Shared Resources

Shared resources are how you create standalone custom AWS resources with Jets. With the Associated Function Resources, you can add custom AWS resources which are associated with Lambda functions. Shared resources are also fully customizable AWS resources, but they are not tightly associated with a Lambda function. Understanding Shared Resources will allow you to customize a Jets application with any custom resource.

SNS Topic Example

Let’s create an SNS Topic as a shared resource. The SNS topic will be used throughout the application to publish messages. Shared resources are defined in the shared/resources folder. You can create the SNS topic like so:

shared/resources/alert.rb

class Alert < Jets::Stack
  sns_topic(:delivery_completed, display_name: "cool topic")
end

This creates an SNS Topic resource. You can then reference the SNS Topic with the Alert.lookup method in your code. For example, here’s a Event that looks up the ARN of the delivery_completed SNS topic and then publishes to it.

app/events/postman_event.rb

class PostmanEvent < ApplicationEvent
  include Jets::AwsServices

  iam_policy("sns")
  def deliver
    topic_arn = Alert.lookup(:delivery_completed) # looks up output from the Alert cfn stack
    sns.publish(
      topic_arn: topic_arn,
      subject: "my subject",
      message: "my message"
    )
  end
end

The lookup method is available to the Alert class as a part of inheriting from the Jets::Stack class. Also note, the code above uses include Jets::AwsServices to provide access to the sns client. Refer to the source for a full list of the clients that are included with the module: jets/aws_services.rb. For services not included, add the gem to your project’s Gemfile and set up the client in the code.

General Resource Form

In the SNS Topic example above we use the sns_topic convenience method to create the resource. Under the hood, the sns_topic method simply performs some wrapper logic and then calls the generalized resource and output method. The code above could have been written like so:

class Resource < Jets::Stack
  resource(
    DeliveryCompleted: {
      Type: "AWS::SNS::Topic",
      Properties: {
        DisplayName: "cool topic"
      }
    }
  )
  output("DeliveryCompleted") # creates CloudFormation output
end

The Jets::Stack resource method is similar to Custom Associated Resources’s resource method. It follows a similar expansion pattern. With it, you can create any AWS resource in your Jets application. You can also create your own convenience wrapper methods and call resource and output as required: Shared Resource Extensions.

IAM Permission

The Jets::Stack lookup method uses a CloudFormation Output that is created as part of the convenience methods. The lookup method requires read permission to the CloudFormation stack. This permission is automatically added to your application default IAM permissions when you are using Shared Resources, given you have not have overridden the application-wide IAM policy.

Understanding the general shared resource method is the key to adding any shared custom resource you require to a Jets application, so hopefully the explanations above help.

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