[gstg] Scale out `redis-cluster-ratelimiting` by resharding the keys to 6th shard
# Production Change
## Change Summary
We are horizontally scaling the `gstg` `redis-cluster-ratelimiting` cluster by moving keyslots from the existing 5 masters onto the newly added shard-06 master, so the cluster ends up balanced across **6 masters (\~2731 slots each)**.
Prerequisite: the shard-06 nodes were provisioned and joined to the cluster in gitlab-com/gl-infra/production#22494, with shard-06-01 a master holding 0 slots.
Target: `16384 / 6 ≈ 2731` slots per master, so move **\~546 slots from each of the 5 existing masters** (5 × 546 = 2730) onto shard-06. This mirrors the validated 5th-shard reshard approach (#22248), adjusted for 5 source masters instead of 4.
Purpose: horizontally scale out `redis-cluster-ratelimiting` to add CPU/ops headroom ahead of the `Rack::Attack` → `Labkit::RateLimit` shadow rollout and expected demand growth. `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 15:15
5. **Time tracking** - 150 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.
### Change steps - steps to take to execute the change
_Estimated Time to Complete (mins)_ - 150 minutes
- [x] Set label ~"change::in-progress" `/label ~change::in-progress`
- [x] Confirm the prerequisite node-add CR #22494 is complete: shard-06 nodes joined, shard-06-01 is a master with **0 slots**.
- [x] Capture the current master node IDs and the new shard-06 master ID, and export them. **Re-validate at execution** — a failover changes which node in a shard is master.
```shell
ssh redis-cluster-ratelimiting-shard-01-01-db-gstg.c.gitlab-staging-1.internal
# List current masters (six total: five WITH slot ranges, plus shard-06-01 with 0 slots)
sudo gitlab-redis-cli cluster nodes | grep master
# Export the FIVE existing masters' IDs (the ones WITH slot ranges):
export M1=<id of existing master 1>
export M2=<id of existing master 2>
export M3=<id of existing master 3>
export M4=<id of existing master 4>
export M5=<id of existing master 5>
# Export the shard-06 master ID and VALIDATE it: must resolve to shard-06-01, flagged "master", with NO slot range (0 slots)
export SHARD06_MASTER_ID=<id of shard-06-01>
sudo gitlab-redis-cli cluster nodes | grep "$SHARD06_MASTER_ID"
# expect: ...redis-cluster-ratelimiting-shard-06-01-db-gstg... master ... (no slot range)
```
- [x] **Batch 1** — move \~546 slots from master 1, starting conservative at `--cluster-pipeline 50`. Observe CPU/memory/error-ratio/redirection metrics; interrupt (ctrl-C) and re-adjust slots/pipeline if it competes with user ops.
```shell
time sudo gitlab-redis-cli --cluster reshard 127.0.0.1:6379 --cluster-from $M1 --cluster-to "$SHARD06_MASTER_ID" --cluster-slots 546 --cluster-pipeline 50 --cluster-yes
```
- [ ] (Optional) If the cluster state breaks mid-migration (a redis process dies or a batch is interrupted), recover before continuing:
```shell
sudo gitlab-redis-cli --cluster fix 127.0.0.1:6379
```
- [x] **Batch 2** — move \~546 slots from master 2 (pipeline can increase, e.g. 100, if batch 1 was clean). Observe metrics.
```shell
time sudo gitlab-redis-cli --cluster reshard 127.0.0.1:6379 --cluster-from $M2 --cluster-to "$SHARD06_MASTER_ID" --cluster-slots 546 --cluster-pipeline 100 --cluster-yes
```
- [x] **Batch 3** — move \~546 slots from master 3. Observe metrics.
```shell
time sudo gitlab-redis-cli --cluster reshard 127.0.0.1:6379 --cluster-from $M3 --cluster-to "$SHARD06_MASTER_ID" --cluster-slots 546 --cluster-pipeline 100 --cluster-yes
```
- [x] **Batch 4** — move \~546 slots from master 4. Observe metrics.
