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HTTP API Design Guidelines

The purpose of this document is to establish a shared vocabulary and a pragmatic (yet robust) approach to design an HTTP API using JSON (JavaScript Object Notation) as a data-interchange format.

Table of Contents

Core Design Principles

An API is a Relationship of Entities

An API (Application Programming Interface) is represented as a relationship of entities.

These entities cooperate together in order to perform actions, manage state and transfer representations of entity data.

Entities have shapes

Clients can interact with entities in two shapes:

  1. A single object: /objects/{id}
  2. A collection of objects: /objects

ID as a Pair

An object uniqueness is defined as a pair of values (entity + id) and not a single value (id).

In other words, this does not define an object:

{
  "id": 1,
  "name": "Max"
}

Valid forms of defining an object:

{
  "entity": "users",
  "id": 1,
  "name": "Max"
}

{
  "entity": "dogs",
  "id": 1,
  "name": "Max"
}

Relationships

There are 3 types of relationship between entities:

  • One-to-One (1-1)
  • One-to-Many (1-N)
  • Many-to-Many (M-N)

This means that, in every type of relationship, an object has possibly:

  • A nested object representing a foreign key relationship ("1-sided" relation)
  • A location for representing the collection of objects ("M/N-sided" relation)

Entities relationships are nested (1-sided relation)

Since the single value of an id does not define an object, entities relationships are nested.

In other words, this does not define a relationship:

{
  "id": 1,
  "name": "Max"
  "owner_id": 1
}

Valid form of defining a relationship:

{
  "entity": "dogs",
  "id": 1,
  "name": "Max"
  "owner": {
    "entity": "users",
    "id": 1
  }
}

The ID Pair is a Location

Now that the uniqueness of object is a pair consisting of both entity and id values, these two values together defines a location inside the API.

GET /dogs/1

{
  "entity": "dogs",
  "id": 1,
  "name": "Max"
  "owner": {
    "entity": "users",
    "id": 1
  }
}

GET /dogs/1/owner redirects to GET /users/1

{
  "entity": "users",
  "id": 1,
  "name": "Max",
  "age": 12,
  "citizen_id": "123.456.789-00"
  "phone_number": "91234-5678"
}

Nested relationship objects are Expandable

Since relationships are represented as nested objects, they are expandable.

GET /dogs/1?expand=owner

{
  "entity": "dogs",
  "id": 1,
  "name": "Max"
  "owner": {
    "entity": "users",
    "id": 1,
    "name": "Max",
    "age": 12,
    "citizen_id": "123.456.789-00"
    "phone_number": "91234-5678"
  }
}

Important

Relationship objects must either return the identification pair (entity + id) or the full object data.

Do not return partial representation of data.

Entities have grammar

Entity endpoints are separated by grammar semantics:

  • A noun endpoint is used for transferring entity state representations (/objects/{id})
  • A verb endpoint is used for performing actions (/objects/{id}/do)

An example for a user_files entity with the following schema:

id: uuid
name: string
extension: string
url: string

POST /user_files will create a new object by transfering a representation:

{
  "name": "gopher"
  "extension": ".png"
  "url": "https://go.dev/blog/gopher"
}

While, for a file upload using multipart/form-data, an action should be used: POST /user_files/upload. Then, after server-side logic was processed, a response object would be:

{
  "entity": "user_files",
  "id": "13728a84-e1dc-4de6-8f88-f0ba574907ad",
  "name": "gopher"
  "extension": ".png"
  "url": "https://storage.com/blobs/gopher.png"
}

Single Responsibility and Concern Separation

Ideally, requests have a single responsibility. This principle promotes the modularization of the API, making it composable.

For instance, imagine that the client-side wants to create a cars object with a photo. Instead of uploading the photo image and submitting other data in a single call, the flow would be:

  1. Create a cars object

POST /cars

{
  "model": "Mazda RX-7",
  "year": 1978
}

Possible response:

{
  "entity": "cars",
  "id": 1,
  "model": "Mazda RX-7",
  "year": 1978,
  "photo": null
}
  1. Upload car image by performing an action:

POST /cars/1/upload_photo (using multipart/form-data)

This way, the logic of uploading the car image is decoupled for its creation, making the API more composable and making server-side logic easier to maintain, debug and reason about.

