What is Parallel Testing? Definition, Approach, Example

Parallel Testing

Parallel Testing is a software testing type in which multiple versions or subcomponents of an application are tested with same input on different systems simultaneously to reduce test execution time. The purpose of parallel testing is finding out if legacy version and new version are behaving the same or differently and ensuring whether new version is more efficient or not.

The below image demonstrate the parallel testing.

Parallel Testing

Parallel Testing Example

When any organization is moving from old system to new system, legacy data is an important part. Transferring this data is a complex process.

In software testing, verifying compatibility of the newly developed system with the old system is done through “parallel testing.”

Parallel Testing Tutorial: Learn in 5 Minutes

Why to do Parallel Testing

Parallel Testing is done due to following reason,

  • To make sure the new version of the application performs correctly
  • To make sure the consistencies are same between new and old version
  • To check if the data format between two versions has changed
  • To check the integrity of the new application

For example- currently users are using 1.0 version of an application and from the month of March users are going to use another version of an application, let’s say 1.1 version.

Parallel Testing Tutorial: Learn in 5 Minutes

In such cases, testers need to do the parallel testing, in order to evaluate that data migration is done successfully. Also to check whether the changes in the new version does not affect the system function. The tester must verify that changes are executed properly, and the user is getting the desired output as per the requirement.

When to do Parallel Testing

Parallel testing can be used extensively when

  • The company moving from old system to new system
  • When synchronization is performed on two systems
  • Legacy data imported from one system to another
  • All the outcomes should be defined more precisely. Example, financial domain or insurance domain where the calculation is a major functionality of the system.

How to do Parallel Testing: Complete Approach

For performing parallel testing, you can simply create several project that will test a different part of the application (Slave Projects) and one project (master project) that will run these projects.

Parallel Testing has two level criteria.

  1. Parallel test entry CriteriaParallel test entry criteria define the tasks that must be satisfied before parallel testing can be efficiently executed.
  2. Parallel test exit CriteriaParallel test exit criteria defines the successful conclusion of the parallel testing stage.

Before performing parallel testing, there are few pre-condition that has to be satisfied.

  • Parallel test cannot begin till environment setup is done.
  • All pre-conditions and scenarios should be defined first
  • Legacy data and new data must be migrated successfully
  • Parallel test is not complete until all of the exit criteria have been satisfied

To perform Parallel Testing, following steps should be followed

Step 1: Run old system against newly developed system

Step 2: Understand different between both the system

Step 3: Go throw complete cycle using same input

Step 4: Measure the output of newly developed system compare to the old system

Step 5: Report cause of bug if found

Good Practices for Parallel Testing

To perform parallel testing here are few tips and tricks, which might be useful.

  • Typical bugs identified in Parallel Testing

Internal logic is changed

Flow of product is changed

Major functionalists are modified

  • How Many Cycles Should Be Required

Number of testing cycle depends upon the complexity of the module.

Run multiple scenario cycles using pre-defined test data, which was pass from the previous system

  • Categorizing DifferenceWhen we run the parallel testing cycle, the results of both the new and legacy systems should be measured line by line with differences highlighted. Every difference which we captured should be defined as per the type of error.
  • Type of error occurred during cyclesFor errors tester should note down following things while performing parallel testing.
    • Entry error
    • Error due to the old system
    • Explainable or acceptable different
    • Unexpected error

What is not a Parallel Testing

It is Parallel Testing It is not Parallel Testing
  • Testing the updated application against the previous application.
  • Run the old scenario with new software with reserved input conditions.
  • The aim is to find out the outcome as per the previous system.
  • Should have knowledge of the old and newly developed system.
  • Testing one software only.
  • Cross-Browser or Cross-Platform Testing.
  • The aim is to find out the designing issue.
  • Know the Difference is not required.

Challenges of Parallel Testing

  • Complete product knowledge is required.
  • Every outcome should be tested
  • Need to concentrate on Data input and flow of product


  • In Software Engineering, Parallel testing is testing multiple applications or subcomponents of one application concurrently to reduce the test time.
  • It ensures that the new system is capable enough to run the software efficiently.