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Currently (March 2021) the Datomic query group CF template has the DesiredCapacity parameter marked as required. Because of this, during a CF template update, CF will always set the DesiredCapacity to the CF parameter for DesiredCapacity. This behavior does not work well in the following situation.

In the query group template I have the DesiredCapacity at 2, and MaxCount at 4. I have an ASG scaling policy to target track 50% average CPU utilization. My query group is currently at the max, because the ASG scaling policy scaled up the group DesiredCapacity to 4 (to match my load). My load has increased to the point where 4 nodes is no longer sufficient. I must update the query group CF template to increase the MacCount. The problem is that when you update the MacCount from 4 to 6 via a CF template update and leave the DesiredCapacity at 2, CF will actually set the DesiredCapacity on the ASG to 2 during the high load situation. This worsens the high load situation.

One option is to query the ASG for its current DesiredCapacity prior to running a CF update and set the CF parameter to the value you retrieved. This approach complicates the update procedure. It also has a "read-modify-write" race condition -- the DesiredCapacity could have changed from the time you read the DesiredCapacity to the time you are performing the CF update.

A second option is to make the DesiredCapacity parameter optional. As per the CF docs, DesiredCapacity is not a required parameter. When it is not present, it defaults to the minimum size of the group. Leaving DesiredCapacity unset will make it so during a CF update, CF will not modify the DesiredCapacity at all -- leaving the ASG "in control."

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