An API (application program interface) is a set of rules that enables different programs to communicate with one another. REST API, also called RESTful API or RESTful web API, is a type of API that follows the principles of Representational State Transfer (REST) architecture. It provides a standard way for web applications to communicate with each other over the internet.

An API outlines the appropriate way for a software developer to compose a program on a server that communicates with various client applications. APIs of different applications can be integrated together to exchange data and perform a specific function, thus, enabling interaction between applications. Various websites like Amazon, Google, Facebook, LinkedIn, and Twitter use REST APIs to allow users to communicate with these cloud services.

This blog takes a deep dive into RESTful APIs, breaking down their core principles, key methods, and real-world applications to help you understand how they power modern web services. cover all its basic aspects, including what REST API stands for, its definition, principles, methods, and more.

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What is a REST API?

In 2000, Roy Fielding defined REST as an architectural style and methodology frequently used in internet services development, such as distributed hypermedia systems.

The full form of REST API is Representational State Transfer Application Programming Interface. It’s more commonly known as REST API web service. It means when a RESTful API is called, the server will transfer a representation of the requested resource’s state to the client system.

Once the client receives the resource representation, it can process, display, or manipulate the data as needed. If it’s a front-end application, it might render the data in the UI. If it’s another service, it could transform the data or trigger additional API calls. Or, if it’s a caching system, it might store the response for future use.

For instance, when a developer requests Twitter’s API to fetch a user’s profile (a resource in RESTful API terms), the API responds with data about that user—such as their name, bio, follower count, and recent tweets. The client application can then display this information in a mobile app, website, or analytics dashboard. This exchange happens through API integration projects, allowing different systems to communicate seamlessly.

(Source: Seobility)

This state representation can be in JSON, XML, or HTML format. What this means is that when a client requests a resource (like a user’s profile in the above example), the API doesn’t just send raw data—it structures the response in a format that the client can understand and work with. The most common format is JavaScript Object Notation (JSON), but APIs can also return Extensible Markup Language (XML) or even HTML in some cases.

For example, a Twitter API response in JSON might look like this:

A sample JSON response from a REST API, showcasing user profile details and a tweet.

The key takeaway is that REST APIs enable clients to interact with resources in a stateless manner—each request contains everything needed, and the response provides the latest state of the resource at that moment.

Understanding the Basic Terminologies

Before diving into the guiding principles of designing REST APIs, let’s briefly discuss three key API terms:

Client

The client is a hardware or software that uses the API made accessible by a server. For instance, when you visit Facebook’s website, your browser is the client that calls Facebook’s API and utilizes data sent back to display info on your screen.

Resource

A resource can be any object the API can offer info about. For instance, in the case of a Twitter API, a resource can be a user, hashtag, or any media type like a picture. Every resource has a distinct identifier which can be a name or number.

The resource is the primary abstraction of information in REST. REST API uses a resource identifier to recognize the specific resource involved in the communication between different elements.

Server

A server is any system that contains resources that the client wants. When it receives client requests, it provides the content to the client using the API interface. The server will only grant a representative state of the source and not complete access to the client.

An excellent example of this is when a mobile app shows YouTube videos through its interface. It uses a REST API to call the video content from YouTube without hosting it on its system.

Design Principles of REST APIs

Now that we’ve covered the basics and learned about the definition of REST APIs, let’s move on to the six REST principles that guide API design:

Client-Server

The REST principle operates on the concept that the client and server should be isolated from each other and allowed to develop independently. This way, you can improve manageability across numerous platforms and increase scalability by streamlining server components as user interface concerns are separate from the data storage concerns.

Stateless

This REST principle dictates that APIs are stateless, allowing for independent calls to be made. Moreover, every call includes the data essential to complete itself effectively.

In other words, every request sent from the client to the server must include all the info needed to comprehend the request.

Cacheable

As a stateless API can upsurge request overhead by managing huge loads of inbound and outbound calls, a REST API design should store cacheable data. According to this API design principle, data within a response should be indirectly or categorized as cacheable or non-cacheable.

