What Is API Testing?

APIs are the buzzword of the software industry these days. They power most modern applications and allow seamless communication between different systems, services, and platforms.

From booking a cab to making online payments, APIs silently work in the background to connect everything. They not only save development time but also enhance scalability, flexibility, and innovation for businesses. In short, APIs act as the backbone of the digital ecosystem we rely on every day.

In this tutorial blog, let’s understand what APIs are, their different types, and then dive deeper to learn how to test them with various testing techniques.

What Are APIs?

API stands for application programming interface. It acts as a bridge that allows two software components to communicate with each other by following a common set of rules and guidelines.

In simple terms, they act like messengers that carry requests and deliver responses between applications. Whether it’s checking the weather on your phone or logging in to a website with your Google account, APIs enable these interactions. They simplify complex integrations, improve efficiency, and make it easier to design fresh and impactful digital experiences.

The following are the day-to-day examples of APIs:

• Online shopping: When you pay with PayPal or another gateway, APIs handle the secure transaction between the store and the payment provider.
• Booking flights and hotels: APIs connect airlines, travel agencies, and payment systems to provide you with real-time availability and prices.
• Social media sharing:  Clicking “Share on Twitter/LinkedIn” uses APIs to post directly from one app to another. 

Types of APIs Based on Their Architectures/Protocols

The following are the popular types of API architectures/protocols:

REST

REST stands for representational state transfer. It is a software architecture that lays out a set of rules and guidelines for designing and building APIs. An application that follows the REST architectural principles is often referred to as RESTful.

When a client makes a request through a RESTful API, it sends back a representation of the resource’s state to the requester or endpoint. This representation can be delivered in various formats over HTTP, such as JSON (JavaScript Object Notation), HTML, XML, plain text, or even language-specific formats like Python or PHP.

Among these, JSON is the most widely used because it’s lightweight, easy for humans to read, simple for machines to parse, and works independently of any programming language.

In RESTful APIs, headers and parameters play a key role by carrying details such as metadata, authorization, URIs, caching, and cookies. Both request and response headers provide essential information about the connection, including status codes and how the request is handled.

SOAP

SOAP stands for Simple Object Access Protocol. It is a protocol that enables different systems to share information in a highly structured manner. Each message is written in XML and placed inside a standard envelope that contains both the request and the necessary details about it.

The rules for this communication are defined in a WSDL (Web Services Description Language), which works like a contract between the sender and the receiver. While SOAP is commonly used over HTTP, it can also run on other protocols such as SMTP or TCP. This flexibility enables it to adapt to various types of infrastructure and communication needs.

SOAP relies on XML as the format for sending and receiving messages between an API client and server. It also defines four key aspects, or dimensions, that shape how the API protocol works:

• Envelope  –  sets the overall structure of the SOAP message
• Encoding  –  explains how different types of data should be represented
• Requests  –define the format and details of what the client sends
• Responses –  define the format and details of what the server sends back

SOAP APIs are often used in industries such as Financial, HealthCare, and Enterprise services, where data needs to be secure, accurate, and trustworthy.

RPC

Remote Procedure Call (RPC) is a protocol that allows a program to execute a procedure (function) on a remote server as if it were a local call, abstracting the underlying network communication. The client sends a request with the procedure name and parameters, and the server executes it and then returns the result.

It’s widely used in microservices, distributed systems, and inter-process communication where low-latency, function-oriented calls are needed.

The following are examples of RPC:

• JSON-RPC: A client calls getBalance(accountId) on a remote banking server via HTTP. The server processes it and returns the balance.
• gRPC: A client calls SayHello(name) on a remote service using Protocol Buffers over HTTP/2; the service responds with a greeting.

GraphQL

GraphQL is a query language for APIs and a runtime for executing those queries. It allows the client to specify exactly what data it needs, which reduces over-fetching(getting more data than needed) and under-fetching (requiring multiple requests to fetch all the data).

In GraphQL, clients can request exactly the data they need, avoiding any unnecessary information. All queries are handled through a single endpoint, which simplifies communication between the client and server. The GraphQL API uses a strongly typed schema that clearly defines the data structure and how different pieces of data relate to each other.

For example, GitHub provides a powerful GraphQL API that lets developers fetch exactly the data they need about repositories, users, issues, pull requests, and more.

WebSocket

The WebSocket API allows the client and server to communicate with each other in real-time. It establishes a two-way connection, allowing messages to be sent and responses to be received instantly, without repeatedly polling the server for updates.

Unlike HTTP APIs, which are request-response (client asks, server responds), WebSockets keep the connection open, so both client and server can send messages anytime.

For example, chat applications, stock trading platforms displaying live prices, multiplayer games where every move counts, or apps that push live notifications use WebSockets to keep information flowing in real-time.

