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HTTP API Design Standards

Guidelines

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

Sources:

JSON API

All endpoints must follow the core JSON API spec: http://jsonapi.org/format/

Changes from JSON API:

  • The primary resource must be keyed by their resource type. The endpoint url must also match the resource type.
  • API errors currently do not 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
  }
}

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.
  • Use plural nouns only for consistency (no singular nouns).
  • Use HTTP verbs (GET, POST, PUT, DELETE) to operate on the collections and elements.
  • Never nest resources - it enforces relationships that could change, and makes clients harder to write.
    • Use filtering: /payments?subscription=xyz rather than /subscriptions/xyz/payments
  • 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

Bad URL examples

HTTP Verbs

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

HTTP METHOD POST GET PUT PATCH DELETE
CRUD OP CREATE READ UPDATE UPDATE DELETE
/plans Create new plan List plans Bulk update Error Delete all plans
/plans/1234 Error Show Plan If exists If exists, full/partial update Plan; If not, error If exists, update Plan using JSON Patch format; If not, error Delete Plan

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 them under a actions prefix. Actions should always be idempotent.

POST /payments/ID/actions/cancel

Responses

Don't set values in keys.

Good:

No values in keys:

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

Bad:

Values in keys:

"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, internal error type (corresponding to some specific internally determined constant, which must be represented as a string), links where developers can find more info.

There must only be one top level error. Errors should be returned in turn. This makes internal error logic simpler. It also makes it easier for API consumers to deal with errors.

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 a sub error. Nested errors may have more specific type errors like invalid_field.

Separate formatting errors from errors the integration should handle

Formatting errors include field presence, length etc. Returned when the data you have passed is wrong.

An error that should be handled in the integration could be when attempting to create a payment against a mandate and the mandate has expired. This is a edge case that needs handling in the API integration. Do not mask these errors as validation errors. Always return these types of errors as a top level error.

Top level error

  • Top level errors MUST implement request_id, type, reason, code,message.
  • type MUST relate to the reason, use it to categorise the error, e.g. api_error.
  • reason MUST be specific to the error.
  • message MUST be specific.
  • Top level errors MAY implement documentation_url, request_url , 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, 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 - Eg. invalid JSON.
  • 401 Unauthorized - No valid API key provided.
  • 402 Request Failed - Parameters were valid but request failed.
  • 403 Forbidden - Missing or invalid permissions.
  • 422 Unprocessable Entity - Parameters were invalid/validation failed.
  • 404 Not Found - The requested item doesn't exist.
  • 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. when passing an array when you should be passing an object).

422 Unprocessable Entity

  • When model validations fail for fields (e.g. name to long).
  • When creating a resource with a related resource being 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.

WebHooks/Server initiated events should not contain serialised versions of resources. Instead provide an id to the resource that changed 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. API version must not be defined in the URL structure (e.g. /v1) this 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, see: http://tools.ietf.org/html/rfc6648

Resource filtering

Resource filters should always be in singular form.

Multiple ids supplied to one filter, as a comma separated list, 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. To enable cursor based pagination, ids should be increasing.

Only support cursor or time based pagination.

Defaults: limit=50 after=NEWEST_RESOURCE before=null

Limits: limit=500

Parameters:

Name Type Description
after string id to start after
before string id to start before
limit string number of records

Response

Paginated results are always enveloped:

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

Updates

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

Example:

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 API: http://jsonapi.org/format/#patch

JSON encode POST, PUT & PATCH bodies

POST, PUT & PATCH expect JSON bodies in request. Content-Type header is required to 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. You can use the values of these headers 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. Also note: 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 headers must be declared in Vary: Vary: Accept, Authorization, Cookie

Any one of these headers can change the representation of the data and should invalidate a cached version. Users might be using different accounts to do admin, all with different privileges and resource visibility. Accept can alter the returned representation.

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 (Date time in UTC). For API calls that allow for a timestamp to be specified, we use that exact timestamp. 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.

You can check the returned HTTP headers of any API request to see your current rate limit status:

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. You can read the CORS W3C working draft, or this intro from the HTML 5 Security Guide.

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. Any non-secure requests will return ssl_required. No redirects are made. Outgoing web hooks must be SSL.

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: Request-Id header.

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