// Module included in the following assemblies:
//
// assembly-cruise-control-concepts.adoc

[id='con-optimization-proposals-{context}']

= Optimization proposals overview

An _optimization proposal_ is a summary of proposed changes that would produce a more balanced Kafka cluster, with partition workloads distributed more evenly among the brokers. 
Each optimization proposal is based on the set of xref:con-optimization-goals-{context}[optimization goals] that were used to generate it.

Use the summary (shown in the `status` of the `KafkaRebalance` resource) to decide whether to:

* Approve the optimization proposal. This instructs Cruise Control to apply the proposal to the Kafka cluster and start a cluster rebalance operation.

* Reject the optimization proposal. You can change the optimization goals and then generate another proposal.

NOTE: All optimization proposals are dry runs: you cannot approve a cluster rebalance without first generating an optimization proposal. 
There is no limit to the number of optimization proposals that can be generated.

[discrete]
== Cached optimization proposal

Cruise Control maintains a _cached optimization proposal_ based on the configured default optimization goals.
Generated from the workload model, the cached optimization proposal is updated every 15 minutes to reflect the current state of the Kafka cluster.
If you generate an optimization proposal using the default optimization goals, Cruise Control returns the most recent cached proposal.

To change the cached optimization proposal refresh interval, edit the `proposal.expiration.ms` setting in the Cruise Control deployment configuration.
Consider a shorter interval for fast changing clusters, although this increases the load on the Cruise Control server.

[#contents-optimization-proposals]
[discrete]
== Contents of optimization proposals

The following table explains the properties contained in an optimization proposal:

[cols="35,65",options="header",stripes="none",separator=¦]
|===

m¦JSON property
¦Description

m¦numIntraBrokerReplicaMovements
¦The total number of partition replicas that will be transferred between the disks of the cluster's brokers.

*Performance impact during rebalance operation*: Relatively high, but lower than `numReplicaMovements`.

m¦excludedBrokersForLeadership
¦Not yet supported. An empty list is returned.

m¦numReplicaMovements
¦The number of partition replicas that will be moved between separate brokers.

*Performance impact during rebalance operation*: Relatively high.

m¦onDemandBalancednessScoreBefore, onDemandBalancednessScoreAfter
¦A measurement of the overall _balancedness_ of a Kafka Cluster, before and after the optimization proposal was generated.

The calculation is 100 minus the sum of the `BalancednessScore` of each violated soft goal. Cruise Control assigns a `BalancednessScore` to every optimization goal based on several factors, including priority (the goal's position in the list of `default.goals` or user-provided goals).

The `Before` score is based on the current configuration of the Kafka cluster.
The `After` score is based on the generated optimization proposal.

m¦intraBrokerDataToMoveMB
¦The sum of the size of each partition replica that will be moved between disks on the same broker (see also `numIntraBrokerReplicaMovements`).

*Performance impact during rebalance operation*: Variable. The larger the number, the longer the cluster rebalance will take to complete. Moving a large amount of data between disks on the same broker has less impact than between separate brokers (see `dataToMoveMB`).

m¦recentWindows
¦The number of metrics windows upon which the optimization proposal is based.

m¦dataToMoveMB
¦The sum of the size of each partition replica that will be moved to a separate broker (see also `numReplicaMovements`).

*Performance impact during rebalance operation*: Variable. The larger the number, the longer the cluster rebalance will take to complete.

m¦monitoredPartitionsPercentage
¦The percentage of partitions in the Kafka cluster covered by the optimization proposal. Affected by the number of `excludedTopics`.

m¦excludedTopics
¦Not yet supported. An empty list is returned.

m¦numLeaderMovements
¦The number of partitions whose leaders will be switched to different replicas. This involves a change to ZooKeeper configuration.

*Performance impact during rebalance operation*: Relatively low.

m¦excludedBrokersForReplicaMove
¦Not yet supported. An empty list is returned.

|===

.Additional resources

* xref:con-optimization-goals-{context}[] 

* xref:proc-generating-optimization-proposals-{context}[] 

* xref:proc-approving-optimization-proposal-{context}[] 