API testing

What is API testing?

API testing is a process that confirms an API is working as expected. There are several types of API tests, and each one plays a distinct role in ensuring that the API's functionality, security, and performance remain reliable. Developers can run API tests manually, or they can automate them with an API testing tool.

Traditionally, API testing has occurred at the end of the development phase, but an increasing number of teams are running tests earlier in the API lifecycle. This approach to API testing, which is known as "shifting left," supports rapid iteration by enabling teams to catch and fix issues as soon as they are introduced.

Here, we'll discuss the role that API testing plays in an API-first world—and clarify the relationship between API testing and API monitoring. We'll also review some of the most common approaches to API testing, as well as some best practices. Finally, we'll discuss how the Postman API Platform enables teams to implement an effective API testing strategy that meets their unique needs.

Why is API testing important in an API-first world?

Today's software landscape is highly competitive, and users are increasingly unwilling to tolerate unreliable applications. An issue at an application's API layer can lead to user-facing errors or latency, which can erode customer trust, lead to churn, and negatively impact the business. This puts enormous pressure on development teams to deliver APIs that are consistently available and highly performant.

Many teams have chosen to tackle this challenge by adopting the API-first development model, in which applications are conceptualized and built as a collection of internal and external services that are delivered through APIs. This strategy treats APIs as crucial infrastructure components, which makes API quality a top priority. API testing plays a central role in the API-first approach, as it enables teams to continuously verify the quality, health, and performance of their endpoints as they work to deliver a seamless digital experience.

What are the different types of API testing?

There are many ways to test an API, and each one serves a unique purpose. The following list represents four of the most common approaches, but there are endless variations within each category that teams can use to build a customized API testing strategy.

Contract testing

An API contract is a human- and machine-readable representation of an API's intended functionality. It establishes a single source of truth for what each request and response should look like—and forms the basis of service-level agreements (SLAs) between producers and consumers. API contract testing helps ensure that new releases don't violate the contract by checking the content and format of requests and responses.

Unit testing

API unit testing is the process of confirming that a single endpoint returns the correct response to a given request. Unit tests may validate that an endpoint handles optional parameters correctly, or that it returns the appropriate error message when sent an invalid request.

End-to-end testing

Whereas unit tests help developers ensure that individual endpoints are working as expected, end-to-end tests are used to validate key user journeys that may involve multiple endpoints and APIs. End-to-end API testing involves chaining requests together and confirming that each one is working properly, which helps teams surface issues in complex workflows before users do.

Load testing

API load testing enables developers to confirm whether their API is able to operate reliably during times of peak traffic. It typically involves using a testing tool to simulate large request volumes and measure the resulting response times and error rates. This type of testing is often performed in anticipation of a significant load increase, such as right before a product launch or yearly sale.

Security testing

API security testing involves identifying and resolving security vulnerabilities within APIs. This type of testing is designed to discover any potential weaknesses that may result in unauthorized access, data breaches, injection attacks, or other security risks.

Integration testing

API integration testing is a critical step in ensuring that different parts of a system are compatible with one another. It helps confirm that APIs can reliably and efficiently communicate and transfer data between one another—even as they evolve over time.

Functional testing

API functional testing verifies that an API meets its specified requirements. This type of testing involves sending specific requests to the API, analyzing the responses, and comparing the actual outcomes with the expected results to ensure the API performs as designed.

What is an example of API testing?

In order to better understand API testing, let's consider a healthcare app that enables users to book appointments with local doctors and view lab results. It connects with healthcare providers' systems via APIs so that patients can check appointment availability, request appointments, and access their medical results.

A crucial test for the app is to make sure it works well with healthcare providers' scheduling systems. Developers need to check that when a user picks a date and time, the app can immediately see if that slot is open, confirm the appointment, or suggest other available times if their first choice isn't open.

