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AWS Lambda function logging in Node.js

AWS Lambda automatically monitors Lambda functions on your behalf and sends function metrics to Amazon CloudWatch. Your Lambda function comes with a CloudWatch Logs log group and a log stream for each instance of your function. The Lambda runtime environment sends details about each invocation to the log stream, and relays logs and other output from your function's code.

This page describes how to produce log output from your Lambda function's code, or access logs using the AWS Command Line Interface, the Lambda console, or the CloudWatch console.

Topics

Creating a function that returns logs

To output logs from your function code, you can use methods on the console object, or any logging library that writes to stdout or stderr. The following example logs the values of environment variables and the event object.

Example index.js file – Logging

exports.handler = async function(event, context) {
  console.log("ENVIRONMENT VARIABLES\n" + JSON.stringify(process.env, null, 2))
  console.info("EVENT\n" + JSON.stringify(event, null, 2))
  console.warn("Event not processed.")
  return context.logStreamName
}

Example log format

START RequestId: c793869b-ee49-115b-a5b6-4fd21e8dedac Version: $LATEST
2019-06-07T19:11:20.562Z	c793869b-ee49-115b-a5b6-4fd21e8dedac	INFO	ENVIRONMENT VARIABLES
{
  "AWS_LAMBDA_FUNCTION_VERSION": "$LATEST",
  "AWS_LAMBDA_LOG_GROUP_NAME": "/aws/lambda/my-function",
  "AWS_LAMBDA_LOG_STREAM_NAME": "2019/06/07/[$LATEST]e6f4a0c4241adcd70c262d34c0bbc85c",
  "AWS_EXECUTION_ENV": "AWS_Lambda_nodejs12.x",
  "AWS_LAMBDA_FUNCTION_NAME": "my-function",
  "PATH": "/var/lang/bin:/usr/local/bin:/usr/bin/:/bin:/opt/bin",
  "NODE_PATH": "/opt/nodejs/node10/node_modules:/opt/nodejs/node_modules:/var/runtime/node_modules",
  ...
}
2019-06-07T19:11:20.563Z	c793869b-ee49-115b-a5b6-4fd21e8dedac	INFO	EVENT
{
  "key": "value"
}
2019-06-07T19:11:20.564Z	c793869b-ee49-115b-a5b6-4fd21e8dedac	WARN	Event not processed.
END RequestId: c793869b-ee49-115b-a5b6-4fd21e8dedac
REPORT RequestId: c793869b-ee49-115b-a5b6-4fd21e8dedac	Duration: 128.83 ms	Billed Duration: 200 ms	Memory Size: 128 MB	Max Memory Used: 74 MB	Init Duration: 166.62 ms	XRAY TraceId: 1-5d9d007f-0a8c7fd02xmpl480aed55ef0	SegmentId: 3d752xmpl1bbe37e	Sampled: true

The Node.js runtime logs the START, END, and REPORT lines for each invocation. It adds a timestamp, request ID, and log level to each entry logged by the function. The report line provides the following details.

Report Log

  • RequestId – The unique request ID for the invocation.
  • Duration – The amount of time that your function's handler method spent processing the event.
  • Billed Duration – The amount of time billed for the invocation.
  • Memory Size – The amount of memory allocated to the function.
  • Max Memory Used – The amount of memory used by the function.
  • Init Duration – For the first request served, the amount of time it took the runtime to load the function and run code outside of the handler method.
  • XRAY TraceId – For traced requests, the AWS X-Ray trace ID.
  • SegmentId – For traced requests, the X-Ray segment ID.
  • Sampled – For traced requests, the sampling result.

You can view logs in the Lambda console, in the CloudWatch Logs console, or from the command line.

Using the Lambda console

You can use the Lambda console to view log output after you invoke a Lambda function. For more information, see Accessing Amazon CloudWatch logs for AWS Lambda.

Using the CloudWatch console

You can use the Amazon CloudWatch console to view logs for all Lambda function invocations.

To view logs on the CloudWatch console

  1. Open the Log groups page on the CloudWatch console.

  2. Choose the log group for your function (/aws/lambda/your-function-name).

  3. Choose a log stream.

Each log stream corresponds to an instance of your function. A log stream appears when you update your Lambda function, and when additional instances are created to handle multiple concurrent invocations. To find logs for a specific invocation, we recommend instrumenting your function with AWS X-Ray. X-Ray records details about the request and the log stream in the trace.

To use a sample application that correlates logs and traces with X-Ray, see Error processor sample application for AWS Lambda.

Using the AWS Command Line Interface (AWS CLI)

The AWS CLI is an open-source tool that enables you to interact with AWS services using commands in your command line shell. To complete the steps in this section, you must have the following:

You can use the AWS CLI to retrieve logs for an invocation using the --log-type command option. The response contains a LogResult field that contains up to 4 KB of base64-encoded logs from the invocation.

