gopkg.in/lni/dragonboat.v2 (go): Package dragonboat is a multi-group Raft implementation.  The NodeHost struct is the facade interface for all features provided by the dragonboat package. Each NodeHost instance, identified by its RaftAddress property, usually runs on a separate host managing its CPU, storage and network resources. Each NodeHost can manage Raft nodes from many different Raft groups known as Raft clusters. Each Raft cluster is identified by its ClusterID Each Raft cluster usually consists of multiple nodes, identified by their NodeID values. Nodes from the same Raft cluster are suppose to be distributed on different NodeHost instances across the network, this brings fault tolerance to node failures as application data stored in such a Raft cluster can be available as long as the majority of its managing NodeHost instances (i.e. its underlying hosts) are available.  User applications can leverage the power of the Raft protocol implemented in dragonboat by implementing its IStateMachine component. IStateMachine is defined in github.com/lni/dragonboat/statemachine. Each cluster node is associated with an IStateMachine instance, it is in charge of updating, querying and snapshotting application data, with minimum exposure to the complexity of the Raft protocol implementation.  User applications can use NodeHost's APIs to update the state of their IStateMachine instances, this is called making proposals. Once accepted by the majority nodes of a Raft cluster, the proposal is considered as committed and it will be applied on all member nodes of the Raft cluster. Applications can also make linearizable reads to query the state of their IStateMachine instances. Dragonboat employs the ReadIndex protocol invented by Diego Ongaro to implement linearizable reads. Both read and write operations can be initiated on any member nodes, although initiating from the leader nodes incurs the lowest overhead.  Dragonboat guarantees the linearizability of your I/O when interacting with the IStateMachine. In plain English, writes (via making proposal) to your Raft cluster appears to be instantaneous, once a write is completed, all later reads (linearizable read using the ReadIndex protocol as implemented and provided in dragonboat) should return the value of that write or a later write. Once a value is returned by a linearizable read, all later reads should return the same value or the result of a later write.  To strictly provide such guarantee, we need to implement the at-most-once semantic required by linearizability. For a client, when it retries the proposal that failed to complete before its deadline during the previous attempt, it has the risk to have the same proposal committed and applied twice into the IStateMachine. Dragonboat prevents this by implementing the client session concept described in Diego Ongaro's PhD thesis.  Dragonboat is a feature complete Multi-Group Raft implementation - snapshotting, membership change, leadership transfer and non-voting members are also provided.  Dragonboat is also extensively optimized. The Raft protocol implementation is fully pipelined, meaning proposals can start before the completion of previous proposals. This is critical for system throughput in high latency environment. Dragonboat is also fully batched, it batches internal operations whenever possible to maximize system throughput.
Keywords: consensus|distributed-consensus|distributed-storage|distributed-systems|go|golang|paxos|raft|raft-algorithm|raft-protocol|replicated-state-machines