API styleguide

A place to define the conventions we use to build APIs

• API styleguide
  • Basic Tools
  • Localization
  • Heroku conventions
    • Foundations
      • Separate Concerns
      • Require Secure Connections
      • Support ETags for Caching
    • Requests
      • Accept serialized JSON in request bodies
      • Use consistent path formats
        • Resource names
      • Minimize path nesting
    • Responses
      • Return appropriate status codes
      • Provide full resources where available
      • Provide standard timestamps
      • Use UTC times formatted in ISO8601
      • Nest foreign key relations
      • Generate structured errors
      • Show rate limit status
      • Keep JSON minified in all responses

Basic Tools

It's quite common in Hyper to build APIs with Ruby on Rails, so most of these conventions are designed to work well with Rails.

Localization

When the responses need to be localized, it's recommended for the client to send the Accept-Language header:

Accept-Language: nb-no, no, en

Heroku conventions

We take in account most of Heroku's conventions for building APIs.

Foundations

Separate Concerns

Keep things simple while designing by separating the concerns between the different parts of the request and response cycle. Keeping simple rules here allows for greater focus on larger and harder problems.

Requests and responses will be made to address a particular resource or collection. Use the path to indicate identity, the body to transfer the contents and headers to communicate metadata. Query params may be used as a means to pass header information also in edge cases, but headers are preferred as they are more flexible and can convey more diverse information.

Require Secure Connections

Require secure connections with TLS to access the API, without exception. It’s not worth trying to figure out or explain when it is OK to use TLS and when it’s not. Just require TLS for everything.

Ideally, simply reject any non-TLS requests by not responding to requests for http or port 80 to avoid any insecure data exchange. In environments where this is not possible, respond with 403 Forbidden.

Redirects are discouraged since they allow sloppy/bad client behaviour without providing any clear gain. Clients that rely on redirects double up on server traffic and render TLS useless since sensitive data will already have been exposed during the first call.

Support ETags for Caching

Include an ETag header in all responses, identifying the specific version of the returned resource. This allows users to cache resources and use requests with this value in the If-None-Match header to determine if the cache should be updated.

Requests

Accept serialized JSON in request bodies

Accept serialized JSON on PUT/PATCH/POST request bodies, either instead of or in addition to form-encoded data. This creates symmetry with JSON-serialized response bodies, e.g.:

$ curl -X POST https://service.com/apps \
    -H "Content-Type: application/json" \
    -d '{"name": "demoapp"}'

{
  "id": "01234567-89ab-cdef-0123-456789abcdef",
  "name": "demoapp",
  "owner": {
    "email": "username@example.com",
    "id": "01234567-89ab-cdef-0123-456789abcdef"
  },
  ...
}

Use consistent path formats

Resource names

Use the plural version of a resource name unless the resource in question is a singleton within the system (for example, in most systems a given user would only ever have one account). This keeps it consistent in the way you refer to particular resources.

Minimize path nesting

In data models with nested parent/child resource relationships, paths may become deeply nested, e.g.:

/orgs/{org_id}/apps/{app_id}/dynos/{dyno_id}

Limit nesting depth by preferring to locate resources at the root path. Use nesting to indicate scoped collections. For example, for the case above where a dyno belongs to an app belongs to an org:

/orgs/{org_id}
/orgs/{org_id}/apps
/apps/{app_id}
/apps/{app_id}/dynos
/dynos/{dyno_id}

Responses

Return appropriate status codes

Return appropriate HTTP status codes with each response. Successful responses should be coded according to this guide:

• 200: Request succeeded for a GET call, for a DELETE or PATCH call that completed synchronously, or for a PUT call that synchronously updated an existing resource
• 201: Request succeeded for a POST call that completed synchronously, or for a PUT call that synchronously created a new resource
• 202: Request accepted for a POST, PUT, DELETE, or PATCH call that will be processed asynchronously
• 206: Request succeeded on GET, but only a partial response returned: see above on ranges

Pay attention to the use of authentication and authorization error codes:

• 401 Unauthorized: Request failed because user is not authenticated
• 403 Forbidden: Request failed because user does not have authorization to access a specific resource

Return suitable codes to provide additional information when there are errors:

• 422 Unprocessable Entity: Your request was understood, but contained invalid parameters
• 429 Too Many Requests: You have been rate-limited, retry later
• 500 Internal Server Error: Something went wrong on the server, check status site and/or report the issue

Refer to the HTTP response code spec for guidance on status codes for user error and server error cases.

