A Comprehensive Guide to Different Types of APIs

APIs, or Application Programming Interfaces, are vital components in today’s software development landscape, providing the building blocks for applications to communicate and interact with each other. Let’s delve into five key types of APIs: REST, SOAP, WebSocket, gRPC, and GraphQL, to gain a clearer understanding of their functions, features, and ideal use-cases.

REST (Representational State Transfer) API

REST APIs are the most common type of API and adhere to a set of constraints that make them stateless, cacheable, and provide a client-server architecture. They are built on standard HTTP protocols and typically return data in JSON format, although they can support other formats.

REST APIs use standard HTTP methods for interaction, including GET, POST, PUT, DELETE, and others. Each method corresponds to a specific type of action that can be performed on the resources of the API.

The simplicity and flexibility of REST APIs have made them the go-to choice for many developers, but they can be inefficient when dealing with large amounts of data, as multiple requests may need to be made to retrieve all necessary data.

SOAP (Simple Object Access Protocol) API

SOAP is a protocol for exchanging structured information in web services using XML. It is highly extensible, allowing communication via several different transport protocols, including HTTP, SMTP, and more.

SOAP APIs are known for their robustness and are often used in enterprise environments. They offer built-in error handling and can be used with a variety of network protocols. However, SOAP’s reliance on XML can lead to large, verbose payloads, making it less efficient than other APIs for some use-cases.

WebSocket API

WebSocket API provides a persistent, full-duplex communication channel between a client and a server. Unlike REST and SOAP, which are request-response based, WebSocket keeps the connection open, allowing real-time data flow. This makes WebSocket APIs ideal for applications that require real-time functionality, like chat apps, online games, and live tracking systems.

While powerful, WebSocket APIs can be more complex to implement and require more resources to maintain the open connections.

gRPC (Google Remote Procedure Call) API

gRPC is a high-performance, open-source framework developed by Google. It uses the HTTP/2 protocol for transport and Protocol Buffers (protobuf), a high-performance binary data format, as its interface definition language.

gRPC supports four types of communication: unary (standard request-response), server streaming, client streaming, and bidirectional streaming. These features, coupled with its efficiency, make gRPC ideal for microservices architectures.


Developed by Facebook, GraphQL is a query language for APIs and a runtime for executing those queries. Unlike REST APIs, where you need to make requests to different endpoints to fetch related data, GraphQL allows you to make a single query to get exactly the data you need.

In GraphQL, clients define the shape and size of the response, which can lead to more efficient data loading and less data over-fetching. It’s a great fit for complex systems and microservices, where data is spread across different services.

Short conclusion

Each of these APIs has its strengths and use-cases, and understanding them can help in making informed design decisions. REST APIs are a popular choice due to their simplicity and wide adoption. SOAP APIs work well in enterprise environments due to their security features. WebSocket APIs excel in real-time applications. gRPC APIs shine in high-performance microservices, and GraphQL APIs offer flexible data retrieval, which is excellent for complex systems.


Data FormatTypically JSON, but can support multiple formatsXMLTypically JSON, can be any data typeProtocol Buffers (binary format)JSON
Communication TypeOne-way communication (request/response)One-way communication (request/response)Two-way communication (full-duplex)Bidirectional streamingOne-way communication (request/response)
Service DiscoveryNot standardized, often uses OpenAPI (Swagger) for documentationStandardized via WSDLNot standardizedNot standardized, but supports server reflectionNot standardized, introspection for schema discovery
PerformanceVaries, generally lower than binary protocols due to textual natureLower due to verbose XML formatHigher due to full-duplex communication and avoidance of HTTP’s overheadHigh (binary protocol, HTTP/2 benefits)Varies, efficient data loading can improve performance
Use CaseGeneral purpose, wide adoptionDistributed enterprise environments, legacy systemsReal-time applications, when server needs to push updatesMicroservices, performance-critical systemsWhen flexible and efficient data loading is needed
ComplexityLow to moderateHighModerateModerate to highModerate

Some general differences in the approach to testing various types of APIs:


  • REST APIs are stateless and can be tested individually for each endpoint.
  • Since REST APIs typically use HTTP, tools like Postman, Curl, or REST-assured (in Java) are commonly used.
  • Each endpoint is tested with different HTTP methods (GET, POST, PUT, DELETE, etc.), and assertions are made on the status code, response time, and response body.
Cheat list for tester:
  1. Endpoint testing: Ensure each endpoint behaves as expected with valid inputs.
  2. HTTP Status Codes: Confirm that the correct status codes are returned for different situations.
  3. Error handling: Send invalid data and check if the API handles errors gracefully.
  4. HTTP Methods: Test the behavior of all relevant HTTP methods (GET, POST, PUT, DELETE, etc.).
  5. Data validation: Make sure the API correctly handles data in different formats, sizes, and structures.
  6. Authorization & Authentication: Verify that the API correctly implements and enforces security measures. 


