feat(demo): Auto Balancing on Scale-Down

[daniel-y]

Task

Create a demo to showcase automatic rebalancing when scaling down the cluster.

Background

When a broker is decommissioned or fails (scale-down), its partitions must be safely and automatically migrated to the remaining active brokers to ensure data availability and distribute the load. This demo will illustrate this process.

Proposed Solution / Demo Plan

1. Environment Setup:

   • Create a docker-compose.yml that starts a 3-node AutoMQ cluster.

2. Initial State:

   • Start the 3-node cluster and produce a balanced load to a topic with several partitions.
   • Instruct the user to verify that partitions are distributed across all three brokers.

3. Scale-Down Event:

   • Instruct the user to simulate a node failure or decommissioning by stopping one of the broker containers: docker compose stop automq-broker-3.

4. Observation and Verification:

   • Allow the auto-balancer and cluster controller a few minutes to react.
   • Instruct the user to re-check the partition distribution.
   • They should observe that the partitions formerly on the stopped broker have been successfully migrated to the two remaining active brokers.

Acceptance Criteria

• A runnable docker-compose.yml is provided in auto-balancing-demos/scale-down/.
• The README.md provides a clear, step-by-step guide.
• The guide explains how to start a 3-node cluster, stop one of the nodes, and verify that its partitions are automatically migrated to the remaining nodes.