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Constructs

Intro

A CDK stack looks like this:

class BaseResources(core.Stack):

    def __init__(self, scope: core.Construct, construct_id: str, **kwargs) -> None:
        super().__init__(scope, construct_id, **kwargs)

        vpc = ec2.Vpc(self, 'vpc', max_azs=2, nat_gateways=1)

The class constructors of both BaseResources and ec2.Vpc (and many other classes in the CDK) have the signature (scope, id, **kwargs). This is because all of these classes are constructs. Constructs are the basic building block of CDK apps. They represent abstract components which can be composed together into higher level abstractions via scopes. Scopes can include constructs, which in turn can include other constructs, etc.

Constructs are always created in the scope of another construct and must always have an identifier which must be unique within the scope it's created. Therefore, construct initializers (constructors) will always have the following signature:

  • scope: the first argument is always the scope in which this construct is created. In almost all cases, you'll be defining constructs within the scope of current construct, which means you'll usually just want to pass self for the first argument.

  • id: the second argument is the local identity of the construct. It's an ID that has to be unique amongst construct within the same scope. The CDK uses this identity to calculate the CloudFormation Logical ID for each resource defined within this scope.

  • kwargs: the last (sometimes optional) arguments is always a set of initialization arguments. Those are specific to each construct. For example, the lambda. Function construct accepts arguments like runtime, code and handler.