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Extension Points

OpenCQRS offers three core extension points for application developers to implement their CQRS applications:

  • CommandHandler definitions encapsulate the command handling logic for changing the system's state by publishing new events
  • StateRebuildingHandler definitions support the reconstruction of write models from events, on which commands are executed
  • EventHandler definitions encapsulate event processing logic

Command Handling

The following diagram shows s simplified command handling sequence involving the application specific extensions (highlighted in red): 

flowchart TB
    Start@{ shape: sm-circ }
    CommandRouter
    EventRepository
    CommandEventPublisher
    CommandHandler
    StateRebuildingHandler@{ shape: procs }
    Stop@{ shape: framed-circle }

    Start---|"(1) send command"|CommandRouter
    CommandRouter---|"(2) read events"|EventRepository
    CommandRouter---|"(3) rebuild state"|StateRebuildingHandler
    CommandRouter---|"(4) execute command"|CommandHandler
    CommandHandler---|"(5a) apply new event(s)"|StateRebuildingHandler
    CommandHandler---|"(5b) capture new event(s)"|CommandEventPublisher
    CommandEventPublisher---|"(6) write event(s)"|EventRepository
    CommandHandler---|"(7) return command result"|Stop

    classDef extension fill:indianred;
    class CommandHandler extension;
    class StateRebuildingHandler extension;

Tip

For details about the command handling process, refer to Command Router.

Event Processing

The following diagram shows a simplified asynchronous event processing loop involving the application specific extensions (highlighted in red):

flowchart TB
    Start@{ shape: sm-circ }
    EventRepository
    EventHandlingProcessor
    ProgressTracker
    EventHandler@{ shape: procs }

    Start---|"(1) start processing loop"|EventHandlingProcessor
    EventHandlingProcessor---|"(1) determine current progress"|ProgressTracker
    EventHandlingProcessor---|"(2) observe events"|EventRepository
    EventHandlingProcessor---|"(3a) handle event"|EventHandler
    EventHandlingProcessor---|"(3b) update progress"|ProgressTracker

    classDef extension fill:indianred;
    class EventHandler extension;

Tip

For details about the event processing loop, refer to Event Handling Processor.