AWS Lambda function logging in C# - AWS Lambda

AWS Lambda function logging in C#

Your Lambda function comes with a CloudWatch Logs log group, with a log stream for each instance of your function. The runtime sends details about each invocation to the log stream, and relays logs and other output from your function's code.

To output logs from your function code, you can use methods on the Console class, or any logging library that writes to stdout or stderr. The following example uses the LambdaLogger class from the Amazon.Lambda.Core library.

Example src/blank-csharp/Function.cs – Logging

public async Task<AccountUsage> FunctionHandler(SQSEvent invocationEvent, ILambdaContext context) { GetAccountSettingsResponse accountSettings; try { accountSettings = await callLambda(); } catch (AmazonLambdaException ex) { throw ex; } AccountUsage accountUsage = accountSettings.AccountUsage; LambdaLogger.Log("ENVIRONMENT VARIABLES: " + JsonConvert.SerializeObject(System.Environment.GetEnvironmentVariables())); LambdaLogger.Log("CONTEXT: " + JsonConvert.SerializeObject(context)); LambdaLogger.Log("EVENT: " + JsonConvert.SerializeObject(invocationEvent)); return accountUsage; }

Example Log format

START RequestId: d1cf0ccb-xmpl-46e6-950d-04c96c9b1c5d Version: $LATEST ENVIRONMENT VARIABLES: { "AWS_EXECUTION_ENV": "AWS_Lambda_dotnetcore2.1", "AWS_LAMBDA_FUNCTION_MEMORY_SIZE": "256", "AWS_LAMBDA_LOG_GROUP_NAME": "/aws/lambda/blank-csharp-function-WU56XMPLV2XA", "AWS_LAMBDA_FUNCTION_VERSION": "$LATEST", "AWS_LAMBDA_LOG_STREAM_NAME": "2020/03/27/[$LATEST]5296xmpl084f411d9fb73b258393f30c", "AWS_LAMBDA_FUNCTION_NAME": "blank-csharp-function-WU56XMPLV2XA", ... EVENT: { "Records": [ { "MessageId": "19dd0b57-b21e-4ac1-bd88-01bbb068cb78", "ReceiptHandle": "MessageReceiptHandle", "Body": "Hello from SQS!", "Md5OfBody": "7b270e59b47ff90a553787216d55d91d", "Md5OfMessageAttributes": null, "EventSourceArn": "arn:aws:sqs:us-west-2:123456789012:MyQueue", "EventSource": "aws:sqs", "AwsRegion": "us-west-2", "Attributes": { "ApproximateReceiveCount": "1", "SentTimestamp": "1523232000000", "SenderId": "123456789012", "ApproximateFirstReceiveTimestamp": "1523232000001" }, ... END RequestId: d1cf0ccb-xmpl-46e6-950d-04c96c9b1c5d REPORT RequestId: d1cf0ccb-xmpl-46e6-950d-04c96c9b1c5d Duration: 4157.16 ms Billed Duration: 4200 ms Memory Size: 256 MB Max Memory Used: 99 MB Init Duration: 841.60 ms XRAY TraceId: 1-5e7e8131-7ff0xmpl32bfb31045d0a3bb SegmentId: 0152xmpl6016310f Sampled: true

The .NET runtime logs the START, END, and REPORT lines for each invocation. 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.

Viewing logs in the AWS Management Console

The Lambda console shows log output when you test a function on the function configuration page. To view logs for all invocations, use the CloudWatch Logs console.

To view your Lambda function's logs

  1. Open the Logs page of the CloudWatch console.

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

  3. Choose the first stream in the list.

Each log stream corresponds to an instance of your function. New streams appear when you update your function and when additional instances are created to handle multiple concurrent invocations. To find logs for specific invocations, you can instrument your function with X-Ray, and record details about the request and log stream in the trace. For a sample application that correlates logs and traces with X-Ray, see Error processor sample application for AWS Lambda.

Using the AWS CLI

To get logs for an invocation from the command line, use the --log-type option. The response includes a LogResult field that contains up to 4 KB of base64-encoded logs from the invocation.

$ aws lambda invoke --function-name my-function out --log-type Tail { "StatusCode": 200, "LogResult": "U1RBUlQgUmVxdWVzdElkOiA4N2QwNDRiOC1mMTU0LTExZTgtOGNkYS0yOTc0YzVlNGZiMjEgVmVyc2lvb...", "ExecutedVersion": "$LATEST" }

You can use the base64 utility to decode the logs.

$ aws lambda invoke --function-name my-function out --log-type Tail \ --query 'LogResult' --output text | base64 -d 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: 100 ms Memory Size: 128 MB Max Memory Used: 73 MB

The base64 utility is available on Linux, macOS, and Ubuntu on Windows. For macOS, the command is base64 -D.

To get full log events from the command line, you can include the log stream name in the output of your function, as shown in the preceding example. The following example script invokes a function named my-function and downloads the last five log events.

Example get-logs.sh Script

This example requires that my-function returns a log stream ID.

#!/bin/bash aws lambda invoke --function-name my-function --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

The script uses sed to remove quotes from the output file, and sleeps for 15 seconds to allow time for the logs to be available. The output includes the response from Lambda and the output from the get-log-events command.

$ ./get-logs.sh { "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: 100 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.