Another important test is to ensure that sensitive medical data is securely transmitted. The API must be tested to guarantee it can safely fetch and display lab results to the user without the risk of data interception or leakage. This includes checking encryption methods and making sure all data exchanges meet HIPAA regulations, which govern the security and privacy of health information.

These tests confirm that the app not only works well for users but also meets the strict security standards required in the healthcare industry.

What are some common bugs found in API testing?

The API testing process can surface a wide range of bugs and issues. Some of the most common ones include:

• Incorrect data formatting: API tests can help uncover responses that return data in the wrong format, such as JSON instead of XML, or vice versa. This can cause parsing errors in the client application.
• Missing data or parameters: API testing can reveal problems with API authentication or authorization, such as incorrect handling of API keys, tokens, or permissions, resulting in unauthorized access or denial of service.
• Performance and scalability problems: API load testing can determine whether an API can perform well under load and scale appropriately. Issues with performance and scalability can lead to slow response times, timeouts, or service disruptions.
• Concurrency issues: API testing can surface race conditions or threading issues in the API implementation, which can lead to unpredictable behavior or data corruption.
• Security vulnerabilities: API security tests can reveal security flaws, such as lack of encryption, exposed sensitive information, or insufficient rate limiting. They can also catch improper validation of input data, which can lead to SQL injection or cross-site scripting (XSS).
• Compatibility issues: API testing can detect when updates in a new API version cause compatibility issues with existing client applications, leading to broken functionality.
• Integration problems: API integration tests help teams uncover instances in which APIs fail to integrate correctly with other systems or services, resulting in data inconsistencies or interoperability issues.
• Cross-Origin Resource Sharing (CORS) misconfigurations: API tests help surface improper CORS configuration, which can cause cross-origin requests to fail and result in client-side issues.

What are the benefits of API testing?

API testing plays a crucial role in modern software development workflows, and its benefits cannot be overstated. These benefits include:

• Quality assurance: API testing helps preserve consumer trust and protect the business's reputation by enabling teams to continuously ensure their API performs as expected.
• Early issue detection and resolution: A shift-left approach to API testing allows teams to identify defects as soon as they are introduced. This makes the development process more predictable and helps teams stay on schedule.
• Resource conservation: More and more teams are automating their approach to API testing, which saves time and allows team members to focus their bandwidth on innovation.
• Rapid iteration: Many teams execute their API tests within CI/CD pipelines, which enables them to automatically validate every code change before it reaches production. This approach supports more frequent releases while reducing the risk of bugs and regressions.

What are some API testing best practices?

There are several best practices that teams should follow to implement an API testing strategy that is efficient and sustainable. These best practices are:

Create a dedicated testing environment

It's crucial for teams to perform API testing in a dedicated environment before they push changes to production. This approach enables them to contain any issues and avoid user-facing downtime. The testing environment should mirror production conditions as closely as possible, but it should include mock data that can be safely manipulated and replaced when necessary.

Automate your API tests

While manual API testing can help developers debug specific problems, test automation enables teams to systematize their approach in order to ensure consistent coverage and reduce the possibility of human error. Teams can use a variety of tools to create test suites and schedule executions to occur at specific times, at specific frequencies, or in CI/CD pipelines after every commit or push.

Run tests throughout the API lifecycle

The traditional approach to API testing, which occurs once the development process is complete, can allow issues to go undetected until they are deeply ingrained and difficult to fix. Teams should therefore run API tests at every stage of the API lifecycle. Certain test types will be more relevant at different stages; for instance, contract tests are typically written at the design stage and executed against all future iterations, while unit tests are usually written and executed during development and in CI/CD pipelines. Running tests early and often helps teams surface and remediate issues as quickly as possible so that they can deliver high-quality APIs to consumers.

Write reusable subtests

While every API endpoint serves a unique purpose and should therefore be tested with custom logic, there may be certain rules that are universally applicable. For instance, teams may wish to specify that every request must return a response in a certain amount of time, or that all responses must be formatted in JSON. Rather than implementing this logic over and over again, they can create subtests that can be reused throughout their test suite. This approach reduces the risk of human error and ensures consistency in how each endpoint is tested.

