API Fundamentals

Rahul Pandey | July 05, 2018

An API is an application programming interface. It defines a way for applications to talk to each other to exchange data. APIs are commonly used when discussing how clients can make requests to a web server, but they are used beyond internet-based apps. Libraries and other systems also expose an API which clients must understand. In this discussion, however, we will focus on RESTful API concepts, which are APIs that deal with resources.

REST APIs are most common, both in the real world and in interview questions. One useful attribute of REST APIs are that calls are stateless, which means that the server does not save any state between requests. This makes it easy to scale REST APIs to handle load changes by simply sending requests to any availble server. Don’t worry about knowing what REST stands for; just know that SOAP is another (less preferred) way of building APIs.



A resource is any information that can be named. Think of a resource as a noun such as a user, payment, or book. If you’re familiar with databases, a resource is roughly an entry in a database table. The resource is the high-level description of the object, and it could contain other resources inside it. A URI is a Uniform Resource Identifier, a way to identify the location of a resource. The idea of a REST API is that the URI represents the resource, and we apply different verbs to this. For example, for the examples above, a REST API may represent the resources as /users, /payments, or /books.

A resource is not a verb or action. For example, the URI /changeToDoList is better described as a PUT on /to-do-lists, and /listOfVehicles is better described as a GET on /vehicles. According to naming conventions, URIs that represent resources should be plural.

HTTP methods

A RESTful API uses the standard HTTP methods:

GET requests usually accept additional data via query parameters in the URL. One common use for query parameters is to filter the result set returned from the server. For example, in /search/vehicles?make=Chevorlet&model=Cruze, the query params represent that we want cars with make of Chevorlet and model of Cruze. The other common use case is pagination for a result set, e.g. /search/vehicles?make=Chevorlet&page_size=20&page=2.

The other HTTP methods accept data in request body rather than in the URL itself. The way to think about it: the request body is used for data that is being uploaded to the server, while query params are used for filtering the data.

Safe and Idempotent

Safe refers to whether the request is modifying the state on the server.

Idempotent refers to the state of the system after the request has completed. A request is idempotent if clients can make the same call repeatedly while producing the same result.

Here is a table of each HTTP method we discussed:

Method Safe Idempotent
GET yes yes
PUT no yes
DELETE no yes
POST no no
PATCH no no

Some things to note here: