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docs: raft cluster status, plans closed, CHANGELOG/README/monitoring

- Add raft-cluster-status overview (C3a/C3b/post-C3b shipped on main)
- Mark C3a/C3b design+plans done; refresh operator docs en/bg
- CHANGELOG 1.2.0 Raft section; README cluster example and status line
- monitoring.md health/metrics match real HTTP port+440 and raft series

2026-07-30 21:41:15 +03:00

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SQL Writes Through Raft (C3b) — Design

Date: 2026-07-30
Status: Done (merged to main). Extended by post-C3b work (DDL, forward, compact, metrics) — see 2026-07-30-raft-cluster-status.md.

Problem

With C3a a real Raft cluster elects a leader over TCP, but SQL writes still bypassed Raft: they went straight to the local LSM. Committed Raft entries could be applied (applyCommand), yet nothing called appendLog from the write path.

Goal (delivered)

When Raft is enabled, SQL writes commit through the Raft log before the client sees success:

Leader: execute locally, append KV pairs (put / delete), wait for majority commitIndex, return.
Followers: originally reject with not leader; leader is '…'; with BARADB_RAFT_CLIENT_PEERS they forward to the leader (post-C3b).
Apply: LSM + secondary B-tree/FTS/HNSW + in-memory graphs (applyReplicatedPut / Delete).
Non-raft: behavior gated on raftNode == nil.

Design (as shipped)

Write interception

core/server.nim executeQuery:

Classify every statement: isWrite / isRaftDdl (not only stmts[0]).
If raft active and write/DDL: require default database + leader (or forward).
After success: pure DML → appendWriteToRaft; any DDL in batch → appendDdlToRaft (full SQL re-exec on apply).

Wait-for-commit

Poll commitIndex outside the storage gate (appendWriteToRaft / waitRaftCommit); timeout → raft commit timeout.

Known v1 limitations (still true)

Multi-DB raft not supported (raft writes only supported on the 'default' database).
CREATE/DROP DATABASE not raft-replicated.
Leader local execute before majority (timeout leaves local write; documented).
No InstallSnapshot full SM dump (safe log compact only).
No membership changes.

Tests

Unit: classification, append/timeout, apply indexes/graphs, compact, metrics
E2E: tests/raft_writes_e2e_test.nim (schema, forward, index SELECT, failover)

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