Custom resources are a way to implement custom provisioning logic in our IaC that will be executed as soon as there is a change in the state of a stack (creation, update, deletion). In the example above, we showed an example of how each of the three CloudFormation update behaviors work. First, AWS may not offer a solution that you need. With a DynamoDB table, it means all the existing items in your table are lost. Forgetting to do so or saving the data incorrectly will cause CloudFormation to hang until it times out. If I return a falsey value, the function will be retriggered in 2 minutes to try again. The documentation for CloudFormation custom resources is here, and gives details on the request object your code should expect, and the responses that CloudFormation accepts. AWS support for Internet Explorer ends on 07/31/2022. In writing a custom resource handler, youll need to handle three different actions: Create: A Create event is invoked whenever a resource is being provisioned for the first time, either because a new stack is being deployed or because it was added to an existing stack; Update: An Update event is invoked when the custom resource itself has a property that has changed as part of a CloudFormation deploy. Supported browsers are Chrome, Firefox, Edge, and Safari. Tomato, to-mah-to the important thing is that we can automate something that was previously manual. Save the changes to your AWS CloudFormation template, and then use the template to update your stack. The custom resource is defined with a service token. webhooks are one of the core use cases for AWS Lambda, instructions in the Github repo for deploying the custom resource. Generally, CloudFormation custom resource behavior falls into one of the following buckets: Provisioning AWS resources that are not supported by CloudFormation. By default, AWS will delete any resources in a stack that are replaced by an update. You implement the creation, update, and deletion logic to define the custom resource deployment. You may also include additional properties to send into your custom resource for configuration. 2022, Amazon Web Services, Inc. or its affiliates. If nothing happens, download GitHub Desktop and try again. What are CloudFormation custom resources and when should I use them? When this happens, there are three potential behaviors for your existing resource: Update with no interruption: Your resource is updated in place without any disruption to normal use. By using custom resources, you can manage all of your resources in one place. Plain and simple. Update: An Update event is invoked when the custom resource itself has a property that has changed as part of a CloudFormation deploy. The timeout can be increased to maximum 12 hours using wait conditions. Figure 4. If a resource or an entire stack is ever removed by accident, setting a Deletion Policy can help make sure you at least retain your data. While custom resources allow you to execute whatever arbitrary code you need during your CloudFormation stack creation, update, or delete, they do have a couple of gotchas you should be aware of. It runs the poll_create() logic again. CreateFlowLogsFunction: Type: 'AWS::Lambda::Function' Properties: Code: ZipFile: | One example isnt quite enough, so lets do another. I can also use it to store the Id for updating or deleting a particular webhook. How to use CloudFormation custom resources, Tips and Tricks for writing Custom Resources. This will allow you to reuse Custom Resources developed by first and third-parties in the community. While it is possible to keep the entire Lambda code inside the template, that is a road that gets painful very quickly so it's best avoid. Perhaps you configured your primary keys incorrectly for your DynamoDB table, or you set up the wrong schema for your User Pool. AWS is the Wal-Mart of the cloud, offering you a wide selection of resources in a single place. Encoding the webhook Id into the Physical Resource Id allows us to identify and update an existing webhook when its input properties change. This is an identifier for the resource you create and is particularly important in Update scenarios. We also saw why the Replacement behavior can result in bad outcomes for your infrastructure. AWS CloudFormation custom resource provides mechanisms to provision AWS resources that dont have built-in support from CloudFormation. With custom resources, you can manage these custom tasks (which are one-off in nature) as deployment stack resources. Examples here include an incident response platform, such as PagerDuty or certain types of database offerings, such as a time-series database (while Timestream is still in preview). This asynchronous model makes it easier and faster for CloudFormation to provision many resources in a stack in parallel, but it also adds complexity. These examples are for the Python flavor, but are easily adaptable to Node.js or Java: If you're looking to get the Java function running, the concepts are the same but the steps differ slightly because everything is harder in Java. The value of the event.PhysicalResourceId during the events: 1) Create Empty. Are you sure you want to create this branch? In this post, well learn when, why, and how to use custom resources. However, it does cause some downtime AWS will need to spin down your existing instance, spin up a new instance, and attach your EBS volume to the new instance. This could be considered provisioning an AWS resource for which there is not CloudFormation support (first bucket), but there is CloudFormation support for creating an ACM Certificate. One of the features of CloudFormation is custom resources. You need to create a new pair for tracking each update/delete operation on the custom resource. This could be for more pet-like EC2 instances but more often is for stateful pieces of your infrastructure. You can then choose to Allow certain updates by adding statements. The certificate still isnt validated, so the function completes without writing a result