# AWS Lambda Logs

Once you connect your AWS account to Baselime, Baselime automatically create CloudWatch Logs subscription filters to automatically ingest logs from your AWS Lambda functions.

Baselime automatically captures logs for newly created AWS Lambda functions, and enables you to query and visualise logs across log groups and log streams.

# How it works

Once Baselime is connected to an AWS Account, it automatically creates Logs subscription filters for all the AWS Lambda functions in the account. Log subscription filters enable Baselime to asynchronously ingest logs from the AWS Lambda functions through Amazon CloudWatch, without any impact on the performance of the AWS Lambda functions.

Sending Lambda Logs to Baselime
Sending Lambda Logs to Baselime

Moreover, Baselime automatically creates new subscription filters for newly deployed AWS Lambda functions. Baselime listens to new AWS Lambda events in Amazon CloudTrail and creates subscription filters for newly created AWS Lambda functions.

AWS Lambda Logs in Baselime
AWS Lambda Logs in Baselime

# Logging best practices

In order to get the most out of Baselime, we recommend adding two log messages to all your AWS Lambda functions:

  • the event which triggered your Lambda function
  • the response your Lambda function returns

These can be added as follows:

exports.handler = async (event, context) => {
  console.log(JSON.stringify({ message: "baselime:trigger", level: "baselime", data: { event }));

  // Your logic
  const response = await yourLogic();

  console.log(JSON.stringify({ message: "baselime:response", level: "baselime", data: { response }));
  return response;

To facilitate this in Node.js runtimes, we maintain a custom logger well suited for AWS Lambda.

npm i @baselime/lambda-logger

It's a 2.5kb JavaScript file with 0 dependencies, and does not have any significant impact on performance or cold-starts.

const { wrap } = require('@baselime/lambda-logger');

exports.handler = wrap(async (event, context) => {
   // Your logic

It also provides an interface to be used as a middy middleware.

import { logger, Baselime } from "@baselime/lambda-logger";
import middy from "@middy/core";

exports.handler = middy()
	.handler(function (e, context) {
		// Your logic

# Logging format

We recommend using structured logging across your application, preferably in JSON format. Feel free to use your favourite logging library; we recommend:

import { logger } from "@baselime/lambda-logger";

  message: "This log is awesome",
  level: "info",
  data: { customer_id: "cus_1234", accountType: "premium" }

logger.info("This log is awesome", { customer_id: "cus_1234", accountType: "premium" });

It is particularly important to format errors and exception correctly to appropriately log stack traces.

const error = new Error("A message for the error");

  message: "Unfortunately something went wrong",
  level: "error",
  data: {
    customer_id: "cus_1234",
    accountType: "premium",
    error: {
      message: error.message,
      name: error.name,
      stack: error.stack

or with the Baselime Lambda Logger for Node.js:

import { logger } from "@baselime/lambda-logger";

logger.info("Unfortunately something went wrong", { customer_id: "cus_1234", accountType: "premium" }, error);

# Discovered Keys

Baselime automatically discovers key - value pairs from your AWS Lambda logs. This enables you to run complex queries and setup alerts on data that otherwise would be difficult to work with from the AWS Lambda service. For instance, from the discovered keys from the Lambda logs, it's possible to set alerts on the maximum memory used by lambda functions during execution, compared to the amount of memory they are assigned at deployment time.

# Lambda Discovered Keys

The Lambda service automatically writes logs at the start and end of every function invocation. These logs are parsed as events in Baselime, and keys are automatically discovered from those messages.

