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What Is Load Testing? Everything You Need to Know

Mercy Kibet
Mercy Kibet
What Is Load Testing? Everything You Need to Know
September 20, 2022
min read

We cannot underestimate the importance of load testing for applications that run on the web. The competitive nature of the web requires that applications and systems respond promptly and constantly. Therefore, developers spend a lot of time and effort developing efficient systems that function smoothly even under high traffic conditions. A developer’s confidence in how much traffic their system can handle is crucial because it enables them to assess whether they have enough resources or need to invest in more hardware. This post defines what load testing is and why it is essential.

What Is Load Testing?

A load test is a set of scripts that automatically creates and simulates a high amount of traffic on your system. You write load tests in an automation language, such as TestComplete, Junit, or Selenium. Load tests are not real-life tests, but they simulate what may happen if your system receives a sudden spike in traffic.

As its name suggests, it’s a type of performance testing conducted to determine how a system behaves when it’s subjected to a significant load. For example, this load might come from many requests or a large amount of data processing. Load testing measures the system’s performance under different utilization levels and determines how much the system can handle. Load testing gives you an overview of how well your system would function with increases in visitors or requests. The testing helps you determine your system’s maximum capacity and, thus, helps you decide when to increase resources (e.g., CPU, memory).

Load Testing Basics

Load testing falls into two categories: synthetic testing and stress testing. These terms are often used interchangeably, but there is a clear difference between the two:

Synthetic Tests

Synthetic tests simulate actual patterns of user requests to uncover issues that typically occur only once in a while during real-life operation of your product or software. One example is simulating random crawlers from different IP addresses that hit your system.

Synthetic tests simulate a situation closely matched to real-life operations (e.g., performing an HTTP request for a website). The problem with synthetic tests is that it’s impossible to simulate all possible operations. For example, you can’t simulate a scenario where a site might suddenly collapse because of errors from multiple external sources.

You create a synthetic test by simulating many users or transactions per second, making them ideal for testing the system’s capabilities under load.

The problem with synthetic tests is that it’s impossible to simulate all possible operations.

Stress Tests

Stress testing simulates a real-life situation. It tests your system’s ability to complete the user-requested operation(s). This test determines the performance and feasibility of your system. Stress testing assesses a system’s ability to handle peak loads without failure. It subjects your system to higher throughput or concurrent users than it typically handles. For example, you can simulate days when you have a sale, like Black Friday.

The stress test samples the performance of your system under high use to determine its strength. A stress test determines the system’s ability to handle frequent and large-scale changes in load without fail.

Does Load Testing Test Functionality?

If you’re using the right approach and tools, then load testing will test performance but not functionality. Load testing determines if the system can handle a certain number of users or transactions per second.

You might think, “If my server can handle all the users, it will function properly.” However, this is not true. Load testing won’t tell you if your application or system is working appropriately or not. You need end-to-end integration tests to guarantee that your system operates a certain way (i.e., it works as intended). Therefore, load testing is not a substitute for integration testing, but it helps find bugs that may occur at the layer of the system you are testing.

How Does a Load Test Work?

You need to understand two critical things about load testing:​

First, how does one define a load? Here, a load refers to the number of concurrent users and requests made to your system.

Load testing has two fundamental processes: initiating and monitoring the load. The professional tester starts the test during the first process, initiating the load. We can do this in several ways, such as:

  • A system administrator starts loads via the command line or an API.
  • A client program sends a custom script to start a request from an external service (e.g., HTTP).

Next, we enter the monitoring phase. During peak loads, the system is usually actively processing requests. However, sometimes the machine is idle and processing lower to zero loads. We simulate this by lowering the load or by stopping all requests.

The second thing you need to understand is how a test script creates a load. Usually, you use test scripts to simulate many visitors to your system (adding as many connections as possible). For example, a load test starts by simulating 1,000 users all at once, but as the test progresses, it decreases that initial user count to 500. It then simulates another 1,000 users for a couple of minutes. The result is that you have many simulated users using your system. You can use this process to see how much traffic the website will handle when at peak use.

What Does a Load Test Measure?

First, how many users at a go can the system handle? We call this metric concurrent user, and we represent it as virtual users (vus) when load testing. This is one of the most critical metrics of your load test. It tells you how well your system performs when activity is high. When you perform a load test, simulate the concurrent user maximum within the first 5% of the total duration of your test. For example, if you allocate twenty-four hours for your test, try to reach your concurrent user maximum within 1.2 hours or less.

We call this metric concurrent user, and we represent it as virtual users (vus) when load testing. This is one of the most critical metrics of your load test. It tells you how well your system performs when activity is high.

Second, how long does it take to respond to the user's request? This is called the Average response time. This is the mean time a system takes to process users' requests after receiving them. The load testing tool calculates this by summing the total response times and dividing it by the requests count. The lower the response time, the better your system handles a high load.

Third, how many functions or virtual transactions per second (TPS) can your system handle? A low TPS rate might reveal serious problems, such as an overloaded database or a slow application server.

What Should You Consider When Choosing a Load Testing Tool?

  • Interface: A good load test tool does not need a complicated interface. The interface should be intuitive and easy for both experienced testers and beginners. A GUI that’s too difficult for inexperienced users or not intuitive enough for experienced users can lead to significant errors in testing because they cannot effectively test or troubleshoot deficiencies.
  • Compatibility: The tool you choose should be compatible with the type of request you want to simulate. The number of protocols, languages, and actions supported by the tool can significantly impact its ability to simulate requests accurately. For example, the load testing tool JMeter doesn’t support JavaScript, and thus, it doesn’t support Ajax requests.
  • Price: You should consider the price of a tool to determine if it’s cost-effective for your testing needs. For example, a tool like LoadRunner Professional might be pricey for a small startup but affordable to a larger startup.


Load testing puts demands on a system and measures its response. Load testing determines a system’s behavior under expected peak load conditions. It helps to identify the system’s maximum operating capacity, identify bottlenecks, and determine which system components are causing poor performance. By understanding how a system responds to increased load, you can be confident that the system will handle expected and unexpected demand. Now that you know what load testing is, you can leverage a tool like Waldo which helps you automate the testing process without the need to write code.

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