# Dqlite internals Dqlite (distributed SQLite) implements a replicated SQLite database by combining the SQLite engine with a Raft-based consensus layer. Each LXD daemon (cluster member) runs a Dqlite node which exposes a SQLite-like API backed by a Raft replicated state machine. A single leader handles writes; followers apply replicated log entries and serve reads depending on configuration. ## Raft [Raft](https://raft.github.io/) is a consensus algorithm that ensures a cluster of nodes can agree on a sequence of state machine commands even in the presence of failures. Raft handles leader election, log replication, safety, and membership changes. ## Dqlite Raft implementation Raft nodes in Dqlite move between four runtime states: `RAFT_UNAVAILABLE`, `RAFT_FOLLOWER`, `RAFT_CANDIDATE` and `RAFT_LEADER`. Followers are passive: they accept `AppendEntries` RPCs (remote procedure calls) from an active leader and reset an election timer; when a follower’s randomized election timeout elapses without leader contact it becomes a candidate, increments its term and sends `RequestVote` RPCs to gather votes. A candidate becomes leader after receiving votes from a majority of voting servers and then starts replicating log entries to followers using `AppendEntries` (heartbeats are empty `AppendEntries` used to maintain authority). The election timeout is randomized, but always shorter than the heartbeat interval. Features such as pre-vote and explicit leadership transfer ensure reliable handover even if all servers cannot vote. Leaders also step down if they lose contact with a majority of voters. The [Dqlite raft roles](#dqlite-internals-raft-roles) control whether a server participates in quorum and elections. For more information on the Canonical Dqlite Raft implementation, see [`dqlite/src/raft.h`](https://github.com/canonical/dqlite/blob/main/src/raft.h) and [Dqlite replication](https://canonical.com/dqlite/docs/explanation/replication). ### Dqlite raft roles 1. `RAFT_VOTER`: Replicates the log and participates in quorum/elections. 2. `RAFT_STANDBY`: Replicates the log but does not participate in quorum/elections. 3. `RAFT_SPARE`: Does not replicate the log and does not participate in quorum/elections. ### LXD cluster roles 1. `database`: Assigned to cluster members with the `RAFT_VOTER` role. 2. `database-standby`: Assigned to cluster members with the `RAFT_STANDBY` role. 3. `database-leader`: Assigned to the current Raft leader.