When trying to perform bulk task operation(start/complete) then it fails with below exception:
 

2020-07-28 15:01:35,221 WARN [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] (default task-1282) SQL Error: -803, SQLState: 23505 2020-07-28 15:01:35,221 ERROR [org.hibernate.engine.jdbc.spi.SqlExceptionHelper] (default task-1282) DB2 SQL Error: SQLCODE=-803, SQLSTATE=23505, SQLERRMC=1;$SCHEMA_NAME.ORGANIZATIONALENTITY, DRIVER=4.19.66 2020-07-28 15:01:35,222 WARN [org.jbpm.services.task.persistence.TaskTransactionInterceptor] (default task-1282) Could not commit session: javax.persistence.PersistenceException: org.hibernate.exception.ConstraintViolationException: 
could not execute statement

This issue occurs when task operation is performed by new users in parallel by multiple threads. This issue can be avoided by these two options:
 

1. Update the database with this new user entry before the bulk task operation
2. Complete a single task operation request first and then send the rest of the requests in parallel.

But when ConstraintViolationException occurs, it makes the Task table and TaskEvent table entries out of sync. 
For same Task_id in Task table state is 'Completed' and in TaskEvent table it shows STARTED(i.e InProgress) status.