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Load Testing vs Stress Testing

Oct 10, 2019
8 min read

Load testing and stress testing are both means of performance testing, so it’s not easy to draw a line of distinction between the two. However, while load testing assesses the software performance under some expected load (regular or peak), stress testing pushes the bar beyond peak conditions to determine the system’s load capacity as well as the point of a complete failure. 

While both types of testing share tools and approaches, there are lots of differences to take into account. This post will walk you through the specifics of load testing vs stress testing and explain why both are crucial for achieving higher performance rate.

What is Load Testing?

Load testing is a complex procedure that includes assessing response times, resource utilization, and system throughput. Thanks to load testing, developers are able to fix issues before releasing a product to ensure it will not become a dealbreaker for end users. 

Objectives of Load Testing

Since attracting new users to the website or the software takes a lot of promotion and marketing efforts, the project team needs to ensure that a product is capable of accommodating a growing number of users — otherwise, they will bounce and all marketing activities will be wasted. 

With that being said, here are some of the most obvious objectives of load testing:

  • To ensure that the system meets performance benchmarks
    Usually, business managers specify the requirements for the product in a service level agreement. Testers need to ensure that the system is prepared to hit those benchmarks and provide end-users with a stable experience.
  • To determine the breaking point of the system
    Through load testing, teams find out the load a product is capable of handling without downtime, functional bottlenecks, or reduced response time.
  • To test the way the product reacts to load-induced downtimes
    Capturing metrics like Mean Time Between Failure (MTBF) or Mean Time To Failure (MTTF) during load testing, QA specialists will not only know for how long the product can work without fail, they will be able to predict the interval between shutdowns as well.
Long story short, load testing helps companies model real life performance conditions and see if the software is capable of providing users with a seamless experience and does not get in the way of completing customer journeys.

Advantages of Load Testing

Business owners and development teams shouldn’t skip load testing activities since the range of insights it provides is tremendous. As for the advantages of load testing, here are the reasons your team should absolutely perform it:

1. Determines performance bottlenecks before the product is released

Load testing gives business managers a better idea of how increase in user flow impacts the system. By identifying performance issues, companies will have room for improving user experience before the product is out.

2. Estimates the system’s productivity 

Knowing the peak point of the software productivity helps development teams during optimization and user capacity planning.

3. Improves scalability

Load testing provides insights that come in handy when writing scalability plans. Also, it helps determine when the software is not ready to scale yet — this way, marketing and PR teams will not work in vain.

4. Increases customer satisfaction

Thanks to the insights provided by load testing, developers can optimize the user experience. This way, people will be much less affected by performance issues as they explore software or a website.

5. Reduces costs of failure

Load testing raises the quality of capacity planning and software tuning. As a result, teams will not have to invest in hardware or server resources that do not improve the overall performance. Instead, business managers can make the most out of the existing infrastructure. 

What to Keep in Mind When Load Testing

While load testing is a powerful way to assess an app’s performance from a user’s point of view, make sure you know what limitations it entails and what exactly to expect from a load testing. 

  • It requires a robust toolkit.
    Load testing measures a handful of metrics — response time and its variation across different system components, CPU, Disk I/O and bandwidth usage, the number of virtual users, and so on. To get insights on all of these points you need to have either all-in-one testing tool or a set of smaller tools. That’s why load testing can be time-consuming and expensive.
  • Keeps software in development for extra time
    Adding load testing to a testing strategy prolongs the amount of time the software stays in development phase. Missing out on favorable market conditions and failing to release the build at the right time can increase opportunity costs.
  • It is short-lived and sensitive to code changes
    To be efficient, load testing has to be re-iterated every time a new feature is added, a front-end is redesigned, there’s an upgrade in back-end technology, etc.

Why Your Software Needs Load Testing

Load testing improves businesses’ efficiency — it gives more awareness of the system’s limit, helps ensure that the product can support the results of marketing and PR efforts, and gives a better sense of awareness when it comes to potential risks. 

Here’s why your software needs load testing:

  • Increases user retention
    Poor software performance is the chief evil that often tanks conversions. Thanks to load testing, you will be able to optimize response time and other usability factors to ensure the software is comfortable to use.
  • Helps prepare the product for seasonal traffic peaks
    Companies run load testing to understand whether their product is able to withstand traffic load during Black Friday, Christmas, the Superbowl, or other significant dates.
  • Increases a developer’s confidence in system performance
    Load testing sessions help teams estimate and protect themselves from sudden shutdowns. It lowers incident-specific costs as well as the cost of failure.

What is Stress Testing?

The difference between load and stress testing is that the latter is used to validate the software’s behavior under extreme conditions. The main objective of this subset of performance testing is to reveal bugs that only occur under extremely high loads. The range of issues, uncovered by stress testing, usually includes memory leaks, synchronization bottlenecks, race conditions, etc. 

