HTTP API Design Standards

Guidelines

This document provides guidelines and examples for GoCardless APIs, encouraging consistency, maintainability, and best practices.

Sources:

• https://github.com/interagent/http-api-design
• https://www.gov.uk/service-manual/making-software/apis.html
• http://www.mnot.net/blog/
• http://www.vinaysahni.com/best-practices-for-a-pragmatic-restful-api
• https://github.com/WhiteHouse/api-standards
• http://apigee.com/about/content/api-fa%C3%A7ade-pattern
• https://pages.apigee.com/web-api-design-ebook.html
• https://groups.google.com/forum/#!forum/api-craft

JSON API

All endpoints must follow the core JSON API spec

Changes from JSON API:

• The primary resource must be keyed by its resource type. The endpoint URL must also match the resource type.
• API errors do not currently follow the JSON API spec.
• Updates should always return 200 OK with the full resource to simplify internal logic.

Example of keying the resource by type:

GET /posts/1

{
  "posts": {
    "id": 1
  }
}

All action calls to an endpoint must be wrapped in a data envelope. This is because actions are carried out on a resource but use a different data type than the core resource itself.

Example of a action call:

POST /posts/action/share

{
  "data": {
    "recipientID": "U123"
  }
}

JSON only

The API should only support JSON.

Reference: http://www.mnot.net/blog/2012/04/13/json_or_xml_just_decide

General guidelines

• A URL identifies a resource.
• URLs should include nouns, not verbs.
• For consistency, only use plural nouns (e.g. "posts" instead of "post").
• Use HTTP verbs (GET, POST, PUT, DELETE) to operate on resources.
• Use filtering instead of nested resources: /payments?subscription=xyz rather than /subscriptions/xyz/payments. Nested resources enforce relationships that could change and makes clients harder to write.
• API versions should be represented as dates documented in a changelog. Version number should not be in the URL.
• API should be behind a subdomain: api.gocardless.com

RESTful URLs

Good URL examples

• List of payments:
  • GET https://api.gocardless.com/payments
• Filtering is a query:
  • GET https://api.gocardless.com/payments?status=failed&sort_field=-created
  • GET https://api.gocardless.com/payments?sort_field=created
• A single payment:
  • GET https://api.gocardless.com/payments/1234
• All amendments in (or belonging to) this subscription:
  • GET https://api.gocardless.com/subscription_amendments?subscription=1234
• Include nested resources in a comma separated list:
  • GET https://api.gocardless.com/payments/1234?include=events
• Include only selected fields in a comma separated list:
  • GET https://api.gocardless.com/payments/1234?fields=amount
• Get multiple resources:
  • GET https://api.gocardless.com/payments/1234,444,555,666
• Action on resource:
  • POST https://api.gocardless.com/payments/1234/actions/cancel

Bad URL examples

• Singular nouns:
  • GET https://api.gocardless.com/payment
  • GET https://api.gocardless.com/payment/123
  • GET https://api.gocardless.com/payment/action
• Verbs in the URL:
  • GET https://api.gocardless.com/payment/create
• Nested resources:
  • GET https://api.gocardless.com/subscriptions/1234/amendments
• Filtering outside of query string:
  • GET https://api.gocardless.com/payments/desc
• Filtering to get multiple resources:
  • GET https://api.gocardless.com/payments?id[]=11&id[]=22

HTTP verbs

Here's an example of how HTTP verbs map to create, read, update, delete operations in a particular context:

Actions

Avoid resource actions. Create separate resources where possible.

Good

POST /refunds?payment=ID&amount=1000

Bad

POST /payments/ID/refund

Where special actions are required, place them under an actions prefix. Actions should always be idempotent.

POST /payments/ID/actions/cancel

Responses

Don’t set values in keys.

Good

"tags": [
  {"id": "125", "name": "Environment"},
  {"id": "834", "name": "Water Quality"}
]

Bad

"tags": [
  {"125": "Environment"},
  {"834": "Water Quality"}
]

String IDs

Always return string ids. Some languages, like JavaScript, don't support big ints. Serialize/deserialize ints to strings if storing ids as ints.

Error handling

Error responses should include a message for the user, an internal error type (corresponding to some specific internally determined constant represented as a string), and links to info for developers.

There must only be one top level error. Errors should be returned in turn. This makes internal error logic and dealing with errors as a consumer of the API easier.

Validation and resource errors are nested in the top level error under errors.

The error is nested in error to make it possible to add, for example, deprecation errors on successful requests.

The HTTP status code is used as a top level error, type is used as a sub error, and nested errors may have more specific type errors, such as invalid_field.

