Most Frequently asked Interview Questions of api(2024)

Question: What is an API and how does it work?

Answer:

An API (Application Programming Interface) is a set of rules and protocols that allow different software applications to communicate with each other. It defines the methods and data formats that applications can use to request and exchange information.

APIs are used to enable the integration of different systems and services, allowing them to work together. In simple terms, an API acts as a “contract” between different software components, specifying how they can interact.

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How Does an API Work?

An API typically works by providing a set of endpoints, which are specific URLs that accept requests from a client (such as a web browser or mobile app) and return responses. These endpoints correspond to various functions of an application or service. The interaction between a client and an API involves the following steps:

1. Client Sends a Request:

   • A client sends an HTTP request to a specific API endpoint, typically using RESTful HTTP methods (GET, POST, PUT, DELETE).
   • The request may include parameters, headers, and a body, depending on the type of request and the data being sent.

2. API Processes the Request:

   • The API takes the incoming request, processes it by executing the corresponding logic, and interacts with the server or database to fetch or modify data.

3. API Sends a Response:

   • After processing the request, the API sends a response back to the client. This response typically contains data (in JSON or XML format) or status information (like success or error messages).
   • The response might include additional metadata like HTTP status codes (e.g., 200 OK, 404 Not Found, 500 Internal Server Error).

4. Client Uses the Response:

   • The client (e.g., a web or mobile application) processes the response and uses the data as required. For example, a web application might display the returned data to the user.

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Types of APIs

1. Web APIs:

   • These are the most common type of APIs. They allow communication over the web using protocols like HTTP/HTTPS.
   • Examples: RESTful APIs, SOAP APIs, GraphQL APIs.

2. Library APIs:

   • These are APIs provided by software libraries that allow applications to interact with the library’s functionality.
   • Examples: Java API, Windows API.

3. Operating System APIs:

   • These provide access to the underlying features of the operating system, such as file management, network services, and hardware interaction.
   • Examples: POSIX API, Windows API.

4. Database APIs:

   • These APIs allow applications to interact with databases to perform CRUD operations (Create, Read, Update, Delete).
   • Examples: SQL-based APIs, MongoDB APIs.

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Common API Formats

1. RESTful APIs:

   • REST (Representational State Transfer) is an architectural style that uses HTTP requests to perform CRUD operations on resources.
   • Resources (e.g., users, products) are represented as URLs, and HTTP methods like GET, POST, PUT, and DELETE are used to interact with them.
   • Data is usually returned in JSON or XML format.

2. SOAP APIs:

   • SOAP (Simple Object Access Protocol) is a protocol for exchanging structured information using XML.
   • It relies on XML messaging and typically operates over HTTP, although other transport protocols can be used.

3. GraphQL APIs:

   • GraphQL is a query language for APIs that allows clients to request only the data they need.
   • Unlike REST, where data is structured in fixed endpoints, GraphQL gives clients the ability to define the structure of the response, enabling more flexible data retrieval.

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Example of a REST API Request and Response

Let’s look at an example of how a REST API works in the context of a simple user management system.

API Endpoint:

GET /users/{id} – This endpoint retrieves the details of a user with a specific id.

Request:

GET /users/123
Host: api.example.com
Authorization: Bearer <token>

Response:

{
    "id": 123,
    "name": "John Doe",
    "email": "[email protected]",
    "age": 30
}

Here:

• The client makes a GET request to the /users/123 endpoint.
• The API retrieves data about user with id = 123 from its database and returns the response in JSON format.
• The client can then use this data to display the user’s details.

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Key Concepts of APIs

• Endpoints: These are the specific paths (URLs) where an API can be accessed. Each endpoint corresponds to a specific resource or action.

• HTTP Methods: APIs often rely on standard HTTP methods to define actions:

  • GET: Retrieve data.
  • POST: Submit data (often used for creating resources).
  • PUT: Update data.
  • DELETE: Delete data.

• Request Headers: These contain metadata, such as authentication tokens (e.g., Bearer tokens), content type (e.g., application/json), and other context information.

• Response Status Codes: These indicate the result of the request:

  • 200 OK: Success.
  • 404 Not Found: Resource not found.
  • 500 Internal Server Error: Server error.

• Authentication: APIs often require an API key or OAuth tokens to ensure that the request is from an authorized user or application.

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Why Are APIs Important?

• Integration: APIs enable different systems and applications to integrate and work together, regardless of their underlying technology.

• Automation: APIs allow for automation of tasks and processes by enabling communication between different applications and systems.

• Flexibility: APIs allow developers to build custom applications using data or services provided by other systems.

