Jest limit concurrency of integration tests

Question

How can I limit the concurrency of test execution in jest? Let's say I only want to run 4 test cases in parallel at the same time.

My current problem is that I have a lot of integration tests using a real database connection. Jest executes too much test cases in parallel, thus often connections timeout or the performance of my test database instance drops significantly because there are too many queries at the same time.

All of my integration test suits have the following structure:

describe('<functionality x>', () => {
    test('case 1', async () => {...})
    test('case 2', async () => {...})
    test('case 3', async () => {...})
})

describe('<functionality y>', () => {
    test('case 1', async () => {...})
    test('case 2', async () => {...})
    test('case 3', async () => {...})
})

I already tried running jest with --maxWorkers=1, but I guess this is the same as --runInBand, which works, but really slows down the overall execution time.

Answer 1

Rather than --maxWorkers=1, you could provide a larger value like --maxWorkers=2 or --maxWorkers=50%.

However, if you are trying to limit the number of tests running in parallel, it seems like you want to use:

jest --maxConcurrency=N

Jest's documentation says:

Prevents Jest from executing more than the specified amount of tests at the same time. Only affects tests that use test.concurrent.

so, note you have to modify your tests by adding the .concurrent:

describe('<functionality x>', () => {
    test.concurrent('case 1', async () => {...})
    test.concurrent('case 2', async () => {...})
    test.concurrent('case 3', async () => {...})
})

describe('<functionality y>', () => {
    test.concurrent('case 1', async () => {...})
    test.concurrent('case 2', async () => {...})
    test.concurrent('case 3', async () => {...})
})