Viewstamped Replication

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viewstamped_replication is a standalone, protocol-generic OTP application for replicating deterministic state machines with Viewstamped Replication (VSR). It does not depend on Concord. Concord depends on this application and uses it as its only replicated engine.

Status

This application implements the pure protocol kernel, supervised replica runtime, client sessions, normal operation, view changes, recovery, state transfer, storage adapters, transport adapters, and telemetry emission. It is the replication runtime for Concord 3.0.

The supported production profile is a fixed, explicitly configured three-replica group that tolerates one crash failure. Current operational constraints are:

  • fixed membership with no reconfiguration protocol;
  • no automatic log-compaction or storage-retention policy;
  • no storage-format migration between incompatible VSR releases;
  • snapshots are explicit rather than managed by a production checkpoint policy.

Fault and quorum model

VSR assumes crash failures, not Byzantine failures. A fixed, ordered configuration contains 2f + 1 replicas and requires a quorum of f + 1. Supported configuration sizes are currently one, three, and five replicas. The first Concord production target is three replicas, which tolerates one crashed replica.

Membership is explicit and fixed. It is never inferred from connected distributed Erlang nodes. The primary for a view is selected deterministically from the view number and the ordered configuration.

Runtime and storage

The application supervisor owns a registry and a dynamic replica supervisor. Multiple independent groups may run in one VM. Public entry points include:

ViewstampedReplication.start_replica(opts)
ViewstampedReplication.stop_replica(group_id, replica_id)
ViewstampedReplication.status(group_id, replica_id)
ViewstampedReplication.primary(group_id, replica_id)
ViewstampedReplication.snapshot(group_id, replica_id)
ViewstampedReplication.command(group_id, operation, client: client)

ViewstampedReplication.Storage.Memory is volatile and intended for tests. ViewstampedReplication.Storage.File provides a checksummed, fsynced WAL, atomic checkpoints, configuration identity validation, and truncation of partial or corrupt WAL tails during recovery. It does not yet provide an automatic compaction or retention policy.

ViewstampedReplication.Transport.Local routes through the local registry and supports an injectable delivery function for deterministic fault tests. ViewstampedReplication.Transport.Distribution routes only through explicit endpoint maps; connected Erlang nodes are never treated as membership.

Architecture

The protocol core is pure and deterministic:

event + protocol state
        |
        v
ViewstampedReplication.Protocol.step/2
        |
        v
new protocol state + ordered effects

The protocol does not send messages, read clocks, schedule timers, access storage, emit telemetry, or invoke the replicated state machine. The supervised replica runtime interprets its ordered effects.

Replicated services implement ViewstampedReplication.StateMachine. Operations must be deterministic and free of arbitrary external side effects.

Terminology

This application uses the VSR terms primary, backup, replica, view, view change, operation number, and commit number.

See the Viewstamped Replication application ADR for the architectural decision and scope.