viewstamped_replication

# Viewstamped Replication

`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 profiles are fixed, explicitly configured groups of
one through six replicas. Current operational constraints are:

- fixed membership with no reconfiguration protocol;
- no automatic log-compaction or storage-retention policy;
- applied client results are retained without eviction, so deployments that
  continually create new client identities need a future replicated
  session-retirement policy to bound the durable client table;
- 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 one through six replicas and requires a strict majority
quorum of `floor(n / 2) + 1`. It tolerates `floor((n - 1) / 2)` crashed
replicas. Even-sized groups therefore have the same failure tolerance as the
preceding odd-sized group while requiring one additional quorum vote.

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:

```elixir
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.read(group_id, operation,
  replica_id: replica_id,
  replicas: members
)
```

`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
incomplete version-two WAL tails whose length header is checksummed. Complete
corrupt records and ambiguous incomplete version-one records fail recovery
without modifying the WAL. A tail ending in a partial magic prefix, or in the
complete magic without a version byte, is treated as an incomplete append and
truncated. It does not yet provide an automatic compaction or retention policy.

WAL records and checkpoints have the same fixed 256 MiB ETF payload limit.
Oversized writes fail before changing the WAL or replacing a checkpoint, so
everything acknowledged to disk remains readable by recovery. State that grows
beyond one checkpoint record requires a future chunked snapshot format; the
adapter does not silently split or truncate it.

Writes default to version one for rollback compatibility. The file storage
option `write_version` controls both new WAL records and rewritten checkpoints.
Set `write_version: 2` only after every deployment has passed the old-reader
rollback window. Once either a version-two WAL record or checkpoint exists,
setting `write_version: 1` does not downgrade it, and opening that directory
with an older reader is unsafe. A later binary rollback requires restoring a
separately verified version-one-compatible backup; no automatic downgrade is
provided.

State transfer must persist the transferred snapshot together with its matching
hard state, log, commit/applied counters, and client table. A custom
`ViewstampedReplication.Storage` adapter must implement
`install_snapshot_state/3` as one crash-atomic operation. The runtime returns
`{:atomic_snapshot_state_install_not_supported, adapter}` instead of attempting
a non-atomic fallback when that callback is absent.

`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:

```text
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.

## Performance benchmark

Committed-prefix validation reuses an equal immutable log and otherwise compares
the retained overlap in a single ordered pass. It never iterates through
compacted historical operation numbers or performs a list-index lookup for each
committed position. Changed logs still reject missing, reordered, or rewritten
retained committed entries, including metadata changes. Snapshot-compacted
positions retain their existing snapshot trust boundary; quorum rules are
unchanged.

The complete protocol invariant check remains linear in retained log size:
it validates contiguity and the uniqueness of retained applied requests. Log
append and indexed fetch also remain linear with the current list representation.
This removes quadratic committed-prefix validation, but does not make protocol
transitions constant-time or introduce automatic retention. Reduction-based
regressions cover 10,000 retained entries and compaction after a large historical
operation number without relying on machine-specific timing deadlines.

Public runtime reads on a ready singleton avoid retained-history traversal.
The replica process exclusively owns its immutable protocol state: initialization
starts recovering, and every state-changing event passes the complete protocol
validation before synchronous effects finish. A normal singleton with all
committed operations applied and no apply in progress can therefore interpret
the existing read effect without revalidating unchanged state. No validation
flag or caller-provided certificate is trusted. Recovery, snapshots, mutations,
heartbeats, not-ready states, and multi-member quorum reads retain the validated
protocol path. The optimization requires no snapshot or log compaction.

The pure `Protocol.step/2` API accepts caller-constructed state and remains
exhaustive. Its ready singleton reads use a derived index to bound process-heap
allocation, but still validate every retained log entry and each applied request's
exact indexed operation number. Missing, malformed, or stale indexes fall back
to the complete uniqueness check; forged metadata cannot conceal duplicates.
Administrative `:sys.replace_state/2` calls and direct OTP callback invocation
bypass runtime state ownership and are not supported protocol-state ingress.

The index takes memory proportional to retained applied requests and is rebuilt
after validated log or applied-position changes, including recovery and snapshot
compaction. Preparation and fallback validation can allocate in proportion to the
log; pure-state reads retain linear CPU work. The index is absent from storage and
wire payloads. Runtime singleton protocol work is bounded independently of retained
history; state-machine query work and waiting behind other mailbox events remain
separate costs. Persistence formats and quorum requirements are unchanged.

Run the explicit local-memory benchmark to compare command throughput and
p50/p95/p99 latency across all supported cluster sizes:

```bash
mix do --app viewstamped_replication cmd mix test \
  test/performance/cluster_size_benchmark.exs --trace
```

The benchmark defaults to four concurrent clients with 100 commands per client
for each cluster size. Override the workload with
`VSR_BENCHMARK_CLIENTS` and `VSR_BENCHMARK_OPERATIONS_PER_CLIENT`. Optional
regression gates are available through `VSR_BENCHMARK_MIN_OPS_PER_SECOND` and
`VSR_BENCHMARK_MAX_P99_US`.

This benchmark uses in-memory storage and local transport so it measures the
protocol and OTP runtime rather than disk or network performance. Production
results must also be measured with the deployment's actual storage, network,
and hardware.

## Terminology

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

See the
[Viewstamped Replication application ADR](https://github.com/gsmlg-dev/concord/blob/main/docs/adr/0001-viewstamped-replication-application.md)
for the architectural decision and scope.