Overview

Custom resources enable you to create resources with custom provisioning logic. This enables users to specify execution logic for resource creation, update, and deletion therefore allowing users to execute any kind of logic during stack deploy/delete. Moreover, it gives users the ability to provision any type of resource that is not included among Stacktape or Cloudformation resources. That way, users can manage all related resources in a single stack.

When to use

Provisioning non-AWS resources - While AWS offers huge amount of services and resources, in some cases, AWS might not offer a solution(resource) for your use-case, and often time a third-party solution better fits your needs. For example: using Auth0 over Cognito or using Algolia over AWS Elasticsearch. These non-AWS resources can be provisioned using custom resources.

Usage

In Stacktape, custom resources are made of two parts:

1. custom-resource-definition - definition is special type of lambda function. It contains the custom logic - the code, which gets executed each time custom-resource-instance is created, updated, or deleted. To put into perspective of standard programming languages: custom-resource-definition is like a class and custom-resource-instance is instance of this class.

2. custom-resource-instance - when you specify instance, you must specify custom-resource-definition definitionName (which you want to instantiate) and resourceProperties(which can be thought of as arguments), which will be passed to the executing code.

All parts of a custom resource are shown in the following snippet. Individual parts are explained in the subsequent sections.

The following example shows a custom resource that is used to seed a mongo cluster.

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resources:
  myMongoCluster:
    type: mongo-db-atlas-cluster
    properties:
      clusterTier: M10

  mongoSeeder:
    type: custom-resource-definition
    properties:
      packaging:
        type: stacktape-lambda-buildpack
        properties:
          entryfilePath: seed-the-mongo-cluster.ts
      connectTo:
        - myMongoCluster

  seedMongoCluster:
    type: custom-resource-instance
    properties:
      definitionName: mongoSeeder
      resourceProperties:
        mongoConnectionString: $Param('myMongoCluster', 'AtlasMongoCluster::SrvConnectionString')

Custom resource definition

Custom resource definition is similar to function definition. This is because on the background custom-resource-definition leverages lambda functions.

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resources:
  myMongoCluster:
    type: mongo-db-atlas-cluster
    properties:
      clusterTier: M10

  mongoSeeder:
    type: custom-resource-definition
    properties:
      packaging:
        type: stacktape-lambda-buildpack
        properties:
          entryfilePath: seed-the-mongo-cluster.ts
      connectTo:
        - myMongoCluster

  seedMongoCluster:
    type: custom-resource-instance
    properties:
      definitionName: mongoSeeder
      resourceProperties:
        mongoConnectionString: $Param('myMongoCluster', 'AtlasMongoCluster::SrvConnectionString')

Code of custom resource

Package configuration allows you to specify a path to your code and other package properties. The code contains a logic of your custom resource.

Code is during deployment packaged and executed as lambda functions. Refer to lambda functions packaging docs.

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resources:
  myMongoCluster:
    type: mongo-db-atlas-cluster
    properties:
      clusterTier: M10

  mongoSeeder:
    type: custom-resource-definition
    properties:
      packaging:
        type: stacktape-lambda-buildpack
        properties:
          entryfilePath: seed-the-mongo-cluster.ts
      connectTo:
        - myMongoCluster

  seedMongoCluster:
    type: custom-resource-instance
    properties:
      definitionName: mongoSeeder
      resourceProperties:
        mongoConnectionString: $Param('myMongoCluster', 'AtlasMongoCluster::SrvConnectionString')

Code example

The following example shows the code of our seeding custom resource in file seed-the-mongo-cluster.ts

See AWS documentation to learn more about:

• the type of requests (events) custom resource receives during stack operations
• response objects which you should return during respective operations

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import mongoose from 'mongoose';
import { SUCCESS, FAILED, send } from 'cfn-response-promise';

