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AWS CloudFormation
User Guide (API Version 2010-05-15)

AWS CloudFormation Best Practices

Best practices are recommendations that can help you use AWS CloudFormation more effectively and securely throughout its entire workflow. Learn how to plan and organize your stacks, create templates that describe your resources and the software applications that run on them, and manage your stacks and their resources. The following best practices are based on real-world experience from current AWS CloudFormation customers.

Organize Your Stacks By Lifecycle and Ownership

Use the lifecycle and ownership of your AWS resources to help you decide what resources should go in each stack. Normally, you might put all your resources in one stack, but as your stack grows in scale and broadens in scope, managing a single stack can be cumbersome and time consuming. By grouping resources with common lifecycles and ownership, owners can make changes to their set of resources by using their own process and schedule without affecting other resources.

For example, imagine a team of developers and engineers who own a website that is hosted on autoscaling instances behind a load balancer. Because the website has its own lifecycle and is maintained by the website team, you can create a stack for the website and its resources. Now imagine that the website also uses back-end databases, where the databases are in a separate stack that are owned and maintained by database administrators. Whenever the website team or database team needs to update their resources, they can do so without affecting each other's stack. If all resources were in a single stack, coordinating and communicating updates can be difficult.

For additional guidance about organizing your stacks, you can use two common frameworks: a multi-layered architecture and service-oriented architecture (SOA).

A layered architecture organizes stacks into multiple horizontal layers that build on top of one another, where each layer has a dependency on the layer directly below it. You can have one or more stacks in each layer, but within each layer, your stacks should have AWS resources with similar lifecycles and ownership.

With a service-oriented architecture, you can organize big business problems into manageable parts. Each of these parts is a service that has a clearly defined purpose and represents a self-contained unit of functionality. You can map these services to a stack, where each stack has its own lifecycle and owners. All of these services (stacks) can be wired together so that they can interact with one another.

Use Cross-Stack References to Export Shared Resources

When you organize your AWS resources based on lifecycle and ownership, you might want to build a stack that uses resources that are in another stack. You can hard-code values or use input parameters to pass resource names and IDs. However, these methods can make templates difficult to reuse or can increase the overhead to get a stack running. Instead, use cross-stack references to export resources from a stack so that other stacks can use them. Stacks can use the exported resources by calling them using the Fn::ImportValue function.

For example, you might have a network stack that includes a VPC, a security group, and a subnet. You want all public web applications to use these resources. By exporting the resources, you allow all stacks with public web applications to use them. For more information, see Walkthrough: Refer to Resource Outputs in Another AWS CloudFormation Stack.

Use IAM to Control Access

IAM is an AWS service that you can use to manage users and their permissions in AWS. You can use IAM with AWS CloudFormation to specify what AWS CloudFormation actions users can perform, such as viewing stack templates, creating stacks, or deleting stacks. Furthermore, anyone managing AWS CloudFormation stacks will require permissions to resources within those stacks. For example, if users want to use AWS CloudFormation to launch, update, or terminate Amazon EC2 instances, they must have permission to call the relevant Amazon EC2 actions.

In most cases, users require full access to manage all of the resources in a template. AWS CloudFormation makes calls to create, modify, and delete those resources on their behalf. To separate permissions between a user and the AWS CloudFormation service, use a service a role. AWS CloudFormation uses the service role's policy to make calls instead of the user's policy. For more information, see AWS CloudFormation Service Role.

Verify Quotas for All Resource Types

Before launching a stack, ensure that you can create all the resources that you want without hitting your AWS account limits. If you hit a limit, AWS CloudFormation won't create your stack successfully until you increase your quota or delete extra resources. Each service can have various limits that you should be aware of before launching a stack. For example, by default, you can only launch 200 AWS CloudFormation stacks per region in your AWS account. For more information about limits and how to increase the default limits, see AWS Service Limits in the AWS General Reference.

Reuse Templates to Replicate Stacks in Multiple Environments

After you have your stacks and resources set up, you can reuse your templates to replicate your infrastructure in multiple environments. For example, you can create environments for development, testing, and production so that you can test changes before implementing them into production. To make templates reusable, use the parameters, mappings, and conditions sections so that you can customize your stacks when you create them. For example, for your development environments, you can specify a lower-cost instance type compared to your production environment, but all other configurations and settings remain the same. For more information about parameters, mappings, and conditions, see Template Anatomy.

Use Nested Stacks to Reuse Common Template Patterns

As your infrastructure grows, common patterns can emerge in which you declare the same components in each of your templates. You can separate out these common components and create dedicated templates for them. That way, you can mix and match different templates but use nested stacks to create a single, unified stack. Nested stacks are stacks that create other stacks. To create nested stacks, use the AWS::CloudFormation::Stack resource in your template to reference other templates.

For example, assume that you have a load balancer configuration that you use for most of your stacks. Instead of copying and pasting the same configurations into your templates, you can create a dedicated template for the load balancer. Then, you just use the AWS::CloudFormation::Stack resource to reference that template from within other templates. If the load balancer template is updated, any stack that is referencing it will use the updated load balancer (only after you update the stack). In addition to simplifying updates, this approach lets you use experts to create and maintain components that you might not be necessarily familiar with. All you need to do is reference their templates.

Do Not Embed Credentials in Your Templates

Rather than embedding sensitive information in your AWS CloudFormation templates, use input parameters to pass in information whenever you create or update a stack. If you do, make sure to use the NoEcho property to obfuscate the parameter value.

