In cases when breaking API changes are deemed unavoidable, the API evangelist and API champion have to be involved in the change. Without their explicit permission breaking changes must not be published.

TODO Establish API evangelist and API champion roles and make teams aware of the roles and how to approach breaking API changes.

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.

Comparing SOA web service interfacing style of SOAP vs. REST, the former tends 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 We apply the RESTful web service principles to all kinds 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: Some interesting reads on the RESTful API design style and service architecture:

A public API doesn’t mean that it is openly accessible by anybody. Depending on the service and the data, the consumer must follow a certain registration and validation process. Furthermore, there will be restricted APIs which will be only available for regulated companies - this will be handled via API audiences (see rule 219)

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.

Understand the specifc 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.

For newly developed APIs or extensions of an existing API you must follow an API first approach, which bascially requires two aspects:

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

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

Elements of 'API First' are also these 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 the development of client applications decoupled from the 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 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 entire 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 Open API 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. For any newly developed services you must use Open API 3.0, existing services that already use Open API 2.0 (a.k.a. Swagger 2) are for now still supported.

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

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

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.

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, GHE 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.

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

Following Open API extension properties must be provided in addition:

Open API 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 Open API specification and must conform to the following definition:

API specifications will evolve and any aspect of an Open API 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:

TODO Establish API audiences suitable for VIG, viesure and twinformatics

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:

Every API endpoint needs to be secured using OAuth 2.0 and an API key. Please refer to the Authentication section of the official Open API specification on how to specify security definitions in your API.

The following code snippet shows how to define the authorization OAuth2 and API Key

The next code snippet applies this security scheme to all API endpoints. The bearer token of the client must have additionally the scopes scope_1 and scope_2.

APIs must define permissions to protect their resources. Thus, at least one permission must be assigned to each endpoint. Permissions are defined as shown in the previous section.

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

APIs should stick to component-specific permissions without resource extension to avoid governance complexity of too many fine grained permissions. For the majority of use cases, restricting access to specific API endpoints using read and write is sufficient for controlling access for client types like merchant or retailer business partners, customers or operational staff. However, in some situations, where the API serves different types of resources for different owners, resource-specific scopes may make sense.

Some examples for standard and resource-specific permissions:

After permission names are defined and the permission is declared in the security definition at the top of an API specification, it must be assigned to each API operation by specifying a security requirement like this:

As long as the functional naming is not supported for permissions, permission names in APIs must conform to the following naming pattern:

This pattern is compatible with the previous definition.

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 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 newer 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:

Service clients should apply the robustness principle:

Service clients must be prepared for compatible API extensions of service providers:

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 a 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.

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.

The Open API 2.0 specification is not very specific on default extensibility of objects, and redefines JSON-Schema keywords related to extensibility, like additionalProperties. Following our overall compatibility guidelines, Open API object definitions are considered open for extension by default as per Section 5.18 "additionalProperties" of JSON-Schema.

When it comes to Open API 2.0, 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.

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.

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.

However, when API versioning is unavoidable, you have to design your multi-version RESTful APIs using media type versioning (instead of URI versioning, see below). Media type versioning is less tightly coupled since it supports content negotiation and hence reduces complexity of release management.

Media type versioning: Here, version information and media type are provided together via the HTTP Content-Type header — e.g. application/x.vig.contract+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. Note: This versioning only applies to the request and response content schema, not to URI or method semantics.

In this example, a client wants only the new version of the response:

A server responding to this, as well as a client sending a request with content should use the Content-Type header, declaring that one is sending the new version:

Using header versioning should:

Hint: Until an incompatible change is necessary, it is recommended to stay with the standard application/json media type.

Further reading: API Versioning Has No "Right Way" provides an overview on different versioning approaches to handle breaking changes without being opinionated.

With URI 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 URI versioning, and go instead with media type versioning and content negotiation (see above).

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 timeline 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.

The minimum lifetime of an API is set to 1 year and the initial shutdown must be communicated at least 6 month in advance. Any deviations from those timeframes have to be cleared by the API Champion.

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.

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 shutdown is planned, the producer must provide a sunset date and document in detail what consumers should use instead and how to migrate.

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 MUST 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 have to stop using a feature the latest. 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, the Warning header has 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.

Property names are restricted to ASCII strings. The first character must be a letter, or an underscore, and subsequent characters can be a letter, an underscore, or a number.

(It is recommended to use _ at the start of property names only for keywords like _links.)

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 so that JSON code looks as if written by the same person.

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.

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 Open API 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:

To indicate they contain multiple values prefer to pluralize array names. This implies that object names should in turn be singular.

Schema-based JSON properties that are boolean by design 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.

Open API 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 the 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 the JSON Merge Patch RFC 7396) which uses null to explicitly indicate property deletion while absent properties are ignored, i.e. not modified.

Optional fields should be declared as nullable: true. The reason behind this is to not differentiate between absent ({}) and null ({"example": null}) properties and therefore, preventing confusion about their semantics. Moreover, Jackson serializes null properties as {"example": null} by default.

