dimgigov
1b3c26123a
CI / test (push) Has been cancelled
CI / verify (push) Has been cancelled
Clients CI / build-server (push) Has been cancelled
Clients CI / test-python (push) Has been cancelled
Clients CI / test-javascript (push) Has been cancelled
Clients CI / test-nim (push) Has been cancelled
Clients CI / test-rust (push) Has been cancelled
- RaftMetrics counters: elections, term changes, appends, commit waits/ms, timeouts, lost leadership, forwards, applies, compactions - Gauges via prometheusText: is_leader, term, log size, commit/applied, apply lag, snapshot index - Wire httpServer.raftNode; extend GET /metrics and GET /health
129 lines
4.8 KiB
Markdown
129 lines
4.8 KiB
Markdown
# Distributed Systems
|
|
|
|
BaraDB supports distributed deployment with Raft consensus, sharding, and replication.
|
|
|
|
> ⚠️ **Multi-Database Limitation**
|
|
> The distributed modules (Raft, sharding, and replication) are currently wired to the **`default`** database only. If you use multiple databases (`CREATE DATABASE`, `USE DATABASE`), distributed features do not yet span across them. Each database would need its own cluster setup.
|
|
|
|
## Raft Consensus
|
|
|
|
Leader election and log replication over TCP. Enable with:
|
|
|
|
| Env | Meaning |
|
|
|-----|---------|
|
|
| `BARADB_RAFT_ENABLED=true` | Turn on Raft |
|
|
| `BARADB_RAFT_NODE_ID` | This node's id |
|
|
| `BARADB_RAFT_PORT` | Raft TCP port |
|
|
| `BARADB_RAFT_PEERS` | Comma-separated `id@host:port` (include self) |
|
|
| `BARADB_RAFT_WRITE_TIMEOUT_MS` | Max wait for majority commit on SQL writes (default 5000) |
|
|
| `BARADB_RAFT_CLIENT_PEERS` | Optional `id@host:clientPort` map for leader write forwarding |
|
|
| `BARADB_RAFT_LOG_MAX_ENTRIES` | Soft cap on in-memory raft log length (default 256); safe prefix compact |
|
|
|
|
When Raft is enabled, SQL DML (`INSERT`/`UPDATE`/`DELETE`/`MERGE` and transactional `COMMIT`) and schema DDL (`CREATE`/`DROP`/`ALTER` table, index, view, graph, …) are accepted only on the leader of the **`default`** database. DML ships as put/delete log entries; DDL ships as a `ddl` entry with the original SQL and is re-executed on every node at apply. Followers that receive a write/DDL **forward** it to the leader when `BARADB_RAFT_CLIENT_PEERS` maps the leader id to a SQL client address; otherwise they return `not leader; leader is '…'`. Writes against any other database name are rejected (`raft writes only supported on the 'default' database`). `CREATE`/`DROP DATABASE` are not raft-replicated (multi-DB is out of scope for v1). Committed DML also updates secondary B-tree/FTS/HNSW indexes and in-memory graphs.
|
|
|
|
**Log compaction (v1):** after apply, each node may drop a fully-safe log prefix once `log.len` exceeds `BARADB_RAFT_LOG_MAX_ENTRIES`. The leader never discards past any peer's `matchIndex` (so lagging followers still catch up via AppendEntries). Snapshot metadata (`lastSnapshotIndex`/`Term`) is persisted in `raft_state.bin`; full InstallSnapshot state-machine payloads are not required while this safe-prefix policy holds.
|
|
|
|
**Metrics:** with raft enabled, `GET /metrics` (HTTP port = `BARADB_PORT + 440`) includes Prometheus lines such as `baradb_raft_is_leader`, `baradb_raft_term`, `baradb_raft_log_entries`, `baradb_raft_apply_lag`, `baradb_raft_commit_wait_ms_total`, `baradb_raft_elections_total`, `baradb_raft_forwards_total`, and `baradb_raft_compactions_total`. `GET /health` embeds a `raft` object (`role`, `term`, `leader_id`, `commit_index`, `apply_lag`, …).
|
|
|
|
```nim
|
|
import barabadb/core/raft
|
|
|
|
var cluster = newRaftCluster()
|
|
cluster.addNode("node1")
|
|
cluster.addNode("node2")
|
|
cluster.addNode("node3")
|
|
|
|
let n1 = cluster.nodes["n1"]
|
|
n1.becomeCandidate()
|
|
n1.becomeLeader()
|
|
let entry = n1.appendLog("SET key1 value1")
|
|
```
|
|
|
|
## Sharding
|
|
|
|
Distribute data across nodes:
|
|
|
|
```nim
|
|
import barabadb/core/sharding
|
|
|
|
var router = newShardRouter(ShardConfig(
|
|
numShards: 4,
|
|
replicas: 2,
|
|
strategy: ssHash
|
|
))
|
|
router.rebalance(@["node1", "node2", "node3"])
|
|
let shard = router.getShard("user_123")
|
|
```
|
|
|
|
### Sharding Strategies
|
|
|
|
| Strategy | Description |
|
|
|----------|-------------|
|
|
| `ssHash` | Hash-based sharding |
|
|
| `ssRange` | Range-based sharding |
|
|
| `ssConsistent` | Consistent hashing |
|
|
|
|
## Replication
|
|
|
|
```nim
|
|
import barabadb/core/replication
|
|
|
|
var rm = newReplicationManager(rmSync)
|
|
rm.addReplica(newReplica("r1", "10.0.0.1", 9472))
|
|
rm.connectReplica("r1")
|
|
let lsn = rm.writeLsn(@[1'u8, 2, 3])
|
|
rm.ackLsn("r1", lsn)
|
|
```
|
|
|
|
### Replication Modes
|
|
|
|
| Mode | Description |
|
|
|------|-------------|
|
|
| `rmSync` | Synchronous replication |
|
|
| `rmAsync` | Asynchronous replication |
|
|
| `rmSemiSync` | Semi-synchronous replication |
|
|
|
|
## Gossip Protocol
|
|
|
|
Membership and failure detection:
|
|
|
|
```nim
|
|
import barabadb/core/gossip
|
|
|
|
var g = newGossipManager()
|
|
g.addNode("node1")
|
|
g.addNode("node2")
|
|
g.tick() # Exchange membership info
|
|
```
|
|
|
|
## Distributed Transactions
|
|
|
|
Two-phase commit across nodes:
|
|
|
|
```nim
|
|
import barabadb/core/disttxn
|
|
|
|
var dt = newDistributedTxn()
|
|
dt.prepare(@["node1", "node2"])
|
|
dt.commit()
|
|
```
|
|
|
|
## Formal Verification
|
|
|
|
Core distributed algorithms are formally specified in TLA+ and model-checked:
|
|
|
|
- **Raft Consensus** — `formal-verification/raft.tla`
|
|
- Verified: ElectionSafety, StateMachineSafety
|
|
- **Two-Phase Commit** — `formal-verification/twopc.tla`
|
|
- Verified: Atomicity, NoOrphanBlocks
|
|
- **Replication** — `formal-verification/replication.tla`
|
|
- Verified: MonotonicLsn, AcksRemovePending
|
|
|
|
Run TLC locally:
|
|
|
|
```bash
|
|
cd formal-verification
|
|
java -cp tla2tools.jar tlc2.TLC -config models/raft.cfg raft.tla
|
|
java -cp tla2tools.jar tlc2.TLC -config models/twopc.cfg twopc.tla
|
|
java -cp tla2tools.jar tlc2.TLC -config models/replication.cfg replication.tla
|
|
``` |