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Gernot Glawe for AWS Community Builders

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AWS Lambda recursive loop detection - but not (yet) for S3

Recursive loops in AWS Lambda are the root of all evil because you create a large bill quickly.

You might think the danger is over because of new AWS recursive loop detection. But what about S3 buckets and Lambda? It's time for a closer look:

A word of caution: Be very careful when you test this. You could create large AWS bills.

TL;DR: Never ever use the same bucket for invoking a Lambda and write the output.

Some testing infrastructure:

1) A S3 bucket (CDK, GO)

    bucky := awss3.NewBucket(stack, aws.String("incoming-ring"), &awss3.BucketProps{
        BlockPublicAccess: awss3.BlockPublicAccess_BLOCK_ALL(),
        RemovalPolicy:     awscdk.RemovalPolicy_DESTROY,
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2) A Lambda Function Resource

    ringFunction := awslambda.NewFunction(stack, aws.String("ring"),
            Description:            aws.String("ring - test recursive loop stop"),
            FunctionName:           aws.String("ring"),
            LogRetention:           awslogs.RetentionDays_THREE_MONTHS,
            MemorySize:             aws.Float64(1024),
            Timeout:                awscdk.Duration_Seconds(aws.Float64(10)),
            Code:                   awslambda.Code_FromAsset(&lambdaPath, &awss3assets.AssetOptions{}),
            Handler:                aws.String("main"),
            Runtime:                awslambda.Runtime_GO_1_X(),
            DeadLetterQueueEnabled: aws.Bool(true),
            DeadLetterQueue:        dlq,
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3) A Lambda Function, which gets the PutObject Event and write to the same bucket:

    someString := "hello world\nand hello go and more"
    myReader := strings.NewReader(someString)

    resp, err := client.PutObject(context.TODO(), &s3.PutObjectInput{
        Bucket: aws.String(bucket),
        Key:    aws.String(s3input),
        Body:   myReader,
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4) An event, which triggers the Lambda function each time an object is Put to the bucket:

    myHandler.AddEventSource(event.NewS3EventSource(bucky, &event.S3EventSourceProps{
        Events:  &[]awss3.EventType{awss3.EventType_OBJECT_CREATED,},
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Now we have:

Ring architecture

And because I am very careful, I decide to create a recursive loop stopper inside the Lambda function:

    if GlobalCounter > 20 {
        log.Fatal("Counter exceeded\n")
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And in the Console, we set the concurrency to 1:


When we test the recursive loop, we always have a browser window with the concurrency open. Because: Setting the concurrency to 0 is the fastest way to stop an endless loop.

The test

Lets test whether S3-Lambda-S3 recursive loops are still possible:

We use saw to tail lambda log:

saw watch /aws/lambda/ring
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And then start the recursive loop:

aws s3 cp s3://ring-incomingring3eb9a9b9-1a8q8cp8qm2ch/
upload: ./ to s3://ring-incomingring3eb9a9b9-1a8q8cp8qm2ch/
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Very fast you get the logs:

[2023-07-17T17:34:39+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) INIT_START Runtime Version: go:1.v18 Runtime Version ARN: arn:aws:lambda:eu-central-1::runtime:ccb68acb59818f9df9b10924cc6c83ca6eaf4067f70ba861c0e211b59e8af729
[2023-07-17T17:34:39+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) START RequestId: 10fba020-0e93-4430-a9d7-728d34b146c9 Version: $LATEST
[2023-07-17T17:34:39+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) Counter: 1
[2023-07-17T17:34:39+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) Etag:  446cce24a7c945503232494e881150ec
[2023-07-17T17:34:39+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) Seq:  0064B55F8E25E34383
[2023-07-17T17:34:39+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) END RequestId: 10fba020-0e93-4430-a9d7-728d34b146c9
[2023-07-17T17:34:39+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) REPORT RequestId: 10fba020-0e93-4430-a9d7-728d34b146c9   Duration: 92.67 ms  Billed Duration: 93 ms  Memory Size: 1024 MB    Max Memory Used: 39 MB  Init Duration: 110.22 ms
[2023-07-17T17:34:40+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) START RequestId: 97cf261e-b272-437f-9637-b7573cfb0fa8 Version: $LATEST
[2023-07-17T17:34:40+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) Counter: 2
[2023-07-17T17:34:40+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) Etag:  a9c4c5f405a3e4cc6ba3b6b355da0e71
[2023-07-17T17:34:40+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) Seq:  0064B55F8FD24FBE14
[2023-07-17T17:34:41+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) END RequestId: 97cf261e-b272-437f-9637-b7573cfb0fa8
[2023-07-17T17:34:41+02:00] (2023/07/17/[$LATEST]e27ec4706f1a4fb0a6d3ca1fdb23bbc0) REPORT RequestId: 97cf261e-b272-437f-9637-b7573cfb0fa8   Duration: 27.20 ms  Billed Duration: 28 ms  Memory Size: 1024 MB    Max Memory Used: 40 MB
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The interesting part is:

Counter: 1
Counter: 2
Counter: 19
Counter: 20
Counter exceeded
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And because AWS BLOG about recursive loops says:
"Lambda now detects the function running in a recursive loop between supported services after exceeding 16 invocations. It returns a RecursiveInvocationException to the caller."

We are now sure that the loop detection for S3 is not working because we counted to 20!

And although I implemented a loop stopper very fast, I started a recursive loop:


So now I set the concurrency to 0 again and delete the Lambda.


It's proven:

Recursive Loop detection does not stop S3 recursive loops, so do use different buckets for event trigger and Lambda output!

So be careful out there.
And checkout my website go-on-aws or - even better :) - use my new udemy course GO on AWS - Coding, Serverless and Infrastructure as Code

Enjoy building!

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