An empty string ("") must be treated the same as any other string value, and is not an indication for an absent or null property.

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

Strings are a reasonable target for values that are enumerations by design.

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. An exception to this suffix is birthdate.

Examples:

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

Use the date and time formats defined by RFC 3339:

Note that the Open API format "date-time" corresponds to "date-time" in the RFC) and 2015-05-28 for a date (note that the Open API format "date" corresponds to "full-date" in the RFC). Both are specific profiles, a subset of the international standard ISO 8601.

A zone offset may be used (both, in request and responses) — this is simply defined by the standards. However, we encourage restricting dates to UTC and without 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 handled correctly. Note also that zone offsets are different from local times that might be including daylight saving time. Localization of dates should be done by the services that provide user interfaces, if required.

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.

Sometimes it can seem data is naturally represented using numerical timestamps, but this can introduce interpretation 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 recommended by ISO 8601(Appendix A of RFC 3339 contains a grammar for durations).

Use JSON-encoded body payloads for transferring structured data. The JSON payload must follow RFC 7159 using a JSON object as top level data structure (if possible) to allow for future extension. This also applies to collection resources, where you would naturally assume an array. See also MUST always return JSON objects as top-level data structures.

Additionally, the JSON payload must comply to RFC 7493), particularly

As a consequence, a JSON payload must

Other media types may be used in following cases:

Exposing binary data using an alternative media type is generally preferred. See the rule above.

If an alternative content representation is not desired, the binary data should be embedded into the JSON document as a base64url-encoded string property following RFC 7493 Section 4.4.

Use the standard media type name application/json (or application/problem+json for MUST use problem JSON).

Custom media types beginning with x bring no advantage compared to the standard media type for JSON, and make automated processing more difficult. They are also discouraged by RFC 6838.

JSON Schema and Open API define several universally useful property formats. The following table contains some additional formats that are particularly useful in an e-commerce environment.

Please notice that the list is not exhaustive and everyone is encouraged to propose additions.

Use the following standard formats for country, language and currency codes:

Whenever an API defines a property of type number or integer, the precision must be defined by the format as follows to 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. E.g. for the following definitions the most specific language types in Java will translate to BigDecimal for Money.amount and int or Integer for the OrderList.page_size:

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:

There exist a variety of field types that are required in multiple places. To achieve consistency across all API implementations, you must use common field names and semantics whenever applicable.

Example:

This applies to specifc path segments and not the names of path parameters. For example {shipment_order_id} would be ok as a path parameter.

Examples:

This is for consistency in your documentation (most other headers follow this convention). Avoid camelCase (without hyphens). Exceptions are common abbreviations like "ID."

Examples:

See also: HTTP Headers are case-insensitive (RFC 7230).

See Common headers and Proprietary headers sections for more guidance on HTTP headers.

Usually, a collection of resource instances is provided (at least API should be ready here). The special case of a resource singleton is a collection with cardinality 1. Exceptions include: * "payment" * "status" * "vat"

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 part of the deployment variant configuration. Therefore, this information has to be declared in the server object.

The trailing slash must not have specific semantics. Resource paths must deliver the same results whether they have the trailing slash or not.

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

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.

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.

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.

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.

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.

To simplify encoding of resource IDs in URLs, their representation must 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.

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. E.g. if /customers/12ev123bv12v/addresses/DE_100100101 is valid, then /customers/12ev123bv12v/addresses, /customers/12ev123bv12v and /customers must be valid as well in principle. E.g.:

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

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:

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.

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 the 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 sub-domains 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.)

Be compliant with the standardized HTTP method semantics 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 patterns 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 a 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 Open API 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 maximum URL length.

We prefer using 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 Open API directly. This 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 implicit filtering must be in the API specification (in its description).

Consider caching considerations when implicitly filtering.

Example:

If an employee of the company Foo accesses one of our business-to-business services 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 non-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 standardized HTTP status codes consistently with their intended semantics. You must not invent new HTTP status codes.

RFC standards define ~60 different HTTP status codes with specific semantics (mainly RFC7231 and RFC 6585) — and there are upcoming new ones, e.g. draft legally-restricted-status. See overview of all error codes on Wikipedia or via https://httpstatuses.com/) also inculding 'unofficial codes', e.g. used by popular web servers like Nginx.

Below we list the most commonly used and best understood HTTP status codes, consistent with their semantic in the RFCs. APIs should only use these to prevent misconceptions that arise from less commonly used HTTP status codes.

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 shortcut 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 may 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 and the media type application/problem+json. Operations should return it (together with a suitable status code) when any problem occurred during processing and you can give more details than the status code itself can supply, whether it be caused by the client or the server (i.e. both for 4xx or 5xx error codes).

For the Open API schema definition of the Problem JSON object see on gitlab. You can reference it by using:

To communicate to the API consumer which parameters cased the problem, you can extend the Problem type by an invalid_params field.

The sub-fields "type", "field", and "message" are mandatory for the schema and additional fields may be added. The possible enum values of type can be modified.