If a response is cacheable, the client cache is provided the right to recycle that response data for similar requests in the future.

Uniform Interface

To decouple a client from the server, you need to have a unified interface that permits autonomous development of the application without tightly coupling its services, models, and actions to the API layer itself. This design principle streamlines the whole system architecture and enhances the visibility of communications. Several architectural controls require guiding the performance of the elements within the REST API architecture to attain a uniform interface.

The REST API architecture defines REST principles by four interface controls, including identifying resources, managing resources through representations, enabling self-descriptive communications, and making hypermedia the engine of the application state.

Layered System

REST API’s architecture includes several layers that operate together to construct a hierarchy that helps generate a more scalable and flexible application. Due to its layered system, an application has better security as components in each layer can’t interact outside the subsequent layer. Moreover, it balances loads and offers shared caches for stimulating scalability.

A layered REST API architecture system has greater stability because it restrains component performance. so that each component can’t ‘see’ further than the immediate layer with which it is intermingling.

Code on Demand

The REST principle enables the communication of coding or applets through the API used within the application.

A REST API definition permits extending client functionality by downloading and implementing coding in the form of applets or scripts. This streamlines clients by decreasing the number of features essential to be pre-implemented.

Most of the time, a server returns static resource representation in XML or JSON format. But when required, servers can deliver executable code to the client.

What are RESTful APIs Used For?

Let’s consider an example to understand the use and functionality of RESTful API.

Suppose you want to watch video tutorials about ‘data integration’ on YouTube. You go to YouTube, type ‘data integration’ into the search field, press enter, and a list of videos about data integration appears. Right?

A REST API works similarly. You seek out something and a list of results returns from your requested service.

With the REST technology, the assumption is that all calls are stateless. This means the REST service can’t retain anything between executions, making it beneficial in cloud applications. Stateless constituents can easily reassign in case of failure and scale to consider load variations because any request can be sent to any instance of a constituent.

The reason why REST is the desired protocol for internet communication is that it does not retain any data that requires recalling by the subsequent transaction. As stated earlier, the REST API technology is also helpful in connecting to cloud applications, as accessing a service via an API needs adjustment in the URL interpretation.

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How Does a REST API Work?

REST determines the structure of an API. Developers oblige to a specific set of rules when designing an API. For instance, one law states that linking to a URL should return some information.

The system knows every URL as a request, and it knows the data returned as a response.

A RESTful API follows a client-server architecture, where a client (such as a web or mobile app) makes requests to a server, which processes those requests and returns the necessary data. Such communications happen over HTTP, using standard methods like GET, POST, PUT, and DELETE to interact with resources.

• Client Sends a Request
  The client initiates an HTTP request to a specific API endpoint. The request includes:

  • A URL that identifies the resource, for example, https://api.twitter.com/users/johndoe
  • An HTTP method that specifies the action (e.g., GET to retrieve data)
  • Optional headers (e.g., authentication tokens, content type)
  • A body (for requests like POST or PUT, where data needs to be sent)

• Server Processes the Request
  The API server receives the request, interacts with the database if needed, and prepares a response.

• Server Sends a Response
  The server returns a response containing:

  • A status code (e.g., 200 OK for success, 404 Not Found for an invalid request)
  • A body containing the requested resource (formatted in JSON, XML, or HTML)

• Client Uses the Response
  The client processes the response and uses the data as needed—displaying it in an app, storing it, or triggering another API call.

REST API breaks a transaction down to generate a sequence of small components. Every component addresses a specific fundamental aspect of a transaction. This modularity makes it a flexible development approach.

RESTful Equivalents of CRUD Operations

A REST API leverages HTTP methods described by the RFC 2616 protocol. It uses the following HTTP requests:

• POST request  to create data (equivalent of CREATE)
• GET request to fetch data (equivalent of READ)
• PUT request to alter the state of data (such as an object, file, or block; equivalent of UPDATE)
• DELETE request to eliminate it (equivalent of DELETE)

Different HTTP verbs or status codes used by REST APIs can be seen here.

