How does the CreateCommandsInvoker work?

[multimeric]

I gather that the CreateCommandsFunction is called once per command deploy, but I can't work out how that happens. The CreateCommandsInvoker depends on both the CreateCommandsFunction and CommandsLayer, which makes sense, but what actually calls the the CreateCommandsFunction?

[multimeric]

The reason I ask is because I want to add some on-setup actions (populating a database), but I'm not sure how to ensure that it runs for each deployment.

[ytausch]

Hi,

CreateCommandsFunction is the lambda backend of an AWS Lambda-backed custom resource. In fact, CreateCommandsInvoker is exactly the name of the custom resource which declares CreateCommandsFunction as its backend. You can see that by looking at the Type attribute of CreateCommandsInvoker in template.yaml - it has the format Custom::XXX.

That means: If CreateCommandsInvoker is created during stack creation, CloudFormation itself makes a request to CreateCommandsFunction which is then responsible for setting up the commands and responding back to CloudFormation that everything went fine. A lot of this request and response logic happens in our dependency cfn-lambda.

The same thing happens when any commands are updated, it is just a little bit more complicated: Updating the commands means updating the commands layer, which then means that the version number of the commands layer is incremented. Since the ARN of the commands layer is passed as property to CreateCommandsInvoker and the ARN of a lambda layer ends with the layer version number, the properties of CreateCommandsInvoker change, which triggers a resource update. CreateCommandsFunction is then again called to handle the update.

If you want to implement a custom setup logic, you have essentially two options:

1. Extend CreateCommandsFunction (note that you can differentiate between creation and update events)
2. Create a new custom lambda-backed stack resource similar to CreateCommandsInvoker that has its own lambda handler similar to CreateCommandsFunction (I recommend also using cfn-lambda there for doing the heavy-lifting).