The main purpose of stress testing is to ensure that user data is not corrupted during system shutdown and the product works as expected once the load goes back to its normal state.

Spike testing — subjecting the system to traffic surges over short time frames — is a well-known subset of stress testing. 

Objectives of Stress Testing

Stress testing is not limited to the expected load — it aims to test the behavior of the system under conditions that far exceed realistic estimates. This type of testing uses Murphy’s Law, where testers presume that anything that can go wrong will eventually do so, and it’s better to be prepared for the worst. 

Stress Testing objectives also include:

  • Analyzing if user data is corrupted after system’s shutdown and finding out which security vulnerabilities might come in the clear once the load skyrockets unexpectedly.
  • Determining the triggers that would signal the risk. Stress testing helps determine the triggers and warning signs of a possible shutdown.
  • Determining what features of hardware influence the endurance of software. Stress testing will expose the vulnerabilities and side effects of a hardware side of a product.
  • Predicting failures connected to high loads. If stress testing is successful, a team should be able to tell how the system will behave once it reaches the breakpoint, what its functional vulnerabilities and security exploits are.

Advantages of Stress Testing

The absence of stress testing can lead to irreparable damage and result in data loss, leaks, or security attacks. It’s not an optional activity, but it’s a must-have process. Here’s why:

1. It prepares the team for extreme situations

After conducting stress tests, your team will know which failures are the most damaging or likely to take place. Such awareness increases the precision of planning as well as the odds of a timely response in case a particular scenario occurs once the software is out. 

2. Ensures the stability of the project

If done right, stress testing provides team with insights and warning signs that help check for most common bugs, like memory leaks, and protect the system from them. After stress testing, developers usually manage to protect software from scripts, bots, and DOS-attacks better. They find out what vulnerabilities do system have and can strengthen the system’s security before bringing the product to the market. 

3. Helps create contingency plans

Stress test sessions focus on detecting a wide range of issues — including deadlocks, synchronization bottlenecks, race attacks, data incoherence, concurrency issues, etc. All the information gathered after a stress testing session forms a solid background for scalability and contingency plans.

4. Checks how the system recovers under stresses

Stress testing offers a full view of the system’s behavior after a load surge. The following metrics help testers understand what the potential damage from a system shutdown might be:

  • Memory consumption
  • Processor utilization
  • Network usage
  • Disk utilization
  • Failed/succeeded transactions
  • Deadlocks
  • Contentions
  • Thread allocation

What to Keep In Mind When Stress Testing

When running stress testing, make sure you understand its nature and limitations. Here are the things to keep in mind when you perform stress testing:

  • Unrealistic estimates
    Stress testing gives results that are never 100% accurate and may take place in real life only to some particular extent. That’s because it is hard to create a test environment that fully corresponds to real-life situations, and get accurate results from there.
  • Requires reiteration
    Similarly to load tests, stress testing has to be constantly reiterated. A back-end change or an added feature can affect the product’s performance and make the system more vulnerable to load increase. To offer relevant insights, stress testing has to be continuously repeated.
  • Expensive sessions
    Stress testing requires simulating high user flows, as well as configuring a highly secure environment, thus team will have to dedicate more resources to it. QA specialists need an advanced toolkit to control stress levels and isolate the test environment. Investing in tools adds to the overall stress and load testing budget.

Difference Between Load Testing and Stress Testing

To have a better idea of which type of testing is right for your project, compare the differences between load vs stress testing. Here’s a table with the most significant distinctions the two share:

Load TestingStress Testing
DefinitionVerifies the performance of the product under expected load conditionsAssesses system performance under a load beyond regular conditions
GoalTo ensure the performance of the product meets the service-level agreementTo ensure that possible system failures won’t result in an irreparable security breach, a data leak, or other disastrous damage
What is being analyzedResponse time, the system’s breakpointError handling, security threats, memory leaks
LimitThe breakpointAbove the breakpoint
TimespanExtended periods of time (e.g. endurance testing)Short periods of time (e.g. spike testing)
ExamplesRunning multiple projects on a single serverUnexpected outages, DOS-attacks


When comparing a stress test vs load test, remember that both are crucial to the product’s capacity. Load testing helps you understand whether the software is reliable under a predicted load whereas stress testing goes beyond peak levels and tests how the system reacts to loads it is not supposed to handle.

If you want to make the most out of stress and load testing, contact a team of experienced performance testing. PFLB has dozens of certified professionals who will thoroughly assess your software project and provide you with comprehensive insights.

Take a look at our cases to see how PFLB contributes to some of the world’s most promising and ambitious projects. Contact us if you want our team of experienced QA testers to run load and stress testing on your project.

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