Reproduce UNABLE_TO_LOCK_ROW error in local environment/sandbox

Question

We have external Systems using an rest-endpoint in our Org. In the called function a certain record is called (some kind of tracker) which gets updated everytime the function runs with an success.

Most of the time everything works fine, but when the load increases (in a certain time of the year, due to more customers) we often get the QueryException UNABLE_TO_LOCK_ROW, because the endpoint is called multiple times and the record is always queried and updated. The Query already uses the "FOR UPDATE" keyword.

I already have some approaches to solve this, but my main problem is how to reproduce the error to verify the solution works.

I guess it is not possible via unittests, but also I tried it via a anonymous script calling the endpoint and also via postman, but it didn't work. There is either no exception or a different one.

I know that there is this question Reproduce UNABLE_TO_LOCK_ROW error but it is 8 years old and the linked youtube video is not accessible anymore.

Can somebody provide some hints or example code to reproduce this?

Endpoint which is called

global static Response post() {  
    Data data = Request //some logic returns data to create an account 
    Account a = insertAccount(data);
}

When the Account is inserted the accountTriggerHandler runs, and the function where the records are locked:

NumberHelper__c numberHelper = [SELECT Id, currentNumber__c FROM NumberHelper__c in :accounts FOR UPDATE LIMIT 1] // there is only 1 record

for (Account a : accounts){
    a.accNumber = numberHelper.currentNumber__c;
    numberHelper.currentNumber__c ++;
}

update autoNumber;  //UNABLE_TO_LOCK_ROW error

In my scripts I just tried make fast calls to the endpoint, but this probably can't work, because of the user context.

Answer 1

I grabbed and edited a testmethod in my code base to show how to mock a selector class to throw an exception. Admittedly, this is based on ApexMocks and relies on fflib Selector classes but you should get the idea

Code Under Test

 class MyClass {
   
    public void doStuff() {
       try {
          Number_Helper__c[] numberHelpers = NumberHelpersSelector.newInstance().selectByAccountIds(accountIds);
       }
       catch (QueryException qe) {
          if (qe.getMessage().contains('UNABLE_TO_LOCK_ROW')) { .. do retry }
       }
     }
  }

Unit Test Class

@IsTest
private static void givenUnableToLockRowVerifyRetry () {
    fflib_ApexMocks mocks = new fflib_ApexMocks();

    
    //  Given mock UNABLE_TO_LOCK_ROW Exception
    Exception mockUnableToLockRowException =
            new System.QueryException('... UNABLE_TO_LOCK_ROW ...');

    // Given mock selector(s)
    NumberHelpersSelector mockNumberHelpersSelector =
           (NumberHelpersSelector) mocks.mock(NumberHelpersSelector.class);
    mocks.startStubbing();
    mocks.when(mockNumberHelpersSelector.SObjectType()).thenReturn(Number_Helper__c.SObjectType);

    // mock throwing exception on call (regardless of arg values)
    ((INumberHelpersSelector)mocks.doThrowWhen(mockUnableToLockRowException,mockNumberHelpersSelector))
            .selectByAccountIds((Set<Id>fflib_Match.anyObject());
    mocks.stopStubbing();
    
    // Given mocks injected
    Application.Selector.setMock(mockNumberHelpersSelector);

    // when code-under-test invoked
    new MyClass().doStuff();

    // verify the retry occurred

}