Parallel testing for PHP

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v6.4.1 2021-12-02 09:12 UTC


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The objective of ParaTest is to support parallel testing in PHPUnit. Provided you have well-written PHPUnit tests, you can drop paratest in your project and start using it with no additional bootstrap or configurations!


Why use paratest over the alternative parallel test runners out there?

  • Code Coverage report combining. Run your tests in N parallel processes and all the code coverage output will be combined into one report.
  • Zero configuration. After composer install, run with vendor/bin/paratest. That's it!
  • Flexible. Isolate test files in separate processes or take advantage of WrapperRunner for even faster runs.


To install with composer run the following command:

composer require --dev brianium/paratest


Only the latest version of PHPUnit is supported, and thus only the latest version of ParaTest is actively maintained.

This is because of the following reasons:

  1. To reduce bugs, code duplication and incompatibilities with PHPUnit, from version 5 ParaTest heavily relies on PHPUnit @internal classes
  2. The fast pace both PHP and PHPUnit have taken recently adds too much maintenance burden, which we can only afford for the latest versions to stay up-to-date


After installation, the binary can be found at vendor/bin/paratest. Run it with --help option to see a complete list of the available options.

Optimizing Speed

To get the most out of paratest, you have to adjust the parameters carefully.

  1. Adjust the number of processes with -p

    To allow full usage of your cpu cores, you should have at least one process per core. More processes allow better resource usage but keep in mind that each process has its own costs for spawning. The default is auto, which means the number of logical CPU cores is set as number of processes. You might try something like logical CPU cores * 2 (e.g. if you have 8 logical cores, you might try 16), but keep in mind that each process generates a little bit of overhead as well.

  2. Use the WrapperRunner if possible

    The default Runner for PHPUnit spawns a new process for each testcase (or method in functional mode). This provides the highest compatibility but comes with the cost of many spawned processes and a bootstrapping for each process. Especially when you have a slow bootstrapping in your tests (like a database setup) you should try the WrapperRunner with --runner WrapperRunner. It spawns one "worker"-process for each parallel process (-p), executes the bootstrapping once and reuses these processes for each test executed. That way the overhead of process spawning and bootstrapping is reduced to the minimum.

  3. Choose between per-testcase- and per-testmethod-parallelization with -f

    Given you have few testcases (classes) with many long running methods, you should use the -f option to enable the functional mode and allow different methods of the same class to be executed in parallel. Keep in mind that the default is per-testcase-parallelization to address inter-testmethod dependencies. Note that in most projects, using -f is slower since each test method will need to be bootstrapped separately.

  4. Tune batch max size --max-batch-size

    Batch size will affect on max amount of atomic tests which will be used for single test method. One atomic test will be either one test method from test class if no data provider available for method or will be only one item from dataset for method. Increase this value to reduce per-process overhead and in most cases it will also reduce parallel efficiency. Decrease this value to increase per-process overhead and in most cases it will also increase parallel efficiency. If amount of all tests less then max batch size then everything will be processed in one process thread so paratest is completely useless in that case. The best way to find the most effective batch size is to test with different batch size values and select best. Max batch size = 0 means that grouping in batches will not be used and one batch will equal to all method tests (one or all from data provider). Max batch size = 1 means that each batch will contain only one test from data provider or one method if data provider is not used. Bigger max batch size can significantly increase phpunit command line length so process can failed. Decrease max batch size to reduce command line length. Windows has limit around 32k, Linux - 2048k, Mac OS X - 256k.


Examples assume your tests are located under ./test/unit.

# Run all unit tests in 8 parallel processes
vendor/bin/paratest -p8 test/unit
# Run all unit tests in 4 parallel processes with WrapperRunner and output html code coverage report to /tmp/coverage
# (Code coverage requires Xdebug to be installed)
vendor/bin/paratest -p4 --runner=WrapperRunner --coverage-html=/tmp/coverage test/unit


If you run into problems with paratest, try to get more information about the issue by enabling debug output via --verbose=1.

When a sub-process fails, the originating command is given in the output and can then be copy-pasted in the terminal to be run and debugged. All internal commands run with --printer [...]\NullPhpunitPrinter which silence the original PHPUnit output: during a debugging run remove that option to restore the output and see what PHPUnit is doing.

Generating code coverage

Beginning from PHPUnit 9.3.4, it is strongly advised to set a coverage cache directory, see PHPUnit Changelog @ 9.3.4.

The cache is always warmed by ParaTest before executing the test suite.

Examples assume your tests are located under ./test/unit.

vendor/bin/paratest --coverage-text test/unit

Running phpunit in 1 process with /codebase/paratest/vendor/phpunit/phpunit/phpunit

Configuration read from /codebase/paratest/phpunit.xml.dist

...............................................................  63 / 155 ( 40%)
............................................................... 126 / 157 ( 80%)

Time: 27.2 seconds, Memory: 8.00MB

OK (163 tests, 328 assertions)

Code Coverage Report:
  2019-01-25 09:41:26

  Classes: 22.86% (8/35)
  Methods: 49.47% (139/281)
  Lines:   59.38% (896/1509)

Caution: Generating coverage is an art in itself. Please refer to our extensive guide on setting up everything correctly for code coverage generation with paratest.


Windows users be sure to use the appropriate batch files.

An example being:

vendor\bin\paratest.bat ...

ParaTest assumes PSR-0 for loading tests.

For convenience paratest windows version use 79 columns mode to prevent blank lines in standard 80x25 windows console.

PHPUnit Xml Config Support

When running PHPUnit tests, ParaTest will automatically pass the phpunit.xml or phpunit.xml.dist to the phpunit runner via the --configuration switch. ParaTest also allows the configuration path to be specified manually.

ParaTest will rely on the testsuites node of phpunit's xml configuration to handle loading of suites.

The following phpunit config file is used for ParaTest's test cases.

<?xml version="1.0" encoding="UTF-8"?>
        <testsuite name="ParaTest Fixtures">

Test token

The TEST_TOKEN environment variable is guaranteed to have a value that is different from every other currently running test. This is useful to e.g. use a different database for each test:

if (getenv('TEST_TOKEN') !== false) {  // Using paratest
    $dbname = 'testdb_' . getenv('TEST_TOKEN');
} else {
    $dbname = 'testdb';


  1. Constants, static methods, static variables and everything exposed by test classes consumed by other test classes (including Reflection) are not supported. This is due to a limitation of the current implementation of WrapperRunner and how PHPUnit searches for classes. The fix is put shared code into classes which are not tests themselves.

For Contributors: Testing paratest itself

Note that The display_errors php.ini directive must be set to stderr to run the test suite.

Before creating a Pull Request be sure to run all the necessary checks with make command.

For an example of ParaTest out in the wild check out the example.