• Scalability: APIs enable scalable, modular systems. Services can be developed and maintained independently, and they can communicate over standardized interfaces.

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Summary

• An API (Application Programming Interface) is a set of protocols and tools that enable different software systems to communicate with each other.
• APIs work by exposing endpoints that handle specific actions or operations, allowing clients to send requests and receive responses.
• REST, SOAP, and GraphQL are common types of APIs used for communication over the web.
• APIs are vital for enabling system integration, automating processes, and creating flexible, scalable applications.

Question: What are the different types of APIs (REST, SOAP, GraphQL)?

Answer:

APIs (Application Programming Interfaces) are used to allow different software systems to communicate with each other. There are several types of APIs, and each type has its own characteristics, use cases, and protocols for interaction. The most common types of APIs are REST, SOAP, and GraphQL.

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1. REST (Representational State Transfer) APIs

Overview:

• REST is an architectural style for building web services that use HTTP methods to perform operations on resources.
• REST APIs are stateless and use standard HTTP methods (GET, POST, PUT, DELETE) to interact with resources.
• Data is typically exchanged in JSON or XML format, with JSON being the most common due to its simplicity and ease of use.

Key Characteristics:

• Stateless: Each request from the client to the server must contain all necessary information (e.g., authentication, data), as the server does not store session information between requests.
• Resources: Resources (like users, products) are identified by URLs. Each resource has a unique endpoint (URL) for interacting with it.
• HTTP Methods: Uses standard HTTP methods:
  • GET: Retrieve data.
  • POST: Create new data.
  • PUT: Update existing data.
  • DELETE: Delete data.
• Uniform Interface: The interaction between the client and server is standardized, which allows clients and servers to communicate in a consistent manner.
• Cacheable: Responses can be marked as cacheable or non-cacheable, improving performance by reducing redundant data requests.

Use Cases:

• Web services, mobile applications, and web apps where simplicity, scalability, and ease of integration are key.
• RESTful APIs are often used for CRUD (Create, Read, Update, Delete) operations.

Example:

• API endpoint: GET /users/123 — This might retrieve user details for the user with ID 123.

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2. SOAP (Simple Object Access Protocol) APIs

Overview:

• SOAP is a protocol for exchanging structured information in the implementation of web services, based on XML.
• SOAP APIs are more rigid than REST, as they use strict standards and formal messaging formats.
• SOAP is often used in enterprise-level applications that require high levels of security, reliability, and ACID-compliant transactions.

Key Characteristics:

• Protocol: SOAP is a protocol with a well-defined specification, including standards for message structure and rules for data exchange.
• Message Format: SOAP messages are always sent in XML format, which can be more verbose than JSON.
• Transport: SOAP can operate over multiple protocols, including HTTP, SMTP, TCP, and more.
• Security: SOAP supports WS-Security, which allows for message encryption, digital signatures, and authentication, making it suitable for applications that require strict security (e.g., financial services).
• Stateful/Stateless: SOAP can be either stateful or stateless, depending on the design.
• Error Handling: SOAP provides standardized error handling using <fault> elements in the response message, which is more structured than the error handling in REST.

Use Cases:

• Legacy systems and applications requiring strong security, high transaction reliability, and ACID properties.
• Enterprises that need to ensure the integrity of messages and deal with complex transactions (e.g., banking, insurance).

Example:

• SOAP API request in XML:

  <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:web="http://www.example.com/webservice">
     <soapenv:Header/>
     <soapenv:Body>
        <web:GetUserInfo>
           <web:userId>123</web:userId>
        </web:GetUserInfo>
     </soapenv:Body>
  </soapenv:Envelope>

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3. GraphQL APIs

Overview:

• GraphQL is a query language for APIs and a runtime for executing queries with your data.
• Unlike REST, which has fixed endpoints for each resource, GraphQL allows clients to request exactly the data they need from a single endpoint.
• GraphQL is flexible and efficient for applications where the structure of data changes frequently or where clients need to request multiple resources at once.

Key Characteristics:

• Single Endpoint: GraphQL uses a single endpoint for all interactions. Unlike REST, which has multiple endpoints for different resources (e.g., /users, /products), GraphQL exposes a single URL to interact with the server.
• Flexible Queries: Clients can request only the data they need, which minimizes over-fetching and under-fetching. This is particularly useful for mobile and web apps where minimizing data transfer is important.
• Strongly Typed: GraphQL schemas are strongly typed, meaning that the structure of the data is defined by the API schema, and clients can query exactly what they need.
• Real-Time Data: GraphQL supports real-time data updates through subscriptions, allowing clients to get real-time information when the data changes.
• Response Format: Like REST, GraphQL usually returns data in JSON format, but the data structure is more flexible.