to S3. In this post, Ill demonstrate how to use AWS Step Functions to implement custom resources using AWS Cloud Development Kit (AWS CDK). One handy feature is you can add a temporary stack policy to be applied only during a particular update. AWS CloudFormation is a powerful tool for provisioning resources in AWS. $, _, or -). Delete CloudFormation stack. By adding an UpdateReplacePolicy of Retain, were indicating that our DynamoDB table should stick around even after its replaced. Rather than returning a simple response in your Lambda function, you need to save your output to S3. Fortunately, there are a number of libraries that ease the burden of writing custom resources. A service token can be an Amazon SNS topic or AWS Lambda function Amazon Resource Name (ARN) from the same AWS region in which you are creating your CloudFormation stack. For more information, see Name type . If the run was unsuccessful, you may include a reason with the Reason property. A few of them are: custom-resource-helper: a Python-based library provided by AWS that uses decorators; cfn-wrapper-python: another Python-based library that was the inspiration for custom-resource-helper. DynamoDB record indicating custom resource update. cfn-custom-resource: Another Python-based library, this one uses classes over decorators. However, sometimes you need more than what CloudFormation currently offers. Or, you can omit the DBInstanceIdentifier property from your template. When deploying a new version of your application, you want to ensure that your database tables are created or that any recent migrations have been applied. Two minutes later, the function is triggered a fourth time. Note: In the following example, the DBInstanceIdentifier property of the MyRDS resource is set to the custom name PRODdb. Think carefully about how far you want to extend CloudFormations capabilities. If the request type is Update or Delete, the payload will also include a PhysicalResourceId parameter. First, let's recap what exactly is the PhysicalResourceId and what is it good for. The max duration for Lambda is only 15 minutes. Step 2: Make sure you have prepared any required items for the stack. Use the custom resource in your CloudFormation template that references the Lambda function or SNS topic. To add the Custom Resource we created to a CloudFormation template, we need to add it as a resource with a type starting with "Custom::". Understand how the Physical Resource Id works. Step Functions workflow prepares the response to be sent back to CloudFormation stack. Once you've decided on which languages you'll want to use, you simply have to implement the logic for the create, read, and update actions that CloudFormation will send you. The profile must have sufficient permissions to run an AWS CDK stack. The beauty (and danger) of custom resources is that you control the code, so you can do anything you please. These concepts can be difficult to understand at an abstract level, so lets explore it with an example. However, there are times when you need to use non-AWS solutions in your architecture. You can't change the type during an update. You can see the handler logic here, and its fairly basic around 120 lines of code. You can specify a custom resource type name up to a maximum length of 60 characters. Protip: CloudFormation resources are always named "AWS::::". If the piece of infrastructure in question is a database or critical EC2 instance, this can be a very unwelcome surprise! It is a parameter that you get with the lifecycle events and you also return it. To prevent a stack failure and avoid the error message, change any resources with custom names to use different names before you update a stack. We can deploy this template using the following command: Make sure you paste in your own unique values for REPO and ENDPOINT in the parameter overrides. You should see a new record with unix epoch timestamp in the DynamoDB table, indicating that the resource was created as shown in the following screenshot. DynamoDB tables), or to prevent all Replacement actions. A more standard approach might use other characters as separators (e.g. By default the CloudFormation stack waits for 1 hour before timing out. That's how CloudFormation works. They allow you to extend CloudFormation to do things it could not normally do. 2. Heres an example CloudFormation template for using our custom webhook: Note that we are provisioning a single resource in the Resources section. My serverless.yml file looks as follows: It deploys a single function, then registers the ARN of that function as a CloudFormation export so that I can import the value into another CloudFormation stack in my account. This repository is design to kickstart building custom resources, having the scaffolding for Python, Node.js, and Java functions ( Ruby coming soon!) The AWS CloudFormation stack can't be successfully updated until the name PRODdb is changed to a different name, such as PRODdb1. The core example here is running relational database initialization or migration scripts. While all of these libraries are solid, the two examples below use the custom-resource-helper library. Fortunately, the mechanisms for updating resources in CloudFormation are well-defined, and AWS provides a number of ways to protect against an accidental configuration error. The flow would look as follows: The initial request would come in and run the create() logic. This error typically occurs when a stack update tries to replace resources that have properties with custom names. You use the AWS Management Console to complete these tasks. That way, AWS CloudFormation generates a unique physical ID to use for the DB instance. Second, AWS may offer a solution in a category but perhaps a third-party solution better fits your needs. In these scenarios, you must look beyond the conventional Lambda function-based approach for custom resources. and examples in both YML and JSON. The ResponseURL parameter includes the presigned S3 URL for you to send