Use isParallel in test classes that can get UNABLE_TO_LOCK_ROW error sometimes

Question

I have enabled Disalbe Parallel Apex Testing to solve UNABLE_TO_LOCK_ROW errors. Let test methods run fast, then I add the isParallel annotation to existing test classes. But that can still cause UNABLE_TO_LOCK_ROW errors event thought each test classes create test data isolate.

I checked the errors which mostly the following one:

System.DmlException: Insert failed. 
First exception on row 0; 
first error: UNABLE_TO_LOCK_ROW, 
unable to obtain exclusive access to this record: []

Looking back to my code that causes the error by creating a new lead record.

My question is why does create a new lead record that can still lock the database? Is there another solution except removing the isParallel annotation from the error test class?

Answer 1

This often happens when you have triggers or other related logic that locks a (possibly unrelated) record as part of the record creation process. For example, if you have implemented the "custom auto-number" pattern (e.g. to use conditional auto-numbers), you may use a Custom Setting that will end up being locked repeatedly. While isolated tests (SeeAllData=false) reduces this possibility, all records created during testing can cause locking errors. Note that a deployment to production automatically disables parallel tests; the setting you specify only affects non-deployment test runs. Ideally, your deployment process should not run more tests than necessary; instead of trying to run in parallel, it is often possible to simply run fewer tests.