Use Cases:

• Applications where the client needs to fetch complex or nested data from multiple resources.
• Mobile apps, modern web apps, and microservices architectures where efficient data fetching and real-time updates are needed.

Example:

A GraphQL query to fetch a user’s name and email:

{
  user(id: "123") {
    name
    email
  }
}

The response might look like:

{
  "data": {
    "user": {
      "name": "John Doe",
      "email": "[email protected]"
    }
  }
}

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Comparison of REST, SOAP, and GraphQL

Feature	REST	SOAP	GraphQL
Protocol	HTTP	XML-based (works over various protocols like HTTP, SMTP, etc.)	HTTP (Single endpoint)
Data Format	JSON (commonly), XML	XML	JSON
Statefulness	Stateless	Can be stateful or stateless	Stateless
Security	Uses HTTP security (HTTPS)	WS-Security (built-in)	Custom security (OAuth, JWT)
Complexity	Simple	Complex (due to strict standards)	Flexible and powerful (but may be complex to implement)
Error Handling	HTTP status codes	SOAP faults (structured errors)	Custom error handling (status codes)
Caching	Cacheable (if properly configured)	No native support for caching	No native support for caching
Use Cases	General web applications, mobile apps	Enterprise-level applications requiring high security and reliability	Complex applications with flexible, efficient data fetching needs

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Summary

• REST: Simple, flexible, and widely used, ideal for web services with basic CRUD functionality. It is stateless, uses standard HTTP methods, and usually returns data in JSON.
• SOAP: A more rigid and formal protocol used for enterprise-level applications requiring higher security, reliability, and complex operations. SOAP uses XML and supports stricter standards like WS-Security.
• GraphQL: A flexible and efficient query language that allows clients to request specific data from a single endpoint. It reduces over-fetching and under-fetching and is ideal for applications with complex data requirements and real-time updates.

Each API type has its own strengths and is suited to different application requirements. REST is the most commonly used API type for web and mobile applications, SOAP is best suited for enterprise-level applications with complex security needs, and GraphQL is gaining popularity for apps that require fine-grained control over data retrieval.

Question: Explain the difference between RESTful and SOAP APIs.

Answer:

RESTful APIs and SOAP APIs are two widely used approaches for building web services, but they differ significantly in terms of their architecture, protocols, communication styles, and use cases. Below is a comparison of the two:

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1. Protocol vs. Architectural Style

• RESTful API:

  • Architectural Style: REST (Representational State Transfer) is an architectural style, not a protocol. It leverages standard web protocols like HTTP for communication between client and server.
  • Stateless: Each request from a client to the server must contain all the information needed to understand and process the request. The server does not maintain any state between requests.

• SOAP API:

  • Protocol: SOAP (Simple Object Access Protocol) is a protocol that defines a strict set of rules for structuring messages, communication, and interactions between applications.
  • Stateful or Stateless: SOAP can be either stateful or stateless, depending on how the service is designed.

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2. Data Format

• RESTful API:

  • Primarily uses JSON for data exchange, though it can also use XML, HTML, and plain text. JSON is the most common due to its lightweight structure and ease of parsing.
  • Human-readable: JSON is easy to read and parse, making it suitable for integration with web and mobile applications.

• SOAP API:

  • Uses XML exclusively for message formatting. SOAP messages are XML-based, which makes them more verbose and complex than JSON.
  • Heavyweight: The XML format used in SOAP adds more overhead compared to the JSON format used in REST, making it less efficient in terms of size and parsing speed.

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3. Communication

• RESTful API:

  • Uses standard HTTP methods for communication:
    • GET: Retrieve data.
    • POST: Create new data.
    • PUT: Update existing data.
    • DELETE: Delete data.
  • Lightweight: REST is more lightweight, simpler to use, and typically faster because it leverages the standard HTTP protocol and does not require additional overhead.

• SOAP API:

  • SOAP uses a more complex message format based on XML wrapped in a SOAP envelope. It can operate over several protocols including HTTP, SMTP, TCP, and JMS.
  • SOAP has a higher overhead because it includes additional elements such as headers and body, and often includes extra security and transaction information.

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4. Security

• RESTful API:

  • Security is typically handled at the transport layer using HTTPS to encrypt the data. For authentication, REST APIs often rely on mechanisms like OAuth, API keys, or JWT tokens.
  • Security is implemented on top of HTTP, and there’s no built-in security specification in REST itself.