Keep your tests organized

It's important for teams to employ a logical and scalable organizational framework for their API test suite—especially as the API grows and changes. For instance, teams should tag each test according to its purpose, which makes it easier to execute batches of related tests with a single command. They should also create distinct test suites for each API resource—and keep unit tests separate from end-to-end tests. Staying organized will help ensure that test logic is not duplicated, that outdated tests are removed, and that new engineers are able to onboard as quickly as possible.

What is the future of API testing?

Postman's State of the API report indicates that developers, product managers, and other technical stakeholders are spending more of their time on API-related work. This trend points to the growing importance of APIs—and, by extension, API testing—to every company's digital strategy. As API testing becomes more ingrained in teams' workflows, it's likely that we'll see increased automation, a greater emphasis on API security testing, and a stronger push towards standardized testing practices within organizations.

Other common questions about API testing

What do you need to begin API testing?

In order to start testing an API, you'll need the API's specification or documentation, which will include detailed information about the available endpoints, parameters, and methods. You'll also need a list of common use cases to capture in your test logic, as well as a testing environment that mirrors the production environment as closely as possible. Finally, you'll need an API testing tool like Postman, which will simplify the testing process while supporting automation and collaboration.

How does API test automation fit in?

API test automation involves executing API tests at predetermined times or frequencies, or within CI/CD pipelines. It improves testing efficiency, reduces the risk of human errors, and enables teams to iterate rapidly and confidently.

What tools do you need for API testing?

The specific tools that you'll need for API testing will depend on your API's use case, architecture, and protocol. That being said, most teams will need an API client like Postman, a code editor like Visual Studio Code, and a collaboration tool like Jira

How do you introduce API testing at your organization?

An organizational approach to API testing requires careful planning and iteration, and it cannot be successfully implemented overnight. You'll need to start by identifying the stakeholders, goals, timelines, required resources, and expected outcomes. You'll also need to choose the types of tests to be performed, as well as the tools to be used. Once your team has written tests and created a testing environment, it's important to monitor the impact and identify opportunities for improvement.

Why use Postman for API testing?

The Postman API Platform includes several API testing features that help teams streamline their testing workflows and catch issues throughout the API lifecycle. With Postman, you can:

• Test any API with pre-configured code snippets: Postman includes a JavaScript-based library of code snippets that enable teams to easily author tests that validate their API's performance, reliability, and behavior. These snippets can be executed against a variety of API architectures, including REST, GraphQL, SOAP, and gRPC.
• Validate complex, end-to-end workflows: Teams can use Postman's Collection Runner to chain requests together, execute them in specific sequences, and log test results. This enables teams to create test suites that validate the most sophisticated, business-critical user journeys.
• Automate test executions on Postman Cloud: With Postman, users can schedule test executions to occur at specified times and frequencies—and get notified when tests fail. These executions will run on Postman's infrastructure, so teams don't have to worry about failures that result from unavailable local resources.
• Execute API tests within CI/CD pipelines: Teams can use Newman or the Postman CLI to run collections and tests within their CI/CD pipeline. This helps ensure backwards compatibility by safeguarding the production environment from breaking changes.
• Monitor test results and troubleshoot bugs: Postman users can debug test failures with the Postman Console, which enables them to inspect every network call—including headers, certificates, requests, and responses. They can also monitor results from manual and automated runs—and access reports to identify testing trends.
• Create a dedicated testing environment: Postman lets users store values in variables at the environment level, which can be used in any request's URL, headers, and body. This allows users to run automated API tests in a dedicated testing environment before deploying code to production.
• Incorporate Postman API tests into incident response workflows: Postman integrates with several third-party monitoring and incident response tools, such as Datadog, New Relic, and Opsgenie, which allows teams to augment their incident response workflows with Postman test data.