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- [Raf] Hi, it's me again, Raf here.

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So, according to everything
you have learned so far,

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CloudFormation is an
infrastructure-as-code service

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that you can rely in order to build

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your infrastructure automations.

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CloudFormation is one
of the core components

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of almost every CI/CD
workflow, and you would like

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to make sure your CloudFormation templates

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are as reliable as possible.

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In most cases, when the
creation of a stack fails,

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you will see a detailed error
from CloudFormation pointing

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out where the issue potentially is.

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The most common error
is due to permissions.

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Resources created or deleted
by your stack get operated

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on behalf of the user who
is operating the stack,

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so if you have a template
that creates an EC2 instance,

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but your user is not authorized to do so,

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that step will fail, and if
the stack creation is set

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to rollback in case of failure,
everything created so far

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will be deleted to
leave the infrastructure

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in a consistent state.

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Another aspect I would like to highlight

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is that CloudFormation is
a service that operates

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the resources in the given Region

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where the stack is running.

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So if you create a stack
in us-east-1, or Virginia,

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the resources will be
created in that Region.

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That being said, some
services require a property

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that only exists in a specific
Region, such as, for example,

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EC2 instances and their AMIs.

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The Amazon Machine Image, or
AMI, is the image containing

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the operating system of the EC2
instance you want to create.

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Each AMI has an ID, and these
IDs are different according

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to the Region you are operating in.

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If you want to create an EC2 instance

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and hardcode the AMI ID into the template,

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you would need to make sure
that the specific AMI ID exists

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in the Region you are creating.

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Otherwise, the creation would
fail because CloudFormation

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would be trying to launch
an instance with an AMI

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that does not exist in that Region.

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CloudFormation has a feature that makes

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the templates more reliable
in terms of cross-Region.

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So, if you want to build
a template that can run

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on any given Region, you can
use a CloudFormation mapping.

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A mapping is a feature that allows you

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to choose different values
according to each Region

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you are running, or any other value.

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Imagine the mapping as an if statement.

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For example, if you are running
the template in us-east-1,

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then use this specific AMI ID.

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If you're running in us-west-2,
then use that other one.

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That would fix that problem and make

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your templates more resilient.

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That can also be applied for parameters,

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such as if a parameter
has environment = prod,

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then choose a specific instance type.

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If the parameter has environments = dev,

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then choose another instance type,

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which could be a smaller one.

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Okay, cool.

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With the use of mappings, you can create

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a more flexible template, but how about

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the resource-creation sequence?

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For example, what if you have
an application that needs

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an RDS database up and
running before creating

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the EC2 instance?

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Another common troubleshooting topic

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is the dependency error.

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As the name suggests, a dependency error

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is when CloudFormation
tries to create a resource

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that depends on another resource

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that hasn't been created yet.

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CloudFormation creates
the resources in parallel

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to optimize speed, but
it may not know the order

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of precedence needed by
your application logic.

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You can add a property called
DependsOn in the creation

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of the EC2 instance.

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That would make CloudFormation
hold the creation

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of the instance, and only
do it after the RDS instance

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has been created.

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In the property, you can specify
one or multiple resources

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you want to depend on.

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If you're doing serverless,
dependency between resources may

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be more frequent, such as
creating an API Gateway endpoint

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and Lambda functions associated with it.

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That's why the usage of SAM,

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or Serverless Application Model,

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brings you good help, because
it is an abstraction layer

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that gives you a serverless-oriented view,

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so it already knows some obvious

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dependencies among resources.

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There are also other
troubleshooting topics described

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in the AWS documentation,
but you guess what?

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Alana told me she will add that
link to the course readings

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so you don't miss anything.