times. Note: The request retry in this context requires to send the exact same request, i.e. updates of the request that would change the result are off-limits. The request hash in the key cache can protection against this misbehavior. The service is recommended to reject such a request using status code 400.

Important: To grant a reliable idempotent execution semantic, the resource and the key cache have to be updated with hard transaction semantics – considering all potential pitfalls of failures, timeouts, and concurrent requests in a distributed systems. This makes a correct implementation exceeding the local context very hard.

The Idempotency-Key header must be defined as follows, but you are free to choose your expiration time:

Hint: The key cache is not intended as request log, and therefore should have a limited lifetime, else it could easily exceed the data resource in size.

Note: The Idempotency-Key header unlike other headers in this section is not standardized in an RFC. Our only reference are the usage in the Stripe API. However, we do not want to change the header name and semantic, and do not name it like the proprietry headers below. The header addresses a generic REST concern and is different from the specific proprietary headers.

As a general rule, proprietary HTTP headers should be avoided. From a conceptual point of view, the business semantics and intent of an operation should always be expressed via the URLs path and query parameters, the method, and the content, but not via proprietary headers. Headers are typically used to implement protocol processing aspects, such as flow control, content negotiation, and authentication, and represent business agnostic request modifiers that provide generic context information (RFC 7231).

However, the exceptional usage of proprietary headers is still helpful when domain-specific generic context information…​

Below, we explicitly define the list of proprietary header exceptions usable for all services for passing through generic context information of our fashion domain (use case 1).

Per convention, non standardized, proprietary header names are prefixed with X-. (Due to backward compatibility, we do not follow the Internet Engineering Task Force’s recommendation in RFC 6648 to deprecate usage of X- headers.) Remember that HTTP header field names are not case-sensitive:

All proprietary headers listed above are end-to-end headers ^[2] and must be propagated to the services down the call chain. The header names and values must remain unchanged.

For example, the values of the custom headers like X-device-Type can affect the results of queries by using device type information to influence recommendation results. Besides, the values of the custom headers can influence the results of the queries (e.g. the device type information influences the recommendation results).

Sometimes the value of a proprietary header will be used as part of the entity in a subsequent request. In such cases, the proprietary headers must still be propagated as headers with the subsequent request, despite the duplication of information.

We strive for a good implementation of REST Maturity Level 2 as it enables us to build resource-oriented APIs that make full use of HTTP verbs and status codes. You can see this expressed by many rules throughout these guidelines, e.g.:

Although this is not HATEOAS, it should not prevent you from designing proper link relationships in your APIs as stated in rules below.

We do not generally recommend to implement REST Maturity Level 3. HATEOAS comes with additional API complexity without real value in our SOA context where client and server interact via REST APIs and provide complex business functions as part of our e-commerce SaaS platform.

Our major concerns regarding the promised advantages of HATEOAS (see also RESTistential Crisis over Hypermedia APIs, Why I Hate HATEOAS and others for a detailed discussion):

When embedding links to other resources into representations you must use the common hypertext control object. It contains at least one attribute:

In API that contain any hypertext controls, the attribute name href is reserved for usage within hypertext controls.

The schema for hypertext controls can be derived from this model:

The name of an attribute holding such a HttpLink object specifies the relation between the object that contains the link and the linked resource. Implementations should use names from the IANA Link Relation Registry whenever appropriate. As IANA link relation names use hyphen-case notation, while this guide enforces snake_case notation for attribute names, hyphens in IANA names have to be replaced with underscores (e.g. the IANA link relation type version-history would become the attribute version_history)

Specific link objects may extend the basic link type with additional attributes, to give additional information related to the linked resource or the relationship between the source resource and the linked one.

E.g. a service providing "Person" resources could model a person who is married with some other person with a hypertext control that contains attributes which describe the other person (id, name) but also the relationship "spouse" between the two persons (since):

Hypertext controls are allowed anywhere within a JSON model. While this specification would allow HAL, we actually don’t recommend/enforce the usage of HAL anymore as the structural separation of meta-data and data creates more harm than value to the understandability and usability of an API.

