High Availability

TDengine TSDB is a distributed time-series database with built-in high availability (HA). By default, it uses a standard three-replica architecture based on the RAFT protocol. To better fit different deployment scenarios, TDengine also offers a two-replica RAFT-based option, as well as an active-active solution using WAL data synchronization for users who prefer a traditional primary-standby setup.

• Three-replica architecture: Stores three copies of each dataset, delivering the highest level of availability at the highest cost.
• Dual-replica with arbitrator: Stores two copies of each dataset, but requires at least three nodes in the cluster. This reduces storage costs compared to three replicas while still ensuring strong consistency and high availability.
• Active-active: Runs with just two nodes. This setup provides good availability but only guarantees eventual consistency.

The key differences between these three approaches are outlined below:

#	Three-Replica	Dual-Replica	Active-Active
Cluster setup	Single cluster	Single cluster	Two independent clusters
Minimum nodes	Three data nodes	Two data nodes and one arbitrator node	Two data nodes
Leader election	Raft algorithm	Arbitrator-managed	Not applicable
Replication method	Raft algorithm	Raft algorithm	taosX
Sync latency	None	None	Depends on taosX; typically seconds
Data durability	No data loss	No data loss	Depends on WAL retention
Data consistency	Strong (Raft)	Strong (Raft)	Eventual
Availability	Service unaffected if any single node fails	Service unaffected if a single node fails, but cannot tolerate consecutive failures	Service remains available as long as one instance is alive