Logging

Every router logs through one shared logger, and your own code can use the same one. Import logger from base and your logs come out in the same shape, at the same level, as the ones the library writes for you.

It stays small on purpose: levels, keys that last for one invocation and JSON output when Lambda asks for it. Install the Powertools logger and the library uses that instead, with nothing to configure.

Logger

This is the logger you get without installing anything. It keeps to the essentials, and you can tune it with environment variables or build your own Logger when you need more.

Default settings

Import logger and start logging. There is nothing to construct, and it writes at INFO and above until you change the level.

import { logger } from '@lambda-event-router/base';

logger.info('My log');

Set the level with your function's application log level, part of Lambda's Advanced Logging Controls, which Lambda exposes to the runtime as AWS_LAMBDA_LOG_LEVEL. You can set that variable directly instead. A level writes itself and everything more severe, so WARN keeps warnings and errors and drops INFO and DEBUG.

The levels, least to most severe: TRACE, DEBUG, INFO, WARN, ERROR, CRITICAL. SILENT turns logging off. AWS's own FATAL is not one of these, so setting the level to it falls back to INFO.

Custom settings

Build your own Logger when the environment variables do not get you far enough, usually to name the service or to put account and region on every line.

import { Logger } from '@lambda-event-router/base';

const logger = new Logger({
  logLevel: 'INFO',
  serviceName: 'serverlessAirline',
  persistentLogAttributes: {
    aws_account_id: '123456789012',
    aws_region: 'eu-west-1',
  },
})

logger.info('My log');

Option	Type	Default	Description
logLevel	LogLevelName	AWS_LAMBDA_LOG_LEVEL, or INFO	The lowest level that gets written. Setting it here wins over the environment variable
serviceName	string	Not set	Added to every log as serviceName
persistentLogAttributes	Record<string, unknown>	{}	Merged into every log, and left alone by the per-invocation reset

A logger you build this way is yours alone. The routers carry on using the shared one, so their logs will not pick up your serviceName or your attributes. Pass it to setLogger to make it the logger everything uses, as in custom logger below.

Per-invocation keys

LambdaRouter calls resetKeys() on the active logger at the start of every invocation. This clears any temporary keys added with appendKeys() during a previous invocation so that state does not leak across warm-container reuse.

If you need attributes that persist for the lifetime of the container (e.g. region, account id), use appendPersistentKeys() / persistentLogAttributes instead. Persistent keys are not affected by the per-invocation reset.

Structured JSON logs

If your Lambda function is configured with log format JSON (Advanced Logging Controls), the runtime sets AWS_LAMBDA_LOG_FORMAT=JSON. The default Logger detects this and emits a single structured object per log call so the runtime can merge its properties into CloudWatch as queryable fields rather than a single stringified message. For heavier structured logging (Lambda context injection, sampling, child loggers) install @aws-lambda-powertools/logger as shown below.

Powertools

Powertools for AWS Lambda (TypeScript) has a logger which "provides an opinionated logger with output structured as JSON".

If you have the Powertools logger installed and you are using the default settings / environment variables, you can use the logger as normal. Under the hood the lambda-event-router will use the powertools logger for any of its logging

npm install @aws-lambda-powertools/logger

import { logger } from '@lambda-event-router/base';

logger.info('My log');

As a note, this is the same as

import { Logger } from '@aws-lambda-powertools/logger';

const logger = new Logger();

logger.info('My log');

Custom logger

import { Logger } from '@aws-lambda-powertools/logger';
import { setLogger, LambdaRouter } from '@lambda-event-router/base';

const customLogger = new Logger({
  serviceName: 'serverlessAirline',
  logLevel: 'INFO',
  persistentLogAttributes: {
    aws_account_id: '123456789012',
    aws_region: 'eu-west-1',
  },
});

setLogger(customLogger);

const lambdaRouter = new LambdaRouter({
  routers: [sqsRouter]
})

export const handler = lambdaRouter.handler()