For example, suppose your stack creates a new database instance. When the database is created, AWS CloudFormation needs to pass a database administrator password. You can pass in a password by using an input parameter instead of embedding it in your template. For more information, see Parameters.

Use AWS-Specific Parameter Types

If your template requires inputs for existing AWS-specific values, such as existing Amazon Virtual Private Cloud IDs or an Amazon EC2 key pair name, use AWS-specific parameter types. For example, you can specify a parameter as type AWS::EC2::KeyPair::KeyName, which takes an existing key pair name that is in the your AWS account and in the region where the you are creating the stack. AWS CloudFormation can quickly validate values for AWS-specific parameter types before creating your stack. Also, if you use the AWS CloudFormation console, AWS CloudFormation shows a drop-down list of valid values, so you don't have to look up or memorize the correct VPC IDs or key pair names. For more information, see Parameters.

Use Parameter Constraints

With constraints, you can describe allowed input values so that AWS CloudFormation catches any invalid values before creating a stack. You can set constraints such as a minimum length, maximum length, and allowed patterns. For example, you can set constraints on a database user name value so that it must be a minimum length of eight character and contain only alpha-numeric characters. For more information, see Parameters.

Use AWS::CloudFormation::Init to Deploy Software Applications on Amazon EC2 Instances

When you launch stacks, you can install and configure software applications on Amazon EC2 instances by using the cfn-init helper script and the AWS::CloudFormation::Init resource. By using AWS::CloudFormation::Init, you can describe the configurations that you want rather than scripting procedural steps. You can also update configurations without recreating instances. And if anything goes wrong with your configuration, AWS CloudFormation generates logs that you can use to investigate issues.

In your template, specify installation and configuration states in the AWS::CloudFormation::Init resource. For a walkthrough that shows how to use cfn-init and AWS::CloudFormation::Init, see Deploying Applications on Amazon EC2 with AWS CloudFormation.

Use the Latest Helper Scripts

The helper scripts are updated periodically. Be sure you include the following command in the UserData property of your template before you call the helper scripts to ensure that your launched instances get the latest helper scripts:

yum install -y aws-cfn-bootstrap

For more information about getting the latest helper scripts, see the CloudFormation Helper Scripts Reference.

Validate Templates Before Using Them

Before you use a template to create or update a stack, you can use AWS CloudFormation to validate it. Validating a template can help you catch syntax and some semantic errors, such as circular dependencies, before AWS CloudFormation creates any resources. If you use the AWS CloudFormation console, the console automatically validates the template after you specify input parameters. For the AWS CLI or AWS CloudFormation API, use the aws cloudformation validate-template command or ValidateTemplate action.

During validation, AWS CloudFormation first checks if the template is valid JSON. If it isn't, AWS CloudFormation checks if the template is valid YAML. If both checks fail, AWS CloudFormation returns a template validation error.

Manage All Stack Resources Through AWS CloudFormation

After you launch a stack, use the AWS CloudFormation console, API, or AWS CLI to update resources in your stack. Do not make changes to stack resources outside of AWS CloudFormation. Doing so can create a mismatch between your stack's template and the current state of your stack resources, which can cause errors if you update or delete the stack. For more information, see Walkthrough: Updating a Stack.

Create Change Sets Before Updating Your Stacks

Change sets allow you to see how proposed changes to a stack might impact your running resources before you implement them. AWS CloudFormation doesn't make any changes to your stack until you execute the change set, allowing you to decide whether to proceed with your proposed changes or create another change set.

Use change sets to check how your changes might impact your running resources, especially for critical resources. For example, if you change the name of an Amazon RDS database instance, AWS CloudFormation will create a new database and delete the old one; you will lose the data in the old database unless you've already backed it up. If you generate a change set, you will see that your change will replace your database. This can help you plan before you update your stack. For more information, see Updating Stacks Using Change Sets.

Use Stack Policies

Stack policies help protect critical stack resources from unintentional updates that could cause resources to be interrupted or even replaced. A stack policy is a JSON document that describes what update actions can be performed on designated resources. Specify a stack policy whenever you create a stack that has critical resources.

During a stack update, you must explicitly specify the protected resources that you want to update; otherwise, no changes are made to protected resources. For more information, see Prevent Updates to Stack Resources.

Use AWS CloudTrail to Log AWS CloudFormation Calls

AWS CloudTrail tracks anyone making AWS CloudFormation API calls in your AWS account. API calls are logged whenever anyone uses the AWS CloudFormation API, the AWS CloudFormation console, a back-end console, or AWS CloudFormation AWS CLI commands. Enable logging and specify an Amazon S3 bucket to store the logs. That way, if you ever need to, you can audit who made what AWS CloudFormation call in your account. For more information, see Logging AWS CloudFormation API Calls in AWS CloudTrail.

Use Code Reviews and Revision Controls to Manage Your Templates

Your stack templates describe the configuration of your AWS resources, such as their property values. To review changes and to keep an accurate history of your resources, use code reviews and revision controls. These methods can help you track changes between different versions of your templates, which can help you track changes to your stack resources. Also, by maintaining a history, you can always revert your stack to a certain version of your template.

Update Your Amazon EC2 Linux Instances Regularly

On all your Amazon EC2 Linux instances and Amazon EC2 Linux instances created with AWS CloudFormation, regularly run the yum update command to update the RPM package. This ensures that you get the latest fixes and security updates.