Cruise control rebalancing behavior during new broker addition and partition movement to new broker

[kspmmitt]

What Does "Existing Partitions Are Moved to the New Brokers" Mean?
In Kafka, each topic is divided into partitions, and those partitions are assigned as replicas to one or more brokers.

When you add new brokers to the cluster, those brokers start empty — they host no partitions by default.

So to use their capacity, Cruise Control will:

Move replicas of existing partitions (from overloaded brokers)

Assign them to the new brokers

This spreads the load — especially disk usage, CPU, and network I/O — more evenly.

How Cruise Control Moves Partitions to New Brokers
Partition Movement = Replica Reassignment
Let’s say:

Topic orders has 6 partitions

Partition orders-3 currently has replicas on Brokers 101, 102, and 103

You added new Broker 104

Cruise Control might propose this:

Move one replica of orders-3 from 101 to 104

So now the new assignment is: orders-3 → [104, 102, 103]

Behind the Scenes: What Happens
Copying the data:
Kafka starts replicating the partition data from one of the existing replicas to the new broker (104).

Syncing to ISR:
Cruise Control waits until the new replica on 104 is in sync (caught up with leaders).

Removing the old replica:
Once synced, Cruise Control removes the replica from the old broker (101) — now that data has "moved".

This is seamless, and Kafka keeps serving reads/writes during this operation.

Key Points
No partition is deleted — just replica locations change

[kyguy]

Hi @kspmmitt, was there a question you wanted to discuss here? Or was this posted accidentally?

[kspmmitt]

This is question only and need confirmation for cruise control behavior w.r.t scenario explained.

[kyguy]

Yes, this is generally correct as well.