Vbucket Map Constraints

Input parameters:

• /num_nodes/ — number of nodes
• /num_vbuckets/ — number of vbuckets
• /num_replicas/ — number of replicas each vbucket has
• /max_slaves/ — maximum number of nodes each node replicates to

Given these input parameters, following are the constraints we try to satisfy:

1. Each node replicates to exactly /max_slaves/ nodes.
2. Each node has replica vbuckets from exactly /max_slaves/ nodes.
3. Number of active vbuckets on each node is either ⌊num_vbuckets / num_nodes⌋ or ⌊num_vbuckets / num_nodes⌋ + 1.
4. Number of replica vbuckets on each node is either ⌊(num_replicas * num_vbuckets) / num_nodes⌋ or ⌊(num_replicas * num_vbuckets) / num_nodes⌋ + 1.
5. For each vbucket its active copy and all the replica copies reside on different tags (racks).
6. For each node for all pairs of its slaves sᵢ and sⱼ, the probability of sⱼ following (potentially indirectly) sᵢ in some replica chain is minimized.
7. For each node its slaves are spread across the tags (racks) as evenly as possible.