  • SOAP APIs are also stateless and can be tested on an operation basis.
  • Given that SOAP APIs use XML, assertions are typically performed on the XML response.
  • Tools like SoapUI are often used for testing. Contract-first testing can be performed as SOAP APIs are well-defined by WSDL.
Cheat list for tester:
  1. WSDL file: Validate the WSDL file for syntax errors or inconsistencies.
  2. SOAP Requests/Responses: Ensure that the SOAP requests and responses are formatted correctly.
  3. Error Handling: Check that the API correctly returns faults for invalid requests.
  4. Data validation: Test the API’s ability to handle data in different formats, sizes, and structures.
  5. Security: Confirm the API correctly implements WS-Security standards.

WebSocket API

  • Since WebSockets provide full-duplex communication, testing can be a bit complex. It’s necessary to test connection, disconnection, message sending, message receiving, and error scenarios.
  • Tools like Postman, WebSocket King, or libraries like websocket-client (Python), WebSocket-Node (Node.js) can be used for testing.

Cheat list for tester:

  1. Connection: Test the WebSocket connection and disconnection functionality.
  2. Message Sending/Receiving: Ensure messages are correctly sent and received in real-time.
  3. Error Handling: Test the API’s ability to handle errors and unexpected disconnections.
  4. Concurrent Connections: Check the behavior with multiple simultaneous connections.
  5. Data Format: Ensure the API correctly handles data in the expected format.


  • Given that gRPC uses HTTP/2, a tool that supports this protocol is needed. The protocol buffers (protobuf) also require special handling.
  • Testing tools include grpccurl and ghz. In addition, unit testing can be done using various language-specific libraries.
  • As gRPC supports four types of communication (unary, server streaming, client streaming, and bidirectional streaming), testing of each of these types is required.

Cheat list for tester:

  1. Protobuf message validation: Ensure that the Protobuf messages are structured and formatted correctly.
  2. Unary & Streaming Calls: Test both unary calls and all three types of streaming calls (server, client, bidirectional).
  3. Error Handling: Test how the API handles invalid requests or data.
  4. Performance: Benchmark the API’s performance, particularly in high-load scenarios.
  5. Security: Validate the API’s implementation of certificate-based authentication and encryption.


  • Unlike REST and SOAP, GraphQL is not based on services but rather on the data schema, leading to a different approach in testing. You’re essentially verifying the data loads correctly according to the queries and mutations.
  • Tools like Postman, Apollo Client Developer Tools, or even browser-based IDEs like GraphiQL can be used for testing.
  • It’s also common to test for over-fetching and under-fetching, as GraphQL allows clients to specify exactly what data they need.

Cheat list for tester:

  1. Query validation: Ensure that queries return correct and expected data.
  2. Mutation testing: Check that mutations correctly create or modify data.
  3. Error Handling: Ensure the API returns errors correctly for invalid queries/mutations.
  4. Data validation: Test the API’s handling of data in different formats, sizes, and structures.
  5. Over-fetching & Under-fetching: Verify that clients can correctly specify the data they want, without over-fetching or under-fetching.

Testing Various APIs with Postman

Postman is a highly popular tool among developers for designing, testing, and documenting APIs. It provides a simple and user-friendly interface to send different types of HTTP requests. Regardless of the API type you’re working with – REST, SOAP, WebSocket, gRPC, or GraphQL – Postman is a tool that can simplify and streamline your testing process.


Testing REST APIs with Postman is straightforward. Given REST APIs utilize HTTP protocols, you can easily send requests to your endpoints using methods like GET, POST, PUT, DELETE, etc. Postman provides options to set headers, request body, query parameters, and allows you to create environments to manage variables.


Although SOAP APIs typically use XML payloads and are more verbose, Postman can handle them as well. To test SOAP APIs, you can send a POST request to the SOAP endpoint with the XML body and headers (like Content-Type: text/xml) set appropriately.

WebSocket API 

Postman introduced support for WebSocket APIs with version 8.0. You can send WS (WebSocket) and WSS (WebSocket Secure) requests, and maintain a history of transmitted messages. With WebSocket connections, Postman allows you to send and receive messages, disconnect and reconnect to the socket, and see status updates.


Postman also allows you to test gRPC by importing a protofile. Testing is similar to REST testing. 

GraphQL API 

Postman has solid support for GraphQL APIs. You can send GraphQL queries in the request body, use GraphQL variables, and even auto-complete queries in Postman thanks to its GraphQL schema support. Postman also allows importing and validating GraphQL schemas.

While Postman does provide support for testing other types of APIs such as SOAP, WebSocket, gRPC, and GraphQL, the full range of its features like pre-request and tests may not be as readily applicable or may require workarounds to function effectively. Therefore, when testing these types of APIs, developers might need to explore other specialized tools or extensions to achieve a similar level of testing depth and automation.


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