ChangeSets are the main way how you should deploy to CloudFormation. They allow you to see exactly what steps CloudFormation would take once you deploy the ChangeSet, so you can find any problems beforehand.
To create a ChangeSet you simply upload a new version of your CloudFormation template. CloudFormation will calculate all the changes which can then be viewed by you. Depending on the complexity of a ChangeSet calculating the necessary actions might take a few seconds. If you’re looking for a great client to help you with creating and deploying ChangeSets check out Formica. Other tools like the AWSCli support this as well.
The default description for ChangeSets is complicated and for typical daily use unimportant. If you want to see it in all its details check out the Changed documentation, bhe main aspects you should focus on (and which Formica as well as AWSInfo provide to you by default) are:
With that information you can get a good understanding of what is about to happen if you deploy the ChangeSet.
In the following example I’ve used AWSIinfo to show me all the details of a particular ChangeSet. In this case I’ve used the stack we deployed before and changed the WebBucketName parameter as well as set the DevBucketName. For each Bucket I’ve also changed the IndexDocument parameter to index.htmls to trigger a change.
# awsinfo cfn change-set tslw -- tslw
Selected Stack tslw-production
Selected CHANGE-SET tslw-production-change-set
-----------------------------------------------------------------------------------------------------------------------------------------
| DescribeChangeSet |
+---------------------+------------------------------------+----------------------+-------------------+---------------------------------+
| 1.Stack | 2.ChangeSetName | 3.Status | 4.Executable | 5.CreatedAt |
+---------------------+------------------------------------+----------------------+-------------------+---------------------------------+
| tslw-production | tslw-production-change-set | CREATE_COMPLETE | AVAILABLE | 2017-10-05T12:06:42.918Z |
+---------------------+------------------------------------+----------------------+-------------------+---------------------------------+
|| 6.Parameters ||
|+--------------------------------------------------------+----------------------------------------------------------------------------+|
|| ParameterKey | ParameterValue ||
|+--------------------------------------------------------+----------------------------------------------------------------------------+|
|| CreateDevBucket | true ||
|| WebBucketName | tslw-test-bucket-name ||
|+--------------------------------------------------------+----------------------------------------------------------------------------+|
|| 7.Changes ||
|+----------+--------------+------------------+----------------+-------------+------------------------------------+--------------------+|
|| 1.Action | 2.LogicalId | 3.Type | 4.Replacement | 5.Scopes | 6.StaticChanges | 7.DynamicChanges ||
|+----------+--------------+------------------+----------------+-------------+------------------------------------+--------------------+|
|| Modify | DevBucket | AWS::S3::Bucket | True | Properties | BucketName, WebsiteConfiguration | ||
|| Modify | WebBucket | AWS::S3::Bucket | True | Properties | BucketName, WebsiteConfiguration | BucketName ||
|+----------+--------------+------------------+----------------+-------------+------------------------------------+--------------------+|
One of the issues of ChangeSets though is that they don’t include a full diff of the templates. They show which actions will be performed, but not enough detail to know exactly whats about to be changed, e.g. in the above example it says the WebsiteConfiguration changed, but not exactly which attribute in that config changed.
To solve this we need a diff between our deployed and local template. This is what Formica provides with its formica diff command.
In the following example we get a full diff between the two templates, giving us more information about the changes we made to our templates.
# formica diff -s tslw-production
+---------------------------------------------------------------------------+-------------+-----------------------------+-----------------------+
| Path | From | To | Change Type |
+===========================================================================+=============+=============================+=======================+
| Resources > DevBucket > Properties > BucketName | Not Present | tslw-test-change-set-bucket | Dictionary Item Added |
+---------------------------------------------------------------------------+-------------+-----------------------------+-----------------------+
| Resources > DevBucket > Properties > WebsiteConfiguration > IndexDocument | index.html | index.htmls | Values Changed |
+---------------------------------------------------------------------------+-------------+-----------------------------+-----------------------+
| Resources > WebBucket > Properties > WebsiteConfiguration > IndexDocument | index.html | index.htmls | Values Changed |
+---------------------------------------------------------------------------+-------------+-----------------------------+-----------------------+
ChangeSet descriptions with AWSInfo (or Formica) as well as Formica diffs together will give you a complete overview on the changes you’re about to make to your infrastructure. This makes it less likely for you to introduce bad configuration into your infrastructure and increases your productivity during development.