What is API Testing?

API testing, also known as Application Programming Interface testing, is a type of software testing that focuses on verifying and validating the functionality, reliability, performance, and security of APIs (Application Programming Interfaces).

Application Program Interface Testing (API) is an integral part of the application development process. Throughout the software development cycle, many aspects of security, performance, functionality and reliability can all be tested via the APIs without the need for any kind of user interface. The testing of any web service primarily involves the sending of requests to the API, fetching and validating output, then verifying the response of the service against expected results for accuracy of data, data format, HTTP status response codes and error codes amongst others. 

Types of API Testing

The primary objectives of API testing are to ensure that the API behaves as expected, conforms to the specifications, and functions correctly under different scenarios. Let’s look at some different types of API testing.

API Unit Testing

In unit testing, you test a single unit of code that’s logically isolated in a system. Units are individually tested for correct operation. This testing is crucial because when applied correctly, it can detect flaws at a very early stage. Bugs detected as a result of a failing unit of code can be more difficult to analyse during the later stages of testing.

Unit tests help to ensure that refactored code does not inadvertently break the specific function or intent of that unit of code. For a single REST service, consider a unit test to be a single call to the API with a fixed set of input parameters and a single expected result. On an Ecommerce application, a REST service for which you can send a GET request to return a single product would be such an example. Tests might assert that a 200 response is returned with a JSON body containing the product ID and some additional product information.

API Contract Testing

API contract testing is undertaken to verify that two separate services can work together in sync and establish seamless communication. In a contract, all interactions made between services are captured and stored, helping to validate messages from both systems and ensure they adhere to a common protocol. 

API Integration Testing

This is a form of API testing most commonly undertaken to check successful integration between internal and external or third-party services. It helps us to ensure that these services, when connected, can work in unison establishing successful data flow and onward communication. This is very important when the services in question have been designed, built and tested by different teams and sometimes different companies.

API End-to-End Testing

As the name suggests, end-to-end API testing is conducted to determine the complete end-to-end flow of data and information between numerous different APIs in a logical functional sequence. We do this to validate that a customer journey can be completed successfully in its entirety.

For example, on an eCommerce website, a functional end-to-end API test might consist of:

• Product search

• Add product to cart

• Register a new customer

• Add a payment method

• Complete a purchase

• Display Order history

  
  
  

This will include a number of services and the capture and onward transmission of data between them. 

API Performance Testing

API Performance testing helps to ascertain whether or not a service or a collection of web services as a whole, is operating within a series of non-functional boundaries as determined by the product owner or the business. There are differing types of API performance and reliability testing and monitoring that teams might wish to undertake but common metrics to review typically include:

• Uptime of services

• CPU and Memory usage

• Requests per minute handled

• Latency

• Errors per minute

Load Testing

By performing load testing in API testing, developers and QA teams can ensure that the API can handle the expected load and provide consistent performance even under high user demands. It helps uncover and address performance-related issues, enabling the API to deliver a smooth and responsive experience to its consumers.

Benefits of API Testing

The widely accepted test automation pyramid states that your unit tests should be more numerous than your API tests, which in turn should be more numerous than your UI tests. API testing offers multiple benefits that help to improve the performance of any application.

Let’s look at a few benefits below:

Find functional and performance quality issues faster without the need for a user interface

The foremost benefit of conducting an API test is that it helps you to validate the correctness of REST service functionality and performance, early in the development lifecycle. You don’t need to wait for the complete development of the UI. API testing helps you to isolate test cases and resultant bugs as early as possible, helping to deliver higher quality applications. It is well understood that the further back in testing bugs are found, the more costly they are to resolve. Good API testing helps to mitigate this.

Reduced test maintenance and quicker test runtime - API tests are not flakey nor inconsistent, as UI tests can be

Application User Interfaces are often under continuous change and as a result, too much automated testing via the frontend can result in brittle tests that fail too often and without the intended test actually revealing anything about the quality of the application at all. A good supporting bed of API tests ensures that you can fully exercise the application business logic without a UI and will run significantly quicker than a test that needs to spin up a browser in order to execute.

Reduced cycle time for bug detection, analysis, fix and redeploy

APIs and particularly microservices, are specifically built to perform a very limited and well documented function. By conducting an API test, you can easily track exactly where your test has failed and the root cause of that failure. As a result of this quick identification of bugs, the time taken to analyse, fix and redeploy a specific API fix to a test environment for retest is significantly reduced.

Before you leave

API testing can be performed using various tools and techniques, such as manual testing, automated testing frameworks, test harnesses, and specialized API testing tools. These tools assist in sending requests to the API, validating responses, capturing and analyzing network traffic, and generating reports.

By conducting comprehensive API testing, developers and QA teams can identify and resolve issues early in the development lifecycle, enhance the overall quality and reliability of the API, and ensure smooth integration and interoperability between different software systems.

Test Evolve Spark supports automated testing of REST and SOAP Services with both Ruby and JavaScript and offers a fully featured FREE evaluation for 30 days.