AWS::CloudFormation::Init

Question 1

What is cfn-init?

Answer 1

cfn-init is a helper script installed on EC2 OS. It is a simple configuration management system (kind of like ansible).

Question 2

What can you do with cfn-init?

Answer 2

You can execute instructions in a procedural way (line by line) but can also be desired state.

• Can specify particular versions of packages. It will ensure things are configured to that end state.
• Can manipulate OS groups and users.
• Can download sources and extract them using authentication.
• Can create files with different contents, permissions and ownerships.
• Can run commands and test if certain conditions are true
• Can control services in an instance

Question 3

How does cfn-initi work?

Answer 3

1. Starts off with a CloudFormation template.
2. This has a logical resource within it which is to create an EC2 instance.
3. This has a specific section called Metadata.
4. This then passes in the information passed in as UserData.
   cfn-init gets variables passed into the user data by CloudFormation.

Question 4

What does it mean that cfn-init can work towards a desired state?

Answer 4

It knows the desired state and can work towards a final configuration.
This can monitor the user data and change things as the EC2 data changes.

Question 5

What is creationPolicy and signals?

Answer 5

A CreationPolicy is something which is added to a logical resource inside a CloudFormation template. You create it and supply a timeout value.

This waits for a signal from the resource itself before moving to a create complete state, verifying if the instance was properly configured.

Question 6

How do dfn-signals work?

Answer 6

The cfn-signal will report the state of the cfn-init to the stack. If the cfn-init was successfully run, then the instance will be moved to a complete state.
If nothing happens in X minutes (timeout) an error will be reported in CloudFormation.