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Rest API Design

History

REST stands for Representational State Transfer protocol. Roy Fielding defined REST in his Phd dissertation in year 2000.

What is it?

REST was developed to provide a uniform interface for

  • Identifying resources
  • Manipulation of resources
  • Self-descriptive messages
  • Using Hypermedia as the Engine of Application State (HATEOS)

Best Practices

  • Basics

Method http://api.co/v2/cars http://api.co/v2/cars/1234
GET List all the cars Retrieve an individual car
POST Create a new car Error
PUT Replace cars collections with new one Replace the car with id 1234
DELETE Delete all cars Delete car with id 1234

Note while PUT operations either client or server can generate id's

  • Nouns are good, Verbs are bad

    • Use nouns to refer resources like cars, fruits etc.
    • Use verbs for action declarations convertMilesToKms, getNutritionalValues

  • Singular or Plural?

    Use correct grammar for declaration

    Avoid /person/145

    Prefer /people/154 Assumed to return 154th person from list of people

  • Use casing

    Use anyone of the below patterns and be consistent!

    Case Styles Example
    UpperCamelCase http://api.fintech.cp/DailyTransactions/Today
    lowerCamelCase http://api.fintech.cp/dailyTransactions/today
    snake_case http://api.fintech.cp/daily_transactions/today

  • Relationships and Resources

    • Resources can have one-to-one, one-to-many, and many-to-many relationships etc. Mapping them correctly is crucial.

    • One-to-One Mapping

      For example, Countries/1/capital suggests a one-to-one relationship between a country and a capital city. This means a country can only have one capital. This type of relationship means that one row relates only to one row in another table (one country, one capital in two different tables). This type of relationship is not common and is often used to break up the amount of data in one row of a table.

    • One-to-Many Mapping

      For example, Tickets/145/messages/4 suggests one-to-many relationship between tickets and messages. Meaning 1 ticket has N messages. Message isn't standalone resource. You can't have /messages/4.

    • Many to Many Mapping

      For example, /usergroups/345/users/56 suggests select 345th user group and get user with id 56. However, one user might be in multiple usergroups i.e. /usergroups/209/users/56 is also valid. In such case so for seperating the dependant resource users into a seperate endpoint like /users/56 and provide resource linking in /usergroups/209/users/56

  • API Parameters

    • PATH : required to access the resource E.g. /cars, /fruits

    • Query Parameters : optional filter the list E.g. /cars?type=SUV&year=2010

    • Body : Resource specific logic. Advance search query. Sometimes it might have both Query and body.

    • Header : Should contain global or platform wide data. E.g. API key parameters, encrypted keys for auth, device type information e.g. mobile or desktop or endpoint, device data type e.g. xml or json. Use header to communicate these parameters

  • HTTP Status Codes

Use correct status codes

Codes Meaning
1xx Request received and understood.
2xx Action requested by client was received, understood and requested.
3xx Client must take additional action to complete the request. Most of these status codes are used in URL Redirection.
4xx Intended for situations where it seems the error was caused by the client.
5xx The server failed to fulfil a request.

Little more on 2xx!

  • 201 Resource Created. POST /cars should return HTTP 201 created with location header stating how to access the resource E.g. location : api.com/cars/124 in header

  • 202 - Accepted

    Use this if the task is huge to run. Tell the client, it has accepted the request and will/is process/processing No payload is returned

  • 204 - No content

    Used when deleted DELETE cars/124 Returns no content. But can also return 200 OK if API intends to send the deleted resource for further processing.

The dangerous 5xx resources!

  • 500 Internal Server Error
  • 501 Not implemented. Server lacks the ability to fulfil the request
  • 504 Gateway Timeout. Server didn't receive timely response

Less known 4xx suggests that you are passing wrong parameter. Can also pass information that is wrong. E.g.

DELETE /cars/MH09234

returns 4xx or message Expecting int car id /car/id got string car/MH09234

Further reading

How to Design Great APIs - Parse Developer Day 2013

The never-ending REST API design debate by Guillaume Laforge

HTTP status codes