Note

If the flow above requires a single call for a specific client (making car metadata and photo upload atomic), a wrapper endpoint can be exposed through a gateway (reverse proxy). This pattern is know as BFF (Backend for Frontend).

Dealing with Collections

A collection in simply an array of entity objects.

Collections have operations

Collections support the following operations:

  • Pagination
  • Sorting
  • Filtering

Pagination

Clients can paginate through a collection using the page_number and page_size query params (if those values are not sent by the client, server defaults will be used).

GET /nations?page_number=1&page_size=2: first page containing two nations objects.

[
  {
    "entity": "nations",
    "id": 1,
    "name": "Brazil",
    "continent": "America"
  },
  {
    "entity": "nations",
    "id": 2,
    "name": "Uruguay",
    "continent": "America"
  }
]

GET /nations?page_number=3&page_size=3: third page containing three nations objects.

[
  {
    "entity": "nations",
    "id": 7,
    "name": "Japan",
    "continent": "Asia"
  },
  {
    "entity": "nations",
    "id": 8,
    "name": "China",
    "continent": "Asia"
  },
  {
    "entity": "nations",
    "id": 9,
    "name": "South Korea",
    "continent": "Asia"
  }
]

Sorting

Sorting order can be either ASCENDING or DESCENDING.

Sort nations by name in ASCENDING order: GET /nations?sort=name

[
  {
    "entity": "nations",
    "id": 122,
    "name": "Afghanistan",
    "continent": "Asia"
  },
  {
    "entity": "nations",
    "id": 83,
    "name": "Albania",
    "continent": "Europe"
  },
  {
    "entity": "nations",
    "id": 57,
    "name": "Algeria",
    "continent": "Africa"
  }
]

Sort nations by name in DESCENDING order: GET /nations?sort=-name (hyphen/minus signal in front of the field name)

[
  {
    "entity": "nations",
    "id": 160,
    "name": "Zimbabwe",
    "continent": "Africa"
  },
  {
    "entity": "nations",
    "id": 45,
    "name": "Zambia",
    "continent": "Africa"
  },
  {
    "entity": "nations",
    "id": 37,
    "name": "Yugoslavia",
    "continent": "Europe"
  }
]

Filtering

Clients can filter the collection by entity properties using the filter query param. The following table shows available filter operators.

Filter operator Description Expression example
Comparison Operators
eq Equal city eq "Redmond"
ne Not equal city ne "London"
gt Greater than price gt 20
ge Greater than or equal price ge 10
lt Less than price lt 20
le Less than or equal price le 100
Logical Operators
and Logical and price le 200 and price gt 3.5
or Logical or price le 3.5 or price gt 200
not Logical negation not price le 3.5
Grouping Operators
( ) Precedence grouping (priority eq 1 or city eq "Redmond") and price gt 100

Just insert the expression as a value for the filter query param:

GET /nations?filter=continent eq "Europe"

Collections represents M/N-sided relations

Recalling, there are 3 types of relationship between entities:

  • One-to-One (1-1)
  • One-to-Many (1-N)
  • Many-to-Many (M-N)

As we've seen, expandable nested objects represents 1-sided relations. For M/N relations (one-to-many and many-to-many), we use collections.

One-to-Many (1-N): an owner has a collection of dogs.

GET /users/1/dogs redirects to /dogs?filter=owner.id eq 1 (fetch dogs collection filtering by the owner)

[
  {
    "entity": "dogs",
    "id": 1,
    "name": "Max",
    "owner": {
      "entity": "users"
      "id": 1
    }
  },
  {
    "entity": "dogs",
    "id": 2,
    "name": "Scott",
    "owner": {
      "entity": "users",
      "id": 1
    }
  }
]

Many-to-Many (M-N): relationship between orders and products. An order has a collection of products, and a product has a collection of orders.

