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1. preserving all the existing partition assignments
2. removing all the partition assignments that have become invalid due to the change that triggers the reassignment
3. assigning the unassigned partitions in a way that balances out the overall assignments of partitions to consumers
4. further balancing out the resulting assignment by finding the partitions that can be reassigned to another consumer towards an overall more balanced assignment.

Upon each partition assignment calculation, the full partition assignments are preserved for each consumer (as user data). This is specially helpful when a consumer group leader dies and the leadership has to be given to some other group member. 

Example 1

Suppose there are three consumers C₀, C₁, C₂, four topics t₀, t₁, t₂, t₃, and each topic has 2 partitions, resulting in partitions t₀p₀, t₀p₁, t₁p₀, t₁p₁, t₂p₀, t₂p₁, t₃p₀, t₃p₁. Each consumer is subscribed to all three topics.

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