API Load Testing : The Ultimate Guide to Load Testing APIs Online

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What is API Load Testing?

API (Application Programming Interface) load testing is the process of testing the performance and scalability of an API under a simulated heavy load. This is done to ensure that the API can handle the expected traffic and provide consistent and reliable performance to its users.

Load testing an API is an important step in the development and deployment process of an API as it helps to identify and resolve any potential performance issues before the API goes live. This can help to prevent disruptions to the service and improve the user experience.

API load testing can be conducted in a variety of ways, depending on the specific goals and objectives of the testing process. Some common goals of API load testing include:

  • Determining the maximum capacity of an API and identifying any potential bottlenecks or vulnerabilities
  • Measuring the performance and efficiency of an API under different load conditions
  • Identifying the breaking point of APIs and determining their ability to handle unexpected spikes in traffic
  • Evaluating the stability and reliability of the API over an extended period of time

Types of API Load Tests

There are several different types of API load tests that can be conducted, depending on the specific goals and objectives of the testing process. Some common types of API load tests include:

Stress testing: This type of test is designed to determine the breaking point of an API by gradually increasing the load until the API fails or becomes unavailable. This helps to identify the maximum capacity of the API and any potential bottlenecks or vulnerabilities.

Soak testing: Also known as endurance testing, this type of test is designed to determine the stability and reliability of an API over an extended period of time. This is done by subjecting the API to a sustained load for a prolonged period of time (e.g. several hours or even days).

Spike testing: This type of test is designed to determine how an API responds to sudden, unexpected spikes in traffic. This can help to identify any issues with the API’s ability to handle sudden increases in load and ensure that it can recover quickly from these types of events.

Performance testing: This type of test is focused on measuring the performance and efficiency of an API under different load conditions. This can include measures such as response time, throughput, and resource utilization.

Factors to Consider When Load Testing an API

There are several factors that should be considered when planning and conducting an API load test. Some of the key considerations include:

Test environment: It is important to ensure that the test environment is as close as possible to the production environment in terms of hardware, software, and network configuration. This will help to ensure that the test results are accurate and representative of the real-world performance of the API.

Test data: It is important to use realistic and representative test data when conducting an API load test. This will help to ensure that the test results accurately reflect the performance of the API under normal conditions.

Test scenarios: It is important to define a set of test scenarios that reflect the expected usage patterns of the API. This can include both positive and negative test cases to ensure that the API is robust and can handle a wide range of input.

Load levels: It is important to define the load levels that will be used during the test and to gradually increase the load over time to simulate real-world usage patterns.

Tools and Techniques for API Load Testing

There are a wide range of tools and techniques available for conducting API load tests. Some of the most commonly used tools and techniques include:

Load testing tools: There are a number of specialized load testing tools that are designed specifically for testing the performance and scalability of APIs. These tools typically allow users to define test scenarios, set load levels, and monitor performance metrics in real-time.

Open-source tools: There are also a number of open-source tools that can be used for API load testing. These tools may not have all of the features of commercial load testing tools, but they can be a good option for developers who are working on a budget. Examples of open-source tools for API load testing include Apache JMeter and Gatling.

Cloud-based services: Another option for conducting API load testing is to use a cloud-based service. These services typically provide a range of tools and features for conducting load tests, and they can be particularly useful for testing APIs that are hosted in the cloud.

Custom scripts: It is also possible to create custom scripts for conducting API load tests. This can be a good option for developers who want to have fine-grained control over the testing process and who have specific requirements that are not met by existing tools. Popular scripting languages for API load testing include Python, Java, and Shell.

Manual testing: In some cases, it may be necessary to conduct manual testing to evaluate the performance and scalability of an API. This can be useful for testing APIs that are difficult to automate or that require a high level of human interaction.

API load testing is an important step in the development and deployment process of an API. It helps to ensure that the API can handle the expected traffic and provide consistent and reliable performance to its users. There are a wide range of tools and techniques available for conducting API load tests, and the most appropriate method will depend on the specific goals and objectives of the testing process.

Load Testing APIs

Load testing an API proves that the API, and the underlying infrastructure, can handle an expected number of simultaneous requests. Sometimes referred to as a volume test, a load test ensures that a system can handle a pre-defined volume of traffic.

Stress Testing APIs

Stress Testing an API tests the upper limits of simultaneous users by increasing the number of requests up to and beyond the theoretical capacity of the service. A stress test will increase load until the resources are overloaded in order to see how the system handles the recovery.

Design API Test Scenarios

Design multiple API test scenarios and gain insights into the overall performance of your systems.

LoadView API Load Testing—RESTful, SOAP, SaaS, & Dynamic

Conduct dynamic API load testing on virtually any type of system or infrastructure.