GET /orders/{id}/products: list products in that order.

GET /products/{id}/orders: list the orders that a given product is present.

Request and Response Cycle

Request HTTP methods

HTTP methods follows the specifications of RFC 9110. A summary is presented below.

HTTP method Common usage
POST Create a new resource by transferring a representation; perform an unsafe action
GET Fetch an object or a collection of objects; perform a safe (read-only) action
PATCH Update an existing object by transferring a partial representation of entity data
PUT Used for upserts
DELETE Client requests the deletion of an object

Response HTTP status codes

Status codes are categorized in four classes:

Status code range Result
2xx Success
3xx Redirection
4xx Client-side errors
5xx Server-side exceptions

Most commonly used response status codes are:

Status code Result
200 OK Fetched an object/collection; updated (PATCH) an object; performed a safe (read-only) action
201 Created Request processing resulted in the creation of an object; performed an unsafe action
202 Accepted Used for asynchronous requests
204 No Content Request succeeded but returned no data
400 Bad Request Malformed request syntax or invalid request semantics
401 Unauthorized Lacking or invalid authentication credentials
403 Forbidden Server understood the request but refuses to fulfill it. Given credentials does not have enough access level to the specified resource
404 Not Found Server did not find a current representation for the target resource
500 Internal Server Error Server encountered an unexpected condition that prevented it from fulfilling the request

JSON Response Document

Unless returning a 204 No Content, requests will return a JSON document. This document have defined fields, making the results predictable and stardandized.

The document have possibly 4 root-level fields:

  • meta: optional object
  • data: only present if errors is absent. Either an object or an array of objects
  • pagination: only present if data is an array
  • errors: only present if data is absent

If the four root-level fields above are absent (meaning a successful request returning no data), it is a 204 No Content status code.

Meta Object

This object represents request metadata. Contains two defaults fields (status and message) and any other API-specific fields.

Example (default fields; mostly for debugging)

{
  "meta": {
    "status": 404,
    "message": "Could not find any user with provided ID."
  }
}

More concrete examples

{
  "meta": {
    "status": 201,
    "message": "Created",
    "request_token": "32606149-b8a2-42b0-b507-92d7f7c22465",
    "remaining_quota": "Your API request limit is current at 69% of your daily usage quota."
  }
}

Data Object/Array

Primary result of your request. The resulting data after requested was processed.

GET /nations/8

{
  "data": {
    "entity": "nations",
    "id": 8,
    "name": "China",
    "continent": "Asia"
  }
}

Pagination Object

If data is an array, pagination is present. This object consists of four fields:

  • total_pages: Total number of pages to represent the collection of objects using the current page size
  • current_page: Current page number
  • page_size: Page size controls how many objects each page will return
  • objects_count: Count of the number of objects existing in this collection

GET /nations?page_number=3&page_size=3

{
  "data": [
    {
      "entity": "nations",
      "id": 7,
      "name": "Japan",
      "continent": "Asia"
    },
    {
      "entity": "nations",
      "id": 8,
      "name": "China",
      "continent": "Asia"
    },
    {
      "entity": "nations",
      "id": 9,
      "name": "South Korea",
      "continent": "Asia"
    }
  ],
  "pagination": {
    "total_pages": 4,
    "current_page": 3,
    "page_size": 3,
    "objects_count": 12
  }
}

Errors array

Array of error objects detailing client-side errors. An error object consists of three fields:

  • code: application-specific error code
  • title: a short, human-readable title of the error caused by the client
  • detail (optional): error message providing details about the error in the current request context
{
  "meta": {
    "status": 400,
    "message": "Failed to upload user profile image."
  },
  "errors": [
    {
      "code": 20202,
      "title": "Invalid media type",
      "detail": "Tried to upload image of content-type 'image/heic' while only 'image/png' is supported."
    },
    {
      "code": 15370,
      "title": "File too large",
      "detail": "File upload limit is 5 MiB. Tried to upload a 17 MiB file."
    }
  ]
}

References

JSON API

Microsoft REST API Guidelines

Heroku Platform API (interagent)