TaskFlow/States of Task and Flow
Flow States
Flow starts its life in PENDING state. Then, when it is run, it transitions to RUNNING state. When all tasks are finished successfully, flow transitions to SUCCESS state. In case of failure, flow is reverted; when it happens, flow first transitions to REVERTING state, then revert happens, then flow transitions to REVERTED state, and then to FAILURE.
Transition from REVERTED to FAILURE state happens immediately. The main purpose of REVERTED state is notifications.
In RUNNING and REVERTING state flow can be interrupted. When this happens, flow transitions to INTERRUPTED state. From that state, flow can run on (transition back to RUNNING) or be reverted (and transition to REVERTED state).
There is also special flow state, RESUMING (not shown on diagram). Any flow is in this state when it is resumed (loaded) from storage. RESUMING state is never saved to storage and exists only for notifications.
Task States
When task just added to the flow, it is in PENDING state, which means it can be executed or waits for all of task it depends on to complete. Then, when task is executed, it transitions to RUNNING state, and stays in it until its execute() method returns. When task is finished, it transitions to SUCCESS state if it was finished successfully, or to FAILURE state if not.
When flow is reverted, all tasks in PENDING state are left as is; tasks from all the other states transition to REVERTING state, and their revert() method is called. When that method returns, task transitions to REVERTED state, and then to PENDING.
Transition from RUNNING to REVERTING state happens when task was interrupted while running, and then revert occurs. This may take place e.g. when daemon is killed while running the task.
Transition from REVERTED to PENDING state happens immediately. The main purpose of REVERTED state is notifications.
Appendix A: Diagrams
The diagrams were drawn using PlantUML. You can re-create them offline or online here.
Source for task state diagram:
[*] --> PENDING PENDING --> RUNNING RUNNING --> SUCCESS RUNNING --> FAILURE RUNNING --> REVERTING SUCCESS --> REVERTING FAILURE --> REVERTING REVERTING -up-> REVERTED REVERTING -up-> FAILURE REVERTED -up-> PENDING
Source for flow state diagram:
[*] --> PENDING PENDING --> RUNNING RUNNING --> SUCCESS RUNNING --> REVERTING : failure RUNNING --> INTERRUPTED REVERTING --> INTERRUPTED INTERRUPTED -l-> RUNNING INTERRUPTED --> REVERTING REVERTING --> REVERTED REVERTED --> FAILURE