For pagination and self-references a simplified form of the extensible common hypertext controls should be used to reduce the specification and cognitive overhead. It consists of a simple URI value in combination with the corresponding link relations, e.g. next, prev, first, last, or self.

See MUST use common hypertext controls and SHOULD use pagination links where applicable for more information and examples.

Links to other resource must always use full, absolute URI.

Motivation: Exposing any form of relative URI (no matter if the relative URI uses an absolute or relative path) introduces avoidable client side complexity. It also requires clarity on the base URI, which might not be given when using features like embedding subresources. The primary advantage of non-absolute URI is reduction of the payload size, which is better achievable by following the recommendation to use gzip compression

For flexibility and precision, we prefer links to be directly embedded in the JSON payload instead of being attached using the uncommon link header syntax. As a result, the use of the Link Header defined by RFC 8288 in conjunction with JSON media types is forbidden.

APIs should support techniques for reducing bandwidth based on client needs. This holds for APIs that (might) have high payloads and/or are used in high-traffic scenarios like the public Internet and telecommunication networks. Typical examples are APIs used by mobile web app clients with (often) less bandwidth connectivity.

Common techniques include:

Each of these items is described in greater detail below.

Compress the payload of your API’s responses with gzip, unless there’s a good reason not to — for example, you are serving so many requests that the time to compress becomes a bottleneck. This helps to transport data faster over the network (fewer bytes) and makes frontends respond faster.

Though gzip compression might be the default choice for server payload, the server should also support payload without compression and its client control via Accept-Encoding request header — see also RFC 7231 Section 5.3.4. The server should indicate used gzip compression via the Content-Encoding header.

Depending on your use case and payload size, you can significantly reduce network bandwidth need by supporting filtering of returned entity fields. Here, the client can explicitly determine the subset of fields he wants to receive via the fields query parameter. (It is analogue to GraphQL fields and simple queries, and also applied, for instance, for Google Cloud API’s partial responses.)

Embedding related resources (also know as Resource expansion) is a great way to reduce the number of requests. In cases where clients know upfront that they need some related resources they can instruct the server to prefetch that data eagerly. Whether this is optimized on the server, e.g. a database join, or done in a generic way, e.g. an HTTP proxy that transparently embeds resources, is up to the implementation.

See MUST stick to conventional query parameters for naming, e.g. "embed" for steering of embedded resource expansion. Please use the BNF grammar, as already defined above for filtering, when it comes to an embedding query syntax.

Embedding a sub-resource can possibly look like this where an order resource has its order items as sub-resource (/order/{orderId}/items):

Caching has to take many aspects into account, e.g. general cacheability of response information, our guideline to protect endpoints using SSL and OAuth authorization, resource update and invalidation rules, existence of multiple consumer instances. As a consequence, caching is in best case complex, e.g. with respect to consistency, in worst case inefficient.

As a consequence, client side as well as transparent web caching should be avoided, unless the service supports and requires it to protect itself, e.g. in case of a heavily used and therefore rate limited master data service, i.e. data items that rarely or not at all change after creation.

As default, API providers and consumers should always set the Cache-Control header set to Cache-Control: no-store and assume the same setting, if no Cache-Control header is provided.

Note: There is no need to document this default setting. However, please make sure that your framework is attaching this header value by default, or ensure this manually, e.g. using the best practice of Spring Security as shown below. Any setup deviating from this default must be sufficiently documented.

If your service really requires to support caching, please observe the following rules:

Hint: For proper cache support, you must return 304 without content on a failed HEAD or GET request with If-None-Match: <entity-tag> instead of 412.

Hint: For ETag source see MAY consider to support ETag together with If-Match/If-None-Match header.

The default setting for Cache-Control should contain the private directive for endpoints with standard OAuth authorization, as well as the must-revalidate directive to ensure, that the client does not use stale cache entries. Last, the max-age directive should be set to a value between a few seconds (max-age=60) and a few hours (max-age=86400) depending on the change rate of your master data and your requirements to keep clients consistent.

The default setting for Vary is harder to determine correctly. It highly depends on the API endpoint, e.g. whether it supports compression, accepts different media types, or requires other request specific headers. To support correct caching you have to carefully choose the value. However, a good first default may be:

Anyhow, this is only relevant, if you encourage clients to install generic HTTP layer client and proxy caches.

