Testing Guides

Load testing (including stress testing, spike testing, and stability testing) is the technique of simulating real users interacting with your site, to see how it performs under heavy load. Proper load testing helps to reduce the risk of site failure in the event of a traffic surge.

We've all seen websites crash under a flood of traffic. When it happens to your site, it's costly and stressful! Preparing your site so it doesn't crash under heavy traffic is often a worthwhile investment, but how can you predict the breaking point at which your site will crash or become slow under heavy load?

Your customers want to build on an API that's fast and reliable. Performance problems present a serious risk to your API's stability, but a little performance testing in the right places can greatly reduce your risk of API failure. Because they're meant to be consumed programmatically, APIs are usually great candidates for automated, including load and performance testing.

Load testing, stress testing, and DDoS attacks all generate heavy traffic against a web server, so the terms get conflated. This guide explains what each one actually means, who does them and why, and how to tell whether a particular test is legitimate QA or something else.

Load testing and stress testing both simulate heavy traffic on a website, API, or application, but the motivations, methods, and expected outcomes are different. This guide explains the difference, when each is appropriate, and how most teams end up doing both.

Performance tuning web applications isn't easy, but it helps to have a methodical and scientific approach. Tuning gets a lot easier when you have a way to generate consistently reproducible traffic and measure the impact of each and every change. Easier said than done? Here's a quick and dirty approach that gets you most of the way there.

Web applications have more moving parts than static sites — authentication, stateful flows, databases, real-time updates, and client-side rendering all interact under load. Stress testing a web application means pushing it past capacity while watching these components for degradation, failure, and recovery.

We make a load testing tool, so it would be tempting to say that you should load test your site early and often, extensively, all the time, no matter what! But the real answer is: it depends. Sometimes it isn't worth it. There are hard and soft costs to load testing that you should weigh against the likelihood and severity of the risk you're trying to mitigate.

The load testing tool landscape is broad. Some tools are free and open source, others are commercial SaaS. Some test at the HTTP protocol level, others automate real browsers. Some are built for developers, others for QA teams. This guide covers the main categories and notable tools, and how to pick one that matches your needs.

Capacity planning means making sure your application and its environment have the capacity to service the expected demand, allowing for fluctuations and traffic spikes. But how can you get the most bang for your buck when capacity planning, without it turning into a dull and cumbersome process? Here is a short, opinionated crash course.

Is the adage true, that 80% of the time that end users spend waiting on a web page is due to front-end factors? Maybe under normal conditions. But under peak traffic, that ratio can flip. Here's a bit of perspective on where to focus your efforts to ensure your site stays fast and delivers a consistent experience to all your users, all the time.

Load testing isn't always easy, but doing it right can greatly reduce your risk of site crashes. We've worked with thousands of testers and performance engineers, and assembled a list of load testing best practices and pitfalls to help you be successful in less time.