```shell
time sudo gitlab-redis-cli --cluster reshard 127.0.0.1:6379 --cluster-from $M4 --cluster-to "$SHARD06_MASTER_ID" --cluster-slots 546 --cluster-pipeline 100 --cluster-yes
```
- [x] **Batch 5** — move the remainder from master 5 so shard-06 lands at \~2731 total. Set `--cluster-slots` to whatever brings shard-06's total to \~2731 (546 if the first four batches each moved exactly 546). Observe metrics.
```shell
time sudo gitlab-redis-cli --cluster reshard 127.0.0.1:6379 --cluster-from $M5 --cluster-to "$SHARD06_MASTER_ID" --cluster-slots 546 --cluster-pipeline 100 --cluster-yes
```
- [x] Recheck that all 16,384 slots are covered and the 6 masters are balanced at \~2731 slots each:
```shell
sudo gitlab-redis-cli cluster info | grep -E 'cluster_state|cluster_slots_ok|cluster_slots_fail'
sudo gitlab-redis-cli --cluster check 127.0.0.1:6379
```
- [ ] Set label ~"change::complete" `/label ~change::complete`
## Rollback
### When to roll back
Resharding is online and reversible, so **first try to mitigate before rolling back** - most issues during a reshard are transient and clear without reversing the migration:
- **Pause first.** If a batch is competing with user operations, interrupt it (ctrl-C) and re-tune (`--cluster-slots` / `--cluster-pipeline` lower), then resume. Do not immediately roll back.
- **Try `--cluster fix`.** For a broken cluster state, stuck migrating/importing slots, or persistent client redirections after a batch completes, run `sudo gitlab-redis-cli --cluster fix 127.0.0.1:6379` first - this resolves most cases without a rollback.
**Roll back (move the moved slots back to their source masters) if, after the mitigations above, any of the following persist:**
- **`cluster_slots_fail` stays non-zero** (slots uncovered/failed) and `--cluster fix` does not clear it — the keyspace is not fully served, so reverse the migration.
- **Sustained redis error-ratio increase or apdex degradation** attributable to the change (e.g. apdex below SLO for \>5 min), not recovering when migration is paused.
- **Set/Get op rate drops, or operations start erroring/timing out**, and it does not recover on pause.
- **Persistent client redirections** that remain after a batch completes and after `--cluster fix`.
- **A redis process crash/failover mid-migration** that leaves the cluster in a state `--cluster fix` cannot recover.
- **Source or destination primary CPU approaches single-core saturation** and drives user-facing latency that re-tuning/pausing does not relieve.
If the issue is isolated to the current batch and the cluster is otherwise healthy, it is safe to simply **stop** (leave the slots already moved in place) rather than fully reverse - the cluster remains valid at any intermediate balance. Full rollback is for returning to the pre-change 5-master state when the change must be abandoned.
### Rollback steps - steps to be taken in the event of a need to rollback this change
_Estimated Time to Complete (mins)_ - 120 minutes
> [!note]
>
> `SHARD06_MASTER_ID` and `M1..M5` must be exported in the shell (see the [Change steps](#change-steps---steps-to-take-to-execute-the-change) for how to determine them) before running the rollback. Each rollback batch moves slots **from** the shard-06 master back **to** the master they came from, in reverse order (5 → 1), returning the cluster to its pre-change 5-master balance.
>
> **Partial migration:** if you abort before all 5 forward batches completed, only run the reverse batches for the ones that actually ran. `$SHARD06_MASTER_ID` only holds the slots moved so far (\~546 per completed batch), so running extra reverse batches will fail or pull slots from the wrong source. Check the current count first with `sudo gitlab-redis-cli cluster nodes | grep "$SHARD06_MASTER_ID"`.
