Improvement/multi az route tables

[caiovfernandes]

Add the variable multi_az_private_rtb, if set, it creates per-az private route-tables.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING.md doc.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.

[adenot]

After looking at this change, i'm not sure how this option is different from multi_nat? Multi_nat already creates multiple private route tables, one per AZ and associate them with the NAT gateway.

[caiovfernandes]

Hi @adenot,

The key difference is that the multi-nat setup creates one NATGateway per Availability Zone (AZ), which increases costs. Roughly an additional $33 per NATGateway per month, even if no data is processed through them. While this cost increase might be relatively minor, it still adds up.

By setting the variable multi_az_private_rtb, users get multi-AZ route tables to be used by a custom NATInstance solution, with a single NATGateway that should be used to failover in case one of the instances fails.