109 lines
4.7 KiB
Markdown
109 lines
4.7 KiB
Markdown
# Cluster Persistence
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The cluser persistence subsystem runs entirely in Server Agents. It handles both read and
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write requests from the [RPC] subsystem. See the [Consul Architecture Guide] for an
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introduction to the Consul deployment architecture and the [Consensus Protocol] used by
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the cluster persistence subsystem.
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[RPC]: ../rpc
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[Consul Architecture Guide]: https://www.consul.io/docs/architecture
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[Consensus Protocol]: https://www.consul.io/docs/architecture/consensus
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<sup>[source](./overview.mmd)</sup>
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## Raft and FSM
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[hashicorp/raft] is at the core of cluster persistence. Raft requires an [FSM], a
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finite-state machine implementation, to persist state changes. The Consul FSM is
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implemented in [agent/consul/fsm] as a set of commands.
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[FSM]: https://pkg.go.dev/github.com/hashicorp/raft#FSM
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[hashicorp/raft]: https://github.com/hashicorp/raft
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[agent/consul/fsm]: https://github.com/hashicorp/consul/tree/main/agent/consul/fsm
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Raft also requires a [LogStore] to persist logs to disk. Consul uses [hashicorp/raft-boltdb]
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which implements [LogStore] using [boltdb]. In the near future we should be updating to
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use [bbolt].
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[LogStore]: https://pkg.go.dev/github.com/hashicorp/raft#LogStore
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[hashicorp/raft-boltdb]: https://github.com/hashicorp/raft-boltdb
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[boltdb]: https://github.com/boltdb/bolt
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[bbolt]: https://github.com/etcd-io/bbolt
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## State Store
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Consul stores the full state of the cluster in memory using the state store. The state store is
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implemented in [agent/consul/state] and uses [hashicorp/go-memdb] to maintain indexes of
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data stored in a set of tables. The main entrypoint to the state store is [NewStateStore].
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[agent/consul/state]: https://github.com/hashicorp/consul/tree/main/agent/consul/state
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[hashicorp/go-memdb]: https://github.com/hashicorp/go-memdb
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[NewStateStore]: https://github.com/hashicorp/consul/blob/main/agent/consul/state/state_store.go
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### Tables, Schemas, and Indexes
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The state store is organized as a set of tables, and each table has a set of indexes.
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`newDBSchema` in [schema.go] shows the full list of tables, and each schema function shows
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the full list of indexes.
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[schema.go]: https://github.com/hashicorp/consul/blob/main/agent/consul/state/schema.go
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There are two styles for defining table indexes. The original style uses generic indexer
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implementations from [hashicorp/go-memdb] (ex: `StringFieldIndex`). These indexes use
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[reflect] to find values for an index. These generic indexers work well when the index
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value is a single value available directly from the struct field, and there are no
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oss/enterprise differences.
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The second style of indexers are custom indexers implemented using only functions and
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based on the types defined in [indexer.go]. This style of index works well when the index
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value is a value derived from one or multiple fields, or when there are oss/enterprise
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differences between the indexes.
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[reflect]: https://golang.org/pkg/reflect/
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[indexer.go]: https://github.com/hashicorp/consul/blob/main/agent/consul/state/indexer.go
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## Snapshot and Restore
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Snapshots are the primary mechanism used to backup the data stored by cluster persistence.
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If all Consul servers fail, a snapshot can be used to restore the cluster back
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to its previous state.
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Note that there are two different snapshot and restore concepts that exist at different
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layers. First there is the `Snapshot` and `Restore` methods on the raft [FSM] interface,
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that Consul must implement. These methods are implemented as mostly passthrough to the
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state store. These methods may be called internally by raft to perform log compaction
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(snapshot) or to bootstrap a new follower (restore). Consul implements snapshot and
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restore using the `Snapshot` and `Restore` types in [agent/consul/state].
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Snapshot and restore also exist as actions that a user may perform. There are [CLI]
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commands, [HTTP API] endpoints, and [RPC] endpoints that allow a user to capture an
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archive which contains a snapshot of the state, and restore that state to a running
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cluster. The [consul/snapshot] package provides some of the logic for creating and reading
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the snapshot archives for users. See [commands/snapshot] for a reference to these user
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facing operations.
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[CLI]: ../cli
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[HTTP API]: ../http-api
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[commands/snapshot]: https://www.consul.io/commands/snapshot
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[consul/snapshot]: https://github.com/hashicorp/consul/tree/main/snapshot
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Finally, there is also a [snapshot agent] (enterprise only) that uses the snapshot API
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endpoints to periodically capture a snapshot, and optionally send it somewhere for
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storage.
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[snapshot agent]: https://www.consul.io/commands/snapshot/agent
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## Raft Autopilot
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[hashicorp/raft-autopilot] is used by Consul to automate some parts of the upgrade process.
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[hashicorp/raft-autopilot]: https://github.com/hashicorp/raft-autopilot
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