consul/docs/persistence/README.md

Cluster Persistence

The cluser persistence subsystem runs entirely in Server Agents. It handles both read and write requests from the RPC subsystem. See the Consul Architecture Guide for an introduction to the Consul deployment architecture and the Consensus Protocol used by the cluster persistence subsystem.

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Raft and FSM

hashicorp/raft is at the core of cluster persistence. Raft requires an FSM, a finite-state machine implementation, to persist state changes. The Consul FSM is implemented in agent/consul/fsm as a set of commands.

Raft also requires a LogStore to persist logs to disk. Consul uses hashicorp/raft-boltdb which implements LogStore using boltdb. In the near future we should be updating to use bbolt.

State Store

Consul stores the full state of the cluster in memory using the state store. The state store is implemented in agent/consul/state and uses hashicorp/go-memdb to maintain indexes of data stored in a set of tables. The main entrypoint to the state store is NewStateStore.

Tables, Schemas, and Indexes

The state store is organized as a set of tables, and each table has a set of indexes. newDBSchema in schema.go shows the full list of tables, and each schema function shows the full list of indexes.

There are two styles for defining table indexes. The original style uses generic indexer implementations from hashicorp/go-memdb (ex: StringFieldIndex). These indexes use reflect to find values for an index. These generic indexers work well when the index value is a single value available directly from the struct field, and there are no ce/enterprise differences.

The second style of indexers are custom indexers implemented using only functions and based on the types defined in indexer.go. This style of index works well when the index value is a value derived from one or multiple fields, or when there are ce/enterprise differences between the indexes.

Snapshot and Restore

Snapshots are the primary mechanism used to backup the data stored by cluster persistence. If all Consul servers fail, a snapshot can be used to restore the cluster back to its previous state.

Note that there are two different snapshot and restore concepts that exist at different layers. First there is the Snapshot and Restore methods on the raft FSM interface, that Consul must implement. These methods are implemented as mostly passthrough to the state store. These methods may be called internally by raft to perform log compaction (snapshot) or to bootstrap a new follower (restore). Consul implements snapshot and restore using the Snapshot and Restore types in agent/consul/state.

Snapshot and restore also exist as actions that a user may perform. There are CLI commands, HTTP API endpoints, and RPC endpoints that allow a user to capture an archive which contains a snapshot of the state, and restore that state to a running cluster. The consul/snapshot package provides some of the logic for creating and reading the snapshot archives for users. See commands/snapshot for a reference to these user facing operations.

Finally, there is also a snapshot agent (enterprise only) that uses the snapshot API endpoints to periodically capture a snapshot, and optionally send it somewhere for storage.

Raft Autopilot

hashicorp/raft-autopilot is used by Consul to automate some parts of the upgrade process.