Specify log group for an AWS lambda?

问题:

Is there a way to specify the CloudWatch log group that an AWS lambda logs to? It seems to be generated directly from the lambda name; however, it would be especially convenient to, for example, aggregate multiple lambdas to a single log group. We are especially interested in specifying the log group when the lambda is created by a CloudFormation template.

回答1:

I don't think that is possible.

Even if it were possible, each AWS Lambda instance would still write to its own log-stream. And though different invocations of the same lambda can write to the same log-stream (when the lambda instance is reused), this will definitely not be the case for different lambdas (since they must use different lambda instances).

As a result, you must have a tool that aggregates multiple log-stream. If so, what's the problem with making it a little more generic, so that it can aggregate log-streams from different log groups?

回答2:

Actually, maybe you can, to an extent at least. I too was in search of the answer and gave this a try. Here's a snippet of two resources; the lambda function and the log group:

"MyLambdaFunction": {
    "Type": "AWS::Lambda::Function",
    "DependsOn": "ReadWriteRole",
    "Properties": {
        //snip
    }
},

"MyLambdaFunctionLogGroup": {
    "Type": "AWS::Logs::LogGroup",
    "DependsOn": "MyLambdaFunction",
    "Properties": {
        "LogGroupName": {"Fn::Join": ["", ["/aws/lambda/", {"Ref": "MyLambdaFunction"}]]},
        "RetentionInDays": 14
    }
},

I found that the log group was created with a retention of 14 days as indicated. When the lambda function runs, it does create log streams in this group. When I deleted the stack, however, it seems that the log groups is not deleted, and the retention is now set to never expire. Perhaps that's good enough so the streams don't get too out of hand...

回答3:

Creating the log group as mentioned as one of the answers works. To keep the retention policy after the stack is deleted, just add a DeletionPolicy.

"MyLambdaFunctionLogGroup": {
  "Type": "AWS::Logs::LogGroup",
  "DependsOn": "MyLambdaFunction",
  "DeletionPolicy": "Retain",
  "Properties": {
    "LogGroupName": {"Fn::Join": ["", ["/aws/lambda/", {"Ref": "MyLambdaFunction"}]]},
    "RetentionInDays": 14
  }
}

回答4:

Interesting. . . when I try this, the log group is created but no streams are being written to it. My Lambda continues to write to the default log stream. . . the Lambda's permissions should allow access to the new group.