Different Types of HTTP Methods in REST APIs

RESTful APIs have multiple HTTP methods, each serving a specific purpose in how clients interact with resources. They allow applications to perform various operations such as fetching, creating, updating, or deleting data. This structured approach ensures consistency and clarity in communication between client and server.

GET

A GET method is the most popular HTTP method used to fetch data from the specified resource or server. Its main purpose is to retrieve the data. GET APIs are considered safe as they do not create, update, or delete any data from the server.

GET APIs are idempotent, meaning executing the same request multiple times will produce the same result; it will not change the state of the server. However, the response might change if the underlying resource changes, for example, fetching the customer details with the GET API after new customers are added.

POST

A POST requests are used for creating a new resource on the server. It usually sends some data to the server in its body. This data can be in different formats, such as JSON, XML, or form data, and the Content-Type header tells the server which format is being used.

A POST request is generally not considered idempotent, as sending the same request multiple times might create duplicate data on the server.

PUT

The PUT method is used to update an existing resource by replacing it completely with new data.

When you use PUT to update a resource, the request body should include the full, updated version of that resource. If some fields are left out, the server usually treats them as removed or resets them to default values.

A PUT request is idempotent, which means sending the same request multiple times has the same effect as sending it once. Even if you repeat the request, the resource will stay in the same updated state.

PATCH

The PATCH method is used to make partial updates to an existing resource. Unlike PUT, which replaces the whole resource, PATCH lets you update only specific fields without affecting the rest. This behaviour of PATCH makes it more flexible and efficient than the PUT method.

DELETE

The DELETE method is used to remove a resource from the server. When a client sends a DELETE request to a specific URL, it permanently deletes that resource from the server. After deleting the resource, the server sends an HTTP response back to the client indicating the outcome of the deletion attempt.

OPTIONS

The OPTIONS method is used to determine the available communication options for a specific resource, such as which HTTP methods and headers are permitted.

It plays an important role in CORS (Cross-Origin Resource Sharing), where browsers send an OPTIONS request before making certain cross-origin requests (like POST) to check if the action is permitted.

Authentication and authorization play a critical role in all HTTP methods, since they involve CRUD operations. It is essential to verify the user’s identity before allowing them to perform any operation on the server.

API Testing Types

There are various ways to test an API, each with its own purpose. Below are eight of the most common approaches; teams can mix and adapt them in multiple ways to create a testing strategy that best fits their needs.

Unit Testing

A unit test checks a small, specific part of your code in isolation, without relying on other parts of the application. In APIs, a unit test can verify that the endpoint returns the correct response with the status code for the given request. It can also check whether an endpoint handles optional parameters correctly or returns the correct error message for an invalid request.

For example, a unit test for a GET API that returns a list of orders for the given Order ID should verify that the correct data is returned in the response along with a 200 status code.

Similarly, for a POST API, a unit test should verify that the API returns an error message with a 400 status code if the request body is not supplied with the request.

Integration Tests

Integration testing is performed to verify that the various components of the software are compatible with each other. In API testing, it helps in validating that different APIs, modules, or services communicate and transfer data efficiently as required between them.

Let’s take an example of an e-commerce application that has an Order API and a Payment API. The Order API creates an order, while the Payment API can process the payment for the orders. The integration testing ensures that the order status is updated correctly after payment.

Consider another example of a travel application where the Booking API creates a reservation for a user; the Notification API is responsible for sending the confirmation. Integration testing helps ensure that once a booking is successful, the user receives the correct notification.

Contract Tests

Contract testing ensures that the consumer (such as a client application) and the provider (such as an API) communicate according to the agreed contract. It verifies that the consumer sends requests in the expected format and the provider returns responses as defined.

Contract testing verifies that both sides send and receive data in the correct format, helping prevent integration problems caused by misaligned expectations.

For example, the Order Service expects the Payment Service to accept payments and return specific fields in the response. While performing Contract testing, the consumer test (Order Service) verifies that when it sends this request, it can correctly handle the expected response structure, whereas the provider test (Payment Service) ensures its API returns the fields exactly as defined in the contract.

JSON Schema Validation

A JSON Schema acts like a blueprint for JSON data. It sets the rules, structure, and constraints the data must follow, removing guesswork and making it easier to understand and predict how the data will look and behave.

JSON Schema validation helps identify problems in the structure of JSON responses and makes it easier to trace and fix the root causes of API failures. JSON Schema validation is widely used for validation related to API data, configuration, and data types.

Functional Tests

Functional testing is a key part of the software testing process. It ensures that APIs work as expected according to the business specifications. This type of testing typically checks field validations, data accuracy, JSON structure, and response correctness.

For example, in a login API, functional testing would verify that valid credentials return a success response with a token, while invalid credentials return the correct error message.

Security Tests

Authentication and Authorization are two key concepts for any application to be secure. Security is important in API testing because APIs often expose sensitive business data and user information.

Authentication ensures that the user or system accessing the API is genuine, while authorization defines what actions or data the user is allowed to access.

