dotnet-logging.md

AWS Lambda Function Logging in C#

Your Lambda function can contain logging statements and, in turn, AWS Lambda writes these logs to CloudWatch Logs.

In the C# programming model, there are three ways to log data in your function:

• Use the static Write or WriteLine methods provided by the C# Console class. Anything written to standard out or standard error - using Console.Write or a similar method - will be logged in CloudWatch Logs.

  public class ProductService
  {
     public async Task<Product> DescribeProduct(DescribeProductRequest request)
      {
         Console.WriteLine("DescribeProduct invoked with Id " + request.Id);
         return await catalogService.DescribeProduct(request.Id);
      }
  }
  

• Use the Log method on the Amazon.Lambda.Core.LambdaLogger class. This is a static class that can be used anywhere in your application. To use this, you must include the Amazon.Lambda.Core library.

  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);
     }
  }
  

  Each call to LambdaLogger.Log results in a CloudWatch Logs event, provided the event size is within the allowed limits. For information about CloudWatch Logs limits, see CloudWatch Logs Limits in the Amazon CloudWatch User Guide.

• Use the logger in ILambdaContext. The ILambdaContext object (if specified) in your method contains a Logger property that represents a LambdaLogger. The following is an example of using this method:

  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);
     }
  }
  

How to Find Logs

You can find the logs that your Lambda function writes, as follows:

• Find logs in CloudWatch Logs. The ILambdaContext object provides the LogStreamName and the LogGroupName properties. Using these properties, you can find the specific log stream where logs are written.
• If you invoke a Lambda function via the console, the invocation type is always RequestResponse (that is, synchronous execution) and the console displays the logs that the Lambda function writes using the LambdaLogger object. AWS Lambda also returns logs from Console.Write and Console.WriteLine methods.
• If you invoke a Lambda function programmatically, you can add the LogType parameter to retrieve the last 4 KB of log data that is written to CloudWatch Logs. For more information, see Invoke. AWS Lambda returns this log information in the x-amz-log-results header in the response. If you use the AWS Command Line Interface to invoke the function, you can specify the --log-type parameter with value Tail.