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The plan is fundamentally sound, but there are some critical risks and best practices you should consider to ensure zero downtime and data safety during the transition. 1. Decommissioning (Nodes 12 $\rightarrow$ 8) Using

ALTER SYSTEM DECOMMISSION BACKEND

is the correct approach as it triggers an asynchronous migration of all tablets to the remaining nodes. * Concurrency Warning: StarRocks documentation generally recommends decommissioning nodes in small batches (no more than 2 at a time) to avoid overwhelming the remaining nodes with migration traffic and to maintain query performance. For 4 nodes, I recommend decommissioning 2, waiting for them to finish, and then decommissioning the next 2. * Check Replicas: Ensure that your data's

replication_num

(default is 3) is less than the remaining 8 nodes. If you have any tables with a high replication factor (e.g., 8+), decommissioning will hang or fail. * Check for Single-Replica Tables: If any tables have a

replication_num

of 1, the decommission process may fail if the target nodes cannot accommodate the data immediately. 2. Monitoring the Decommission * Verification: Don't just wait for them to disappear. Use

SHOW BACKENDS;

and check the

SystemDecommissioned

column. Once the node is gone from the list, StarRocks has safely moved all data. * Tablet Health: It is a good idea to run

SHOW PROC '/statistic';

to ensure there are no "Unhealthy" tablets before proceeding to the Terraform phase. 3. Terraform & Instance Upgrades (The Risky Part) Your plan to

terraform apply

to upgrade the instance types of the remaining 8 nodes is the most dangerous step. * Avoid Simultaneous Restarts: Most cloud providers (AWS/GCP/Azure) require a stop/start or a replacement to change an instance type. If your Terraform configuration applies this change to all 8 nodes at once, your entire cluster will go offline. * Recommended Approach: 1. Do not let Terraform update all 8 nodes at once. 2. Either use a Terraform module that supports rolling updates (e.g., an AWS Auto Scaling Group with an Instance Refresh) or—more commonly—update the instance type manually one by one. 3. For each node: Stop the BE $\rightarrow$ Change Instance Type $\rightarrow$ Start BE $\rightarrow$ Wait for it to show as

Alive

in

SHOW BACKENDS

$\rightarrow$ Move to the next. * IP Pinning: Pinning private IPs is excellent. StarRocks uses the IP/Port as the unique identifier for BEs. If the IP changes, the FE will treat it as a new, empty node. 4. Refined Plan Strategy 1. Phase 1 (Scale In): Decommission 2 BEs, wait for completion. Decommission the next 2 BEs, wait for completion. 2. Phase 2 (Clean State): Run

terraform apply

to only remove the 4 decommissioned nodes from the state/infrastructure. 3. Phase 3 (Vertical Upgrade): Perform a rolling upgrade of the instance types for the 8 remaining nodes. If using Terraform for this, do it one node (or one small group) at a time by updating variables/targets specifically, or perform it manually to ensure the cluster stays up. Summary Checklist * [ ]

SHOW BACKENDS

shows all remaining nodes are healthy. * [ ]

SHOW PROC '/statistic'

shows 0 unhealthy tablets. * [ ] Verify

replication_num

on all tables is $\le 8$. * [ ] Ensure Terraform won't reboot all 8 nodes simultaneously. References • administration/management/Scale_up_down.md • deployment/plan_cluster.md • knowledge/usage/best-practice.md • best_practices/query_tuning/schema_tuning.md • deployment/upgrade.md