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[1] Affinity based rebalancing
In the first case, the reason for the rebalancing is a change in the list of nodes - owners of the partition. If a new node has joined the topology then part of the partitions will be reassigned to it due to the affinity function property of the uniform distribution of partitions among the nodes. If the node is left then the rebalancing will be triggered to ensure the availability of the required number of copies of the partition in the grid. Such kind of rebalancing can only happen when baseline is changed.

For example, let's consider adding new node s4 to topology. The assignment will look like:

F(0) = s1, s2, s3, s4

F(1) = s2, s3, s4, s1

F(2) = s3, s4, s1, s2

F(3) = s4, s1, s2, s3

F(4) = s1, s2, s3, s4

F(5) = s2, s3, s4, s1

Node s4 now owns all 6 partitions and will load them during rebalancing.

[2] Updates counter based rebalancing.
In the second case , which is relevant only for the persistence mode, when baseline is not changed rebalancing is used to restore the consistency of the data if the node was absent for some time in the topology under the load.
When the node re-joins the topology, it will have stale data, and the partition update counter will be analyzed to determine how much the partitions have “lagged” from other copies.
Depending on how strong the partitions are lagging behind, the second type of rebalance can use the write-ahead log as a data source for updates history. In this case, the rebalancing can be performed without complete reloading of the partition and is called “historical”.
Another scenario for the occurrence of [2] is the activation of the grid having nodes in topology with stale data.Rebalancing

Technically rebalancing is the exchange of supply and demand messages between nodes. A demand message is sent from node where partition has stale data to node with actual data. Supply messages message containing data is send in response to demand message, after . After processing a supply message next demand message is send and so on until nothing more to rebalance.

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