## Examples

Let's say we make a simple change to a template, like changing the default InstanceType from `t2.small` to `t2.micro`.  Here's what happens when you run `lono cfn update`

    lono cfn update ec2

The output should look similiar to this:

<img src="/img/tutorials/ec2/cfn-update-instance-type.png" alt="Stack Update" class="doc-photo">

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](https://medium.com/boltops/a-simple-introduction-to-cloudformation-part-4-change-sets-dry-run-mode-c14e41dfeab7) 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

## Conventions: template and param

Lono follows some [coventions](https://lono.cloud/docs/conventions/) that helps keep the commands short.  We'll go through an example to explain:

Provided that you are in a lono project and have a `demo` lono blueprint that contains a `demo` template.  To update a stack you can run:

    lono cfn update demo

The above command will generate:

* template:   output/demo/templates/demo.yml
* parameters: output/demo/params/development.json

By [convention]({% link _docs/conventions/cli.md %}), the blueprint name is the same as the stack name. In turn, template name is the same as the blueprint name. Lastly, the param name will default to the template name.

Here are examples of overriding the template and params name conventions.

    lono cfn update demo --template different1

The template used is `app/templates/different1.rb` and the parameters used is `configs/demo/params/development/demo/different1.txt`.

    lono cfn update demo --param different2

The template used is `app/templates/demo.rb` and the parameters used is `configs/demo/params/development/demo/different2.json`.

    lono cfn update demo --template different3 --param different4

The template used is `app/templates/different3.rb` and the parameters used is `configs/demo/params/different3/different4.json`.