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Isolated Testing for SWEs


Also See: //docs/workflow/

This page explains how to convert a googletest executable into a fully isolated test. This is done by describing runtime dependencies for a test.

For information about the infrastructure itself and the roadmap, see Isolated Testing Infrastructure.

Note: If you're a Googler who needs to perform a manual test on a platform for which you cannot readily build (e.g. Windows Developer with no development environment on your Mac book), follow these instructions to get the build from a try run.

What's "Isolate"?

Goal: describe the exact list of files needed to run a executable.

The isolate project is all the file formats, tools and server code to make this work fast and efficiently. Every test executable list the data files it depends on, which allows the testers bots to only download the files they need instead of the whole Chromium source tree.

They are described in files.

What's "Swarming"?

Goal: distribute tasks fast and efficiently in an heterogeneous fleet of bots.

The swarming project is all the tools, server and bot code to run a step (like a unit test) on a remote bot and get results back. It has native google-test sharding capability.

What are the advantages?

By reducing the amount of data that we need to transfer to the tester machines, it becomes much easier to increase the number of tester bots that we have using Swarming (part of LUCI).

Adding a new test

Expectations of the tests:


Download binaries from a try job

Run a test isolated locally

echo gn > out/Release/mb_type  # Must be done once to avoid from performing a clobber
tools/mb/ run //out/Release base_unittests  # Builds, generates .isolate (via gn desc runtime_deps), and runs (via " run")

Run a test built locally on Swarming

1. Eligibility and login

Right now, only users Chromium team members can use the infrastructure directly. Note that the whole Swarming infrastructure is open source so if any other company would help to recreate the same infrastructure internally, send us a note at

By login first, you have access tokens so that the following commands do not have to constantly prompt for your identity. You only need to do this once:

luci-auth login

If you are running through a text only session on a remote machine, append argument --auth-no-local-webserver

2. One step build & run

python3 tools/mb/ run --swarmed out/Release \
base_unittests \
 -- --gtest_filter=<test filter>

This will perform the steps below, picking defaults based on your local setup. If it fails, you can re-run the last command run by to see the cause of the failure. You may need to login again.

If you want to pick different defaults, e.g., run tests on Windows-11 from a Windows 10 machine, you need to use --no-default-dimensions and explicitly specify some dimensions, e.g.,


python3 tools/mb/ run \
  --swarmed out/Release \
  -d os Windows-11 \
  -d pool chromium.tests \
  --no-default-dimensions base_unittests \
  -- --gtest_filter=<test filter>

Or, you can run the individual steps below manually.

3. Build & Generate .isolate file

The isolate file describes what are the files that needs to be mapped on the Swarming bot. It is generated via GN "data" and "data_deps" statements, and is processed by

ninja -C out/Release base_unittests.exe
echo gn > out/Release/mb_type  # Must be done once to avoid from performing a clobber
python3 tools\mb\ isolate //out/Release base_unittests # Creates out/Release/base_unittests.isolate

4. Compute .isolated file and upload it

The isolated file contains the SHA-1 of each input files. It is archives along all the inputs to the Isolate server. Since the isolate server is a content-addressed cache, only the ones missing from the cache are uploaded. Depending on your connection speed and the size of the executable, it may take up to a minute:

tools\luci-go\isolate archive \
  -i out\Release\base_unittests.isolate \
  -cas-instance chromium-swarm

This will output a digest string that you use in step 5.

5. Trigger the task

That's where a task is requested, specifying the isolated (tree of SHA-1 of files to map in):

tools\luci-go\swarming trigger \
  -digest <digest from step 4> \
  -server \
  -d os=Windows-10-19042 \
  -d pool=chromium.tests \
  -d cpu=x86-64 -- out\Release\base_unittests.exe

Wait for results. OS currently available:

For other available --dimension values, look at Swarming bots e.g.:

That's it. Feel free to contact the team at for any chromium open source specific questions.


Additional Notes


I run a task on Swarming and it hangs there

It is possible that all the bots are currently fully utilized. In this case, the task is in PENDING state.

It seems tedious to list each test data file individually, can I just list src/ ?

In theory yes, in practice please don't and keep the list to the strict minimum. The goal is not to run the tests more slowly and having the bots download 20 gb of data. Reasons includes:

  1. Isolate Server is optimized for < 50000 files scenario. There's a 2ms/file cost per cache hit. So for example, layout tests are currently out of the use case since there's > 80000 files.
  2. It's always possible to go coarser but it's much harder to get back stricter.

Where can I find the .isolated file?

The .isolated files are generated when you build the isolation specific version of a target, e.g. out/Debug or out/Release. The isolation target name is just the normal target name with a _run added to the end.

If you have a test that passes locally and fails on some trybots, then it could be related.

This error can be seen when a browser test js file is not found:

TypeError: Cannot read property 'testCaseBodies' of undefined

Where should I file bugs?

Swarming specific bugs can be filed on at in component Infra>LUCI>TaskDistribution.