Note: generic client and proxy caching on HTTP level is hard to configure. Therefore, we strongly recommend to attach the (possibly distributed) cache directly to the service (or gateway) layer of your application. This relieves from interpreting the Vary header and greatly simplifies interpreting the Cache-Control and ETag headers. Moreover, is highly efficient with respect to caching performance and overhead, and allows to support more advanced cache update and warm up patterns.

Anyhow, please carefully read RFC 7234 before adding any client or proxy cache.

Access to lists of data items must support pagination to protect the service against overload as well as for best client side iteration and batch processing experience. This holds true for all lists that are (potentially) larger than just a few hundred entries.

There are two well known page iteration techniques:

The technical conception of pagination should also consider user experience related issues. As mentioned in this article, jumping to a specific page is far less used than navigation via next/prev page links (See SHOULD use pagination links where applicable). This favours cursor-based over offset-based pagination.

Note: To provide a consistent look and feel of pagination patterns, you must stick to the common query parameter names defined in MUST stick to conventional query parameters.

Cursor-based pagination is usually better and more efficient when compared to offset-based pagination. Especially when it comes to high-data volumes and/or storage in NoSQL databases.

Before choosing cursor-based pagination, consider the following trade-offs:

The cursor used for pagination is an opaque pointer to a page, that must never be inspected or constructed by clients. It usually encodes (encrypts) the page position, i.e. the identifier of the first or last page element, the pagination direction, and the applied query filters - or a hash over these - to safely recreate the collection (see also Cursor-based pagination in RESTful APIs).

For iterating over collections (result sets) we propose to either use cursors (see SHOULD prefer cursor-based pagination, avoid offset-based pagination) or simple hypertext controls (see SHOULD use pagination links where applicable). To implement these in a consistent way, we have defined a response page object pattern with the following field semantics:

Pagination responses should contain the following additional array field to transport the page content:

To simplify user experience, the applied query filters may be returned using the following field (see also GET with body):

As Result, the standard response page using cursors or pagination links may be defined as follows:

Note: While you may support cursors for next, prev, first, last, and self, it is best practice to replace these with links in favor of SHOULD use pagination links where applicable.

To simplify client design, APIs should support simplified hypertext controls for paginating over collections whenever applicable as follows (see also [pagination-fields] for details):

Remark: You should avoid providing a total count unless there is a clear need to do so. Very often, there are significant system and performance implications when supporting full counts. Especially, if the data set grows and requests become complex queries and filters drive full scans. While this is an implementation detail relative to the API, it is important to consider the ability to support serving counts over the life of a service.

Change APIs, but keep all consumers running. Consumers usually have independent release lifecycles, focus on stability, and avoid changes that do not provide additional value. APIs are contracts between service providers and service consumers that cannot be broken via unilateral decisions.

There are two techniques to change APIs without breaking them:

We strongly encourage using compatible API extensions and discourage versioning (see SHOULD avoid versioning and MUST use media type versioning below). The following guidelines for service providers (SHOULD prefer compatible extensions) and consumers (MUST prepare clients to accept compatible API extensions) enable us (having Postel’s Law in mind) to make compatible changes without versioning.

Note: There is a difference between incompatible and breaking changes. Incompatible changes are changes that are not covered by the compatibility rules below. Breaking changes are incompatible changes deployed into operation, and thereby breaking running API consumers. Usually, incompatible changes are breaking changes when deployed into operation. However, in specific controlled situations it is possible to deploy incompatible changes in a non-breaking way, if no API consumer is using the affected API aspects (see also Deprecation guidelines).

Hint: Please note that the compatibility guarantees are for the "on the wire" format. Binary or source compatibility of code generated from an API definition is not covered by these rules. If client implementations update their generation process to a new version of the API definition, it has to be expected that code changes are necessary.

API designers should apply the following rules to evolve RESTful APIs for services in a backward-compatible way:

Designers of service provider APIs should be conservative and accurate in what they accept from clients:

Not ignoring unknown input fields is a specific deviation from Postel’s Law (e.g. see also
The Robustness Principle Reconsidered) and a strong recommendation. Servers might want to take different approach but should be aware of the following problems and be explicit in what is supported:

In specific situations, where a (known) input field is not needed anymore, it either can stay in the API definition with "not used anymore" description or can be removed from the API definition as long as the server ignores this specific parameter.

