Creating a function that returns logs Tools and libraries Using Powertools for AWS Lambda (.NET) and AWS SAM for structured logging Viewing logs in the Lambda console Viewing logs in the CloudWatch console Viewing logs using the AWS Command Line Interface (AWS CLI)Deleting logs

Log and monitor C# Lambda functions

AWS Lambda automatically monitors Lambda functions and sends log entries 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 and other output from your function's code to the log stream. For more information about CloudWatch Logs, see Using CloudWatch Logs logs with Lambda.

Sections

Creating a function that returns logs
Using logging tools and libraries
Using Powertools for AWS Lambda (.NET) and AWS SAM for structured logging
Viewing logs in the Lambda console
Viewing logs in the CloudWatch console
Viewing logs using the AWS Command Line Interface (AWS CLI)
Deleting logs

Creating a function that returns logs

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 .NET runtime logs the START, END, and REPORT lines for each invocation. The report line provides the following details.

REPORT line data fields

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. When invocations share an execution environment, Lambda reports the maximum memory used across all invocations. This behavior might result in a higher than expected reported value.
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.

Using logging tools and libraries

Powertools for AWS Lambda (.NET) is a developer toolkit to implement Serverless best practices and increase developer velocity. The Logging utility provides a Lambda optimized logger which includes additional information about function context across all your functions with output structured as JSON. Use this utility to do the following:

Capture key fields from the Lambda context, cold start and structures logging output as JSON
Log Lambda invocation events when instructed (disabled by default)
Print all the logs only for a percentage of invocations via log sampling (disabled by default)
Append additional keys to structured log at any point in time
Use a custom log formatter (Bring Your Own Formatter) to output logs in a structure compatible with your organization’s Logging RFC

Using Powertools for AWS Lambda (.NET) and AWS SAM for structured logging

Follow the steps below to download, build, and deploy a sample Hello World C# application with integrated Powertools for AWS Lambda (.NET) modules using the AWS SAM. This application implements a basic API backend and uses Powertools for emitting logs, metrics, and traces. It consists of an Amazon API Gateway endpoint and a Lambda function. When you send a GET request to the API Gateway endpoint, the Lambda function invokes, sends logs and metrics using Embedded Metric Format to CloudWatch, and sends traces to AWS X-Ray. The function returns a hello world message.

Prerequisites

To complete the steps in this section, you must have the following:

.NET 6 or .NET 8
AWS CLI version 2
AWS SAM CLI version 1.75 or later. If you have an older version of the AWS SAM CLI, see Upgrading the AWS SAM CLI.

Deploy a sample AWS SAM application

Initialize the application using the Hello World TypeScript template.


sam init --app-template hello-world-powertools-dotnet --name sam-app --package-type Zip --runtime dotnet6 --no-tracing

Build the app.
```
cd sam-app && sam build
```
Deploy the app.
```
sam deploy --guided
```
Follow the on-screen prompts. To accept the default options provided in the interactive experience, press Enter.

Note
For HelloWorldFunction may not have authorization defined, Is this okay?, make sure to enter y.

Get the URL of the deployed application:


aws cloudformation describe-stacks --stack-name sam-app --query 'Stacks[0].Outputs[?OutputKey==`HelloWorldApi`].OutputValue' --output text

Invoke the API endpoint:
```
curl -X GET <URL_FROM_PREVIOUS_STEP>
```
If successful, you'll see this response:
```
{"message":"hello world"}
```

To get the logs for the function, run sam logs. For more information, see Working with logs in the AWS Serverless Application Model Developer Guide.


sam logs --stack-name sam-app

The log output looks like this:


