suaveplan/packages/observability/monitor/logger/src/event/event-logger.md

event-logger

(no submodules)

Purpose

EventLogger routes each log entry through an EventBusLike<LoggerEvents> emitter after the standard AbstractLogger pipeline (level filtering, redaction, sampling, OTel). It exists for environments where other parts of an application need to react to log events — for example, a UI component that surfaces recent errors, a test harness that asserts on emitted entries, or an aggregator that combines logs from multiple services. The emitter is injected so any conformant EventBusLike works (including @suaveplan/events' EventBus).

Features

• Emits "log" for every accepted record
• Emits "log:debug", "log:info", "log:warn", "log:error" per level
• Emits "metrics:updated" after every log with the current LoggerMetrics snapshot
• on*/off* helper methods for typed subscription/unsubscription
• Proper unsubscription via the () => void handle returned by EventBusLike.on
• Compatible with any EventBusLike<LoggerEvents<TMetadata>> — no hard dep on @suaveplan/events

Usage: Basic Example

import { EventBus } from "@suaveplan/events";
import { EventLogger } from "@suaveplan/logger/event";

const bus = new EventBus();
const logger = new EventLogger(bus, { level: "info" });

logger.onError((entry) => {
  alerting.report(entry.message, entry.metadata);
});

logger.error("Payment gateway unreachable");

Usage: Advanced Example

import { EventBus } from "@suaveplan/events";
import { EventLogger } from "@suaveplan/logger/event";

const bus = new EventBus();
const logger = new EventLogger(bus, { level: "debug" });

// Subscribe
const handleInfo = (entry) => console.log("[INFO]", entry.message);
logger.onInfo(handleInfo);

// Log
logger.info("Started");

// Unsubscribe
logger.offInfo(handleInfo);

API Reference

class EventLogger<TMetadata> extends AbstractLogger<TMetadata>

Constructor: new EventLogger(emitter, config?)

Parameter	Type	Description
emitter	LoggerEventBus<TMetadata>	Any EventBusLike conformant bus
config	EventLoggerConfig<TMetadata>?	Inherits all AbstractLoggerConfig options

Subscription helpers (all return this)

onLog / offLog, onDebug / offDebug, onInfo / offInfo, onWarn / offWarn, onError / offError, onMetricsUpdate / offMetricsUpdate

getEmitter(): LoggerEventBus<TMetadata>

Returns the injected bus.

interface LoggerEvents<TMetadata>

{
  "log":             LogEntry<TMetadata>;
  "log:debug":       LogEntry<TMetadata>;
  "log:info":        LogEntry<TMetadata>;
  "log:warn":        LogEntry<TMetadata>;
  "log:error":       LogEntry<TMetadata>;
  "metrics:updated": LoggerMetrics;
}

Implementation Notes

Unsubscription is implemented by storing the () => void cleanup function returned by emitter.on() in a WeakMap<listener, cleanup>. Calling off* looks up and invokes the cleanup. The WeakMap avoids memory leaks when the listener function is garbage collected without explicit unsubscription.