Lambda can't find the file when code version inserted into Handler property in CloudFormation template

Question

The artefact that I create and deploy has a version number in it. It is of the form:

universe-0.0.1-SNAPSHOT.zip

where the 0.0.1-SNAPSHOT is the version.

Now the CloudFormation template has the Handler mapped through a Fn::Join function:

  UFunctionCelestial:
    Type: AWS::Lambda::Function
    Properties:
      Code:
        S3Bucket: !Ref ArtefactRepositoryBucket
        S3Key: !Join [ '', [!Sub '${AWS::StackName}-', !Ref CodeVersion, '.zip' ] ]
      Handler: !Join [ '', [!Sub '${AWS::StackName}-', !Ref CodeVersion, '/src/lambdas/celestial_persist_function.handler'] ]
      Role: !GetAtt [ UIAMRoleFunctionServiceRoleCelestial, Arn ]
      Runtime: python3.7
      Environment:
        Variables:
          CELESTIAL_TABLE_NAME: !Ref UTableCelestial
          PRIMARY_KEY: id
    DependsOn:
      - UIAMRoleFunctionServiceRolePolicyCelestial
      - UIAMRoleFunctionServiceRoleCelestial

...SNIP...

Parameters:
  ArtefactRepositoryBucket:
    Type: String
    Description: S3 bucket for asset "foundry-cdk/CelestialHandler/Code"
  CodeVersion:
    Type: String
    Description: S3 key for asset version "foundry-cdk/CelestialHandler/Code"

I guess most of this is irrelevant, except the Handler property

Now the !Ref CodeVersion code version for that line seems to evaluate as 0/0/1-SNAPSHOT for some reason, even though I call this template with the command:

aws2 cloudformation deploy \
    --template-file ${CF_TEMPLATE_FILE} \
    --region ${ACCOUNT_REGION} \
    --stack-name ${PROJECT_NAME} \
    --force-upload \
    --capabilities CAPABILITY_IAM \
    --parameter-overrides \
        ArtefactRepositoryBucket=${S3_AWS_RELEASES_BUCKET} \
        CodeVersion=${APPLICATION_VERSION}

I echoed out the ${APPLICATION_VERSION} and, yep, it evaluates to 0.0.1-SNAPSHOT

and yet when I go to the console and look at my Lambda I get the message:

Lambda can't find the file universe-0/0/1-SNAPSHOT/src/lambdas/celestial_persist_function.py. Make sure that your handler upholds the format: file-name.method.

So my question is, why is CloudFormation turning my dots into slashes and giving the Lambda a bum reference?

actually, that's what I started with. I noticed that I had ${APPLICATION_VERSION} double quoted and ${S3_AWS_RELEASES_BUCKET} not double quoted, when I saw this error, I removed the double quotes as one of my early tries at fixing this. The quoting (or not quoting) ended up making no difference. — Michael Coxon

Arun Kamalanathan Arun Kamalanathan · Accepted Answer · 2020-02-17T19:47:14

It's not cloudformation what doing it. The behavior you are seeing is normal behavior.

Generally, When you define the handler as "folder1.folder2.file.handler", it will look for a folder1, then look for folder2 inside that, then look for the file file.py inside. Finally, the file.py is expected to have a function named handler.

I am assuming that your zip file has a top folder called src. In that case, your handler should be defined as src.lambdas.celestial_persist_function.handler since you have already mentioned where your zip file is using the S3Key.

hope this helps.

Lambda can't find the file when code version inserted into Handler property in CloudFormation template

1 Answers