For example, in an e-commerce API, a logged-in customer (authenticated) can view their own orders, but only an admin (authorized) can access all customer orders.

Performance Tests

Performance testing is equally important in API testing to ensure the application can handle real-world traffic and workloads efficiently. It helps identify bottlenecks, response time delays, and scalability issues under different load conditions. By testing how APIs behave under stress, spikes, and concurrent requests, teams can proactively prevent system crashes or slowdowns.

For example, testing a payment API under heavy load ensures transactions remain reliable even during peak shopping seasons.

End-to-End Tests

End-to-end tests verify the entire workflow of an application, considering all connected services. The basic idea behind end-to-end testing is to ensure everything works together as expected.

In API testing, these tests validate real user scenarios rather than isolated endpoints. They help catch integration issues, problems related to data integrity, and gaps that unit or integration tests might miss. For example, an end-to-end test for an e-commerce application API could verify that a user can log in, add items to the cart, place an order, and receive a confirmation successfully.

Now that we’ve explored different testing techniques for APIs, let’s look at some additional strategies to test them more efficiently.

How to Test APIs?

When it comes to API testing, a structured approach helps uncover issues early and ensures smooth communication between services. The following checklist could prove helpful for you while testing APIs:

Understanding Requirements

• Understand the API requirement, and ask for clarifications on what and how to test.
• Seek clarity on the request and response fields, along with their data types, to ensure accurate validation and avoid misunderstandings during API testing.
• For POST, PUT, and PATCH requests, always clarify the structure of the request body and its JSON schema to ensure your tests align with the expected input format.
• API integration testing — review the functional specifications to understand field mappings and how data flows between APIs, ensuring smooth communication and accurate data handling across systems.
• It’s helpful to request the Swagger link so the API documentation can be easily accessed, reviewed, and understood.

Test Execution

• Verify that the GET, POST, PUT, PATCH, and DELETE APIs retrieve the correct responses as per the given request.
• Verify that the GET request retrieves the correct responses for the given request with query or path parameters. Verify that the data requested is correctly retrieved in the response.
• Verify for negative scenarios such as sending blank data in mandatory fields, incomplete request body, invalid data, etc., in POST, PUT, and PATCH APIs.
• Validate that the request body sent in POST, PUT, and PATCH APIs correctly creates/updates the specified data in the system.
• Verify the Data Type of Fields and respective values — Date Value, String, numbers, etc., in the response. The data should be as per the functional requirement specification document. Also, verify the JSON schema of the response, which should be as per the specification provided.
• Verify that test cases related to different Status Codes — 200, 201, 400, 401, 403, 404, 500, 503. Appropriate messages should be displayed for the respective status codes. Inquire if any redirection-related Status Code needs to be verified explicitly for any API.
• Validate the response data and its format to ensure it matches the request accurately and contains the expected values.
• Validate the response schema to ensure the API returns data in the expected structure and adheres to the defined JSON or XML format.
• Monitor API response time to ensure the service is performing efficiently and meets the expected speed under various conditions. Ideally, the response should be retrieved within milliseconds. Pay attention to response times that exceed 2–3 seconds, as this may indicate performance bottlenecks that need to be addressed.
• Verify that the APIs have correct Authorization and Authentication to ensure that only legitimate users can access them and that they have the proper permission for each action.
• Further diving into functional validation testing, verify the valid, invalid, and edge cases for the smooth functioning of the APIs.
• Verify data integrity to ensure that the data sent and received through the API stays correct and consistent throughout the process.
• Perform a round of end-to-end testing to ensure that all the integrated APIs are working properly. An edge case would be to hit the GET API after creating a record using POST to verify that the data created appears in the GET request. Similarly, hitting the GET API and expecting a 404 Status Code after deleting the record to check that the record was deleted successfully.

To provide an outline and assist with basic test scenarios, I have prepared some sample test cases and uploaded them to GitHub.

Tools for API Testing

The following is the list of tools that are currently available and used by many for API Testing:

• Swagger UI
• Postman
• Insomnia
• Bruno
• SoapUI
• Hoppscotch
• Rest-Assured
• SuperTest
• Axios
• Playwright
• Cypress
• OkHttp3
• Requests — Python
• Karate DSL
• CURL

Feel free to add more tools in the comments.

Final Words

API Testing is a buzzword in the software industry today. It is crucial to understand the requirements and clarify any doubts before proceeding with API testing. Every API serves a specific purpose, which must be accurately tested and thoroughly verified before it is released to production. There is no one-size-fits-all approach to testing APIs; the verification process depends on the API, its functionality, and its usage.

In my experience, performing functional, performance, security, and end-to-end testing provides a strong foundation for understanding the API’s behavior. While conducting these tests, you naturally gain deeper insights and identify additional scenarios that may require testing. Ask as many questions as necessary and continue testing until you are confident that the API is production-ready.