Load testing is the process of putting demand on a system or device and measuring its response. Load testing is performed to determine a system’s behavior under both normal and anticipated peak load conditions. It helps to identify the maximum operating capacity of an application as well as any bottlenecks and determine which element is causing degradation. When the load placed on the system is raised beyond normal usage patterns, in order to test the system's response at unusually high or peak loads, it is known as stress testing. The load is usually so great that error conditions are the expected result, although no clear boundary exists when an activity ceases to be a load test and becomes a stress test.
There is little agreement on what the specific goals of load testing are. The term is often used synonymously with concurrency testing, software performance testing, reliability testing, and volume testing. Load testing is a type of non-functional testing.
Software load testing 
||It has been suggested that Scalability testing be merged into this article. (Discuss) Proposed since July 2012.|
The term load testing is used in different ways in the professional software testing community. Load testing generally refers to the practice of modeling the expected usage of a software program by simulating multiple users accessing the program concurrently. As such, this testing is most relevant for multi-user systems; often one built using a client/server model, such as web servers. However, other types of software systems can also be load tested. For example, a word processor or graphics editor can be forced to read an extremely large document; or a financial package can be forced to generate a report based on several years' worth of data. The most accurate load testing simulates actual use, as opposed to testing using theoretical or analytical modeling.
Load testing lets you measure your website's QOS performance based on actual customer behavior. Nearly all the load testing tools and frame-works follow the classical load testing paradigm, which is listed in Figure 1. When customers visit your web site, a script recorder records the communication and then creates related interaction scripts. A load generator tries to replay the recorded scripts, which could possibly be modified with different test parameters before replay. In the replay procedure, both the hardware and software statistics will be monitored and collected by the conductor, these statistics include the CPU, memory, disk IO of the physical servers and the response time, throughput of the System Under Test (short as SUT), etc. And at last, all these statistics will be analyzed and a load testing report will be generated.
Load and performance testing analyzes software intended for a multi-user audience by subjecting the software to different amounts of virtual and live users while monitoring performance measurements under these different loads. Load and performance testing is usually conducted in a test environment identical to the production environment before the software system is permitted to go live.
As an example, a web site with shopping cart capability is required to support 100 concurrent users broken out into following activities:
- 25 Virtual Users (VUsers) log in, browse through items and then log off
- 25 VUsers log in, add items to their shopping cart, check out and then log off
- 25 VUsers log in, return items previously purchased and then log off
- 25 VUsers just log in without any subsequent activity
A test analyst can use various load testing tools to create these VUsers and their activities. Once the test has started and reached a steady state, the application is being tested at the 100 VUser load as described above. The application’s performance can then be monitored and captured.
The specifics of a load test plan or script will generally vary across organizations. For example, in the bulleted list above, the first item could represent 25 VUsers browsing unique items, random items, or a selected set of items depending upon the test plan or script developed. However, all load test plans attempt to simulate system performance across a range of anticipated peak workflows and volumes. The criteria for passing or failing a load test (pass/fail criteria) are generally different across organizations as well. There are no standards specifying acceptable load testing performance metrics.
A common misconception is that load testing software provides record and playback capabilities like regression testing tools. Load testing tools analyze the entire OSI protocol stack whereas most regression testing tools focus on GUI performance. For example, a regression testing tool will record and playback a mouse click on a button on a web browser, but a load testing tool will send out hypertext the web browser sends after the user clicks the button. In a multiple-user environment, load testing tools can send out hypertext for multiple users with each user having a unique login ID, password, etc.
The popular load testing tools available also provide insight into the causes for slow performance. There are numerous possible causes for slow system performance, including, but not limited to, the following:
- Application server(s) or software
- Database server(s)
- Network – latency, congestion, etc.
- Client-side processing
- Load balancing between multiple servers
Load testing is especially important if the application, system or service will be subject to a service level agreement or SLA.
User Experience Under Load test 
In the example above, while the device under test (DUT) is under production load - 100 VUsers, run the target application. The performance of the target application here would be the User Experience Under Load. It describes how fast or slow the DUT responds, and how satisfied or how the user actually perceives performance.
Many performance testers are running this test, but they call it different names. This name was selected by the Panelists and many Performance Testers in 2011 Online Performance Summit by STP.
There are already many tools and frameworks available to do the load testing from both commercial and open source.
