docs: ✏️ instance and node group changes

runbooks/source/node-group-changes.html.md.erb

@@ -0,0 +1,33 @@
+---
+title: Handling Node Group and Instance Changes
+weight: 54
+last_reviewed_on: 2023-13-12
+review_in: 6 months
+---
+
+# <%= current_page.data.title %>
+
+## Why?
+
+You may need to make a change to an EKS [cluster node group] or [instance type config]. We can't just let terraform apply these changes because terraform doesn't gracefully rollout the old and new nodes. Terraform will bring down all of the old nodes immediately, which will cause outages to users.
+
+## How?
+
+The method to avoid bringing down all the nodes at once is to follow these steps:
+
+1. add a new node group with your [updated changes]
+1. lookup the old node group name (you can find this in the aws gui)
+1. once merged in you can drain the old node group using the following command:
+> cloud-platform pipeline cordon-and-drain --cluster-name <cluster_name> --node-group <old_node_group_name>
+1. raise a new [pr deleting] the old node group
+
+notes:
+
+- When making changes to the default node group in live, it's handy to pause the pipelines for each of our environments for the duration of the change.
+- the `cloud-platform pipeline` command [cordons and drains nodes] in a given node group waiting 2mins between each drained node. This command runs remotely in concourse.
+
+[cluster node group]: https://github.com/ministryofjustice/cloud-platform-infrastructure/blob/97768bfd8b4e25df6f415035acac60cf531d88c1/terraform/aws-accounts/cloud-platform-aws/vpc/eks/cluster.tf#L60
+[instance type config]: https://github.com/ministryofjustice/cloud-platform-infrastructure/blob/97768bfd8b4e25df6f415035acac60cf531d88c1/terraform/aws-accounts/cloud-platform-aws/vpc/eks/cluster.tf#L43
+[pr deleting]: https://github.com/ministryofjustice/cloud-platform-infrastructure/pull/2663
+[updated changes]: https://github.com/ministryofjustice/cloud-platform-infrastructure/pull/2657
+[cordons and drains nodes]: https://github.com/ministryofjustice/cloud-platform-terraform-concourse/blob/main/pipelines/manager/main/cordon-and-drain-nodes.yaml