myxce/.kiro/steering/ha-async-io.md

HA Protocol & Async I/O

Philosophy

The HA layer combines Raft-style consensus election with a single-writer replication model. The async I/O framework (iomgr/vtaskmgr/delaymgr) enforces strict threading rules: non-blocking dispatch on the epoll thread, thread-pool handler execution, and timer-thread blocking I/O.

Single-writer constraint: only the primary writes WAL entries. This avoids Raft log replication complexity while the election mechanism inherits Raft's safety guarantees (term-based voting, freshness gates, monotonic commits).

Process-lifecycle invariant (applies to ALL controller code): a controller MUST NEVER self-terminate. It exits only on a genuine crash or an admin/supervisor kill — never via exit()/abort()/tlog_exit() as a designed response to an operational condition (unrecoverable/corrupt WAL, lost quorum, unreachable peer, failed catch-up). "Fail loud" means log a loud operator-visible ERROR + enter a permanent non-promotable/fenced state + stay alive and keep retrying catch-up (so the node self-heals when a peer returns, without manual admin recovery) — it does NOT mean exit. See src/controller/CLAUDE.md → "Process Lifecycle (DESIGN INVARIANT)" for the authoritative statement.

Async I/O Model

Three components in src/lib/ plus a dedicated slow-lane executor:

Component	Thread model	May do	Must NOT do
iomgr	Single-threaded epoll loop	non-blocking socket I/O; endpoint lifecycle; dispatch to vtask	any blocking call; any inline message processing (dispatch it)
vtaskmgr	N-thread pool (default 8, --vcpus CLI override; cluster.c:1082)	bounded handler work; bounded blocking ONLY under admission reservation	long/unbounded blocking I/O (snapshot transfer, config-push) — starves the pool
delaymgr	Single timer thread	fire timers; trigger (enqueue) work; return fast	connect(), file I/O, any blocking — it drives heartbeats + election timeouts
slow lane	dedicated executor / admission-capped pool	long blocking I/O: reconnect, snapshot transfer, config-push	— (this is where blocking ops go, NOT delaymgr)

Thread-role contract (DESIGN INVARIANT — applies to ALL components: controller, worker, scheduler). No thread that carries liveness signals (iomgr, delaymgr) may execute blocking work inline:

1. iomgr does only non-blocking I/O + dispatch. It never blocks and never processes a message inline — it hands off to a vtask.
2. delaymgr only fires timers and triggers work, then returns. It NEVER blocks (no connect(), no file I/O) — because the same single thread drives heartbeats and election timeouts, so any block there stalls liveness for the whole cluster and can cause spurious failover.
3. vtask pool runs bounded handler work. Bounded blocking (e.g. the commit-wait barrier) is permitted ONLY under an admission reservation (COMMIT_WAIT_RESERVED, commit.c:195-203) that keeps workers free for liveness-critical messages. Long/unbounded blocking I/O does NOT belong here.
4. Long blocking I/O (reconnect, snapshot transfer, config-push) runs on a dedicated slow lane / admission-capped executor, never on iomgr or delaymgr.

SUPERSEDES (2026-06-25): the former rule "delaymgr: connect()/file I/O = Permitted; blocking calls belong in delaymgr tasks." That guidance was unsafe — delaymgr is also the heartbeat/election-timeout driver (see Heartbeat below), so a permitted blocking connect() there stalls heartbeats to healthy peers → spurious election. Blocking ops move to the slow lane; delaymgr becomes trigger-only. A blocking audit (2026-06-25) found three live violations of the corrected contract — snapshot send + persist_checkpoint inline on iomgr (ha.c:2702), push_config_transport_only busy-poll+join on iomgr (config_repl.c:1332), and blocking gethostbyname+connect on the delaymgr thread (netio.c:289/302) — tracked as the raft-drive-async / P2 work in roadmap.md.

Outgoing sends use xce_request(ep, &req) which enqueues to the endpoint's send buffer — always non-blocking. Replies use xce_reply(ep, host, &reply) — also non-blocking.

Connection Ownership

Inbound vote connections (accepted on the listener): Used exclusively for vote RPC exchange (RequestVote + reply). After the reply is sent, the connection is left open but NOT reused for state push. Owned by the remote peer (caller closes).

Outbound control channels (ha->standby_endpoints): Carry all persistent state from primary to standby — ctrlhello, config push, WAL replication, heartbeat NOPs. Owned by vote_ep_err_handler (promoted vote endpoints) and standby_ep_err_handler_primary (late-joiner outbound connections). Freed on disconnect.

Late joiners: primary_heartbeat_fn scans the peers list and opens a fresh outbound connection to any peer absent from standby_endpoints. On connect, on_standby_connected sends ctrlhello, registers the endpoint, and calls repl_add_standby.

Raft Election

The Raft protocol logic lives in the library src/lib/raft/raft.c, consumed verbatim (election, logOk freshness, (term,index) truncation, current-term commit, UpdateTerm). The controller (ha.c/cluster.c) is a thin adapter over it via five callbacks (on_persist_entry, on_get_entry_term, on_send_append_entries, on_truncate_from, on_commit_advanced) and the entry points (raft_log_append, raft_on_append_entries_request/reply, raft_advance_commit). The controller does NOT re-implement Raft protocol decisions — landed by spec ha-term-safety (closes bug-851: silent committed-prefix divergence from term-blind hdr.term==0 logs). When wiring HA code, drive these entry points; never hand-roll a parallel election/commit/truncate path.

