[KAFKA-1792] change behavior of --generate to produce assignment config with fair replica distribution and minimal number of reassignments

• Type: Sub-task
• Status: Patch Available
• Priority: Major
• Resolution: Unresolved
• Component/s: tools
• Assignee: Dmitry Pekar
• Reporter: Dmitry Pekar
• Reviewer: Neha Narkhede
• Created: 21/Nov/14 13:04
• Updated: 15/Sep/15 07:32

Description

Current implementation produces fair replica distribution between specified list of brokers. Unfortunately, it doesn't take
into account current replica assignment.

So if we have, for instance, 3 brokers id=[0..2] and are going to add fourth broker id=3,
generate will create an assignment config which will redistribute replicas fairly across brokers [0..3]
in the same way as those partitions were created from scratch. It will not take into consideration current replica
assignment and accordingly will not try to minimize number of replica moves between brokers.

As proposed by charmalloc this should be improved. New output of improved --generate algorithm should suite following requirements:

• fairness of replica distribution - every broker will have R or R+1 replicas assigned;
• minimum of reassignments - number of replica moves between brokers will be minimal;

Example.
Consider following replica distribution per brokers [0..3] (we just added brokers 2 and 3):

• broker - 0, 1, 2, 3
• replicas - 7, 6, 0, 0

The new algorithm will produce following assignment:

• broker - 0, 1, 2, 3
• replicas - 4, 3, 3, 3
• moves - -3, -3, +3, +3

It will be fair and number of moves will be 6, which is minimal for specified initial distribution.

The scope of this issue is:

• design an algorithm matching the above requirements;
• implement this algorithm and unit tests;
• test it manually using different initial assignments;

Issue Links

• relates to: KAFKA-2106 Partition balance tool between borkers