Repository navigation
Expose an id for concurrent test runners (like JEST_WORKER_ID) #55842
Description
Activity
We could add an environment variable fairly easily, but I have a question:
if (process.env.STAGE === "test" && process.env.JEST_WORKER_ID) { process.env.DB_DATABASE = `myapp_tests_${process.env.JEST_WORKER_ID}`; }
There is already a
NODE_TEST_CONTEXTenvironment variable that can be used to determine that the test runner is being used (if you actually need something like that). Why not use something likecrypto.randomUUID()to create a unique ID for each file?Reacted by Moshe AtlowThe problem is that, as an external step before the test runner is launched, I:
- Start up the postgres instance via
docker compose - Run an init script to create
ndatabases (based on the parallelism I run the tests with) - Run my migration logic (
node-pg-migrate) against all the test databases - Create a
flush_database()function in each test database to runbeforeEachtest
I don't want to do this for each test file because the migration logic is quite slow. It's much faster to migrate once and have a fast
flush_databasefunction to reset database state between tests.So a UUID, for my use case, would not be appropriate. Having a sequential integer between 1 and
n(wherenis the max concurrency) is required for the database setup logic that occurs outside the test runner context.- Start up the postgres instance via
Got it. Thanks for the explanation. I think we should add support for this.
Reacted by Ben Limmer, Pietro Marchini, Stephen Haberman and Moshe AtlowI can work on this! will pick this up!
Reacted by Ben LimmerReacted by Pietro MarchiniThanks @cu8code. Please be sure to handle both cases of test isolation. When
--experimental-test-isolation=process, each child process should get an environment variable from 1 to N. When--experimental-test-isolation=none, only the value of 1 should be used.Reacted by 0hm☘️ and Moshe AtlowHi @cu8code, are you working on this? If not I can give it a try.
I am working on this 👍
Is
worker.threadIdpossibly what you're looking for?18 remaining items
I've opened PR #61394 to address this issue.
The PR adds
NODE_TEST_WORKER_IDenvironment variable andcontext.workerIdproperty, which are set based on the test isolation mode:--test-isolation=process(default): Each test file gets a unique worker ID from 1 to N, where N is the concurrency level--test-isolation=none: All tests get worker ID 1 (since they run in the same process)
Worker IDs are managed through a pool that allocates and reuses IDs as test files complete, so with 16 test files and concurrency of 8, IDs 1-8 get reused twice.
Why Sequential IDs (1, 2, 3...) vs UUIDs:
I considered using
crypto.randomUUID()as suggested in the comments, but went with sequential IDs for consistency with other test frameworks:- Jest uses
JEST_WORKER_ID(sequential) - Vitest uses
VITEST_POOL_ID(sequential) - Mocha uses
MOCHA_WORKER_ID(sequential)
Sequential IDs also make it easier to pre-allocate resources.
This is available at import time, so it works for module-level initialization without needing to pass context around.
Looking forward to feedback on the implementation!Reacted by Jacob Smith- added a commit that references this issue
on Feb 1, 2026 I'd like to work on this issue.
- added a commit that references this issue
on Feb 24, 2026 - added a commit that references this issue
on Feb 28, 2026 - added 6 commits that reference this issue
on Apr 4, 2026
What is the problem this feature will solve?
When running tests concurrently (the default setting in the Node test runner), it's common practice to split concurrent test runs across multiple local resources (like locally running servers, databases, etc).
Many other popular test runners offer this feature via an environment variable:
jest:JEST_WORKER_IDmocha:MOCHA_WORKER_IDvitest:VITEST_POOL_IDThis makes it very easy to set up the proper database connection. For example, here's a snippet from my codebase:
Then, in my database docker container, I create
ndatabases, one for each worker. For example, if I runjestwith--maxWorkers 8, I createmyapp_tests_1->myapp_tests_8. During tests, the parallel test suites talk to a different database to avoid deadlocks.What is the feature you are proposing to solve the problem?
The simplest solution for users coming from other frameworks would be to provide an environment variable just like the other test runners (e.g.,
NODE_TEST_WORKER_ID).However, if that's not feasible, adding a
workerIdto theTestContextcould also be a good solution. This solution is not as ideal as the environment variable because people would need to pass the test context to whatever establishes their db/server/etc connection. In my case, at least, this would require refactoring of code. Right now, I rely on that variable being set at file import time.What alternatives have you considered?
I considered trying to find a solution based on
pid, but it's not reliable enough. @redyetidev mentioned in Slack that there might be a solution usinghooks. I have not dug into this yet.