[gstg] Add new nodes for the 6th Shard to redis-cluster-ratelimiting
# Production Change
## Change Summary
This change request adds a **6th shard** (master + 2 replicas) to the existing `gstg` `redis-cluster-ratelimiting` cluster, growing it from 5 → 6 shards (15 → 18 nodes). This is the node-provisioning step only; migration of key(slot)s onto the new master is a follow-up CR (gitlab-com/gl-infra/production#22495).
Purpose: horizontally scale out `redis-cluster-ratelimiting` to add CPU/ops headroom ahead of the `Rack::Attack` → `Labkit::RateLimit` shadow rollout (both rate limiters run in parallel during comparison, roughly doubling counter operations) and expected demand growth. Adding the 5th shard previously brought per-primary CPU saturation down as expected. This is being done in `gstg` first; `gprd` will follow in a separate CR.
## Change Details
1. **Services Impacted** - ~"Service::RedisClusterRateLimiting"
2. **Change Technician** - @haseeb_ahmed
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3. **Change Reviewer** - @thisisshreya
{+DRI for the review of this change+}
4. **Scheduled Date and Time (UTC in format YYYY-MM-DD HH:MM)** - 2026-07-10 12:15
5. **Time tracking** - 60 minutes
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6. **Downtime Component** - No downtime expected
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> [!important]
>
> If your change involves scheduled maintenance, add a step to set and [unset maintenance mode](https://gitlab.com/gitlab-com/runbooks/-/blob/master/docs/monitoring/set_maintenance_window.md) per our runbooks. This will make sure SLA calculations adjust for the maintenance period.
## Preparation
> [!note]
>
> The following checklists must be done in advance, before setting the label ~"change::scheduled"
### Change Reviewer checklist
<!--To be filled out by the reviewer.-->
~C4 ~C3 ~C2 ~C1:
- [x] Check if the following applies:
- The **scheduled day and time** of execution of the change is appropriate.
- The [change plan](#detailed-steps-for-the-change) is technically accurate.
- The change plan includes **estimated timing values** based on previous testing.
- The change plan includes a viable [rollback plan](#rollback).
- The specified [metrics/monitoring dashboards](#key-metrics-to-observe) provide sufficient visibility for the change.
~C2 ~C1:
- [ ] Check if the following applies:
- The complexity of the plan is appropriate for the corresponding risk of the change. (i.e. the plan contains clear details).
- The change plan includes success measures for all steps/milestones during the execution.
- The change adequately minimizes risk within the environment/service.
- The performance implications of executing the change are well-understood and documented.
- The specified metrics/monitoring dashboards provide sufficient visibility for the change.
- If not, is it possible (or necessary) to make changes to observability platforms for added visibility?
- The change has a primary and secondary SRE with knowledge of the details available during the change window.
- The change window has been agreed with Release Managers in advance of the change. If the change is planned for APAC hours, this issue has an agreed pre-change approval.
- The labels ~"blocks deployments" and/or ~"blocks feature-flags" are applied as necessary.
### Change Technician checklist
- [x] The [Change Criticality](https://handbook.gitlab.com/handbook/engineering/infrastructure-platforms/change-management/#change-criticalities) has been set appropriately and requirements have been reviewed.
- [x] The [change plan](#detailed-steps-for-the-change) is technically accurate.
- [x] The [rollback plan](#rollback) is technically accurate and detailed enough to be executed by anyone with access.
- [x] This Change Issue is linked to the appropriate Issue and/or Epic
- [ ] Change has been tested in staging and results noted in a comment on this issue.
- [ ] A dry-run has been conducted and results noted in a comment on this issue.
- [x] The change execution window respects the [Production Change Lock periods](https://handbook.gitlab.com/handbook/engineering/infrastructure-platforms/change-management/#production-change-lock-pcl).