Service clients should apply the robustness principle:

The OpenAPI specification is not very specific on default extensibility of objects, and redefines JSON-Schema keywords related to extensibility, like additionalProperties. Following our compatibility guidelines, OpenAPI object definitions are considered open for extension by default as per Section 5.18 "additionalProperties" of JSON-Schema.

When it comes to OpenAPI, this means an additionalProperties declaration is not required to make an object definition extensible:

API formats must not declare additionalProperties to be false, as this prevents objects being extended in the future.

Note that this guideline concentrates on default extensibility and does not exclude the use of additionalProperties with a schema as a value, which might be appropriate in some circumstances, e.g. see SHOULD define maps using additionalProperties.

When changing your RESTful APIs, do so in a compatible way and avoid generating additional API versions. Multiple versions can significantly complicate understanding, testing, maintaining, evolving, operating and releasing our systems (supplementary reading).

If changing an API can’t be done in a compatible way, then proceed in one of these three ways:

As we discourage versioning by all means because of the manifold disadvantages, we strongly recommend to only use the first two approaches.

Tip: versioning should always be the last resort and decided with peers.

However, when API versioning is unavoidable, you have to design your multi-version RESTful APIs using media type versioning (see MUST not use URL versioning). Media type versioning is less tightly coupled since it supports content negotiation and hence reduces complexity of release management.

Version information and media type are provided together via the HTTP Content-Type header — e.g. application/vendor.cart+json;version=2. For incompatible changes, a new media type version for the resource is created. To generate the new representation version, consumer and producer can do content negotiation using the HTTP Content-Type and Accept headers.

With URL versioning a (major) version number is included in the path, e.g. /v1/customers. The consumer has to wait until the provider has been released and deployed. If the consumer also supports hypermedia links — even in their APIs — to drive workflows (HATEOAS), this quickly becomes complex. So does coordinating version upgrades — especially with hyperlinked service dependencies — when using URL versioning. To avoid this tighter coupling and complexer release management we do not use URL versioning, instead we MUST use media type versioning with content negotiation.

In a response body, you must always return a JSON object (and not e.g. an array) as a top level data structure to support future extensibility. JSON objects support compatible extension by additional attributes. This allows you to easily extend your response and e.g. add pagination later, without breaking backwards compatibility. See SHOULD use pagination links where applicable for an example.

Maps (see SHOULD define maps using additionalProperties), even though technically objects, are also forbidden as top level data structures, since they don’t support compatible, future extensions.

Enumerations are per definition closed sets of values, that are assumed to be complete and not intended for extension. This closed principle of enumerations imposes compatibility issues when an enumeration must be extended. To avoid these issues, we strongly recommend to use an open-ended list of values instead of an enumeration unless:

To specify an open-ended list of values use the marker x-extensible-enum as follows:

Note: x-extensible-enum is not JSON Schema conform but will be ignored by most tools.

See SHOULD declare enum values using UPPER_SNAKE_CASE string about enum value naming conventions.

The API deprecation must be part of the API specification.

If an API endpoint (operation object), an input argument (parameter object), an in/out data object (schema object), or on a more fine grained level, a schema attribute or property should be deprecated, the producers must set deprecated: true for the affected element and add further explanation to the description section of the API specification. If a future shut down is planned, the producer must provide a sunset date and document in details what consumers should use instead and how to migrate.

Before shutting down an API, version of an API, or API feature the producer must make sure, that all clients have given their consent on a sunset date. Producers should help consumers to migrate to a potential new API or API feature by providing a migration manual and clearly state the time line for replacement availability and sunset (see also SHOULD add Deprecation and Sunset header to responses). Once all clients of a sunset API feature are migrated, the producer may shut down the deprecated API feature.

If the API is consumed by any external partner, the API owner must define a reasonable time span that the API will be maintained after the producer has announced deprecation. All external partners must state consent with this after-deprecation-life-span, i.e. the minimum time span between official deprecation and first possible sunset, before they are allowed to use the API.

