Multi-Leader Replication

A natural extension of the leader-based replication model is to allow more than one node to accepts, which requires Multi-Leader Replication. Within each datacenter, regular leader-follower replication is used; between datacenters, each datacenter's leader replicates its changes to the leaders in other datacenters.

Use Cases for Multi-Leader Replication

• Multi-datacenter operation
  • Inter-datacenter network delay is hidden from users, thus better perceived performance.
  • Tolerance of datacenter outages
  • Tolerance of network problems (traffic between datacenters usually goes over the public internet)
• Clients with offline operation
• Collaborative-editing

Handling Write Conflicts

• Conflict avoidance (writes for a particular record go through the same leader)
• Converging toward a consistent state
  • Give each write a unique ID
  • Give each replica a unique ID
  • Merge the result together (with application code to resolve the conflict or not)
• Custom conflict resolution logic
  • On Write, typically runs in background
  • On Read, the application may prompt the user or automatically resolve the conflict and write the result back to db.

Automatic Conflict Resolution

• Conflict-free replicated datatypes
• Mergeable persisitent data structures
• Oporation transformation

Multi-Leader Replication Topologies

• Circular topology (single point of failure)
• Star topology (single point of failure)
• All-to-all topology (Dependent update arrives before insert issue, use version vectors to solve)