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Flow

Workflow processing flow

Below diagram demonstrates how a workflow is processed by Cadence workers and backend components. Note that this is a partial diagram that only highlights selected components and their interactions. There is more happening behind the scenes.

flowchart TD
    User((Domain X Admin)) -->|"0. Deploy worker with config: domain, tasklist(s)"| Worker[[\nDomain X Worker\n\n]]
    ExternalInitiator(("Domain X\nAnother Service/CLI")) -->|"1. Start/Signal Workflow"| Frontend
    Worker -->|2. Poll tasks for domain: X, tasklist: tl1| Frontend[[\nCadence Frontend\n\n]]
    Frontend -->|3. Forward Poll request| Matching[[\nCadence Matching\n\n]]
    Matching -->|4. Poll| TaskListManager(Tasklist Manager for tl1)
    TaskListManager -->|5. Long Poll| WaitUntilTaskFound
    WaitUntilTaskFound --> |6. Return task to worker| Worker
    Worker -->|7. Generate decisions\n& respond| Frontend
    Frontend -->|8. Respond decisions| History[[\nCadence History\n\n]]
    History -->|9. Mutable state update| ExecutionsStore[(Executions Store)]
    History -->|10.a. Notify| HistoryQueue(History Queues - per shard)
    HistoryQueue -->|Periodic task fetch| ExecutionsStore
    HistoryQueue -->|11.b. Execute transfer task| TransferTaskExecutor(Transfer Task Executor)
    HistoryQueue -->|11.a. Execute timer task| TimerTaskExecutor(Timer Task Executor)
    HistoryQueue -->|Periodic per-shard offset save| ShardsStore[(Shards Store)]
    TransferTaskExecutor -->|12.b. Add task| Matching
    TimerTaskExecutor -->|12.a. Add task| Matching
    Matching -->|13. Add task| TaskListManager
    TaskListManager -->|14. Add task| SyncMatch{Check sync match}
    SyncMatch -->|15.b. Found poller - Do sync match| SyncMatched
    SyncMatch -->|15.a. No pollers - Save task| TasksStore[(Tasks Store)]
    TaskListManager -->|Periodic task fetch| TasksStore
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Cassandra Notes:

In order to leverage Cassandra Light Weight Transactions, Cadence stores multiple types of records in the same executions table. (ref)

  • Shards Store: Shard records are stored in executions table with type=0.
  • Executions Store: Execution records are stored in executions table with type=1
  • Tasks Store: Task records are stored in executions table with type={2,3,4,5}