使用官方CloudFormation模板在EC2中缩减MarkLogic集群节点（无数据丢失）的方案咨询

I've run into this exact scenario before with MarkLogic's CloudFormation templates—they don't support direct scaling down via stack updates out of the box because of underlying cluster dependencies and network resource cleanup. Here's a step-by-step, safe approach to scale your cluster back to 3 nodes without losing data:

Step 1: Prepare the MarkLogic Cluster (Critical for Data Safety) #

Before touching CloudFormation, you need to properly remove the target node from the MarkLogic cluster itself to protect your data:

• Identify the node to remove: Log into your MarkLogic Admin interface, go to the Hosts page, and note the hostname/IP of the 4th node you added.
• Migrate all Forests off the target node:
  • For every database (including the 2 new ones you created), navigate to its Forests configuration.
  • For each Forest hosted on the target node, click Modify and reassign its replicas to the remaining 3 nodes. Ensure each Forest has at least one active replica on a node you're keeping.
  • Wait for the Forest migration to complete—check the Status column on the Forests page to confirm all are marked Open and no rebalancing tasks are running.
• Remove the node from the cluster: Return to the Hosts page, select the target node, click Remove, and confirm the action. Let the cluster finish the removal process (this updates cluster metadata and ensures no pending tasks are tied to the node).

Step 2: Clean Up Unused Network Interfaces (Fix the CloudFormation Error) #

The error message explicitly calls out unused network interfaces—here's how to resolve that:

• Go to the AWS EC2 Console, locate the EC2 instance for the node you just removed from MarkLogic.
• Navigate to the Network Interfaces tab for that instance, or search for network interfaces tagged with your CloudFormation stack name.
• Detach any unused network interfaces associated with the node, then delete them. Double-check that these interfaces aren't attached to any other resources before deleting.

Step 3: Update the CloudFormation Stack to Scale Down #

Now you can safely adjust the node count in CloudFormation:

• Open the CloudFormation Console, select your stack, and click Update Stack.
• Choose to use the existing template (or re-upload the same official template if needed), then update the NodesPerZone parameter to 3.
• Proceed with the stack update—this time it should succeed, as the cluster and network resources are now properly prepared for the node removal.

Key Notes to Avoid Issues #

• Backup first: Always take a full backup of your MarkLogic databases before performing any scaling operations. Better safe than sorry!
• Verify Forest health: After migrating Forests, confirm that all data is accessible and no Forests are in a degraded state.
• Don't skip manual steps: The CloudFormation custom resource enforces these checks because scaling down a MarkLogic cluster requires more than just terminating an EC2 instance—you have to preserve data integrity first.

内容的提问来源于stack exchange，提问作者Vikram