Load testing tools 
|Tool Name||Company Name||Notes|
|Apache JMeter||An Apache Jakarta open source project||Java desktop application for load testing and performance measurement.|
|BlazeMeter||BlazeMeter Ltd.||BlazeMeter is a JMeter compatible, self-service, load testing platform for websites, web apps, mobile and databases, supporting any user scenario. Scalable load up to 200,000 concurrent simulated browser users from across eight geographical locations. Can also be used for integration and functional testing.|
|Blitz||Spirent Communications||Blitz is a service for load and performance testing of websites, mobile, web apps and REST APIs in the cloud. It allows to simulate up to 50,000 simultaneous virtual users from different worldwide locations.|
|Gatling||Open Source||JVM application with scenarios as code and portable HTML reports.|
|Load Impact||Load Impact - AB||Cloud based large scale performance testing SaaS tool primarily used for executing large volume performance tests. Simple UI caters to developers and novices. Licensed.|
|LoadRunner||HP||Performance testing tool primarily used for executing large numbers of tests (or a large number of virtual users) concurrently. Can be used for unit and integration testing as well. Licensed.|
|Load Test (included with Soatest)||Parasoft||Performance testing tool that verifies functionality and performance under load. Supports SOAtest tests, JUnits, lightweight socket-based components. Detects concurrency issues.|
|loadUI||SmartBear Software||Open source and cross-platform load testing tool, targeted mainly at web services. Integrates with soapUI.|
|Login VSI||Login VSI B.V.||Performance testing software for Windows based virtualized desktops by simulating user workloads. Licensed.|
|NeoLoad||Neotys||Load testing tool for web and mobile applications. Load can be generated from local agents or from the cloud. Licensed.|
|OpenSTA||Open System Testing Architecture||Open source web load/stress testing application, licensed under the Gnu GPL. Utilizes a distributed software architecture based on CORBA. OpenSTA binaries available for Windows.|
|Rational Performance Tester||IBM||Eclipse based large scale performance testing tool primarily used for executing large volume performance tests to measure system response time for server based applications. Licensed.|
|Silk Performer||Borland||Application performance tool with cloud and local virtual agents. Supports most protocols and applications. Licensed.|
|Visual Studio Ultimate edition||Microsoft||Visual Studio Ultimate edition includes a load test tool which enables a developer to execute a variety of tests (web, unit etc...) with a combination of configurations to simulate real user load.|
Mechanical load testing 
The purpose of a mechanical load test is to verify that all the component parts of a structure including materials, base-fixings are fit for task and loading it is designed for.
The Supply of Machinery (Safety) Regulation 1992 UK state that load testing is undertaken before the equipment is put into service for the first time.
Load testing can be either Performance,Static or Dynamic.
Performance testing applies a safe working load (SWL), or other specified load, for a designated time in a governing test method, specification, or contract.
Static testing is when a load at a factor above the SWL is applied. The item is not operated through all configurations as it is not a requirement of this test.
Dynamic testing is when a load at a factor above the SWL is applied. The item is then operated fully through all configurations and motions. Care must be taken during this test as there is a great risk of catastrophic failure if incorrectly carried out.
The design criteria, relevant legislation or the Competent Person will dictate what test is required.
Under the Lifting Operations and Lifting Equipment Regulations 1998 UK load testing after the initial test is required if a major component is replaced, if the item is moved from one location to another or as dictated by the Competent Person
The loads required for a test are stipulated by the item under test, but here are a few to be aware off. Powered lifting equipment Static test to 1.25 SWL and dynamic test to 1.1 SWL. Manual lifting equipment Static test to 1.5 SWL
For lifting accessories. 2 SWL for items up to 30 tonne capacity. 1.5 SWL for items above 30 tonne capacity. 1 SWL for items above 100 tonnes.
Car charging system 
|This section does not cite any references or sources. (January 2011)|
A load test can be used to evaluate the health of a car's battery. The tester consists of a large resistor that has a resistance similar to a car's starter motor and a meter to read the battery's output voltage both in the unloaded and loaded state. When the tester is used, the battery's open circuit voltage is checked first. If the open circuit voltage is below spec (12.6 volts for a fully charged battery), the battery is charged first. After reading the battery's open circuit voltage, the load is applied. When applied, it draws approximately the same current the car's starter motor would draw during cranking. Based on the specified cold cranking amperes of the battery, if the voltage under load falls below a certain point, the battery is bad. Load tests are also used on running cars to check the output of the car's alternator.
See also 
- "Load Testing ASP.NET Applications with Visual Studio 2010". Eggheadcafe.com. Retrieved 2013-01-13.
- Modeling the Real World for Load Testing Web Sites by Steven Splaine