Example retrieve a log ID
The following example shows how to retrieve a log ID from the LogResult field for a function named my-function.

aws lambda invoke --function-name my-function out --log-type Tail

You should see the following output:

{
    "StatusCode": 200,
    "LogResult": "U1RBUlQgUmVxdWVzdElkOiA4N2QwNDRiOC1mMTU0LTExZTgtOGNkYS0yOTc0YzVlNGZiMjEgVmVyc2lvb...",
    "ExecutedVersion": "$LATEST"
}

Example decode the logs
In the same command prompt, use the base64 utility to decode the logs. The following example shows how to retrieve base64-encoded logs for my-function.

aws lambda invoke --function-name my-function out --log-type Tail \
--query 'LogResult' --output text |  base64 -d

You should see the following output:

START RequestId: 57f231fb-1730-4395-85cb-4f71bd2b87b8 Version: $LATEST
"AWS_SESSION_TOKEN": "AgoJb3JpZ2luX2VjELj...", "_X_AMZN_TRACE_ID": "Root=1-5d02e5ca-f5792818b6fe8368e5b51d50;Parent=191db58857df8395;Sampled=0"",ask/lib:/opt/lib",
END RequestId: 57f231fb-1730-4395-85cb-4f71bd2b87b8
REPORT RequestId: 57f231fb-1730-4395-85cb-4f71bd2b87b8  Duration: 79.67 ms      Billed Duration: 80 ms         Memory Size: 128 MB     Max Memory Used: 73 MB

The base64 utility is available on Linux, macOS, and Ubuntu on Windows. macOS users may need to use base64 -D.

Example get-logs.sh script
In the same command prompt, use the following script to download the last five log events. The script uses sed to remove quotes from the output file, and sleeps for 15 seconds to allow time for the logs to become available. The output includes the response from Lambda and the output from the get-log-events command.
Copy the contents of the following code sample and save in your Lambda project directory as get-logs.sh.
The cli-binary-format option is required if you are using AWS CLI version 2. You can also configure this option in your AWS CLI config file.

#!/bin/bash
aws lambda invoke --function-name my-function --cli-binary-format raw-in-base64-out --payload '{"key": "value"}' out
sed -i'' -e 's/"//g' out
sleep 15
aws logs get-log-events --log-group-name /aws/lambda/my-function --log-stream-name $(cat out) --limit 5

Example macOS and Linux (only)
In the same command prompt, macOS and Linux users may need to run the following command to ensure the script is executable.

chmod -R 755 get-logs.sh

Example retrieve the last five log events
In the same command prompt, run the following script to get the last five log events.

./get-logs.sh

You should see the following output:

{
    "StatusCode": 200,
    "ExecutedVersion": "$LATEST"
}
{
    "events": [
        {
            "timestamp": 1559763003171,
            "message": "START RequestId: 4ce9340a-b765-490f-ad8a-02ab3415e2bf Version: $LATEST\n",
            "ingestionTime": 1559763003309
        },
        {
            "timestamp": 1559763003173,
            "message": "2019-06-05T19:30:03.173Z\t4ce9340a-b765-490f-ad8a-02ab3415e2bf\tINFO\tENVIRONMENT VARIABLES\r{\r  \"AWS_LAMBDA_FUNCTION_VERSION\": \"$LATEST\",\r ...",
            "ingestionTime": 1559763018353
        },
        {
            "timestamp": 1559763003173,
            "message": "2019-06-05T19:30:03.173Z\t4ce9340a-b765-490f-ad8a-02ab3415e2bf\tINFO\tEVENT\r{\r  \"key\": \"value\"\r}\n",
            "ingestionTime": 1559763018353
        },
        {
            "timestamp": 1559763003218,
            "message": "END RequestId: 4ce9340a-b765-490f-ad8a-02ab3415e2bf\n",
            "ingestionTime": 1559763018353
        },
        {
            "timestamp": 1559763003218,
            "message": "REPORT RequestId: 4ce9340a-b765-490f-ad8a-02ab3415e2bf\tDuration: 26.73 ms\tBilled Duration: 27 ms \tMemory Size: 128 MB\tMax Memory Used: 75 MB\t\n",
            "ingestionTime": 1559763018353
        }
    ],
    "nextForwardToken": "f/34783877304859518393868359594929986069206639495374241795",
    "nextBackwardToken": "b/34783877303811383369537420289090800615709599058929582080"
}

Deleting logs

Log groups aren't deleted automatically when you delete a function. To avoid storing logs indefinitely, delete the log group, or configure a retention period after which logs are deleted automatically.