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Child Logger Overrides ​

Problem ​

A sub-module needs its own log level or namespace without losing the parent's configuration. Creating a child logger via child() or withBindings() scopes the override without affecting other callers.

Solution ​

Use child(overrides) to change level or namespace on a derived logger, or withBindings(fields) to pin request-scoped context on every call.

ts
import { createLogger } from '@vielzeug/rune';

const base = createLogger({ logLevel: 'info', namespace: 'app' });

// config override — change level for one path
const verbose = base.child({ logLevel: 'debug' });
base.info('base flow');
verbose.debug('debug details for one path');

// context binding — pin fields to every call
const reqLog = base.withBindings({ requestId: 'abc-123', userId: 42 });
reqLog.info('processing'); // emits requestId + userId on every line
reqLog.warn({ slow: true }, 'query took 2s');

Pitfalls ​

  • withBindings() returns a new logger instance — it does not mutate the parent. Subsequent calls on the parent use the original bindings.
  • Overriding the log level on a child via child({ logLevel }) does not affect the parent. Remote forwarding still uses the child logger's own resolved remote threshold.
  • Passing a mutable object as a binding captures a reference. Subsequent mutations to that object appear in all future log entries from the child logger.