IT & Engineering

What is a RESTful API, how it works, advantages, and examples

RESTful APIs are APIs that adhere to the six constraints of the REST architecture. In this post, we'll learn how they work, their uses, and their advantages.



RESTful APIs are the most commonly used APIs in the world of web services. They use Hypertext Transfer Protocol (HTTP) requests to create, read, update, and delete (CRUD) data. They are popular because of their simplicity, scalability, speed, and ability to handle all data types.

In this article, we will take a deep dive into the world of RESTful APIs and discuss how they work, their uses, their advantages, and how you can use Mailgun’s Email API to send, receive, and track emails.

What is an API? 

An Application Programming Interface (API) is a set of rules two software programs use to communicate with each other and exchange data.

Think of the last time you ordered a Subway Footlong. They gave you a number of options in terms of the type of bread, the sauces, the vegetables, and so on. You told them your preferences, and they prepared the sandwich as per your requirements. They are able to do the same for every customer because they have set a standard of communication between themselves and the customer and described all the options available to the customer. The standards they have set serve as the API, which helps you when placing your order and them when making the sandwich exactly how you like it.

Now think of the “log in with Google” or “sign up with Google” button you see on a lot of websites. Google has defined a standard for communication between itself and all the websites in the world through the “Google Sign-In for Websites” API. The web developers take the API and apply it to the login/signup buttons on their sites, and the API springs to action when the button is clicked, gets your data from Google, and helps you log in by using that data.

Now that we understand what an API does, let’s quickly define some of the terms we will use throughout the article.


A client is a system that makes requests to access data on a server. In the Subway analogy, the client is the person who came in to get a sandwich. In the example of you using your Google ID to sign up or log into a website, the website is the client.


A server is a system that has the resources needed by the client. In the Subway example, the server is the restaurant. In the Google ID example, Google is the server.


A resource is any data that the server can provide. In the Subway example, the sandwich is the resource. In the Google example, your user ID is the resource.

When discussing APIs, you may hear the term “resource identifier.” A resource identifier is not the resource itself; it is a unique ID assigned to the resource. Think of an employee database with thousands of employees. Chances are, some of them will share the same first and last name. That’s why in large companies, everyone is assigned a unique employee ID, so there are no mix-ups when it comes to payroll and vacation days. Similarly, by using resource identifiers, APIs make sure that the client and the server are on the same page about the resources being called by the client.

Types of API requests

There are four types of API requests:

  • DELETE: to delete existing data

  • PUT/PATCH: to update existing data/to modify and replace existing data

  • GET: to retrieve data

  • POST: to create new data

What is a RESTful API and how does it work?

A RESTful API is an API that follows the Representational State Transfer (REST) architectural constraints. These development constraints – basically a series of rules APIs must follow – allow for faster, scalable APIs that support all data types. Because of this, RESTful APIs have become the most common APIs in the world, especially for web services.

The concept of REST and its six constraints was first introduced by Roy Fielding in his dissertation – Architectural Styles and the Design of Network-based Software Architectures – in 2000. The parameters help developers by outlining what an effective API includes and excludes. So, although there are constraints, RESTful APIs are actually easier to develop, use, and connect than their non-RESTful counterparts.

Let’s discuss the six in detail.

The six architectural constraints of the REST framework

1. Client–server

Client-server means that the roles of the server and the client are clearly defined and distinct. The server handles data storage exclusively, and the client reads data (and modifies it if it has permission to do so). This way, the server and the client can scale and evolve independently of each other. 

2. Uniform interface

Having a uniform interface means that the client should receive information from the server in a consistent and readable format. The uniform interface must adhere to the following guiding principles:

  • Hypermedia as the Engine of Application State (HATEOAS): RESTful APIs are driven by hypermedia. This means that to understand the response sent by the server, the client only needs to understand hypermedia. In other types of APIs, you need an alternate language called interface description language (IDL); in RESTful APIs, that’s not the case.

  • Self-descriptive messages: a message can be defined as each request from the client to the server and each response from the server to the client. Each message going back and forth between the client and the server should contain enough information to process the message. This means that the request going from the client to the server should identify the resource it is trying to access, as well as say exactly what it wants to do with it (create, read, update, or delete).

  • Identification of resources: the resources mentioned in the messages going back and forth between the client and the server must be identifiable through representations. This is achieved by adhering to the Uniform Resource Identifier (URI) standard. In other words, a message from the server to the client might not contain an actual file from its database but an HTML representation of it along with some metadata. Since the HTML representation adheres to the URI standard, the client can easily read it.

  • Manipulation of resources through representations: the client should be able to manipulate (create, read, update, delete) resources by sending the server a resource representation of what the final version of the resource should look like. If the client has sufficient permission to manipulate data, the server should comply with the request.

3. Stateless

RESTful APIs are stateless, which means that the server does not store any information about the client’s session. During any session, a client may make multiple queries and send multiple requests to the server. For the API to be RESTful, each request must be self-contained and must not contain any information related to a past, future, or simultaneous request. This way, the load on the server is reduced greatly.

4. Cacheability

When a user makes a request to get data from the server, the request travels from the client to the server through a cache of stored information. Data already present in the cache can be retrieved from it without putting any burden on the server.