Provide full resources where available

Provide the full resource representation (i.e. the object with all attributes) whenever possible in the response. Always provide the full resource on 200 and 201 responses, including PUT/PATCH and DELETE requests, e.g.:

$ curl -X DELETE \  
  https://service.com/apps/1f9b/domains/0fd4

HTTP/1.1 200 OK
Content-Type: application/json;charset=utf-8
...
{
  "created_at": "2012-01-01T12:00:00Z",
  "hostname": "subdomain.example.com",
  "id": "01234567-89ab-cdef-0123-456789abcdef",
  "updated_at": "2012-01-01T12:00:00Z"
}

202 responses will not include the full resource representation, e.g.:

$ curl -X DELETE \  
  https://service.com/apps/1f9b/dynos/05bd

HTTP/1.1 202 Accepted
Content-Type: application/json;charset=utf-8
...
{}

Provide standard timestamps

Provide created_at and updated_at timestamps for resources by default, e.g:

{
  // ...
  "created_at": "2012-01-01T12:00:00Z",
  "updated_at": "2012-01-01T13:00:00Z",
  // ...
}

These timestamps may not make sense for some resources, in which case they can be omitted.

Use UTC times formatted in ISO8601

Accept and return times in UTC only. Render times in ISO8601 format, e.g.:

"finished_at": "2012-01-01T12:00:00Z"

Nest foreign key relations

Serialize foreign key references with a nested object, e.g.:

{
  "name": "service-production",
  "owner": {
    "id": "5d8201b0..."
  },
  // ...
}

Instead of e.g.:

{
  "name": "service-production",
  "owner_id": "5d8201b0...",
  // ...
}

This approach makes it possible to inline more information about the related resource without having to change the structure of the response or introduce more top-level response fields, e.g.:

{
  "name": "service-production",
  "owner": {
    "id": "5d8201b0...",
    "name": "Alice",
    "email": "alice@heroku.com"
  },
  // ...
}

Generate structured errors

Generate consistent, structured response bodies on errors. Include a machine-readable error id, a human-readable error message, and optionally a url pointing the client to further information about the error and how to resolve it, e.g.:

HTTP/1.1 429 Too Many Requests

{
  "id":      "rate_limit",
  "message": "Account reached its API rate limit.",
  "url":     "https://docs.service.com/rate-limits"
}

Document your error format and the possible error ids that clients may encounter.

Show rate limit status

Rate limit requests from clients to protect the health of the service and maintain high service quality for other clients. You can use a token bucket algorithm to quantify request limits.

Return the remaining number of request tokens with each request in the RateLimit-Remaining response header.

Keep JSON minified in all responses

Extra whitespace adds needless response size to requests, and many clients for human consumption will automatically "prettify" JSON output. It is best to keep JSON responses minified e.g.:

{"beta":false,"email":"alice@heroku.com","id":"01234567-89ab-cdef-0123-456789abcdef","last_login":"2012-01-01T12:00:00Z","created_at":"2012-01-01T12:00:00Z","updated_at":"2012-01-01T12:00:00Z"}

Instead of e.g.:

{
  "beta": false,
  "email": "alice@heroku.com",
  "id": "01234567-89ab-cdef-0123-456789abcdef",
  "last_login": "2012-01-01T12:00:00Z",
  "created_at": "2012-01-01T12:00:00Z",
  "updated_at": "2012-01-01T12:00:00Z"
}