• SOAP API:

  • SOAP has WS-Security, which is a built-in standard for providing message-level security features such as encryption, digital signatures, and authentication.
  • SOAP is often used in applications requiring robust security features like banking, financial services, and enterprise systems.

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5. Error Handling

• RESTful API:

  • Relies on HTTP status codes to indicate the status of a request. Common HTTP status codes include:
    • 200 OK: The request was successful.
    • 404 Not Found: The requested resource was not found.
    • 500 Internal Server Error: The server encountered an error while processing the request.
  • Errors are often returned in a simple message, typically in JSON format, with additional error details if needed.

• SOAP API:

  • Uses SOAP Faults to handle errors. A SOAP Fault is a standardized way of communicating errors in the message.
  • SOAP Fault messages provide detailed error information with specific error codes, making them useful in enterprise systems where clear error reporting is required.

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6. Caching

• RESTful API:

  • Supports caching of responses as part of the HTTP protocol. Responses from REST APIs can be cached on the client-side or server-side using HTTP headers, which can significantly improve performance in some cases.
  • REST is cacheable when designed properly, which can optimize performance by reducing server load.

• SOAP API:

  • SOAP does not have native support for caching, and as a result, it is less flexible in terms of optimizing performance through caching. SOAP requests are often not cacheable due to their complexity and structure.

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7. Flexibility and Use Cases

• RESTful API:

  • REST is more flexible and suitable for web services where the interaction is typically stateless and resources (data) are easily represented as URLs. It’s widely used for:
    • Web services that interact with client-side applications.
    • Mobile app backends.
    • Microservices architectures.
    • Public APIs.

• SOAP API:

  • SOAP is rigid and more suited for applications that require strong standards for data exchange, security, and reliability. It’s ideal for:
    • Enterprise applications (e.g., banking, financial systems).
    • Systems that require ACID transactions and high reliability.
    • Applications needing complex security and reliable messaging.

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8. Performance

• RESTful API:
  • Because REST uses simpler HTTP messages and often returns data in lightweight JSON format, it generally has better performance than SOAP, especially in scenarios where high throughput and low latency are important.
• SOAP API:
  • SOAP tends to have higher overhead due to the verbose XML format, the additional SOAP envelope, and the complexity of processing the messages. As a result, SOAP is typically slower and consumes more bandwidth than REST.

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Summary of Key Differences

Feature	RESTful API	SOAP API
Protocol	Architectural style (uses HTTP)	Protocol (can use various transport protocols like HTTP, SMTP)
Data Format	JSON (commonly), XML, HTML, Text	XML only
Communication	Uses standard HTTP methods (GET, POST, PUT, DELETE)	Uses a predefined XML message format in SOAP envelope
Security	Handled by HTTPS, OAuth, API keys, JWT	WS-Security (built-in message security features)
Error Handling	HTTP status codes	SOAP Faults with detailed error information
Caching	Cacheable (with proper HTTP headers)	No native support for caching
Performance	Lightweight and efficient	Heavyweight due to XML and SOAP envelope
Flexibility	More flexible, used for modern web and mobile applications	More rigid, used for enterprise-level applications
Use Cases	Web services, mobile apps, microservices	Enterprise, secure transactions, complex systems

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Conclusion

• RESTful APIs are simpler, faster, and more flexible, making them ideal for modern web and mobile applications, where scalability, performance, and ease of use are key.
• SOAP APIs are more robust and feature-rich, particularly when enterprise-level security, reliability, and ACID transactions are necessary. They are typically used in legacy or complex systems that require strong data integrity and message security.

The choice between RESTful and SOAP APIs depends on the specific requirements of the application, including performance, security, and complexity needs.

Question: What are HTTP methods, and how do they relate to REST APIs?

Answer:

HTTP methods (also known as HTTP verbs) are a set of request types that define the action to be performed on a specified resource in a web application or web service. These methods are used to communicate with servers and are fundamental to the design and operation of RESTful APIs.

In the context of REST APIs, HTTP methods map directly to the standard CRUD operations (Create, Read, Update, Delete) used in databases. Below is an overview of the most commonly used HTTP methods and their corresponding actions in a REST API.

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1. GET (Retrieve Data)

• Purpose: The GET method is used to retrieve data from the server without making any changes to it. It is a read-only operation.

• Action: GET requests are used to fetch a resource or a collection of resources from the server.

• Typical Use Case: Fetching a list of items, details of a specific item, or a resource in general.

  Example:

  • GET /users: Retrieve a list of all users.
  • GET /users/{id}: Retrieve details of a specific user identified by id.