Formating errors vs integration errors

Formatting errors should be separate from errors handled by the integration.

Formatting errors include things like field presence and length, and are returned when incorrect data is sent to the API.

An example of an error that should be handled by the integration is attempting to create a payment against a mandate that has expired. This is an edge case that the API integration needs to handle. Do not mask these errors as validation errors. Return them as a top level error instead.

Top level error

• Top level errors MUST implement request_id, type, code and message.
• type MUST relate to the reason. For example, use it to categorise the error: reason: api_error.
• message MUST be specific.
• Top level errors MAY implement documentation_url, request_url and id.
• Only return id for server errors (5xx). The id should point to the exception you track internally.

{
  "error": {
    "documentation_url": "https://api.gocardless.com/docs/beta/errors#access_forbidden",
    "request_url": "https://api.gocardless.com/requests/REQUEST_ID",
    "request_id": "REQUEST_ID",
    "id": "ERROR_ID",
    "type": "access_forbidden",
    "code": 403,
    "message": "You don't have the right permissions to access this resource"
  }
}

Nested errors

• Nested errors MUST implement reason and message.
• reason MUST be specific to the error.
• Nested errors MAY implement field.

{
  "error": {
    "top level errors": "...",

    "errors": [
      {
        "field": "account_number",
        "reason": "missing_field",
        "message": "Account number is required"
      }
    ]
  }
}

HTTP status code summary

• 200 OK - everything worked as expected.
• 400 Bad Request - e.g. invalid JSON.
• 401 Unauthorized - no valid API key provided.
• 402 Request Failed - parameters were valid but request failed.
• 403 Forbidden - missing or invalid permissions.
• 404 Not Found - the requested item doesn’t exist.
• 422 Unprocessable Entity - parameters were invalid/validation failed.
• 500, 502, 503, 504 Server errors - something went wrong on GoCardless’ end.

400 Bad Request

• When the request body contains malformed JSON.
• When the JSON is valid but the document structure is invalid (e.g. passing an array when an object should be passed).

422 Unprocessable Entity

• When model validations fail for fields (e.g. name too long).
• Trying to create a resource when a related resource is in a bad state.

What changes are considered “backwards-compatible”?

• Adding new API resources.
• Adding new optional request parameters to existing API methods.
• Adding new properties to existing API responses.
• Changing the order of properties in existing API responses.
• Changing the length or format of object IDs or other opaque strings.
  • This includes adding or removing fixed prefixes (such as ch_ on charge IDs).
  • You can safely assume object IDs we generate will never exceed 128 characters, but you should be able to handle IDs of up to that length. If for example you’re using MySQL, you should store IDs in a VARCHAR(128) COLLATE utf8_bin column (the COLLATE configuration ensures case-sensitivity in lookups).
• Adding new event types. Your webhook listener should gracefully handle unfamiliar events types.

Versioning changes

The versioning scheme is designed to promote incremental improvement to the API and discourage rewrites.

Server initiated events such as webhooks should not contain serialised resources. If a resource changed, provide its id instead and let the client request it using a version.

Format

Versions should be dated as ISO8601 (YYYY-MM-DD)

• Good: 2014-05-04
• Bad: v-1.1, v1.2, 1.3, v1, v2

Version maintenance

Maintain old API versions for at least 6 months.

Implementation guidelines

The API version must be set using a custom HTTP header. The API version must not be defined in the URL structure (e.g. /v1) because it makes incremental change impossible.

HTTP Header

GoCardless-Version: 2014-05-04

Enforce the header on all requests.

Validate the version against available versions. Do not allow dates up to a version.

The API changelog must only contain backwards-incompatible changes. All non-breaking changes are automatically available to old versions.

Reference: https://stripe.com/docs/upgrades

X-Headers

The use of X-Custom-Header has been deprecated.

Resource filtering

Resource filters MUST be in singular form.

Multiple ids should be supplied to a filter as a comma separated list, and should be translated into an OR query. Chaining multiple filters with & should be translated into an AND query.

Good

GET /refunds?payment=ID1,ID2&customer=ID1

Bad

GET /refunds?payments=ID1,ID2&customer=ID1

Pagination

All list/index endpoints must be paginated by default. Pagination must be reverse chronological.

Only support cursor or time based pagination.

Defaults

limit=50 after=NEWEST_RESOURCE before=null

Limits

limit=500

Parameters:

Response

Paginated results are always enveloped:

{
  "meta": {
    "cursors": {
      "after": "abcd1234",
      "before": "wxyz0987"
    },
    "limit": 50
  },
  "payments": [{
    …
  },
  …]
}

Updates

Full or partial updates using PUT should replace any parameters passed and ignore fields not submitted.