# START Log Message

The following keys are discovered from the START message:

  • @type: is always START
  • @requestId: the request ID of the Lambda invocation
  • @version: the invoked version of the Lambda function
 START RequestId: d5d410e5-7406-5a1b-888e-4aa7492a313d Version: $LATEST

  @type: "START",
  @requestId: "d5d410e5-7406-5a1b-888e-4aa7492a313d",
  @version: "$LATEST"

# END Log Message

The following keys are discovered from the END message:

  • @type: is always END
  • @requestId: the request ID of the Lambda invocation
 END RequestId: d5d410e5-7406-5a1b-888e-4aa7492a313d

  @type: "END",
  @requestId: "d5d410e5-7406-5a1b-888e-4aa7492a313d",

# REPORT Log Message

The following keys are discovered from the REPORT message:

  • @type: is always REPORT
  • @requestId: the request ID of the Lambda invocation
  • @duration: the duration in milliseconds
  • @billedDuration: the billed duration in milliseconds
  • @memorySize: the total memory available to the invocation, in MB
  • @maxMemoryUsed: the max memory used, in MB
  • @initDuration: the duration of the lambda initialisation in milliseconds (cold starts)

If the Lambda function is instrumented with XRAY, additional keys are discovered:

  • @xRAYTraceId: the XRAY trace ID
  • @segmentId: the XRAY segment ID
  • @sampled: always true
REPORT RequestId: 8fc6f963-411b-58b5-8483-a32130c0f45d Duration: 201.67 ms Billed Duration: 202 ms Memory Size: 2048 MB Max Memory Used: 81 MB Init Duration: 480.25 ms XRAY TraceId: 1-61c86fa4-59b3c6d959653c527fd10966 SegmentId: 107ea78a5750a811 Sampled: true

  @type: "REPORT",
  @requestId: "8fc6f963-411b-58b5-8483-a32130c0f45d",
  @duration: 201.67,
  @billedDuration: 202,
  @memorySize: 2048,
  @maxMemoryUsed: 81,
  @initDuration: 480.25,
  @xRAYTraceId: "1-61c86fa4-59b3c6d959653c527fd10966",
  @segmentId: "107ea78a5750a811",
  @sampled: true

# Timeout Invocations

If your async Lambda invocation times out, Additional keys are automatically discovered:

  • @timedOut: always true
  • @timeout: the duration after which the invocation timed-out in seconds
  • @message: always Task timed out after {@timeout} seconds
  • @timestamp: the timestamp at the moment the invocation timed out.
 2021-12-26T13:15:48.760Z 5b252591-51c6-5d15-87a9-7fd33ce32be4 Task timed out after 15.01 seconds

  @timestamp: "2021-12-26T13:15:48.760Z",
  @requestId: "5b252591-51c6-5d15-87a9-7fd33ce32be4",
  @timedOut: true,
  @message: "Task timed out after 15.01 seconds",
  @timeout: 15.01

# console.log Log Message

For Node.js environments, AWS Lambda uses a modified version of console.log (and other console logging functions) to write to stdout and stderr. These add fields to the log message which are parsed as follows:

  • @timestamp: the timestamp at the moment the log message was written
  • @requestId: the request ID of the Lambda invocation
  • LogLevel: the log level (INFO, DEBUG, WARN, ERROR)
  • @message: the message.
 2021-12-26T14:00:19.258Z e578aebe-054e-4844-835c-66a75d3112cf ERROR Error doing something.

  @timestamp: "2021-12-26T14:00:19.258Z",
  @requestId: "e578aebe-054e-4844-835c-66a75d3112cf",
  LogLevel: "ERROR",
  @message: "Error doing something."

# Troubleshooting

If you're having trouble sending data from your AWS Lambda logs to Baselime, here are a few things to check:

  • Verify that your AWS account is correctly connected to Baselime and you receive data in other datasets such as CloudWatch Metrics or CloudTrail Events
  • Check that your Lambda functions are not already using the maximum number of subscription filters allowed per log group. AWS limits each log group to 2 subscription filters at most. If you're already at the limit, you can remove subscription filters with the cloudwatch-subscription-filters-remover to delete the ones you don't need anymore.
  • Make sure that your AWS Lambda functions are being invoked and you can view the logs in the CloudWatch section of the AWS Console