Why People opt for REST APIs

Here are a few benefits that have contributed to the increase in demand for REST APIs:

Scalability

REST API offers excellent scalability. As clients and servers are separated, a product can be scaled by a team of developers without much trouble.
Plus, it is easier to integrate REST with present sites without refactoring website infrastructure. This allows developers to work faster instead of spending time reworking a website from scratch. As an alternative, they can merely add extra functionality. This makes it the most used method of integration.

Flexibility and Portability

Users can easily communicate even if the REST client-server is hosted on different servers, offering an essential benefit from management’s perspective.

Independence

Thanks to the parting between client and server, the REST protocol makes it easy for developments across the different areas to occur autonomously. Moreover, the REST API is adjustable to the operational syntax and platform, offering the prospect to test numerous environments during development.

REST vs. SOAP APIs

Typical data transfer protocols, such as SOAP (Simple Object Access Protocol), offer excellent data security and integrity capabilities. Moreover, SOAP offers in-built retry logic to compensate for unsuccessful communications. But such protocols are also tough to work with. RESTful API is a simpler alternative that has developed exponentially in the past few years. People often get confused regarding REST standards. Compared to SOAP, older web services, REST is more flexible and easier to implement. Here are some of the difference between REST and SOAP:

1. Protocol: Soap uses XML as its message format and often relies on other protocols such as HTTP and SMTP for message transmission. Rest, on the other hand, is an architectural style that uses standard HTTP methods (GET, POST, PUT, DELETE) and can support various message formats, such as JSON or XML.
2. Message Format: The XML format that SOAP uses for structuring messages an be more verbose and complex compared to other formats while REST supports various message formats, with JSON being the most common due to its simplicity and ease of parsing.
3. Statefulness: SOAP can be designed either stateful or stateless, depending on the requirements. Rest, however, is inherently stateless. Each request from a client to a RESTful service contains all the information needed to fulfill that request.
4. Transport Protocol: REST primarily relies on HTTP for communication. SOAPS  uses protocols like HTTP, but it can work over other transport protocols as well.
5. Standards: As REST relies on standard HTTP methods and status codes, it is less prescriptive, and more flexible in implementation. SOAP follows specific standards like WS-Security for security features and has a standardized set of rules.
6. Performance: SOAP is usually less efficient due to the XML parsing overhead and the verbosity of the XML format. While REST performs better especially with smaller payloads and when using more lightweight data formats like JSON.
7. Error Handling: SOAP has standardized fault elements for error handling and REST uses standard HTTP status codes for error handling, providing a simpler and more consistent approach.
8. Tool Support: SOAP has well-established tools and frameworks, especially in enterprise-level environment, while REST has widespread support with various REST API tools and libraries, making it a popular choice for web and mobile applications.
9. Integration: SOAP is often used for enterprise-level integration and in scenarios where strict standards compliance is required while Rest is more suitable for web and mobile applications, providing a lightweight and flexible approach to integration.

Read about the differences between REST vs. SOAP in detail.

Benefits of REST API over SOAP

Bandwidth Usage

REST is usually preferred over the more robust SOAP as the former uses lesser bandwidth, making it more appropriate for the world’s comprehensive web services. It uses HTTP protocol to fetch data or carry out operations in several data formats (like XML and JSON); it allows for quicker processes. Consequently, SOAP uses XML data transfer, defining operations as unidirectional WSDL ports with several process instances sharing the same procedures. In REST, operations are described in messages themselves. Moreover, there’s a single direction for each process instance.

Coupling Method

SOAP and REST protocols have a difference in their coupling method. Specifically, SOAP has a tight coupling, while REST has a weak coupling. The tight coupling in SOAP means that modules are interdependent, and any changes to one can disrupt the operation of others. Weak coupling means that modules are independent and variations in one module don’t affect the operation of others. This provides flexibility and reusability when adding, substituting, or adjusting modules. On the other hand, tight coupling means that modules tend to be codependent. So, variations in one module can have a system-wide effect. All these differences are what make API RESTful.