You may optionally extend the Problem JSON by the field error_code. This field a system wide unique code for machines that identifies this problem type.

The sub-fields "type", "description", and "x-extensible-enum" are mandatory for the schema and additional fields may be added. The possible enum values of type can be modified.

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

Problem type identifiers in our APIs are not meant to be resolved. The RFC encourages that custom problem types are URI references that point to human readable documentation, but we deliberately decided against that. URLs tend to be fragile and not very stable over a longer period. Hosting documentation often requires to bind to a specific tool or have DNS records that contain volatile organization structures, e.g. team names. Another reason is that all the important parts of an API must be documented using OpenAPI anyway.

In order to stay compatible the proposed pattern for custom problem types is to use relative URI references:

Examples of problem types that do not satisfy our criteria:

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.

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 the network bandwidth needs by supporting filtering of returned entity fields. Here, the client can explicitly determine the subset of fields it wants to receive via the fields query parameter. (It is analog 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 the best case complex, e.g. with respect to consistency, in the 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 change rarely or not at all 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 attaches 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 requires caching support, 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 navigating 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.

When using pagination make sure to conform to the standardized pagination. A copy of the schema can be found on gitlab or below. All fields except sort are mandatory, but you may extend the schema with custom fields.

This may look as follows:

To simplify client design, APIs should support simplified hypertext controls for pagination over collections whenever applicable. Beside next this may comprise the support for prev, first, last, and self as link relations (see also Link relation fields for details).

The page content is transported via items, while the query object may contain the query filters applied to the collection resource as follows:

Note: In case of complex search requests, e.g. when GET With Body is required, the cursor may not be able to encode all query filters. In this case, it is best practice to encode only page position and direction in the cursor and transport the query filter in the body - in the request as well as in the response. To protect the pagination sequence, in this case it is recommended, that the cursor contains a hash over all applied query filters for pagination request validation.

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.

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 the rules below.

Links to other resources must always use full, absolute URIs.

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 sub-resources. The primary advantage of non-absolute URIs is reduction of the payload size, which is better achievable by following the recommendation to use gzip compression

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:

Use this list and mention its support in your Open API definition.

The Content-Location header is optional and 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:

As the correct usage of Content-Location with respect to semantics and caching is difficult, we discourage the use of Content-Location. In most cases it is sufficient to direct clients to the resource location by using the Location header instead without hitting the Content-Location-specific ambiguities and complexities.

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

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 accomplished best by supporting the ETag header together with the If-Match or If-None-Match conditional header. The content 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 conforms 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 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 receiving 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 protect 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, as it fits not into our section about Proprietary headers and we did not want to change the header name and semantic, we decided to treat it as any other common header.

As a general rule, proprietary HTTP headers should be avoided. However, they can still be useful in cases where context needs to be passed through multiple services in an end-to-end fashion. As such, a valid use case for a proprietary header is providing context information, which is not a part of the actual API, but is needed by subsequent communication.

From a conceptual point of view, the semantics and intent of an operation should always be expressed by URLs path and query parameters, the method, and the content. Headers are more often used to implement functions close to the protocol considerations, such as flow control, content negotiation, and authentication. Thus, headers are reserved for general context information (RFC 7231).

X- headers were initially reserved for unstandardized parameters, but the usage of X- headers is deprecated (RFC 6648). This complicates the contract definition between consumer and producer of an API following these guidelines, since there is no aligned way of using those headers. Because of this, these guidelines restrict which X- headers can be used and how they are used.

The Internet Engineering Task Force’s states in RFC 6648 that company-specific header names should incorporate the organization’s name. We aim for backward compatibility, and therefore keep the X- prefix.

The following proprietary headers have been specified by this guideline for usage so far. Remember that HTTP header field names are not case-sensitive.

Exception: The only exception to this guideline are the conventional hop-by-hop X-RateLimit- headers which can be used as defined in MAY use code 429 with headers for rate limits.

All service applications must publish Open API 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.

Owners of APIs used in production must monitor API services to get information about the clients' usage. This information, for instance, is useful to identify potential review partners for API changes.

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

Versioning changes are generally related to the increase or reduction of the quantity or granularity of constraints.

The use of the terms “fine-grained” and “coarse-grained” is highly subjective. What may be a fine-grained constraint in one contract may not be in another. The point is to understand how these terms can be applied when comparing parts of a message definition or when comparing different message definitions with each other.

The number one concern when developing and deploying a new version of a service contract is the impact it will have on other parts of the enterprise that have formed or will form dependencies on it. This measure of impact is directly related to how compatible the new contract version is with the old version and its surroundings in general. This section establishes the fundamental types of compatibility that relate to the content and design of new contract versions.

In the contract-first web service, the "contract" (a WSDL definition of operations and endpoints and XML schema of the messages) is created first, without actually writing any service code.

As described in Strategy #2: The Flexible Strategy (Backward Compatibility)

As described in Strategy #3: The Loose Strategy (Backward and Forward Compatibility)

As described in Strategy #1: The Strict Strategy (New Change, New Contract)

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

The Swagger/Open API 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):