What is API Testing?

API testing is a type of software testing that analyzes an application programming interface (API) to verify that it fulfills its expected functionality, security, performance and reliability. The tests are performed either directly on the API or as part of integration testing.

An API is code that enables the communication exchange of data between two software programs. An application typically consists of multiple layers, including an API layer. API layers focus on the business logic in applications, defining requests such as how to create them and the data formats used.

As opposed to user interface (UI) testing, which validates the application's look and feel, API testing focuses on analyzing the application's business logic as well as security and data responses. An API test is generally performed by making requests to one or more API endpoints and comparing the responses with expected results.

API testing is frequently automated and used by DevOps, quality assurance and development and testing teams for continuous testing practices. API testing is generally performed by using software to send calls to API endpoints to validate the system's response.

Why is API testing important?

UI tests are often inefficient for validating API service functionality and usually don't cover all the necessary aspects of back-end testing. This can result in bugs left within the server or unit levels -- a costly mistake that can delay the product release and could require large amounts of code to be rewritten.

API testing lets developers start testing early in the development cycle before the UI is ready. Any request that doesn't produce the appropriate value at the server layer won't be displayed on the UI layer. This lets developers eliminate at least half of the existing bugs before they become more serious problems. It also lets testers make requests that might not be possible through the UI -- a necessity for exposing security flaws.

Many companies use microservices for their software applications, as they enable software to be deployed more efficiently. If one area of the app is being updated, the other areas can continue functioning without interruption. Each application section has a separate data store and different commands for interacting with that data store. Most microservices use APIs; therefore, as more businesses adopt the use of microservices, API testing will become increasingly necessary to ensure all parts are working correctly.

API testing is also integral to Agile software development, in which instant feedback is necessary for the process flow. In Agile environments, unit tests and API tests are preferred over graphical user interface (GUI) tests because they're easy to maintain and more efficient. GUI tests often require intense reworking to keep pace with the frequent changes in an Agile environment.

Overall, incorporating API tests into the test-driven development process can benefit engineering and development teams across the entire development lifecycle. These benefits are then passed along to customers in the form of improved services and software products.

How to approach API testing

An API testing process should begin with a clearly defined scope of the program as well as a full understanding of how the API should work. It's crucial to incorporate DevOps practices throughout API development and testing, while also aiming to embrace both test-driven development and behavior-driven development methodologies. Early testing rather than testing later in the development lifecycle should also be considered, as it facilitates rapid iteration and enables team members to detect and resolve issues promptly after they're introduced.

Some questions to consider include the following:

• What is the API functionality?
• What endpoints are available for testing?
• What response codes are expected for successful requests
• What response codes are expected for unsuccessful requests?
• Which error message is expected to appear in the body of an unsuccessful request?
• What API testing tools should be used?

Once factors such as these and other testing requirements are addressed, testers can begin applying various testing techniques and writing code to test the API, if needed.

Test cases are written for the API and should define conditions or variables with which testers can determine if a specific system responds appropriately and performs correctly. Once the test cases have been specified, testers can perform them and compare the expected results with the actual results. Tests analyze responses such as the following:

• Reply time.
• Data quality.
• Confirmation of authorization.
• Hypertext Transfer Protocol (HTTP) status codes.
• Error codes.

The API testing process analyzes multiple endpoints, such as web services, databases or web UIs. Testers should watch for failures or unexpected inputs. Response time should be within a defined limit that teams deem acceptable and the API should be secured against potential attacks.

Tests should also be constructed to ensure users can't affect the application unexpectedly, the API can handle the expected user load and work across multiple browsers and devices. The test should also analyze the results of nonfunctional tests including performance and security.

Types of API tests

Various types of tests can be performed to ensure the API is working appropriately. Tests range from general to specific analyses of the software and testing can be automated or conducted manually. Manual testing is typically done without using test scripts but automated testing uses tools, frameworks and test scripts.

The following are some examples of manual testing:

• Exploratory testing. Testers actively explore the application to discover bugs. During black box testing, testers send requests to the API to confirm the expected output. Conversely, in white box API testing, they scrutinize and validate API functions using the source code.
• UI testing. While it isn't specific to an API, UI testing is equally important, as it evaluates user experience and verifies seamless integration with the interface.
• Ad-hoc testing. Ad-hoc testing occurs after formal testing, involving random checks to uncover vulnerabilities and assess overall API integration stability. It's employed when making minor tweaks or adding new features, enabling testers to promptly evaluate updates without predefined test case planning or documentation.

The following are some examples of automated tests:

1. Validation testing

Validation testing analyzes API projects based on three distinct sets of criteria: the API's usability as a product, its transactional behavior and its operational efficiency. The following are some typical questions asked during validation testing:

• Is the API designed in a way that meets its product goals or solves the problem it's supposed to?
• Were there any major coding missteps that would push the API in an unsustainable direction?
• Is the API accessing data in accordance with predefined policies?
• Is the API storing data in accordance with security or compliance rules?
• Would any code alterations improve the API's overall functionality?

2. Functional testing

Functional testing ensures the API performs exactly as it's supposed to. This test analyzes specific functions within the codebase to guarantee the API functions within its expected parameters and can handle errors when the results are outside the designated parameters.

3. Load testing

Load testing is used to see how many calls an API can handle. This test is often performed after a specific unit or codebase is completed to determine whether the theoretical solution can also work as a practical solution when acting under a given load.

4. Reliability testing

Reliability testing ensures the API can produce consistent results and that the connection between platforms is reliable.

5. Security testing

Security testing attempts to validate the encryption methods the API uses as well as the access control design. It includes the validation of authorization checks for resource access and user rights management.

6. Penetration testing

Penetration testing builds on security testing. In this type of test, the API is attacked by a person with limited or no knowledge of the software. This lets testers analyze the attack vector from an outside perspective. The attacks used in penetration testing can be limited to specific elements of the API or can target the API in its entirety.