This is for internal use by the PaaS team. Public-facing documentation is located at docs.cloud.service.gov.uk.

How to disable a single AZ on GOV.UK PaaS

There are scenarios, where we will need to manually disable a single AZ on AWS, prompting the cells, traffic, scheduler and auctioneer to redistribute the applications across the remaining AZs.

Scenario like this may be when we see an AZ being flappy in network connectivity terms.

We’ve developed a way, to accomplish that, with either your dev machine and Terraform, or simply running the appropriate Concourse job.

The paas-cf repository contains a terraform module, which will deny all traffic, to subnets in one particular AZ.

It also has a Concourse job, in create-cloudfoundry pipeline, and the operator groups, to disable and enable particular AZs.

Please note: Disabling the AZ a, will likely bring the Concourse down, meaning the re-enabling action will need to be done by hand from op’s machine.

This will involve you having to download the Terraform state file for that particular region. See the pipeline for more details.

How to run

This is accomplished with Terraform. It is designed, to interacively ask for the values it needs, so you don’t have to remember to set them at all times.

The values it is asking for are: - AZ - eu-west-1a, eu-west-1b, eu-west-1c - Region - eu-west-1 - VPC ID - vpc-1234567890

The following will disable an AZ:

terraform apply

The following will re-enable an AZ:

terraform destroy