Triggers

Triggers allows you to execute code during deployments. This can be used for a variety of use cases such as:

  • Self tests: validate something after a resource/construct been provisioned

  • Data priming: add initial data to resources after they are created

  • Preconditions: check things such as account limits or external dependencies before deployment.

Usage

The TriggerFunction construct will define an AWS Lambda function which is triggered during deployment:

# Example automatically generated from non-compiling source. May contain errors.
import aws_cdk.aws_lambda as lambda_
import aws_cdk.triggers as triggers
from aws_cdk import Stack

# stack: Stack


triggers.TriggerFunction(stack, "MyTrigger",
    runtime=lambda_.Runtime.NODEJS_14_X,
    handler="index.handler",
    code=lambda_.Code.from_asset(__dirname + "/my-trigger")
)

In the above example, the AWS Lambda function defined in myLambdaFunction will be invoked when the stack is deployed.

Trigger Failures

If the trigger handler fails (e.g. an exception is raised), the CloudFormation deployment will fail, as if a resource failed to provision. This makes it easy to implement “self tests” via triggers by simply making a set of assertions on some provisioned infrastructure.

Order of Execution

By default, a trigger will be executed by CloudFormation after the associated handler is provisioned. This means that if the handler takes an implicit dependency on other resources (e.g. via environment variables), those resources will be provisioned before the trigger is executed.

In most cases, implicit ordering should be sufficient, but you can also use executeAfter and executeBefore to control the order of execution.

The following example defines the following order: (hello, world) => myTrigger => goodbye. The resources under hello and world will be provisioned in parallel, and then the trigger myTrigger will be executed. Only then the resources under goodbye will be provisioned:

# Example automatically generated from non-compiling source. May contain errors.
from constructs import Construct, Node
import aws_cdk.triggers as triggers

# my_trigger: triggers.Trigger
# hello: Construct
# world: Construct
# goodbye: Construct


my_trigger.execute_after(hello, world)
my_trigger.execute_before(goodbye)

Note that hello and world are construct scopes. This means that they can be specific resources (such as an s3.Bucket object) or groups of resources composed together into constructs.

Re-execution of Triggers

By default, executeOnHandlerChange is enabled. This implies that the trigger is re-executed every time the handler function code or configuration changes. If this option is disabled, the trigger will be executed only once upon first deployment.

In the future we will consider adding support for additional re-execution modes:

  • executeOnEveryDeployment: boolean - re-executes every time the stack is deployed (add random “salt” during synthesis).

  • executeOnResourceChange: Construct[] - re-executes when one of the resources under the specified scopes has changed (add the hash the CloudFormation resource specs).