Process variables turn unexpectedly null when jobs are executed in parallel

[tasso94]

Environment (Required on creation)

Most likely, all Camunda versions are affected.

Description (Required on creation; please attach any relevant screenshots, stacktraces, log files, etc. to the ticket)

Process variables turn unexpectedly null when jobs are executed in parallel.

Steps to reproduce (Required on creation)

1. Deploy a process with a flow node (script/service task) that is asyncBefore, which first reads process variable A and second updates process variable A.
2. State
   • There is a process variable A of type Object.
   • The flow node has two tokens within the same process instance during runtime.
3. The job executor executes both resulting jobs in parallel.

Observed Behavior (Required on creation)

Process variables turn null.

Expected behavior (Required on creation)

Process variables don't turn null.

Root Cause (Required on prioritization)

1. T1 executes job 1 that starts executing token 1.
2. T2 executes job 2 that starts executing token 2, and selects the process variable A, including the referenced byte array row from the database.
3. T1 continues and updates the process variable A as defined in the script/service task.
   • Since the variable is of type Object, the value is stored in the byte array table, and the row in the runtime variable table references the row in the byte array table.
   • Updating the variable value leads to deleting the byte array row, inserting a new one, and updating the runtime variable row with the new reference.
4. T1 is done and commits all modifications to the database.
5. T2 continues and selects the byte array row from the previously selected process variable A.
   • The process variable A was selected before the byte array reference was updated => T2 still uses the state of the process variable A with the outdated byte array reference.
   • The variable A returns null since it doesn't exist anymore.

Solution Ideas

Reuse the byte array row by updating it. Like this, the variable value cannot turn null.

Hints

See the prototypical implementation that was validated with the customer: #4313

Links

• https://jira.camunda.com/browse/SUPPORT-21494

Breakdown

Pull Requests

Beta Give feedback

1. fix(engine): don't delete byte array when updating var #4313
   ci:all-db +1
   

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