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

Conduct dynamic API load testing on virtually any type of system or infrastructure.
Get actionable load test data—see where the problems lie and solve them fast.

What is API Testing?

Many web-based platforms and Software as a Service (SaaS) organizations have created different APIs (Application Programming Interfaces) to allow their customers to interact with and consume data from the platform on their own terms. APIs typically allow machine-to-machine communication using industry standard languages or file formats. APIs free end users from the confines of using a default interface. Additionally, users can integrate controls and outputs into dashboards and custom applications, as well as automate common functions and procedures.

APIs have become an incredible tool for sharing data between different software platforms across the Internet. As businesses become more sophisticated and require real-time access to more data, many companies are building APIs to allow customers and partners to interact with and consume data in a manner that best suits their needs, but what is API load testing and how is it done?

In order to understand how API load testing services fit into your overall testing needs, we should first establish a basic understanding of what is API testing, why API testing is necessary, and how API testing is performed.

Whether the API is a RESTful application that uses XML or JSON, or a SOAP-based XML container, you should build test scripts that check response times and accuracy of the service. Once you’ve established the responsiveness and the accuracy of the API, it is important to perform two additional tests on the system – API load testing and stress testing.

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 Test APIs?

While 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 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 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 testing?

There are several API 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 test an API?

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

What are the main challenges of API testing?

Six main challenges to API 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 tools?

API 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 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.

How Do You Test a REST API?

REST APIs are more common due to the standard of the Resource Description Framework (RDF). Subsequently, more APIs are available for integrating one system with another. At their simplest, REST APIs consist of Uniform Resource Identifier (URI) requests, such as GET, POST and DELETE. While the complexity of a given API may be as simple as a single GET request, they are more complicated, requiring secure credentials to authenticate and supplying a list of different commands that can be executed, all with multiple variables.
A basic API test uses GET and POST commands to script authentication, read data from a system, POST new data to the system, and confirm the expected response. Once created, users can re-use them for a single-use test, as well as a load testing.

How Do You Test SOAP APIs?

Unlike REST APIs, which are considered a software architecture style, a SOAP API is considered a type of protocol. A SOAP API may use a Web Services Description Language (WSDL) to specify the available endpoints and data formats. Load testing SOAP APIs by defining the GET and POST parameters within HTTP/S requests. SOAP API load testing can scale up from one user to thousands of simultaneous users by increasing the user count in a load curve.

How to Test Web APIs

Web APIs are empowering external systems to tie into existing applications every day. For example, most common social media platforms have APIs that are used to connect users of one web application to another. Many platforms utilize multiple APIs to let developers of other applications interact with their own systems.

There are many tools available to perform testing on a web API (such as Selenium). It’ important to find a solution that can generate external commands from servers outside your network and validate the responsiveness and effectiveness of the API. For applications that expect to have hundreds of thousands of simultaneous users, simultaneous user load testing is an extremely important component of a web API testing process.

API Testing from Geographic Regions

While an API can be tested in-house to ensure it responds properly to commands and requests, in-house testing does not guarantee the API will be able to respond effectively to external requests across the Internet. Once an API has been deployed, it would ideally be tested using an external load testing service with nodes dispersed throughout the geographic regions of the target market.

Load Testing APIs with LoadView

The Loadview API platform allows you to perform comprehensive API load testing to ensure speed, response time, and scalability of your APIs. The platform can simulate thousands of users hitting your API every second to generate 100,000+ calls to your server per load testing session. Conduct dynamic API load testing on virtually any type of system or infrastructure and get actionable load test data to see where the problems lie so you can solve them fast.

On-demand and Configurable API Testing for Your Requirements

LoadView provides users with a powerful, on-demand API testing solution where you can input constant or dynamic variables into API requests and scale the number of simultaneous users up from locations around the world easily and with little overhead. LoadView can test virtually any type of API in existence. If necessary, custom C# scripts can be written to assist in the load test by generating random variables or passing parameters from one step of the API load test to the next.

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

Easily adjust virtual user allocation between different geo-zones.