GET /items/id_123
{
  "id": "id_123",
  "meta": {
    "created": "date",
    "published": false
  }
}

PUT /items/id_123 { "meta": { "published": true } }
{
  "id": "id_123",
  "meta": {
    "published": false
  }
}

PATCH Updates

PATCH is reserved for JSON Patch operations.

JSON encode POST, PUT & PATCH bodies

POST, PUT and PATCH expect JSON bodies in the request. Content-Type header MUST be set to application/json. For unsupported media types a 415 (Unsupported Media Type) response code is returned.

Caching

Most responses return an ETag header. Many responses also return a Last-Modified header. The values of these headers can be used to make subsequent requests to those resources using the If-None-Match and If-Modified-Since headers, respectively. If the resource has not changed, the server will return a 304 Not Modified. Note that making a conditional request and receiving a 304 response does not count against your rate limit, so we encourage you to use it whenever possible.

Cache-Control: private, max-age=60 ETag: <hash of contents> Last-Modified: updated_at

Vary header

The following header values must be declared in the Vary header: Accept, Authorization and Cookie.

Any of these headers can change the representation of the data and should invalidate a cached version. This can be useful if users have different accounts to do admin, each with different privileges and resource visibility.

Reference: https://www.mnot.net/cache_docs/

Compression

All responses should support gzip.

Result filtering, sorting & searching

See JSON-API: http://jsonapi.org/format/#fetching-filtering

Pretty printed responses

JSON responses should be pretty printed.

Time zone/dates

Explicitly provide an ISO8601 timestamp with timezone information (DateTime in UTC). Use the exact timestamp for API calls that allow a timestamp to be specified. These timestamps look something like 2014-02-27T15:05:06+01:00. ISO 8601 UTC format: YYYY-MM-DDTHH:MM:SSZ.

HTTP rate limiting

All endpoints must be rate limited. The current rate limit status is returned in the HTTP headers of all API requests.

Rate-Limit-Limit: 5000
Rate-Limit-Remaining: 4994
Rate-Limit-Reset: Thu, 01 Dec 1994 16:00:00 GMT
Content-Type: application/json; charset=utf-8
Connection: keep-alive
Retry-After: Thu, 01 May 2014 16:00:00 GMT

RateLimit-Reset uses the HTTP header date format: RFC 1123 (Thu, 01 Dec 1994 16:00:00 GMT)

Exceeding rate limit:

// 429 Too Many Requests
{
    "message": "API rate limit exceeded.",
    "type": "rate_limit_exceeded",
    "documentation_url": "http://developer.gocardless.com/#rate_limit_exceeded"
}

CORS

Support Cross Origin Resource Sharing (CORS) for AJAX requests.

Resources:

• CORS W3C working draft
• HTML5 Rocks

Any domain that is registered against the requesting account is accepted.

$ curl -i https://api.gocardless.com -H "Origin: http://dvla.com"
HTTP/1.1 302 Found
Access-Control-Allow-Origin: *
Access-Control-Expose-Headers: ETag, Link, RateLimit-Limit, RateLimit-Remaining, RateLimit-Reset, OAuth-Scopes, Accepted-OAuth-Scopes
Access-Control-Allow-Credentials: false

// CORS Preflight request
// OPTIONS 200
Access-Control-Allow-Origin: *
Access-Control-Allow-Headers: Authorization, Content-Type, If-Match, If-Modified-Since, If-None-Match, If-Unmodified-Since, Requested-With
Access-Control-Allow-Methods: GET, POST, PATCH, PUT, DELETE
Access-Control-Expose-Headers: ETag, RateLimit-Limit, RateLimit-Remaining, RateLimit-Reset
Access-Control-Max-Age: 86400
Access-Control-Allow-Credentials: false

TLS/SSL

All API request MUST be made over SSL, including outgoing web hooks. Any non-secure requests return ssl_required, and no redirects are performed.

HTTP/1.1 403 Forbidden
Content-Length: 35

{
  "message": "API requests must be made over HTTPS",
  "type": "ssl_required",
  "docs": "https://developer.gocardless.com/errors#ssl_required"
}

Include related resource representations

See JSON-API: http://jsonapi.org/format/#fetching-includes

Limit fields in response

See JSON-API: http://jsonapi.org/format/#fetching-sparse-fieldsets

Unique request identifiers

Set a Request-Id header to aid debugging across services.