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Auto-Rebalance & Scheduling

MatrixCube is a framework to implement distributed system. For a distributed system, data are stored across many machines. When the number of machines changes, with cluster scaling out or machine crash for example, data need to be moved across machines.

Prophet is the key component of MatrixCube for Auto-Rebalance and Scheduling. An Etcd Server is embedded inside Prophet for storing the metadata of the cluster.

It has three main objectives to reach:

  • Keep the storage level of each Store balanced.
  • Keep the write/read requests balanced.
  • Keep the logical table distribution balanced.

We designed a mechanism of Heartbeat and Event Notify to achieve these objectives. Each Store and Leader Replica will send Hearbeat information to Prophet, Prophet will make scheduling decision based on the information. We need to configure certain Stores to take the duty of Prophet.

Store Hearbeat

Each Store sends Heartbeat periodically to Prophet, the Heartbeat includes:

  • At the moment, how many Replicas are in this Store.
  • At the moment, how much storage space does this Store have, how much space is already used, how much space remaining.

Prophet collects all Heartbeats, and Prophet will understand a global Replica mapping and the storage space of each Store. Based on this information, Prophet sends scheduling orders, moving some Replicas to proper Stores, in order to balance the Replica numbers for each Store. Since each Replica of a Shard is the same size, the storage space is thus equalized.

Replica Hearbeat

For each Shard, it has several Replicas distributed in several Stores. These Replicas form a Raft-Group and a Leader is elected. This Leader sends periodically Heartbeats to Prophet. This Heartbeat has information as:

  • At the moment, how many Replicas a Shard has, and the latest active time of each Replica.
  • At the moment, who is the Leader Replica.

Prophet collects all Hearbeats and constructs a global Shard Replica and Replica Leader mapping. With this information, Prophet launches scheduling orders under the following circumstances:

  • Add Replica order: if the number of Shard Replica is not enough, look for appropriate Stores to add Replicas.
  • Remove Replica order: if the number of Shard Replica exceeds the limit, delete Replicas in appropriate Stores.
  • Move Replica order: if the number of Shard Replica is not balanced, some Replicas will be moved to achieve the balance.
  • Transfer Leader order: if the Leader number is not balanced in the cluster, some Leaders will be transferred.

Event Notify

The Heartbeat information collected will be synchronized to all MatrixCube Stores. Each Store will form the global routing table.