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When to do load vs stress testing

stress testing
Load testing and stress testing are two important techniques used to evaluate the performance and reliability of software applications and systems. Determining when to perform each type of test is crucial for delivering high-quality software that meets user expectations. This article provides a detailed analysis of the key factors to consider when deciding between load testing and stress testing.

Overview of Load and Stress Testing

Load testing evaluates how an application performs under normal expected usage. It determines if the system can handle the expected number of concurrent users and requests while still meeting service level agreements (SLAs) for response times and throughput.

Load tests incrementally increase the load on the system until performance degrades or fails. This helps determine the maximum operating capacity of an application. Common load test metrics include:
  • Response time for each request
  • Throughput (requests per second)
  • Resource utilization (CPU, memory, network, etc.)
  • Error rate percentage

In contrast, stress testing pushes an application beyond normal load conditions to the breaking point. The goal is to uncover bugs, bottlenecks, and potential failures under extreme workloads.
Stress tests overload the system with requests using high user loads, large complex queries, insufficient resources, and other techniques. This provides confidence that the system will not crash under heavy use.

Key stress test metrics include:
  • Stability under extreme peak concurrent users
  • Maximum capacity before system failure
  • Error rates and failures under high loads
  • Resource saturation points
While load tests focus on normal conditions, stress tests reveal problems that may occur during traffic spikes or when systems are pushed to the limits. Used together, they provide a comprehensive view of system performance and reliability.

Key Factors in Choosing Between Load and Stress Testing

There are several key considerations when determining whether to perform load testing or stress testing first on a system:

Stage of Development
In early development, focus on uncovering fundamental bugs and flaws using stress testing. There is little point measuring performance when basic functionality may still be broken.
As the system matures, shift to more targeted load tests based on expected usage patterns. Address any issues discovered under regular load conditions first before testing extreme scenarios.

Type of System
Stress testing is especially critical for systems that must maintain high reliability during traffic spikes or high concurrency. Examples include e-commerce sites during flash sales, voting systems on election day, and financial trading systems during volatile markets.
Meanwhile, load testing may take priority for systems that need to handle a predictable amount of sustained usage without downtime. Internal enterprise tools often fall into this category.

Risk Tolerance
If failures or downtime carry high risks, then rigorous stress testing is warranted to uncover any weaknesses. The cost of dealing with potential incidents under load is much less than if they occur in production.
For lower risk or experimental systems, standard load testing may be sufficient since the impact of any failures is limited. However, avoid skimping on stress testing for any mission critical system.

Resource Constraints
Since stress tests require simulating potentially millions of users, they consume far more resources and time to set up. Load tests with fewer concurrent users are generally easier to implement and execute.
If resources are scarce, focus on critical use cases under expected load. Less risky or important flows can be tested later with stress tests when time allows.

Use Cases
Carefully evaluate which user scenarios, workflows, integrations and interfaces to focus on for load vs stress testing.
Focus load testing on the primary use cases under normal conditions. Meanwhile, stress test riskier areas like computationally intensive backend processes, third party service integrations, and APIs with unpredictable usage spikes.

Best Practices for Load and Stress Testing

Follow these best practices when implementing load and stress tests:
  • Set objective pass/fail criteria - Define quantitative thresholds for response time, throughput, error rates and other metrics under load and stress conditions. These SLAs determine whether the system passes or fails each test.
  • Isolate the environment - Conduct tests in a staging environment identical to production to get accurate results without impacting live systems or users.
  • Validate at scale - Run load tests with the expected number of concurrent users and data volume. Stress tests should overload resources to their limits.
  • Monitor resources - Watch for bottlenecks in CPU, memory, network bandwidth and other constrained resources that degrade performance under load.
  • Identify root causes - Analyze logs, metrics and failures to trace problems to their source during testing. Refactor error handling and components at fault.
  • Test early, test often - Run load and stress tests continuously throughout development, especially after major code changes, to catch any regressions.
  • Automate execution - Use CI/CD pipelines to run performance tests automatically on every build. This also facilitates frequent regression testing.

When to Run Load vs Stress Tests
Given the above considerations, here are two general workflows for when to focus on load testing versus stress testing:

Waterfall Development
  1. Run initial stress tests during the design phase to validate architecture and technology choices.
  2. As core features are built, perform stress tests on each component and integration point under extreme conditions to surface bugs.
  3. Later in development, execute broader stress tests to reveal performance issues and stability risks under high usage especially for key areas identified in use case analysis.
  4. In QA before release, do final load tests simulating expected production usage to verify the system meets SLAs for normal traffic patterns.

Agile Development
  1. Start with exploratory stress testing of main user workflows in each sprint to shake out defects rapidly as the system evolves.
  2. Once a feature is coded, subject it to stress tests that purposely break it before functional testing to uncover weaknesses.
  3. Run basic load tests on completed features/stories at the end of each sprint to catch performance regressions as the system grows.
  4. Every few sprints, conduct fuller load and stress tests on the integrated system. Fix any issues where usage SLAs are not met.
  5. Prior to production release, do final comprehensive load testing against specifications. Follow up with stress tests using excessive loads beyond those expected in real-world conditions.

The agile workflow surfaces bugs earlier through continuous stress testing, while still verifying load capabilities through periodic broader tests.


Determining whether to load test or stress test a system depends on the development phase, risk tolerance, use cases and available resources. Load tests evaluate normal performance, while stress tests reveal flaws under extreme conditions.

Follow these best practices for both types of testing:
  • Set clear, measurable pass/fail criteria based on SLAs
  • Isolate test environments from production
  • Profile hardware bottlenecks under load
  • Automate test execution as part of CI/CD

By leveraging a combination of disciplined load and stress testing, organizations can deliver robust, high-quality software able to meet demands under both regular and peak usage. Testing early and often improves customer satisfaction by preventing performance issues or outages once an application is live.