- [x] Once all boxes above are checked, mark the change request as scheduled: `/label ~"change::scheduled"`
- [ ] For ~C1 and ~C2 change issues, the change event is added to the [GitLab Production](https://calendar.google.com/calendar/embed?src=gitlab.com_si2ach70eb1j65cnu040m3alq0%40group.calendar.google.com) calendar by the [change-scheduler bot](https://gitlab.com/gitlab-com/gl-infra/ops-team/toolkit/change-scheduler). It is schedule to run every 2 hours.
- [ ] For ~C1 and ~C2 change issues, Platform Leadership provides approval with the ~platform_leadership_approved label on the issue.
- [ ] For ~C1, ~C2, or ~"blocks deployments" change issues, confirm with Release managers that the change does not overlap or hinder any release process.
- [ ] For ~C1 change issues or ~C2 change issues happening during weekend, SREs on-call must be informed at least 2 weeks in advance.
## Detailed steps for the change
### Pre-execution steps
> [!note]
>
> The following steps should be done right at the scheduled time of the change request.
- [x] Make sure all tasks in [Change Technician checklist](#change-technician-checklist) are done
- [x] There are currently no [active incidents](https://gitlab.com/gitlab-com/gl-infra/production/-/issues/?sort=created_date&state=opened&label_name%5B%5D=Incident%3A%3AActive&or%5Blabel_name%5D%5B%5D=severity%3A%3A1&or%5Blabel_name%5D%5B%5D=severity%3A%3A2&first_page_size=20) that are ~severity::1 or ~severity::2
- [ ] If the change involves doing maintenance on a database host, an appropriate silence targeting the host(s) should be added for the duration of the change.
- [x] Verify `rails` users are disallowed to execute `debug` command for `redis-cluster-ratelimiting`.
### Change steps - steps to take to execute the change
_Estimated Time to Complete (mins)_ - 60 minutes
Set label ~"change::in-progress" `/label ~change::in-progress`
- [x] Merge chef-repo MR creating the role for the new shard (`role[gstg-base-db-redis-cluster-ratelimiting-shard-06]`, mirrors shard-05): https://gitlab.com/gitlab-com/gl-infra/chef-repo/-/merge_requests/7651
- [x] Using Atlantis, merge the config-mgmt MR that bumps the shard count on the `redis-cluster-ratelimiting` module (`count = 5 -> 6` in `environments/gstg/redis-cluster.tf`): https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/merge_requests/14748
- Verify the atlantis plan shows **3 new** instances created — `redis-cluster-ratelimiting-shard-06-{01,02,03}-db-gstg` — and **no changes/recreation** on the existing 15 shard-01..05 instances.
- [x] Wait for successful completion of `startup-script.sh` in serial-port logs (first converge runs \~15 min/node). Confirm the zone for each node from the atlantis plan before running (expected: 06-01 → `us-east1-c`, 06-02 → `us-east1-d`, 06-03 → `us-east1-b`):
```shell
gcloud compute --project=gitlab-staging-1 instances get-serial-port-output redis-cluster-ratelimiting-shard-06-01-db-gstg --zone us-east1-c --port 1 | grep -aE 'Bootstrap finished|startup-script exit status'
gcloud compute --project=gitlab-staging-1 instances get-serial-port-output redis-cluster-ratelimiting-shard-06-02-db-gstg --zone us-east1-d --port 1 | grep -aE 'Bootstrap finished|startup-script exit status'
gcloud compute --project=gitlab-staging-1 instances get-serial-port-output redis-cluster-ratelimiting-shard-06-03-db-gstg --zone us-east1-b --port 1 | grep -aE 'Bootstrap finished|startup-script exit status'
```
- [x] Verify Chef converge completed (Roles/Recipes populated, Cinc version is `16.18.30`):