Why Load Test APIs?

While load testing an API from within your network should theoretically help discover any problems with your system, it is best practice to perform additional tests emulating the end user experience from outside of your network. Testing your API externally can identify response time averages from the perspective of an end user or third-party system. These average response time values server as a baseline performance metrics to which you can compare future responsiveness. Typically, external testing results are more representative of a customer’s experience than a low latency test from within your firewall. External API tests also can help identify problems that you may not experience while testing behind your firewall.

API Performance Testing Questions

There are a few questions you need to answer when setting up API performance tests, including the following:

• Who is your end user or target audience?
• Why are they using your API?
• What is the user trying to achieve with the API?
• How critical is the API to your users?
• What happens if the API is unavailable or unreliable?
• How fast do users expect to receive feedback from the API?
• How will you test each of these assumptions?

You’ve Answered the Questions, Now Create Your API Test Case

Once you’ve answered these questions, build API test cases to verify the needs of each test case are met. Depending on the answers these questions, different types of tests might be necessary to validate the test case assumptions. For example, submitting data to an API might only look for a “successful” response from the API. Likewise, sending a query may elicit certain keywords or values in the response from the server. The results of API testing also differ based upon the reason for testing. There are many different times during the development process, and even post-production, that you would want to test an API. Each of these instances may need to be setup differently.

What Will You Load Test in the API?

Now that you’ve answered some of the initial questions around the requirements, how will you know if the API has succeeded or failed? You need to design your test cases by specifying parameters like the following:

• Input parameters
• Expected resulting outputs
• Maximum time to receive a response
• Parsing inputs
• Error handling
• Proper response formatting

After each new code build, include each test case in a testing script and ensure that it runs successfully. Moreover, include each test case in a scheduled load test to verify that the API can handle simultaneous load.

Ways to Load Test APIs: Types of API Performance Testing

As you can see below, there are many additional names for tests and types of tests that can accomplish additional API testing goals. Due to the nature of the LoadView platform, we generally focus more on functional testing and load or stress testing APIs.

Integration Testing

Integration testing ensures that new changes to the API do not cause problems/bugs in other modules or system.

Load Testing

Load testing ensures that the production infrastructure can handle the expected number of simultaneous users accessing the system.

Regression Testing

Regression testing determines whether any new changes cause negative effects in previously successful tests for existing functionality.

Scalability Testing

Scalability testing is utilized to find how a system responds to changes in the number of simultaneous users. Systems are expected to scale up or down depending on the number of concurrent users, and adjusting the resources being utilized for consistent and stable user experience.

Security Testing

Security testing attempts to exploit potential vulnerabilities in a system or the underlying framework.

UI Testing

UI testing makes sure every aspect of the user interface functions as expected by testing every case possible using the GUI to ensure it is successful.

Functional Testing

Functional testing takes the system requirements and user stories and tests each use case to be sure the system can handle all necessary scenarios.

Stress Testing

Stress testing is like load testing in that it may take common use cases and run many simultaneous instances of the case at the same time. Stress testing takes the test one step further than load testing because it continues to push additional simultaneous users through the system until the system reaches a failure point. Perform stress tests on both a system-wide level, as well as on very specific components of a system. We’ve got a great article that examines the differences of load testing versus stress testing.

What tool is used for API load testing?

There are several API load testing tools available. Examples include Postman and SoapUI. Tools like these offer pipeline integration, asynchronous testing, GUIs, team collaboration, and, most importantly, the direct generation of API documentation.

How do you load test an API?

Load testing an API commences with a clear definition of the API. We next establish the scope of testing, followed by the application of testing techniques. These techniques can include value analysis, error guessing, and test cases.

Why do we test API?

Although API load testing covers sending data to meet requests and receiving output, the most crucial reason is data validation. The overall process can help ensure your API interacts correctly with users and produces the right results.

What are the main challenges of API load testing?

Six main challenges to API load testing span almost the entire testing sequence. It starts with the initial setup of the API test, then runs through the API function cycle toward data validation. An additional challenge lies in ensuring the API test tracks data accurately.

What are API load testing tools?

API load testing tools are various software programs or web applications that are used to run a variety of tests on APIs for uptime, load, and performance. API load testing tools are generally used by companies to verify the performance of their APIs, and also by end users to ensure that APIs which they are paying for (or using) are performing as expected.

Multiple Load Curve Tests for API Testing


Depending on the requirements of your API testing, the LoadView platform allows you to choose from multiple load curves. Define your API load tests to simulate traffic at the pace you need by implementing a load curve that increases the number of simultaneous users as necessary to properly test your system.

Load Step Curve

The Load Step Curve option generates load with a pre-defined number of concurrent users, allowing you to check the response time as the number of concurrent users increases over a specified time.