• Idempotent: A GET request can be safely repeated multiple times without changing the state of the resource.

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2. POST (Create Data)

• Purpose: The POST method is used to create a new resource on the server.

• Action: POST requests typically submit data to the server to create a new record in a database or perform some other creation operation.

• Typical Use Case: Creating a new resource, like adding a new user, submitting a form, or uploading data.

  Example:

  • POST /users: Create a new user by sending user data in the request body.

• Not Idempotent: A POST request may create a new resource or trigger an action that changes the state of the system, so repeating the request could result in different outcomes (e.g., creating multiple resources).

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3. PUT (Update Data)

• Purpose: The PUT method is used to update an existing resource or create a new resource if it does not already exist.

• Action: PUT requests are used to send data to the server to update a resource or replace it entirely with the new data.

• Typical Use Case: Updating a resource’s properties, like changing a user’s email address or modifying a product’s details.

  Example:

  • PUT /users/{id}: Update the details of a user identified by id (e.g., change their email or name).

• Idempotent: Repeating a PUT request with the same data will not change the result. It ensures the resource’s state is always the same after each request.

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4. PATCH (Partial Update)

• Purpose: The PATCH method is used to partially update an existing resource.

• Action: Unlike PUT, which typically requires the complete resource to be sent for updating, PATCH allows updating only specific fields or properties of a resource.

• Typical Use Case: Updating a specific property of a resource, such as changing a user’s phone number without affecting other data.

  Example:

  • PATCH /users/{id}: Update a specific field, like the user’s phone number or address, without changing the entire user object.

• Not necessarily Idempotent: Depending on the operation, the PATCH request might not be idempotent, though it can be made so in some cases.

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5. DELETE (Delete Data)

• Purpose: The DELETE method is used to remove a resource from the server.

• Action: DELETE requests are used to delete an existing resource, which is typically removed from the database or data store.

• Typical Use Case: Deleting a user, removing an order, or clearing a record from a database.

  Example:

  • DELETE /users/{id}: Delete the user with the specified id.

• Idempotent: A DELETE request is idempotent. If the same DELETE request is made multiple times, the outcome is the same — the resource is removed (or already absent).

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6. OPTIONS (Request Supported Methods)

• Purpose: The OPTIONS method is used to retrieve the allowed methods that a server supports for a specific resource or endpoint.

• Action: It allows a client to determine which HTTP methods are supported for a specific resource, without making a request to interact with the resource itself.

  Example:

  • OPTIONS /users: Request the allowed methods (e.g., GET, POST, PUT, DELETE) for the /users endpoint.

• Not Idempotent: This method is used to retrieve metadata and does not alter the state of any resource.

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7. HEAD (Retrieve Headers)

• Purpose: The HEAD method is similar to GET, but it only retrieves the headers of a resource, not the body content.

• Action: It is useful when you want to check if a resource exists, check metadata, or test a resource without downloading the full content.

• Typical Use Case: Checking whether a resource exists or testing caching policies without downloading the full resource.

  Example:

  • HEAD /users/{id}: Check the metadata (headers) of the user identified by id without downloading the actual user data.

• Idempotent: Like GET, the HEAD method is idempotent and can be repeated without changing the resource.

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How HTTP Methods Relate to RESTful APIs:

• CRUD Operations Mapping: In RESTful APIs, HTTP methods map directly to CRUD operations:

  • Create: POST
  • Read: GET
  • Update: PUT (or PATCH for partial updates)
  • Delete: DELETE

• Resource Representation: REST is based on the idea of resources (data entities), and HTTP methods define how those resources should be manipulated. A resource is typically represented in the form of a URL (e.g., /users, /products/{id}). The HTTP methods define what kind of action you want to perform on that resource.

• Statelessness: RESTful APIs rely on HTTP methods to interact with the server, and each request is stateless — the server does not store any client state between requests. Each request must contain all the information necessary for the server to understand and process it.

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Example of RESTful API and HTTP Methods:

Consider a RESTful API for managing users:

• GET /users: Retrieve a list of all users.
• GET /users/{id}: Retrieve details of a specific user.
• POST /users: Create a new user.
• PUT /users/{id}: Update details of an existing user.
• PATCH /users/{id}: Partially update a user’s details.
• DELETE /users/{id}: Delete a specific user.

Each of these operations uses a different HTTP method to perform different actions on the same set of resources (users).

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Conclusion:

HTTP methods are integral to RESTful APIs because they define the operations (CRUD actions) that can be performed on resources. The appropriate use of these methods ensures the proper functioning of the API and adheres to REST principles such as statelessness, simplicity, and scalability.

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