Owners of an API, API version, or API feature used in production that is scheduled for sunset must monitor the usage of the sunset API, API version, or API feature in order to observe migration progress and avoid uncontrolled breaking effects on ongoing consumers. See also SHOULD monitor API usage.

During the deprecation phase, the producer should add a Deprecation: <date-time> (see draft: RFC Deprecation HTTP Header) and - if also planned - a Sunset: <date-time> (see RFC 8594) header on each response affected by a deprecated element (see MUST reflect deprecation in API specifications).

The Deprecation header can either be set to true - if a feature is retired -, or carry a deprecation time stamp, at which a replacement will become/became available and consumers must not on-board any longer (see MUST not start using deprecated APIs). The optional Sunset time stamp carries the information when consumers latest have to stop using a feature. The sunset date should always offer an eligible time interval for switching to a replacement feature.

If multiple elements are deprecated the Deprecation and Sunset headers are expected to be set to the earliest time stamp to reflect the shortest interval consumers are expected to get active.

Note: adding the Deprecation and Sunset header is not sufficient to gain client consent to shut down an API or feature.

Hint: In earlier guideline versions, we used the Warning header to provide the deprecation info to clients. However, Warning header has a less specific semantics, will be obsolete with draft: RFC HTTP Caching, and our syntax was not compliant with RFC 7234 — Warning header.

Clients should monitor the Deprecation and Sunset headers in HTTP responses to get information about future sunset of APIs and API features (see SHOULD add Deprecation and Sunset header to responses). We recommend that client owners build alerts on this monitoring information to ensure alignment with service owners on required migration task.

Hint: In earlier guideline versions, we used the Warning header to provide the deprecation info (see hint in SHOULD add Deprecation and Sunset header to responses).

Clients must not start using deprecated APIs, API versions, or API features.

All service applications must publish OpenAPI specifications of their external APIs. While this is optional for internal APIs, i.e. APIs marked with the component-internal API audience group, we still recommend to do so to profit from the API management infrastructure.

Note: To publish an API, it is still necessary to deploy the artifact successful, as we focus the discovery experience on APIs supported by running services.

Owners of APIs used in production should monitor API service to get information about its using clients. This information, for instance, is useful to identify potential review partner for API changes.

Hint: A preferred way of client detection implementation is by logging of the client-id retrieved from the OAuth token.

The following frameworks were specifically designed to support the API First workflow with OpenAPI YAML files (sorted alphabetically):

The Swagger/OpenAPI homepage lists more Community-Driven Language Integrations, but most of them do not fit our API First approach.

These utility libraries support you in implementing various parts of our RESTful API guidelines (sorted alphabetically):

Cursor-based pagination is a very powerful and valuable technique (see also SHOULD prefer cursor-based pagination, avoid offset-based pagination, that allows to efficiently provide a stable view on changing data. This is obtained by using an anchor element that allows to retrieve all page elements directly via an ordering combined-index, usually based on created_at or modified_at. Simple said, the cursor is the information set needed to reconstruct the database query to retrieves the minimal page information from the data storage.

The cursor itself is an opaque string, transmitted forth and back between service and clients, that must never be inspected or constructed by clients. Therefore, it is good practice to encode (encrypt) its content in a non-human-readable form.

The cursor content usually consists of a pointer to the anchor element defining the page position in the collection, a flag whether the element is included or excluded into/from the page, the retrieval direction, and a hash over the applied query filters (or the query filter itself) to safely re-create the collection. It is important to note, that a cursor should be always defined in relation to the current page to anticipate all occurring changes when progressing.

The cursor is usually defined as an encoding of the following information:

Note: In case of complex and long search requests, e.g. when GET with body is already required, the cursor may not be able to include the query because of common HTTP parameter size restrictions. In this case the query filters should be transported via body - in the request as well as in the response, while the pagination consistency should be ensured via the query_hash.

Remark: It is also important to check the efficiency of the data-access. You need to make sure that you have a fully ordered stable index, that allows to efficiently resolve all elements of a page. If necessary, you need to provide a combined index that includes the id to ensure the full order and additional filter criteria to ensure efficiency.

MUST publish OpenAPI specification, Removed Zalando specific part