Goal-based Curve

The Goal-based Curve allows you to automatically adjust users to reach a required rate of transactions. This type of test is typically used to validate Service Level Agreements (SLAs) in production environments.

Dynamic Adjustable Curve

The Dynamic Adjustable Curve allows you to change the user load, in real-time, during a test. You start with a pre-determined number of concurrent users and can be adjusted between a defined minimum and maximum.

Worldwide API Load Testing Options

Test API availability under heavy load from different geographic regions around the world. LoadView allows you to distribute the load between from over 20 geographic regions in any manner that you want. For example, if most of your customers are in the East Coast of the United States, you may select to send 60 percent of your load test from the East Coast servers and distribute the other 40 percent between other Google locations. Why would you do this? Because speed matters and choosing the closest locations to your actual customers will provide the most effective emulation of real users during a load test.

API Load Testing

API Load Testing Options: Behind the Firewall

We touched on this briefly above in the Why Test APIs section, but the LoadView platform also has the flexibility to be utilized to test APIs that are not publicly available, from behind your firewall. The platform offers a few different options, depending on the requirements of your business. For example, for a typical load test, the load injectors are initiated dynamically, meaning that the IP addresses are not static and vary from load test to load test. However, if you require testing an API target behind your firewall, LoadView provides a proxy option to run tests with a static IP address, with the advantage of keeping these whitelisted for additional future tests.

Or, if due to internal security policies your organization cannot open your network to outside load injectors, LoadView provides an On-site Agent feature to execute load tests from within your own network, which does not require an organization to make their network accessible to outside traffic. For more information about setting up and configuring load tests from behind your firewall, visit our Knowledge Base page.

API Performance Testing Under Load

Most APIs are tested for accuracy and flexibility, but do you know how many concurrent or simultaneous connections your API can support? This question is often more difficult to answer than verifying the accuracy of the results from your API. Due to constraints such as simultaneous database users, RAM availability, pagefile management, and CPU usage, you may not be able to support as many simultaneous users as you think.

Load testing an API with LoadView can be as simple as creating a script that sends several calls to the API in a sequence and scaling up the number of simultaneous users to the upper limits of expected traffic. The scripts are reusable and can be used to monitor the system throughout the service period. And this is something you definitely can’t to with a tool like JMeter!

Finding API Performance Bottlenecks

Once you’re able to generate bottleneck conditions, you want to be able to identify why the system is grinding to a halt. Using LoadView in conjunction with MetricsView is an excellent way to identify the cause of such slowdowns.

API Keyword Validation

You can also specify keywords expected from the API server to establish a successful response. If the expected keywords are not found in the response, the individual testing session will be flagged with an error. You can also setup maximum response threshold timeouts, so if the API takes longer than the set threshold, the test will return an error.

API Response Times

Load tests will also record server response codes, such as 400 and 500 server error response codes. When reviewing load test results, you can see average test response times displayed in a chart, as well as the ability to dig deeper into reports to see individual testing sessions. This helps to visualize how the average response time is affected as you increase the number of simultaneous users.

Drill-down into API Reporting Data

Examine individual load test sessions to see error codes and utilize additional troubleshooting tools such as DNS traceroutes and waterfall charts, for single devices or tasks within a specific time period, along with metrics of the server response.

API Performance Metrics

By installing the MetricsView Private Agent behind your firewall on a system that can monitor the API servers, you can monitor the Windows Performance Counters related to the API functionality. Measuring metrics such as CPU usage, available memory, bandwidth usage, SQL database transactions, and Storage I/O can help determine if there is a hardware or software bottleneck preventing the API from completing transactions more efficiently. Gather performance metrics consisting of the response times of each session and validation that the API returns the expected results in each response.

RESTful API Load Testing with LoadView


LoadView testing for REST applications lets you define a list of steps to perform while interacting with an API through a series of GET/POST requests to the RESTful API server or URL. The series of API requests is saved as a script in the Dotcom-Monitor cloud and can be queued up to run simultaneously by hundreds or thousands of load injectors around the world.

By running a load test using nodes in multiple regions, you can increase simultaneous connections in the load test to the point that the API begins slowing down the average response time. Furthermore, adding more simultaneous users to the API load test should eventually stress the API server to the point that your RESTful requests begin to time out. When the API begins consistently timing out you will able to identify a bottleneck in the system.

Load testing an API with LoadView can be as simple as creating a script that sends several calls to the API in a sequence and scaling up the number of simultaneous users to the upper limits of expected traffic. The scripts are reusable and can be used to monitor the system throughout the service period.

HTTPS Load Task
Global Configuration

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LoadView API Load Testing

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