CloudFormation to detach root device from stopped instance - aws-cloudformation

Question

My actual task is:
Create AMI outside CloudFormation
Use CloudFormation to launch instance from AMI
Detach all existing volumes (including root and data volumes)
Attach the new volumes (root and data) which are created from latest snapshot.
I am stuck at building CloudFormation script to detach volumes from the instance.
Any suggestions or any one written CF script to detach the volumes (root & data)?

Answer 1

Here's one way you can approach this:
Create an AMI outside of AWS CloudFormation
Use AWS CloudFormation to launch an instance from that AMI
Create a Lambda-backed custom resource that takes a reference of the instance that was just launched as input, uses the AWS SDK to find out the volumes that are currently attached to the EC2 instance, and detach them one by one.
Attach whatever volumes you want to attach using resource of type AWS::EC2::VolumeAttachment
Key takeaway: If you want to perform a custom operation that CloudFormation doesn't natively support, try Lambda-backed custom resources.

Answer 2

Using the BlockDeviceMappings of EC2Instance you can specify all the EBS volumes for the instance. One of the EBS properties is SnapshotId. If that changes the volumes will be replaced on update. Just pass in the snapshotId as a parameter.
Example:
{
"Type": "AWS::EC2::Instance",
"Properties": {
"BlockDeviceMappings": [
{
"DeviceName": "/dev/sda1",
"Ebs" : { "SnapshotId" : {"Ref": "Snapshot1Id"} }
},
{
"DeviceName": "/dev/sda2",
"Ebs" : { "SnapshotId" : {"Ref": "Snapshot2Id"} }
},
{
"DeviceName": "/dev/sda3",
"Ebs" : { "SnapshotId" : {"Ref": "Snapshot3Id"} }
}
],
"ImageId": { "Ref": "InstanceAmi" }
}