EventSourcingDB

EventSourcingDB & ESModel Modules (src/EventSourcingDB)

These modules implement the core event sourcing pattern for the database.

Core Components

EventSourcingDB.js (EventSourcingDB class)

• Purpose: Orchestrates the event sourcing process. It manages the flow of events from the EventQueue to the ESModel instances, ensuring atomic and ordered processing.
• Key Features:
  • Event Loop: Polls or waits for new events from the EventQueue.
  • Event Processing Pipeline: For each event:
    1. Preprocessing: Calls preprocessor static methods on all registered ESModels. ESModel.preprocessor assigns IDs to new objects.
    2. Reduction: Calls reducer static methods on all ESModels. ESModel.reducer calculates the necessary change (ins, upd, rm, esFail) based on the event type and current state.
    3. Application: Within a database transaction:
       • Calls applyResult (via model.applyResult(result) which calls the imported applyResult) on each model with its calculated result, making the actual database changes using the underlying JsonModel methods (in writable mode).
       • Calls deriver methods on models for post-processing (also in writable mode).
    4. Commit: Commits the transaction and updates the database user_version.
  • Sub-events: Handles events dispatched during the processing of another event, processing them recursively within the same transaction.
  • Error Handling: Logs errors and halts processing if a reducer or applicator fails. Stores error information in the event's result field in the EventQueue.
  • State Management: Manages database connections (potentially separate R/W and RO), tracks the current processed version, and uses AsyncLocalStorage for context.
  • Lifecycle Events: Emits events ('begin', 'result', 'error', 'processed') for monitoring.
• Dependencies: DB, EventQueue, ESModel, debug, lodash, AsyncLocalStorage.