API - Basics
Last modified on Fri 09 Apr 2021

What did the router say to the doctor? It hurts when IP.

API

API (application programming interface) is a general term used in all sorts of contexts. In an abstract sense, it describes a relationship between two actors. One actor provides a certain function, while the other wants to execute it without having to know what happens under the hood.

However, this article is not about interfaces or APIs in general. It will focus on web APIs.

What is a web API, anyway?

The age-old analogy is that of a waiter and a customer. The customer orders a pizza from the waiter. The waiter disappears into the kitchen and 10 minutes later, he appears and delivers a pizza on a plate.

Now let's just exchange the roles and say:

Those are all the ingredients you need to describe a slow, albeit functional web API. :)

Let's ditch the analogies

The Wiki article on web APIs states the following:

A server-side web API is a programmatic interface consisting of one or more publicly exposed endpoints to a defined request–response message system, typically expressed in JSON or XML, which is exposed via the web—most commonly by means of an HTTP-based web server.

Let's dissect that quickly.

Let's expand our thinking a bit. This is how a typical flow would look like if we wanted to fetch all the Pokémon from a certain web API:

If you want to try it out, just open your trusty terminal and execute the following: curl https://pokeapi.co/api/v2/pokemon

HTTP Requests

I was a bit sly and mentioned a GET request without really explaining what it is. Let's dive deeper into HTTP requests.

Generally, these are the basic ingredients that make up an HTTP request:

HTTP methods

Each endpoint does not necessarily support all types of actions. Actions can be fetching data, creating data, updating data, deleting data, etc.

We call these actions HTTP methods and this is the full list:

See this article on HTTP Request Methods for more info.

URI

The URI specifies the resource ("type of data" in layman's terms) you want to interact with.

For instance:

Query params

Query params are additions to the URI via which you can pass some information so the response can be prepared for you in a specific way. It's like telling the waiter you want extra mozzarella on your pizza. :)

For instance:

The query param here is limit=100. Consult your API docs to know when you can use which parameter and for what purpose.

Request headers

Headers usually contain metadata like authorisation tokens, the content type, etc.

Here's an example of request headers:

Content-Type: application/json
User-Agent: PostmanRuntime/7.26.8
Accept: */*
Cache-Control: no-cache
Host: myapi.foo
Accept-Encoding: gzip, deflate, br
Connection: keep-alive
Content-Length: 63

Read up on request headers here.

Request body

When creating a resource using POST or other methods that send data, the request body will contain a full or partial representation of the resource to be created or updated. The body will often be specified in JSON or XML.

If you are not sending any data, the body should remain empty.

Here's an example of a request body in JSON:

{
    "name":  "John",
    "surname": "Doe"
    "occupation": "Tester"
}

Example

Here's what a full request might look like:

POST /api/person HTTP/1.1
Content-Type: application/json
User-Agent: PostmanRuntime/7.26.8
Accept: */*
Cache-Control: no-cache
Host: myapi.foo
Accept-Encoding: gzip, deflate, br
Connection: keep-alive
Content-Length: 63

{
    "name":  "John",
    "surname": "Doe"
    "occupation": "Tester"
}

HTTP responses

A response is what you get back after making a request. Obviously. :)

Simply put, it consists of:

You're already familiar with headers and the body, so let's look closely into HTTP status codes.

Status codes

Status codes are found at the very top of the response and they are issued to briefly signify what exactly happened with the request.

They usually have a three-digit code and a short description. There are five classes of status codes:

Here are some you might already be familiar with:

200 OK
201 Created
304 Not Modified
400 Bad Request
401 Unauthorized 
403 Forbidden
404 Not Found
405 Method Not Allowed
500 Internal Server Error
503 Service Unavailable

A list of all status codes can be found here.

Example

This is the response to GET /api/v2/pokemon?limit=1 (some response headers are omitted for brevity):

HTTP/1.1 200 OK
Date: Fri, 09 Apr 2021 15:22:19 GMT
Content-Type: application/json; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
access-control-allow-origin: *
cache-control: public, max-age=86400, s-maxage=86400
etag: W/"a8-VCBC7cGvhjUog0edXFvoOrl6iko"

{"count":1118,"next":"https://pokeapi.co/api/v2/pokemon?offset=1&limit=1","previous":null,"results":[{"name":"bulbasaur","url":"https://pokeapi.co/api/v2/pokemon/1/"}]}

Interacting with APIs

You can interact with APIs or even test them with a variety of tools:


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