How to Test Web APIs with LoadView

There are many tools available to perform testing on a web API. LoadView focuses on generating external commands from servers outside your network and validating the responsiveness and effectiveness of the API. For applications that expect to have hundreds of thousands of simultaneous users, simultaneous user load testing is an extremely important component of a web API testing process.

Load Testing SaaS APIs

As your SaaS product becomes more and more popular, users will begin demanding access to the services directly through APIs. As soon as users gain access to your APIs, they will likely start testing the limits of the SaaS system by sending many requests, one after the other, as fast as possible in order to create a real-time data feed into their own environment.

Proactively Testing your APIs is Key

Whether you expect your customers to perform such requests so frequently or not, they will eventually do so, to the point that it may overload your systems. Setting up and performing regular load tests on your API before your customers do it for you is a proactive way of knowing the limits of your system, at which point you can build in some safeguards to protect your users from accidentally making too many calls. Often, API managers will limit the number of requests per minute from an IP address, or limit the number of requests based upon the user account. LoadView makes it possible to load test SaaS APIs with ease!

Dynamic API Load Generation

Using context parameters, LoadView allows you to specify a dynamic list of variables to use within subsequent API calls during a load test. If necessary, you can also use C# to create a randomly generated value within specified constraints. LoadView support is available 24/7 to help you with any scripting issues and to help generate scripts for dynamic variables.

API Testing Automation

While testing APIs is clearly essential for both software and websites, it is often overlooked. APIs are easy targets for cyber-attackers, which is why continual testing is important. For example, are you sure that your API prevents unauthorized queries or submissions? Are you sure that someone can’t start guessing other user’s authentication tokens? Does your API provide error message when there’s a problem or are these properly hidden? There are a lot of security considerations when it comes to API usage. If you fail to automate testing, user data could be at stake.

Don’t Forget About API Security

It is recommended that in addition to API load and capacity testing, firms should conduct API security testing as part of their organization’s security protocol. An error in an API can bring down an entire network infrastructure, both internally for your organization and externally for the users who rely on your API for their systems.

API Monitoring

How can you fix this problem? By simply automating your API testing. This allows you to check for security problems without having to remember to test manually. There are a variety of solutions to do this, with Postman being one of the most popular open-source options. If you’re looking for a robust, commercially available option that comes with all the bells and whistles you’d need for true enterprise API testing, then you should consider LoadView and Dotcom-Monitor’s full suite of automated API testing and monitoring tools.

Why Monitor APIs?

APIs provide secondary interfaces for your application users to interact with a system. For example, if a system must be online 24/7, associated APIs should adhere to a similar Service Level Agreement (SLA). Third-party, external API monitoring is the easiest way to provide a non-biased verification that the API is performing within the SLA requirements. Even after you have built and performed tests to verify the API is working, it is best practice to setup ongoing monitoring to verify continuous service.

Ensure API SLAs are Being Met

The SLA report is a special report within the platform where you can review the SLA status of a single device over a period that you specify, for example daily, weekly, or monthly, so you can track your provider’s adherence to SLA requirements. By viewing your reports through a single interface, you can see why, and from where, SLA goals are not being met. These reports can also be shared with service providers.

LoadView API Load Testing—Beyond Cutting Edge

Unlike other tools on the market today, users don’t have to worry about specific hardware or software requirements with LoadView. It is completely cloud-based, so developers and engineers can focus their time creating, executing, and analyzing their API tests. LoadView is an outstanding API testing tool – from SOAP APIs and REST APIs, to performing other Web API level testing and validation – LoadView has you covered.

Not only does the Dotcom-Monitor suite cover REST, SOAP, and other APIs, but it also allows for nearly limitless configuration options and reporting tools. Having access to a performance testing tool like LoadView can literally change the way you run your website or business.

Try LoadView for free today and see how you can automate your API testing in minutes!

LoadView API Load Testing

LoadView isn’t your average load testing tool—it’s revolutionary dynamic testing software
that will change the way you look at your system and infrastructure.