Allow NAT instance launch before terminating #167
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This change makes it possible to allow new NAT instances to launch before terminating old ones by setting the ASG instance maintenance policy.
With
allow_launch_before_terminatingenabled the routes will only be replaced once (directly from old to new) when instances are refreshed. This behavior is also a prerequisite for syncing conntrack tables as mentioned in #67.As two instances will briefly run concurrently a spare Elastic IP is needed for the new instance to launch. I refactored the existing Elastic IP calculations, and added a validation for the existing
nat_instance_eip_idsvariable in the form of a precondition block. This does require Terraform v1.2 or later, which I think should be fine after more than three years.Depends on #169.