Parallel Testing

Parallel testing is a testing of multiple applications or subcomponents of one application concurrently to reduce the test time.

When an 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.”

In parallel testing, tester runs two different versions of software concurrently with the same input. The aim to find out whether the legacy system and the new system are behaving the same or different. It ensures that the new system is capable enough to run the software efficiently.

The below image demonstrates parallel testing.


Why do Parallel Testing

Parallel Testing is done due to the following reason,

  • To make sure the new version of the application performs correctly
  • To make sure the consistencies are the same between new and old version
  • To check if the data format between the 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.

In such cases, testers need to do 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.

Test Strategy for Parallel Testing

For performing parallel testing, you can simply create several projects 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.

Parallel test entry Criteria

Parallel test entry criteria define the tasks that must be satisfied before parallel testing can be efficiently executed.

Parallel test exit Criteria

Parallel test exit criteria define the successful conclusion of the parallel testing stage. Before performing parallel testing, there are few pre-condition that has to be satisfied.

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

To perform Parallel Testing, following steps should be followed

  1. Run the old system against newly developed system
  2. Understand different between both the system
  3. Go throw complete cycle using the same input
  4. Measure the output of a newly developed system compare to the old system
  5. Report cause of bug if found

Good Practices for Parallel Testing

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

Typical bugs identified in Parallel Testing

  • Internal logic is changed
  • The flow of the product is changed
  • Major functionalists are modified

How Many Cycles Should Be Required

The number of testing cycles depends upon the complexity of the module.

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

Categorizing Difference

When 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 cycles

For 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. Testing one software only.
Run the old scenario with new software with reserved input conditions. Cross-bower or Cross-Platform Testing.
The aim is to find out the outcome as per the previous system. The aim is to find out the designing issue.
Should have knowledge of the old and newly developed system. 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


  • 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.

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