 Developers should label the data traveling from the server to the client as cacheable or non-cacheable. Cacheable data is stored on the client cache and can be accessed as needed, which makes access to this data faster, and there’s no burden on the server if the client wants to access that data again.

5. Layered system

In a layered system, there are multiple layers, each with one high-level purpose. Think of a standard four-layer system with a database layer that handles data, a persistence layer that handles how the data is kept in the database, a business layer that handles all types of business logic, and a presentation layer that transforms the data into an understandable format.

The layers are organized in such a way that they only interact with the layers above and below them. Sometimes, instead of going with four layers, developers decide to go with three and use one layer to serve as the database and persistence layers combined.

After setting up the layers, developers add proxy and gateway components and divide different functions between the different layers. The number of layers and how the developers choose to arrange them depends on the system’s requirements. 

6. Code-on-demand

Code-on-demand is the only optional architectural constraint for an API to be considered as RESTful. The developer should build the API in a way that the client would have the option to demand executable code from the server. This executable code is often in the form of applets or scripts. Upon receiving said code from the server, the client executes it completely on its side.

For example, think of the olden days when we needed Adobe Flash Player to run certain animated sections on most web pages, and if you didn’t have Flash Player, those sections didn’t load and showed an error.

The components of a RESTful API

RESTful APIs consist of the following components:

1. The endpoints

The endpoint describes the location of the data on the server. The endpoints are URLs of the resources you are trying to access through the API.

2. The method

We have already discussed the four HTTP methods (GET, PUT, POST, DELETE) APIs use to manipulate data. An API request must use one of these methods for the server to understand what needs to be done.

3. The headers

RESTful APIs contain HTTP headers that contain information, such as metadata, proxies, and HTTP connection types. In a request message, the header contains information on the nature of the request as well as the types of valid responses.

In a response message, the header also contains information on the status of the request along with status codes. “404,” for example, means the API was unsuccessful at retrieving requested data from the server.

4. The data (or body)

The data (or body) of the RESTful API consists of further information on the resources requested by the client. In the case of a simple GET request, further information is not needed. In a POST request, the client will declare the type of content in the header, and the actual content will be in the body. This could be a new resource the client is submitting to the server. The server will see whether the content type is acceptable or not and proceed to find the resource in the body.   

Advantages of RESTful APIs

RESTful APIs are fast, flexible, scalable, and versatile. Here are some of the key advantages of this type of API:

1. Supports all types of data formats

In other types of APIs, you are limited in your choice of data formats. However, RESTful APIs support all data formats. 

2. Works wonders with web browsers

In RESTful APIs, you can send an HTTP request, get JavaScript Object Notation (JSON) data in response, and parse that data to use it on the client applications. This makes it the best choice for web browsers. Furthermore, you can easily integrate these APIs with your existing website. 

3. Uses less bandwidth

Thanks to JSON, RESTful APIs use less bandwidth as compared to other types of APIs. However, this stands true for JSON-based web APIs only. An XML-based web API will have a similar payload to its non-RESTful counterpart whether it adheres to the REST architectural constraints or not.

4. Does not need to be designed from scratch

In most cases, you can get models that you can modify and use. For example, NetApp and Mailgun provide a complete tutorial and source code for building a private API. In some cases, maybe when developing a private API, you will need to design the API from scratch, and for that, you can get a lot of support from Stack Overflow

5. Easy for most developers

RESTful APIs use HTTP methods for communication. You can use Python, JavaScript (Node.js), Ruby, C#, and a number of other languages to develop these APIs, making them easier to work with for most developers.

What are RESTful APIs used for?

RESTful APIs are popular in the SaaS industry, as they are great for web services. These APIs are used as: 

Public APIs for access to widely used data

APIs provided by Twitter, Facebook, and Google are the best examples of public APIs. These APIs are available to everyone, and anyone can simply take the API code and implement it on their website so that users can log in using their social media accounts.

Private APIs for access to data within an organization

RESTful APIs are also used for private and internal communication within the software programs used in an organization.  Think of an agency that offers web development services to its customers. It will have HR, finance, sales, and support, marketing, as well as production and QA departments. All of these departments will use software applications specific to them, as well as some common software applications to manage payroll, PTOs, performance reviews, etc. All of these software programs will need to talk to a central data repository that provides the leadership with a high-level view. The central data repository will be able to talk to all of these programs through RESTful APIs. 

Third-Party APIs for access to paid data and resources

Many organizations use third-party APIs for communication between their software programs and those of their partners or customers. A good example of this would be the Mailgun Email API. You can use this API to integrate email into your marketing and sales applications.

For the average user of your in-house software, nothing has really changed except some new functionalities are now available to them: they can send email campaigns using the same software they already used for content management, marketing, customer engagement, and sales and support. However, on the back end, Mailgun’s RESTful API is taking requests from your software application to Mailgun and bringing back responses. 

Use Mailgun’s API to take your email to the next level

Mailgun’s API integrates with your existing software and opens the door to easier and more powerful email campaigns. The API can collect and modify customer data from your CRM and other tools, allowing you to send emails in bulk to segments of your audience, modify and manage email lists, and dig into detailed analytics regarding customer email behavior. Mailgun’s Email API is RESTful, which means it integrates with all email providers. It’s also built with security and fidelity in mind to mitigate the risks surrounding user data and ensuring emails end up in inboxes, not spam folders.

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