Ease of Implementation

RESTful API is easier to implement than SOAP, thanks to its simpler data format and architecture. RESTful API doesn’t require a separate messaging layer to communicate between systems, making it a quicker alternative. Additionally, RESTful API is platform-independent, making it flexible and accessible across different programming languages.

Applications of RESTful API

Several applications and projects are using REST APIs to transfer data, and businesses are increasingly embracing RESTful web services to enjoy horizontal growth.

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Benefits of using REST APIs

REST APIs are the most commonly used APIs due to the several benefits they offer: Here is why developers prefer working with REST APIs:

1. Simplicity and Ease of Use:
   • REST APIs are relatively simple to understand and use as they follow standard HTTP methods (GET, POST, PUT, DELETE) and use standard conventions for resource representation (usually JSON or XML).
2. Scalability:
   • RESTful services can be easily scaled horizontally, as they are stateless. Each request from a client contains all the information needed to fulfill that request, which makes it easier to distribute and load balance.
3. Flexibility:
   • REST allows for a wide range of data formats, but JSON is most commonly used due to its simplicity and ease of parsing. This flexibility makes REST APIs suitable for various types of clients and applications.
4. Statelessness:
   • Each request from a client to a REST API is independent and stateless. The server does not need to store any information about the client between requests, simplifying the design and implementation of both the client and server.
5. Interoperability:
   • REST APIs are platform-independent and can be implemented in any programming language. Clients can easily consume them in different technologies, which leads to increased interoperability.
6. Cacheability:
   • REST supports caching mechanisms, allowing clients to cache responses. This improves performance and reduces the load on the server, especially for resources that don’t change frequently.
7. Uniform Interface:
   • It is easier for developers to work with RESTful APIs as they have a uniform and consistent interface. This uniformity can be attributed to standardization of resource URIs, HTTP methods, and representation formats.
8. Reduced Latency:
   • The stateless nature of REST eliminates the need for the server to store information about the client,  which reduces overall latency. Clients can include all the necessary information in each request, and servers respond with the required data.
9. Ease of Integration:
   • The development process with RESTful APIs is pretty straight forward as they can be easily integrated with different systems.
10. Security:
    • You can easily secure REST APIs with standard HTTPS protocols and establish a secure communication channel between clients and servers. Additionally, you can also implement authentication and authorization mechanisms to control access to resources.

Challenges of working with REST APIs

There is no doubt that REST APIs offer a host of benefits. However, that doesn’t mean that they don’t come with their own set of challenges. Here are some of the common challenges associated with using REST APIs:

1. Over-fetching or Under-fetching of Data: Clients may receive more data than needed (over-fetching) or not enough data (under-fetching) for a particular operation, which can lead to inefficient use of bandwidth and affect performance.
2. Limited Support for Real-time Communication: RESTful APIs are based on a request-response model due to which they are not ideal for real-time communication. You can employ techniques like long polling or WebSocket, but they are not inherently supported by REST.
3. Versioning: You need to implement changes as APIs evolve. However, managing backward compatibility and versioning can be challenging, especially when dealing with a large user base and multiple versions of clients.
4. Lack of Discoverability: Discovering available resources and their capabilities can be challenging without proper documentation. REST APIs often rely on external documentation, and there is no standard way to discover resources dynamically.
5. Security Concerns: While you can secure REST APIs using HTTPS and authentication mechanisms, security remains a concern. You must implement proper authentication, authorization, and encryption to ensure the confidentiality and integrity of data.
6. Statelessness: While statelessness is a benefit, it can also be a challenge in certain scenarios. Some applications may require server-side state management, which is not inherently supported by REST.
7. Complex Nested Resource Structures: When dealing with complex relationships between resources, designing clean and intuitive URIs can be challenging. Deeply nested resource structures can result in long and complex URIs, making the API less user-friendly.
8. Inadequate Support for Transactions: RESTful APIs typically lack built-in support for transactions that involve multiple operations. Coordinating multiple requests to ensure atomicity can be complex and may require additional design considerations.
9. Performance Overhead: REST APIs can have performance overhead, especially when dealing with a large number of small requests. This can be mitigated to some extent with techniques like batching or pagination.

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