// incoming event is in following form
// {
//   "RequestType" : "Create" || "Update" || "Delete",
//   "RequestId" : "9db53695-b0a0-47d6-908a-ea2d8a3ab5d7",
//   "ResponseURL" : "https://...",
//   "ResourceType" : "AWS::Cloudformation::CustomResource",
//   "LogicalResourceId" : "...",
//   "StackId" : "arn:aws:cloudformation:...",
//   "ResourceProperties" : {
//      ... properties of custom-resource-instance
//   }
// }

export default async (event, context) => {
  // custom resource definition code
  let success = true;
  let dataToReturn = {};

  try {
    // we are only seeding database if the operation is Create
    if (event.RequestType === 'Create') {
      // we are using the "mongoConnectionString" property passed by custom-resource-instance to create connection
      const connection = await mongoose.connect(event.ResourceProperties.mongoConnectionString, {
        authMechanism: 'MONGODB-AWS',
        authSource: '$external',
        useNewUrlParser: true,
        useUnifiedTopology: true,
        dbName: 'my-test-database'
      });
      // code with seeding the database ...
      // ...
    }
  } catch (err) {
    success = false;
  }
  await send(event, context, success ? SUCCESS : FAILED, dataToReturn, 'customresourceid');
};

// function must respond to "ResponseURL" with response in following form
// we are using "cfn-response-promise" library which formats response for us
// {
//   "Status" : "SUCCESS" || "FAILED",
//   "RequestId" : "9db53695-b0a0-47d6-908a-ea2d8a3ab5d7",
//   "LogicalResourceId" : "...",
//   "StackId" : "arn:aws:cloudformation:...",
//   "PhysicalResourceId" : "...",
//   "Data" : {
//     ... attributes which can be queried in template using $Param
//   }
// }

Accessing other resources

• For most of the AWS resources, resource-to-resource communication is not allowed by default. This helps to enforce security and resource isolation. Access must be explicitly granted using IAM (Identity and Access Management) permissions.

• Access control of Relational Databases is not managed by IAM. These resources are not "cloud-native" and have their own access control mechanism (connection string with username and password). They are accessible by default, and you don't need to grant any extra IAM permissions. If the default, connection-string-based access-control is not sufficient for your use case, you can restrict connection to only resources in the same VPC. In that case, your function must join that VPC to access them.

• Stacktape automatically handles IAM permissions for the underlying AWS services that it creates (i.e. granting function permission to write logs to Cloudwatch, or allowing to communicate with their event source and many others).

• If your compute resource needs to communicate with other infrastructure components, you need to add permissions manually. You can do this in 2 ways listed below.

Using connectTo

• List of resource names or AWS services that this function will be able to access (basic IAM permissions will be granted automatically). Granted permissions differ based on the resource.
• Works only for resources managed by Stacktape in resources section (not arbitrary Cloudformation resources)
• This is useful if you don't want to deal with IAM permissions yourself. Handling permissions using raw IAM role statements can be cumbersome, time-consuming and error-prone. Moreover, when using connectTo property, Stacktape automatically injects information about resource you are connecting to as environment variables into your custom resource runtime.

In this example, we are granting access to the mongo cluster. This will inject credentials for access which are used in code.

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resources:
  myMongoCluster:
    type: mongo-db-atlas-cluster
    properties:
      clusterTier: M10

  mongoSeeder:
    type: custom-resource-definition
    properties:
      packaging:
        type: stacktape-lambda-buildpack
        properties:
          entryfilePath: seed-the-mongo-cluster.ts
      connectTo:
        - myMongoCluster

  seedMongoCluster:
    type: custom-resource-instance
    properties:
      definitionName: mongoSeeder
      resourceProperties:
        mongoConnectionString: $Param('myMongoCluster', 'AtlasMongoCluster::SrvConnectionString')

Environment

Use environment variables for injecting information that resource needs during code execution.

Custom resource instance

Custom-resource-instance, creates instance of specified custom-resource-definition. Depending on the action (create/update/delete) stack is undergoing, the instance receives event (see code example) and executes the code.

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resources:
  myMongoCluster:
    type: mongo-db-atlas-cluster
    properties:
      clusterTier: M10

  mongoSeeder:
    type: custom-resource-definition
    properties:
      packaging:
        type: stacktape-lambda-buildpack
        properties:
          entryfilePath: seed-the-mongo-cluster.ts
      connectTo:
        - myMongoCluster

  seedMongoCluster:
    type: custom-resource-instance
    properties:
      definitionName: mongoSeeder
      resourceProperties:
        mongoConnectionString: $Param('myMongoCluster', 'AtlasMongoCluster::SrvConnectionString')

API reference