- [ ] Run migrations in reverse order:
```shell
ssh redis-cluster-ratelimiting-shard-01-01-db-gstg.c.gitlab-staging-1.internal
# Rollback batch 5: shard-06 -> master 5
sudo gitlab-redis-cli --cluster reshard 127.0.0.1:6379 --cluster-from "$SHARD06_MASTER_ID" --cluster-to $M5 --cluster-slots 546 --cluster-pipeline 50 --cluster-yes
# Rollback batch 4: shard-06 -> master 4
sudo gitlab-redis-cli --cluster reshard 127.0.0.1:6379 --cluster-from "$SHARD06_MASTER_ID" --cluster-to $M4 --cluster-slots 546 --cluster-pipeline 50 --cluster-yes
# Rollback batch 3: shard-06 -> master 3
sudo gitlab-redis-cli --cluster reshard 127.0.0.1:6379 --cluster-from "$SHARD06_MASTER_ID" --cluster-to $M3 --cluster-slots 546 --cluster-pipeline 50 --cluster-yes
# Rollback batch 2: shard-06 -> master 2
sudo gitlab-redis-cli --cluster reshard 127.0.0.1:6379 --cluster-from "$SHARD06_MASTER_ID" --cluster-to $M2 --cluster-slots 546 --cluster-pipeline 50 --cluster-yes
# Rollback batch 1: shard-06 -> master 1
sudo gitlab-redis-cli --cluster reshard 127.0.0.1:6379 --cluster-from "$SHARD06_MASTER_ID" --cluster-to $M1 --cluster-slots 546 --cluster-pipeline 50 --cluster-yes
```
- [ ] Set label ~"change::aborted" `/label ~change::aborted`
## Monitoring
### Key metrics to observe
- Metric: Redis service error ratio
- Location: [view (gstg)](https://dashboards.gitlab.net/d/redis-cluster-ratelimiting-main/redis-cluster-ratelimiting3a-overview?orgId=1&var-PROMETHEUS_DS=mimir-gitlab-gstg&var-environment=gstg&var-shard=$__all&viewPanel=3422679610&from=now-6h%2Fm&to=now%2Fm&timezone=utc)
- What changes to this metric should prompt a rollback: Sustained increase in the error ratio of redis server
- Metric: Redis Cluster Slots Failed
- Location: [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=$__all&from=now-6h%2Fm&to=now%2Fm&timezone=utc&viewPanel=panel-102)
- What changes to this metric should prompt a rollback: A non-zero failed slots
Metric: Set/Get commands issued to the cluster
* Location: [explore view](https://dashboards.gitlab.net/explore?schemaVersion=1&panes=%7B%22tl9%22:%7B%22datasource%22:%22mimir-gitlab-gstg%22,%22queries%22:%5B%7B%22refId%22:%22A%22,%22expr%22:%22sum%20by%20%28cmd%29%20%28rate%28redis_commands_total%7Benvironment%3D%5C%22gstg%5C%22,%20type%3D%5C%22redis-cluster-ratelimiting%5C%22,%20cmd%3D~%5C%22set%7Cget%5C%22%7D%5B$__rate_interval%5D%29%29%22,%22range%22:true,%22instant%22:true,%22datasource%22:%7B%22type%22:%22prometheus%22,%22uid%22:%22mimir-gitlab-gstg%22%7D,%22editorMode%22:%22code%22,%22legendFormat%22:%22__auto%22%7D%5D,%22range%22:%7B%22from%22:%22now-6h%22,%22to%22:%22now%22%7D,%22compact%22:false%7D%7D&orgId=1)
* What changes to this metric should prompt a rollback: A drop in the set/get operation rate, or operations starting to error/time out
Metric: Redirections for the clients
- Location: [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-2d&to=now&timezone=utc&viewPanel=panel-104)
- What changes to this metric should prompt a rollback: Persistent redirections after a batch completes. First try `sudo gitlab-redis-cli --cluster fix 127.0.0.1:6379` to resolve.
- Redis logs: https://nonprod-log.gitlab.net/app/r/s/TxVaz
issue
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