Logs and metrics

Logs

The function's stdout and stderr go to the container output, unbuffered. Around each invocation, the runtime writes the platform lines AWS Lambda™ writes:

START RequestId: 3f2a… Version: $LATEST
END RequestId: 3f2a…
REPORT RequestId: 3f2a…	Duration: 12.34 ms	Billed Duration: 13 ms	Memory Size: 256 MB	Max Memory Used: 61 MB	

Field	Content
Init Duration	On the first REPORT after an init
Status: timeout	The invocation timed out; a Task timed out after N seconds line comes before END
Status: error, Error Type	The invocation failed
Max Memory Used	The container's peak since it started, not the invocation's

With AWS_LAMBDA_LOG_FORMAT=JSON, these lines become JSON records (platform.start, platform.runtimeDone, platform.report). The runtimes of the AWS images also format the application logs from this variable.

The runtime's own logs are JSON on stderr. Set their level with LOCH_RUNTIME_LOG_LEVEL (debug, info, warn, error). They never contain events or secrets.

docker logs -f fn

Health

GET /_loch/health needs no credential. It answers 200 {"status":"ready","queueDepth":0} when the function is ready, 503 with initializing, error or stopping otherwise. In the container, /loch/lambda-runtime health checks it and exits 0 or 1.

Metrics

GET /_loch/metrics returns Prometheus text. It needs the credential, except from the loopback.

curl -s -H "Authorization: Bearer $TOKEN" localhost:8080/_loch/metrics

Main series: loch_lambda_invocations_total{outcome}, loch_lambda_invocation_duration_seconds, loch_lambda_queue_depth, loch_lambda_state{state}. Full list: Metrics reference.

Good to know

• The function's code can print fake START, END or REPORT lines. Never derive billing or health from the logs: use the metrics, the invoke envelope or /_loch/health.
• The runtime does not rate-limit logs: bound their volume with the Docker log driver options (max-size, max-file).
• X-Amz-Log-Type: Tail (--log-type Tail in the AWS CLI) returns the last 4 KB of the invocation logs in X-Amz-Log-Result.

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"AWS" and "AWS Lambda" are trademarks of Amazon.com, Inc. or its affiliates. This project is not affiliated with or endorsed by Amazon. It implements the publicly documented AWS Lambda Runtime and Invoke APIs and contains no code from the AWS Lambda Runtime Interface Emulator.