2023/02/20/[$LATEST]4eaf8445ba7a4a93b999cb17fbfbecd8 2023-02-20T14:15:27.988000 INIT_START Runtime Version: dotnet:6.v13        Runtime Version ARN: arn:aws:lambda:ap-southeast-2::runtime:699f346a05dae24c58c45790bc4089f252bf17dae3997e79b17d939a288aa1ec
2023/02/20/[$LATEST]4eaf8445ba7a4a93b999cb17fbfbecd8 2023-02-20T14:15:28.229000 START RequestId: bed25b38-d012-42e7-ba28-f272535fb80e Version: $LATEST
2023/02/20/[$LATEST]4eaf8445ba7a4a93b999cb17fbfbecd8 2023-02-20T14:15:29.259000 2023-02-20T14:15:29.201Z        bed25b38-d012-42e7-ba28-f272535fb80e    info   {"_aws":{"Timestamp":1676902528962,"CloudWatchMetrics":[{"Namespace":"sam-app-logging","Metrics":[{"Name":"ColdStart","Unit":"Count"}],"Dimensions":[["FunctionName"],["Service"]]}]},"FunctionName":"sam-app-HelloWorldFunction-haKIoVeose2p","Service":"PowertoolsHelloWorld","ColdStart":1}
2023/02/20/[$LATEST]4eaf8445ba7a4a93b999cb17fbfbecd8 2023-02-20T14:15:30.479000 2023-02-20T14:15:30.479Z        bed25b38-d012-42e7-ba28-f272535fb80e    info   {"ColdStart":true,"XrayTraceId":"1-63f3807f-5dbcb9910c96f50742707542","CorrelationId":"d3d4de7f-4ccc-411a-a549-4d67b2fdc015","FunctionName":"sam-app-HelloWorldFunction-haKIoVeose2p","FunctionVersion":"$LATEST","FunctionMemorySize":256,"FunctionArn":"arn:aws:lambda:ap-southeast-2:123456789012:function:sam-app-HelloWorldFunction-haKIoVeose2p","FunctionRequestId":"bed25b38-d012-42e7-ba28-f272535fb80e","Timestamp":"2023-02-20T14:15:30.4602970Z","Level":"Information","Service":"PowertoolsHelloWorld","Name":"AWS.Lambda.Powertools.Logging.Logger","Message":"Hello world API - HTTP 200"}
2023/02/20/[$LATEST]4eaf8445ba7a4a93b999cb17fbfbecd8 2023-02-20T14:15:30.599000 2023-02-20T14:15:30.599Z        bed25b38-d012-42e7-ba28-f272535fb80e    info   {"_aws":{"Timestamp":1676902528922,"CloudWatchMetrics":[{"Namespace":"sam-app-logging","Metrics":[{"Name":"ApiRequestCount","Unit":"Count"}],"Dimensions":[["Service"]]}]},"Service":"PowertoolsHelloWorld","ApiRequestCount":1}
2023/02/20/[$LATEST]4eaf8445ba7a4a93b999cb17fbfbecd8 2023-02-20T14:15:30.680000 END RequestId: bed25b38-d012-42e7-ba28-f272535fb80e
2023/02/20/[$LATEST]4eaf8445ba7a4a93b999cb17fbfbecd8 2023-02-20T14:15:30.680000 REPORT RequestId: bed25b38-d012-42e7-ba28-f272535fb80e  Duration: 2450.99 ms   Billed Duration: 2451 ms Memory Size: 256 MB     Max Memory Used: 74 MB  Init Duration: 240.05 ms
XRAY TraceId: 1-63f3807f-5dbcb9910c96f50742707542       SegmentId: 16b362cd5f52cba0

This is a public API endpoint that is accessible over the internet. We recommend that you delete the endpoint after testing.
```
sam delete
```

Managing log retention

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 CloudWatch automatically deletes the logs. To set up log retention, add the following to your AWS SAM template:


Resources:
  HelloWorldFunction:
    Type: AWS::Serverless::Function
    Properties:
      # Omitting other properties

  LogGroup:
    Type: AWS::Logs::LogGroup
    Properties:
      LogGroupName: !Sub "/aws/lambda/${HelloWorldFunction}"
      RetentionInDays: 7

Viewing logs in the Lambda console

You can use the Lambda console to view log output after you invoke a Lambda function.

If your code can be tested from the embedded Code editor, you will find logs in the execution results. When you use the console test feature to invoke a function, you'll find Log output in the Details section.

Viewing logs in 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

Open the Log groups page on the CloudWatch console.
Choose the log group for your function (/aws/lambda/your-function-name).
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.

Viewing logs 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 --cli-binary-format raw-in-base64-out | base64 --decode

The cli-binary-format option is required if you're using AWS CLI version 2. To make this the default setting, run aws configure set cli-binary-format raw-in-base64-out. For more information, see AWS CLI supported global command line options in the AWS Command Line Interface User Guide for Version 2.

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.


#!/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 stream1 --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.

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Document Conventions

Context

Tracing

Log and monitor C# Lambda functions

Sections

Creating a function that returns logs

REPORT line data fields

Using logging tools and libraries

Using Powertools for AWS Lambda (.NET) and AWS SAM for structured logging

Prerequisites

Deploy a sample AWS SAM application

Note

Managing log retention

Viewing logs in the Lambda console

Viewing logs in the CloudWatch console

To view logs on the CloudWatch console

Viewing logs using the AWS Command Line Interface (AWS CLI)

Example retrieve a log ID

Example decode the logs

Example get-logs.sh script

Example macOS and Linux (only)

Example retrieve the last five log events

Deleting logs