AWS CloudFormation

AWS CloudFormation is a service that allows you to manage, configure and provision your AWS infrastructure using code in a template.

AWS CloudFormation provides a common language for you to describe and provision all the infrastructure resources in your cloud environment.

Resources are defined using an AWS CloudFormation template.

CloudFormation interprets the template and makes the appropriate API calls to create the resources you have defined.

CloudFormation supports YAML or JSON.

CloudFormation can be used to provision a broad range of AWS resources.

Think of CloudFormation as deploying “infrastructure as code”.

CloudFormation has some similarities with AWS Elastic Beanstalk though they are also quite different as detailed in the table below:

CloudFormation vs Elastic Beanstalk

Deployment and Provisioning

Key Concepts

The following table describes the key concepts associated with AWS CloudFormation:

CloudFormation Deployment

Templates

A template is a YAML or JSON template used to describe the end-state of the infrastructure you are either provisioning or changing.

After creating the template, you upload it to CloudFormation directly or using Amazon S3.

CloudFormation reads the template and makes the API calls on your behalf.

The resulting resources are called a “Stack”.

Logical IDs are used to reference resources within the template.

Physical IDs identify resources outside of AWS CloudFormation templates, but only after the resources have been created.

Template elements

Mandatory:

  • List of resources and associated configuration values.

Not mandatory:

  • Template parameters (limited to 60).
  • Output values (limited to 60).
  • List of data tables.

Template components

Resources – the required Resources section declares the AWS resources that you want to include in the stack, such as an Amazon EC2 instance or an Amazon S3 bucket.

  • Mandatory.
  • Represent AWS components that will be created.
  • Resources are declared and can reference each other.

The following example YAML code declares an EC2 instance as a resource:

 Resources:
  MyEC2Instance:
    Type: "AWS::EC2::Instance"
    Properties:
      ImageId: "ami-0ff8a91507f77f867"

Parameters – use the optional Parameters section to customize your templates. Parameters enable you to input custom values to your template each time you create or update a stack.

  • Provide inputs to your CloudFormation template.
  • Useful for template reuse.

The following example declares a parameter named InstanceTypeParameter. This parameter lets you specify the Amazon EC2 instance type for the stack to use when you create or update the stack.

Note: the InstanceTypeParameter has a default value of t2.micro. This is the value that AWS CloudFormation uses to provision the stack unless another value is provided.

Parameters: 
  InstanceTypeParameter: 
    Type: String
    Default: t2.micro
    AllowedValues: 
      - t2.micro
      - m1.small
      - m1.large
    Description: Enter t2.micro, m1.small, or m1.large. Default is t2.micro.

Mappings – the optional Mappings section matches a key to a corresponding set of named values.

  • Fixed variables.
  • Good for differentiating between regions, environments, AMIs etc.
  • Need to know the values in advance.
  • For user-specific values use parameters instead.

The following example has region keys that are mapped to two sets of values: one named HVM64 and the other HVMG2.

RegionMap: 
    us-east-1:
      HVM64: ami-0ff8a91507f77f867
      HVMG2: ami-0a584ac55a7631c0c
    us-west-1:
      HVM64: ami-0bdb828fd58c52235
      HVMG2: ami-066ee5fd4a9ef77f1

Exam tip: with mappings you can, for example, set values based on a region. You can create a mapping that uses the region name as a key and contains the values you want to specify for each specific region.

Outputs – the optional Outputs section declares output values that you can import into other stacks (to create cross-stack references), return in response (to describe stack calls), or view on the AWS CloudFormation console.

  • Outputs can be imported into other stacks.
  • Can view the outputs in the console or using the AWS CLI.
  • Cannot delete a Stack if it’s outputs are being referenced by another CloudFormation Stack.

In the following example YAML code, the output named StackVPC returns the ID of a VPC, and then exports the value for cross-stack referencing with the name VPCID appended to the stack’s name

Outputs:
  StackVPC:
    Description: The ID of the VPC
    Value: !Ref MyVPC
    Export:
      Name: !Sub "${AWS::StackName}-VPCID"

Conditions – the optional Conditions section contains statements that define the circumstances under which entities are created or configured.

  • Control the creation of resources based on a condition.
  • Applied to resources and outputs.

In the sample YAML code below, resources are created only if the EnvType parameter is equal to prod:

Conditions: 
  CreateProdResources: !Equals [ !Ref EnvType, prod ]

Stacks and Stack Sets

Stacks:

  • Deployed resources based on templates.
  • Create, update and delete stacks using templates.
  • Deployed through the Management Console, CLI or APIs.

Stack creation errors:

  • Automatic rollback on error is enabled by default.
  • You will be charged for resources provisioned even if there is an error.

Updating stacks:

  • AWS CloudFormation provides two methods for updating stacks: direct update or creating and executing change sets.
  • When you directly update a stack, you submit changes and AWS CloudFormation immediately deploys them.
  • Use direct updates when you want to quickly deploy your updates.
  • With change sets, you can preview the changes AWS CloudFormation will make to your stack, and then decide whether to apply those changes.

