Running Tests in Parallel

If your project has a large number of tests, it will need more time to run them on one machine. To reduce this time, you can run tests in parallel by spreading them across multiple machines. This requires specifying a parallelism level. You can use either the CircleCI CLI to split test files, or use environment variables to configure each parallel machine individually.

Specifying a Job’s Parallelism Level
Using the CircleCI CLI to Split Tests
Using Environment Variables to Split Tests
Running Split Tests
See Also

Specifying a Job’s Parallelism Level

Test suites are conventionally defined at the job level in your .circleci/config.yml file. The parallelism key specifies how many independent executors will be set up to run the steps of a job.

To run a job’s steps in parallel, set the parallelism key to a value greater than 1.

# ~/.circleci/config.yml
version: 2
jobs:
  test:
    docker:
      - image: circleci/<language>:<version TAG>
    parallelism: 4

For more information, see the Configuring CircleCI document.

Using the CircleCI CLI to Split Tests

CircleCI supports automatic test allocation across your containers. The allocation is file-based. It requires the CircleCI CLI, which is automatically injected into your build at run-time.

To install the CLI locally, see the Using the CircleCI Local CLI document.

Globbing Test Files

The CLI supports globbing test files using the following patterns:

* matches any sequence of characters (excluding path separators)
** matches any sequence of characters (including path separators)
? matches any single character (excluding path separators)
[abc] matches any character (excluding path separators) against characters in brackets
{foo,bar,...} matches a sequence of characters, if any of the alternatives in braces matches

To glob test files, pass one or more patterns to the circleci tests glob command.

circleci tests glob "tests/unit/*.java" "tests/functional/*.java"

To check the results of pattern-matching, use the echo command.

# ~/.circleci/config.yml
version: 2
jobs:
  test:
    docker:
      - image: circleci/<language>:<version TAG>
    parallelism: 4
    steps:
      - run:
          command: |
            echo $(circleci tests glob "foo/**/*" "bar/**/*")
            circleci tests glob "foo/**/*" "bar/**/*" | xargs -n 1 echo

Splitting Test Files

The CLI supports splitting tests across machines when running parallel builds. To do this, pass a list of filenames to the circleci tests split command.

circleci tests split test_filenames.txt

The CLI looks up the number of containers, along with the current container index. Then, it uses deterministic splitting algorithms to split the test files across all available containers.

By default, the number of containers is specified by the parallelism key. You can manually set this by using the --total flag.

circleci tests split --total=4 test_filenames.txt

Similarly, the current container index is automatically picked up from environment variables, but can be manually set by using the --index flag.

circleci tests split --index=0 test_filenames.txt

Splitting by Name

By default, circleci tests split expects a list of filenames and splits tests alphabetically by test name. There are a few ways to provide this list:

Create a text file with test filenames.

circleci tests split test_filenames.txt

Provide a path to the test files.

circleci tests split < /path/to/items/to/split

Or pipe a glob of test files.

circleci tests glob "test/**/*.java" | circleci tests split

Splitting by Filesize

When provided with filepaths, the CLI can also split by filesize. To do this, use the --split-by flag with the filesize split type.

circleci tests glob "**/*.go" | circleci tests split --split-by=filesize

Splitting by Timings Data

On each successful run of a test suite, CircleCI saves timings data to a directory specified by the path in the store_test_results step. If you do not use store_test_results, there will be no timing data available for splitting your tests.

To split by test timings, use the --split-by flag with the timings split type.

circleci tests glob "**/*.go" | circleci tests split --split-by=timings

The CLI expects both filenames and classnames to be present in the timing data produced by the testing suite. By default, splitting defaults to filename, but you can specify classnames by using the --timings-type flag.

cat my_java_test_classnames | circleci tests split --split-by=timings --timings-type=classname

If you need to manually store and retrieve timing data, use the store_artifacts step.

Using Environment Variables to Split Tests

For full control over parallelism, CircleCI provides two environment variables that you can use in lieu of the CLI to configure each container individually. CIRCLE_NODE_TOTAL is the total number of parallel containers being used to run your job, and CIRCLE_NODE_INDEX is the index of the specific container that is currently running. See the built-in environment variable documentation for more details.

Running Split Tests

Globbing and splitting tests does not actually run your tests. To combine test grouping with test execution, consider saving the grouped tests to an environment variable, then passing this variable to your test runner.

TESTFILES=$(circleci tests glob "spec/**/*.rb" | circleci tests split --split-by=timings)
bundle exec rspec -- ${TESTFILES}

The TESTFILES var will have a different value in each container, based on $CIRCLE_NODE_INDEX and $CIRCLE_NODE_TOTAL.

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