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Update the Stack

Now is a good time to introduce lono cfn update command before moving on. In the previous section, we used the lono cfn create command and assumed that the template magically worked the very first time. In practice, it usually takes a few iterations before you get the CloudFormation template working right. The CloudFormation development flow more commonly goes:

CloudFormation Development Flow

Let's make a simple change to test out the lono cfn update command. We'll change the default InstanceType from t2.small to t2.micro and then run:

lono cfn update ec2

The output should look similiar to this:

Stack Update

Notice that before the update gets performed lono provides a preview of the changes that are about to take place. There are two types of "diffs" in the preview.

  1. Source code diff of the templates. This is generated by downloading the current CloudFormation template and comparing it with the locally generated one using colordiff.
  2. CloudFormation Change Set list of changes. This is generated using AWS CloudFormation Change Set feature.

You are prompted with an "Are you sure?" confirmation before lono continues.

By default, the update command will display a preview of the stack changes before applying the update and prompt to check if you are sure. If you want to bypass the are you sure prompt, use the --sure option.

lono cfn update ec2 --sure

You'll find the lono cfn update command helpful in the sections.

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