your output. If you're thinking developing and maintaining Custom Resources is challenging/you would like to reuse great resources that others have developed, AWS have now developed and released a Cloudformation Registry. This request . CloudFormation has lots of defined resources that you can use to provision AWS resources. And with custom logic, you can do anything you want. Custom resources are basically just Lambda functions that get called by CloudFormation. For more information, see Replacement. Step 1: Pick a template. Step Functions allow complex deployment tasks to be orchestrated as a step-by-step workflow. Delete: Deletes an existing resource. #5 Open This should deploy the solution. This helps you save your existing data to be used in a migration. Meanwhile, CloudFormation is constantly polling for changes on the bucket. To deploy the custom resource, I use the Serverless Framework. To use a CloudFormation custom resource, you'll need to do three things: Write the logic for your custom resource; Make your custom resource logic available by deploying to an AWS Lambda function or by subscribing to an SNS topic. Below are a few key tips for writing resilient custom resources: Catch every exception to prevent hanging CloudFormation stacks. CloudFormation stack then goes into waiting mode where it waits for a SUCCESS or FAILURE signal to continue. Note that youll need to provision your own Github token before deploying. Your handler will receive this webhook and run any logic you want. P.S. Now that we know what custom resources are and when you might use them, lets see how to use custom resources. But obviously that did mean what I thought it meant. Weve done a lot of background on custom resources, but theres no substitute for actually walking through some examples. This falls into the second use case we discussed for when to use custom resources Provisioning non-AWS resources with CloudFormation. This time, the certificate is ready. To handle this state, I used the PhysicalResourceId property that is returned by the custom resource to our CloudFormation template. Theres no subsitute for hands-on learning, so this post also includes two walkthroughs of creating custom resources: Adding third-party resources to CloudFormation by provisioning a Github webhook, Extending AWS offerings and handling slow resources by provisioning an ACM certificate. Fortunately, the custom-resource-helper library makes it easy. With Lambda, this is a problem. This property is important, as it can be used to identify a created resource apart from its input properties. I recommend removing the profile when youre finished with this walkthrough. Once your function is deployed and registered, you can use the following stack to test it out: It takes DOMAIN and RECORD parameters to indicate the certificate you want to provision. While creating, updating or deleting a custom resource, the CloudFormation will wait until you return a response to the provided ResponseURL in the request object. Im particularly grateful for Ben Bridts CloudFormation Gaps repository on Github. Update with some interruption: Your resource is still the same resource, but there will be some downtime as the update is applied. Feel free to check out all the custom resource code here. In the Github webhook example below, we use the Physical Resource Id to encode the Id of the GitHub webhook. I update the CloudFormation Custom Resources template, hit "Retry". Once the resource provider puts something into the resource response bucket, its job is done. By default, none of the resources in your CloudFormation stack are protected from updates. As long as the value changes the call to update it will be made. These use cases are neat but remember that with great power comes great responsibility. His areas of expertise include architecting serverless solutions, CI/CD and automation. There was a problem preparing your codespace, please try again. AWS resource and property types reference. Replacement of certain resources can cause data loss without proper backup. You can limit this behavior by attaching a stack policy to your CloudFormation stack. With custom resources, you could write a script in a Lambda function that is triggered after your RDS database is configured to execute any migration scripts needed. Its possible you have a stateful resource, like a DynamoDB table or a Cognito User Pool, that needs a fundamental change in its structure. Im only going to highlight the important parts of the logic here. For an update behavior to be relevant, two things need to be true: You have an existing resource that is managed by a CloudFormation stack; and. This time it runs the poll_create() function. In addition to running its logic, it will also create a CloudWatch Scheduled Event that will re-trigger my function in two minutes. For example, you might have a DynamoDB table resource as follows: With an UpdateReplace Policy, you can specify what should happen to an existing resource in the event it is replaced due to an update. Its important to understand what that means for your custom resource and its failure modes. Alex DeBrie on Twitter, The three kinds of update behaviors with CloudFormation, Using Stack Policies to prevent undesirable updates to existing resources, Using UpdateReplace Policies to manage resources that are replaced, three potential behaviors for your existing resource, CloudFormation reference page for an EC2 instance, resize your existing EBS-backed instances without a replacement. The custom-resource-helper tears down the CloudWatch Scheduled Event so that it wont trigger again, then it writes the custom resources output to the presigned S3 URL. Put it more in the post include CloudFormation, you can run migration. Directions assume you have to rename the resource in your CloudFormation stack override your blanket policy of no updates specific. Function and a poll_create ( ) function, you will implement an AWS CDK.! 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