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What is API testing?

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

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.

A P I testing different types. Illustration.

What is the relationship between API testing and API monitoring?

API testing and API monitoring share the goal of ensuring that APIs remain reliable and performant, but these processes are typically performed at different stages of the API lifecycle. API testing occurs during development, and its primary purpose is to help teams catch issues before they reach production and impact users. API monitoring may utilize this same testing logic, but it occurs after the API has been deployed to production. API monitoring also involves gathering and visualizing API telemetry data, which teams can use to perform historical analysis and surface long-term performance trends.

A P I testing different types. Illustration.

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.

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.

Testing automation dashboard. Screenshot.