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You can monitor the progress of a stack update by viewing the stack's events. The console's *Events* tab displays each major step in the creation and update of the stack sorted by the time of each event with latest events on top.
The start of the stack update process is marked with an UPDATE_IN_PROGRESS event for the stack:
2011-09-30 09:35 PDT AWS::CloudFormation::Stack MyStack UPDATE_IN_PROGRESS
Next are events that mark the beginning and completion of the update of each resource that was changed in the update template. For example, updating an AWS::RDS::DBInstance resource named MyDB would result in the following entries:
2011-09-30 09:35 PDT AWS::RDS::DBInstance MyDB UPDATE_COMPLETE 2011-09-30 09:35 PDT AWS::RDS::DBInstance MyDB UPDATE_IN_PROGRESS
The UPDATE_IN_PROGRESS event is logged when AWS CloudFormation reports that it has begun to update the resource. The UPDATE_COMPLETE event is logged when the resource is successfully created.
When AWS CloudFormation has successfully updated the stack, you will see the following event:
2011-09-30 09:35 PDT AWS::CloudFormation::Stack MyStack UPDATE_COMPLETE
During stack update operations, if CloudFormation needs to replace an existing resource, it first creates a new resource and then deletes the old resource. However, there may be cases where CloudFormation cannot delete the old resource (e.g., if the user does not have permissions to delete a resource of a given type).
CloudFormation makes three attempts at deleting the old resource. If CloudFormation cannot delete the old resource, it removes the old resource from the stack and continues updating the stack. When the stack update is complete, CloudFormation issues an `UPDATE_COMPLETE` stack event, but includes a `StatusReason` that states that one or more resources could not be deleted. CloudFormation also issues a `DELETE_FAILED` event for the specific resource, with a corresponding `StatusReason` providing more detail on why CloudFormation failed to delete the resource.
The old resource still exists and will continue to incur charges, but is no longer accessible through CloudFormation. To delete the old resource, access the old resource directly using the console or API for the underlying service.
This is also true for resources that you have removed from the stack template, and so will be deleted from the stack during the stack update.
If an update of a resource fails, AWS CloudFormation reports an UPDATE_FAILED event that includes a reason for the failure. For example, if your update template specified a property change that is not supported by the resource such as reducing the size of AllocatedStorage for an AWS::RDS::DBInstance resource, you would see events like these:
2011-09-30 09:36 PDT AWS::RDS::DBInstance MyDB UPDATE_FAILED Size cannot be less than current size; requested: 5; current: 10 2011-09-30 09:35 PDT AWS::RDS::DBInstance MyDB UPDATE_IN_PROGRESS
If a resource update fails, AWS CloudFormation rolls back any resources that it has updated during the upgrade to their configurations before the update. Here is an example of the events you would see during an update rollback:
2011-09-30 09:38 PDT AWS::CloudFormation::Stack MyStack UPDATE_ROLLBACK_COMPLETE 2011-09-30 09:38 PDT AWS::RDS::DBInstance MyDB UPDATE_COMPLETE 2011-09-30 09:37 PDT AWS::RDS::DBInstance MyDB UPDATE_IN_PROGRESS 2011-09-30 09:37 PDT AWS::CloudFormation::Stack MyStack UPDATE_ROLLBACK_IN_PROGRESS The following resource(s) failed to update: [MyDB]
https://docs.aws.amazon.com/cli/latest/reference/cloudformation/describe-stack-events.html