Somewhat of a prerequisite for automating things in AWS is the ability to add tags to resources. Tags are a key/value pair that you can add to supported resources. They allow you to create an arbitrary piece of data that is attached to a resource, such as an EC2 instance, a security group, or a Lambda function. There's a long list of resources you can tag, so check the AWS documentation for further details. 

The reason you would want to add a tag to a resource is that tags are returned when you request information about the resource. For example, the describe-instances operation for EC2 API returns any tags associated with an EC2 instance. You can also filter the query down to display just the tags for a given instance ID:

aws ec2 describe-instances 
    --instance-id i-123456780 
    --query "Reservations[*].Instances[*].[Tags[*]]"

A few examples of a tag could be as follows:

• owner: Scott
• cost-center: Technology
• backup: true

You can use this contextual information to drive the logic in your automation. You might want to assign an owner to a machine so you know who is responsible for it, or attribute the usage cost to a particular billing center, or flag that the instance should be added to the standard backup policy.

Tags are easy to add. Here's an example of tagging Lambda functions:

• This is how you tag using the Lambda management console:

• Now for tagging using the AWS CLI.

When creating a new function, you can use the --tags option. You can include multiple tags separated by a comma.

aws lambda create-function 
    --function-name hello-world
    --handler index.py
    --runtime python3.6
    --role arn:aws:iam::123456789012:role/lambda_basic_execution
    --tags "Owner=Scott,Environment=UAT"

Alternatively, you can achieve the same result by adding tags to an existing function using the tag-resource action of the Lambda CLI, as follows:

aws lambda tag-resource 
    --resource arn:aws:lambda:us-east-1:123456789012:function:hello-world
    --tags "Business Unit=Research & Development"

• Adding tags to a CloudFormation template is also possible by specifying the Tags property in a function declaration. See the following for an example:

HelloFunction: 
  Type: "AWS::Lambda::Function"
  Properties: 
    Handler: "index.handler"
    Code: 
      S3Bucket: "lambda-functions"
      S3Key: "function.zip"
    Runtime: "java8"
    Tags: 
    - Key: "Owner"
      Value: "Scott"
    - Key: "DeployedByPipeline"
      Value: "serverless-hello-world-pipeline"

• When adding tags to your resources in the Serverless Framework, you can add tags just to the functions that you create:

functions:
  hello:
    handler: handler.hello
    tags:
      DeployedBy: Serverless

• Or—and this is extremely useful—you can add tags that attach to every resource that gets created by deploying the CloudFormation stack as part of a serverless deployment. This declaration is made at the provider level:

provider:
  name: aws
  runtime: nodejs10.x
  stackTags:
    DeployedBy: Serverless

Okay; let's move on to a reason where we may be able to leverage tags, and then move on to an example.