Stack Sets:

  • AWS CloudFormation StackSets extends the functionality of stacks by enabling you to create, update, or delete stacks across multiple accounts and regions with a single operation.
  • Using an administrator account, you define and manage an AWS CloudFormation template, and use the template as the basis for provisioning stacks into selected target accounts across specified regions.
  • An administrator account is the AWS account in which you create stack sets.
  • A stack set is managed by signing in to the AWS administrator account in which it was created.
  • A target account is the account into which you create, update, or delete one or more stacks in your stack set.

Before you can use a stack set to create stacks in a target account, you must set up a trust relationship between the administrator and target accounts.

CloudFormation Nested Stacks:

  • Nested stacks allow re-use of CloudFormation code for common use cases.
  • For example standard configuration for a load balancer, web server, application server etc.
  • Instead of copying out the code each time, create a standard template for each common use case and reference from within your CloudFormation template.

User data with EC2

  • User data can be included in CloudFormation.
  • The script is passed into Fn::Base64
  • The user data script logs are stored in /var/log/cloud-init-output.log
  • Binary is available on Amazon EC2 at /opt/aws/bin/cfn-init

CloudFormation Helper Scripts

cfn-init:

  • The cfn-init helper script reads template metadata from the AWS::CloudFormation::Init key and acts accordingly to:
  • Fetch and parse metadata from AWS CloudFormation
  • Install packages
  • Write files to disk
  • Enable/disable and start/stop services
  • cfn-init does not require credentials, so you do not need to use the –access-key–secret-key–role, or –credential-file options.
  • Logs go to /var/log/cfn-init.log

cfn-signal:

  • The cfn-signal helper script signals AWS CloudFormation to indicate whether Amazon EC2 instances have been successfully created or updated.
  • If you install and configure software applications on instances, you can signal AWS CloudFormation when those software applications are ready.
  • You use the cfn-signal script in conjunction with a CreationPolicy or an Auto Scaling group with a WaitOnResourceSignals update policy.
  • When AWS CloudFormation creates or updates resources with those policies, it suspends work on the stack until the resource receives the requisite number of signals or until the timeout period is exceeded. 
  • You can signal a creation policy (CreationPolicy) or a wait condition handle (WaitOnResourceSignals).

Troubleshooting errors:

  • Make sure the AMI has the CloudFormation helper scripts included.
  • Check that the cfn-init and cfn-signal commands have run successfully.
  • Verify internet connectivity.

Creation Policies and Wait Conditions

  • CreationPolicy attribute:
  • Use the CreationPolicy attribute when you want to wait on resource configuration actions before stack creation proceeds.
  • You can associate the CreationPolicy attribute with a resource to prevent its status from reaching create complete until AWS CloudFormation receives a specified number of success signals or the timeout period is exceeded.
  • To signal a resource, you can use the cfn-signal helper script or SignalResource API.
  • AWS CloudFormation publishes valid signals to the stack events so that you track the number of signals sent.

The following CloudFormation resources support creation policies:

DeletionPolicy attribute:

  • With the DeletionPolicy attribute you can preserve or (in some cases) backup a resource when its stack is deleted.
  • You specify a DeletionPolicy attribute for each resource that you want to control.
  • If a resource has no DeletionPolicy attribute, AWS CloudFormation deletes the resource by default.

DependsOn attribute:

  • With the DependsOn attribute you can specify that the creation of a specific resource follows another.
  • When you add a DependsOn attribute to a resource, that resource is created only after the creation of the resource specified in the DependsOn attribute.

WaitCondition:

  • Note: For Amazon EC2 and Auto Scaling resources, AWS recommends that you use a CreationPolicy attribute instead of wait conditions.
  • You can use a wait condition for situations like the following:
  • To coordinate stack resource creation with configuration actions that are external to the stack creation.
  • To track the status of a configuration process.

UpdatePolicy Attribute (WaitOnResourceSignals)

Use the UpdatePolicy attribute to specify how AWS CloudFormation handles updates to the following resources:

UpdateReplacePolicy attribute:

  • Use the UpdateReplacePolicy attribute to retain or (in some cases) backup the existing physical instance of a resource when it is replaced during a stack update operation.

Rollbacks and Creation Failures

Stack creation failures:

  • By default everything will be deleted.
  • You can optionally disable rollback (good for troubleshooting failures).

Stack update failures:

  • The stack will automatically roll back to the previous known working state.
  • The logs can assist with understanding what issue occurred.

Monitoring and Reporting

You can monitor the progress of a stack update by viewing the stack’s events. The console’s Events tab displays each major step in the creation and update of the stack sorted by the time of each event with latest events on top.

For resources created by CloudFormation, use AWS monitoring and reporting tools applicable to the service.

Authorization and Access Control

You can use IAM with AWS CloudFormation to control what users can do with AWS CloudFormation, such as whether they can view stack templates, create stacks, or delete stacks.

In addition to AWS CloudFormation actions, you can manage what AWS services and resources are available to each user.

That way, you can control which resources users can access when they use AWS CloudFormation.

For example, you can specify which users can create Amazon EC2 instances, terminate database instances, or update VPCs. Those same permissions are applied anytime they use AWS CloudFormation to do those actions.

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