```shell
knife node show redis-cluster-ratelimiting-shard-06-01-db-gstg.c.gitlab-staging-1.internal
knife node show redis-cluster-ratelimiting-shard-06-02-db-gstg.c.gitlab-staging-1.internal
knife node show redis-cluster-ratelimiting-shard-06-03-db-gstg.c.gitlab-staging-1.internal
knife node show redis-cluster-ratelimiting-shard-06-01-db-gstg.c.gitlab-staging-1.internal -a chef_packages.chef.version # expect 16.18.30
```
- [x] Verify SSH access works (i.e. `gitlab_users` recipe ran):
```shell
ssh redis-cluster-ratelimiting-shard-06-01-db-gstg.c.gitlab-staging-1.internal
```
- [x] Add the new master (`shard-06-01`) and its replicas to the redis cluster via `redis-cli` on the first node in the cluster:
```shell
ssh redis-cluster-ratelimiting-shard-01-01-db-gstg.c.gitlab-staging-1.internal
export ENV=gstg
export PROJECT=gitlab-staging-1
export DEPLOYMENT=redis-cluster-ratelimiting
# This adds the master node
sudo gitlab-redis-cli --cluster add-node \
$DEPLOYMENT-shard-06-01-db-$ENV.c.$PROJECT.internal:6379 \
$DEPLOYMENT-shard-01-01-db-$ENV.c.$PROJECT.internal:6379
# This adds the replica nodes
for j in {02,03}; do
node_id="$(sudo gitlab-redis-cli cluster nodes | grep $DEPLOYMENT-shard-06-01-db-$ENV.c.$PROJECT.internal | awk '{ print $1 }')";
sudo gitlab-redis-cli --cluster add-node \
$DEPLOYMENT-shard-06-$j-db-$ENV.c.$PROJECT.internal:6379 \
$DEPLOYMENT-shard-01-01-db-$ENV.c.$PROJECT.internal:6379 \
--cluster-slave --cluster-master-id $node_id
done
```
- [ ] Verify cluster health after the add: `cluster_state:ok`, `cluster_slots_fail:0`, and `cluster_known_nodes` grew by 3 (15 → 18):
```shell
sudo gitlab-redis-cli cluster info | grep -E 'cluster_state|cluster_slots_fail|cluster_known_nodes|cluster_size'
sudo gitlab-redis-cli cluster nodes | grep shard-06 # master shows in cluster; replicas show "slave <master-id> ... connected"
```
- [ ] Set label ~"change::complete" `/label ~change::complete`
## Rollback
### Rollback steps - steps to be taken in the event of a need to rollback this change
_Estimated Time to Complete (mins)_ - 30 minutes
- [ ] At this stage the newly added nodes hold no keyslots, so no data-path rollback is required.
- [ ] If a node misbehaves, remove it from the cluster with `sudo gitlab-redis-cli --cluster del-node <first-node>:6379 <node-id>` (replicas first, then the empty master).
- [ ] The config-mgmt (`count = 6 -> 5`) and chef-repo role MRs can be reverted to de-provision the instances.
- [ ] Set label ~"change::aborted" `/label ~change::aborted`
## Monitoring
### Key metrics to observe
- Metric: redis-cluster-ratelimiting Apdex and RPS
- Location: [Redis-cluster-ratelimiting dashboard (gstg)](https://dashboards.gitlab.net/d/redis-cluster-ratelimiting-main/redis-cluster-ratelimiting3a-overview?orgId=1&var-PROMETHEUS_DS=mimir-gitlab-gstg&var-environment=gstg&var-shard=$__all&from=now-6h%2Fm&to=now%2Fm&timezone=utc)
- What changes to this metric should prompt a rollback: Degradation in Apdex for over 5 minutes
- Metric: Redis Cluster Slots Failed
- Location: [Redis cluster slots failed view (gstg)](https://dashboards.gitlab.net/d/redis-cluster-ratelimiting-main/redis-cluster-ratelimiting3a-overview?orgId=1&var-PROMETHEUS_DS=mimir-gitlab-gstg&var-environment=gstg&var-shard=$__all&from=now-6h%2Fm&to=now%2Fm&timezone=utc&viewPanel=panel-102)
- What changes to this metric should prompt a rollback: Non-zero `Redis Cluster Slots Failed`
issue
GitLab AI Context
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