EventQueue

EventQueue Module (src/EventQueue.js)

This module provides an event queue implementation, likely used for the event sourcing mechanism.

Core Components

EventQueue.js

• Purpose: Implements a persistent event queue backed by a SQLite table (defaulting to history). It extends JsonModel, suggesting it leverages the JSON storage capabilities.
• Key Features:
  • Stores events with v (auto-incrementing version/ID), type, ts (timestamp), data (JSON), result (JSON), and size (calculated size of data/result).
  • Provides methods for adding events (add), retrieving the latest version (getMaxV), and fetching the next event after a specific version (getNext), including waiting for new events.
  • Includes database migrations for adding indices and helper views (_recentHistory, _historyTypes).
  • Manages internal state (currentV, knownV) for optimization.
  • Uses debug for logging.
  • Handles concurrency within the same process using Promises (_addP).
  • Optionally keeps the Node.js process alive while waiting (forever: true).
  • Allows customization of table name and columns.
• Dependencies: JsonModel, debug

Usage

• Instantiate EventQueue (likely via new EventQueueImpl(...) although the export might simplify this).
• Use add(type, data, [ts]) to append new events.
• Use getMaxV() to get the highest event version number.
• Use getNext(v, [noWait]) to retrieve events sequentially, potentially waiting for new ones.

Schema (Default history table)

• v: INTEGER PRIMARY KEY AUTOINCREMENT (Event version/ID)
• type: TEXT (Event type identifier)
• ts: INTEGER (Timestamp in milliseconds since epoch, indexed)
• data: JSON (Payload of the event)
• result: JSON (Result associated with processing the event, optional)
• size: INTEGER (Calculated size of data + result JSON strings)

Dependencies

• debug: Used for logging.
• JsonModel: Base class providing JSON handling and DB interaction.