OptimisticFailureCause

The RollbackCause for a NestingLevel whose optimistic execution was invalid, and that should be retried. The specific situations in which an optimistic failure can occur are specific to the STM algorithm, but may include:

• the STM detected that the value returned by a previous read in this nesting level is no longer valid;
• a cyclic dependency has occurred and this nesting level must be rolled back to avoid deadlock;
• a transaction with a higher priority wanted to write to a Ref written by this transaction;
• the STM decided to switch execution strategies for this atomic block; or
• no apparent reason (*).

(*) - Some STMs perform validation, conflict detection and deadlock cycle breaking using algorithms that are conservative approximations. This means that any particular attempt to execute an atomic block might fail spuriously.