fix: Specify the number of read replicas as one less that total (#190)

[emillbrandt]

Description

replica_scale_min and replica_count are used incorrectly as the total cluster node count.

Motivation and Context

#190
Currently setting read replicas to 0 deletes the writer instance.
Once extra auto scaling read replicas gets created with the auto-scaling policy, wasting money.

Breaking Changes

None.

How Has This Been Tested?

Created a cluster with zero read replicas, one writer and zero read replicas were created
Created a cluster with one read replicas, one writer and one read replicas were created
Created a cluster with two read replicas, one writer and two read replicas were created

[svenlito]

@emillbrandt Hey, thank you for the PR! Would you mind resolving the conflict here?

[antonbabenko]

This is a breaking change to my mind.

I wonder how bad is this breaking change for users who are using this module already. If this PR is merged they will suddenly have 1 more instance, right? This is not what people expect.

@bryantbiggs What do you think?

[emillbrandt]

@svenlito Done

[bryantbiggs]

yes I agree @antonbabenko - this would fall under breaking change I think since it will create 1 additional instance in everyones cluster. its not as severe, but still a potentially disruptive change. maybe we hold off for a bit, there are a few things that we know of that we would like to fix at the next breaking change (i.e. -

terraform-aws-rds-aurora/main.tf

Line 10 in a109e82

# TODO - remove coalesce() at next breaking change - adding existing name as fallback to maintain backwards compatibility

, maybe set min TF to 0.13.x, change some variable wording, etc.)

[antonbabenko]

Agree, let's put this on hold and make a bigger release with the changes you just described.

[github-actions]

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