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 logs the request ID for an invocation.
public class ProductService
{
public async Task<Product> DescribeProduct(DescribeProductRequest request)
{
Console.WriteLine("DescribeProduct invoked with Id " + request.Id);
return await catalogService.DescribeProduct(request.Id);
}
}
Lambda also provides a logger class in the Amazon.Lambda.Core library. Use the Log method on the Amazon.Lambda.Core.LambdaLogger class to write logs.
using Amazon.Lambda.Core;
public class ProductService
{
public async Task<Product> DescribeProduct(DescribeProductRequest request)
{
LambdaLogger.Log("DescribeProduct invoked with Id " + request.Id);
return await catalogService.DescribeProduct(request.Id);
}
}
An instance of this class is also available on the context object.
public class ProductService
{
public async Task<Product> DescribeProduct(DescribeProductRequest request, ILambdaContext context)
{
context.Logger.Log("DescribeProduct invoked with Id " + request.Id);
return await catalogService.DescribeProduct(request.Id);
}
}
Example Log Format
START RequestId: 29d229ff-xmpl-49d8-8d86-b1f89b06bdc9 Version: $LATEST
END RequestId: 29d229ff-xmpl-49d8-8d86-b1f89b06bdc9
REPORT RequestId: 29d229ff-xmpl-49d8-8d86-b1f89b06bdc9 Duration: 576.36 ms Billed Duration: 600 ms Memory Size: 512 MB Max Memory Used: 69 MB Init Duration: 205.64 ms
XRAY TraceId: 1-5e34a44f-a125xmpl3603a4cdf526103e SegmentId: 5f41xmpl4b1664d9 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.
Topics
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
-
Open the Logs page of the CloudWatch console.
-
Choose the log group for your function (/aws/lambda/function-name).
-
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.
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.
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"
}
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.