• RequestVote + RequestVoteReply over Cap'n Proto
• Freshness gate ((term,lsn), term-primary): a voter grants iff cand_term > local_term ∨ (cand_term == local_term ∧ cand_index >= local_len). The term axis is authoritative — a longer-but-stale-term divergent tail can NOT out-rank a node holding the committed prefix. This only works because the term axis is now correct (per-entry terms, below).
• Votes persisted atomically with term increment (raft_cb_persist)
• Majority wins: quorum = ceil(n_peers / 2)
• safe_committed_lsn derived from voter RequestVoteReply messages — no round-trip needed after winning. The granting voter's advertised (tag, durable-floor LSN) MUST be recorded into the election context so the safe-commit reflects the true voted majority (a discarded vote LSN degrades safe-commit to self_lsn and truncates committed entries — bug-828).
• WAL truncation rule (load-bearing — bug-828): the ONLY truncates are (a) the votes-derived post-election persist_truncate_wal_above(safe_committed_lsn) in raft_cb_become_leader, and (b) the library on_truncate_from (term,lsn) conflict reconcile — both run before the new leader commits anything. NEVER truncate on the lagging learned-committed watermark (last_known_committed_lsn) — it lags the true commit point, so a pre-election/heuristic truncate on it physically destroys committed entries (clamps durable_lsn down). This generalizes persistence.md's "never truncate on a stale ctrlhello".

Key safety invariants:

• currentTerm is monotonic (never decreases)
• votedFor updated atomically with term increment before granting any vote
• commitIndex never decreases (monotonic)

Heartbeat Model

Bidirectional heartbeat with Raft-style AppendEntries semantics:

• Primary → standby: OPCode_nop every heartbeat_interval_ms (3s) via self-perpetuating delaymgr one-shot (ha_start_primary_heartbeat). Re-arms only while role is PRIMARY.
• Standby → primary: Each heartbeat reply carries (applied_lsn, config_tag) via RequestReply.stateReport, giving the primary continuous visibility into standby convergence.
• Standby liveness watchdog: ha_arm_heartbeat_timeout_vtask armed on entering CTRL_STANDBY. If no message arrives from the primary within heartbeat_timeout_ms (10s), triggers re-election via ha_on_primary_ctrl_fd_rdhup.

Replication Consistency (Two-Tier)

Tier 1 — Majority (service gate): Every state-changing operation is committed on a majority before acknowledgment. Post-election write safety comes from Raft's own-term commit rule (§5.4.2): an election winner assumes CTRL_PRIMARY directly and, at promotion, appends a WAL_NOOP entry of its current term and arms the commit engine's writable-LSN gate (commit_set_writable_lsn). Confirmation of any operation then waits behind committed_lsn >= writable_lsn — no wall-clock path changes leadership or confirms operations. Single-node mode returns -ENODEV (degraded).

Tier 2 — Full peers (continuous best-effort): Primary continuously pushes config and WAL to all standbys via ha_dispatch_repl_action_for_standby on each heartbeat reply. The dispatch gate in CTRL_INIT and CTRL_ELECTING allows replication opcodes so campaigning nodes never reject state push.

Per-Entry Term & (term,lsn) Reconcile (ha-term-safety)

Every WAL entry carries the leader's real Raft term in wal_entry_hdr_t.term (a value fix — the 40-byte header layout is unchanged). The primary stamps s->term via raft_log_append → on_persist_entry; the live replication carries that term (the legacy term-0 repl_broadcast was retired — a standby persists the leader's term, not 0 or a local substitute). Standbys intake every heartbeat/replication through raft_on_append_entries_request, so UpdateTerm (adopt a higher term + step down) and (term,lsn) conflict-truncate happen on every RPC. Reconcile by (term, lsn), never by LSN alone — a same-LSN entry with a different term is divergent and is truncated, then re-accepted from the leader. Recovery reads back hdr.term per LSN (legacy pre-feature term-0 maps to a single "lowest-known" term for matching). Configuration changes ride the same unified log as a WAL_CONFIG_MARKER (a {version_tag, sha256} reference, no config bytes) that commits + reconciles by (term,lsn) exactly like a job entry; the config bytes travel out-of-band and are hash-verified against the committed marker before activation (mismatch ⇒ loud serve-fence, stay alive + retry).

Commit-Source Ownership (a deliberate decision — do not re-add a library commit ceiling)

The durable-majority barrier + the own-term WAL_NOOP writable gate (commit.c, see persistence.md) is authoritative for advancing committed_lsn. The raft library's commit_index (fed via on_commit_advanced after raft_advance_commit) is kept advisory only — it is NOT folded in as a hard ceiling (committed = min(durable-majority, raft_commit_index) was tried and removed). Rationale: under the single-writer model the durable-majority watermark is already a current-term (term,lsn)-safe quorum (term-carrying replication + reconcile-before-durable + the own-term gate), so the library index is redundant for safety; and because the library index advances asynchronously and lags whenever a live path (e.g. catch-up delivery) under-drives it, a min() ceiling starves commit below the safe durable majority (jobs acked-durable on a majority but never committed). Keep driving the library (for its own match_index/next_index/send state) but never let its commit index gate committed_lsn. The Figure-8 "current-term only" guarantee is realized by single-writer + the own-term NOOP gate (a new leader cannot ack a prior-term entry as committed until its own-term NOOP is majority-durable), not by the library ceiling.

Dispatch Gate

ha_dispatch_gate controls which opcodes are accepted per role:

Role	Allowed
CTRL_PRIMARY	All opcodes (client ops subject to write quorum; under no quorum, the fact allowlist OPCode_notify/update/setpreason/clrpreason is still admitted)
CTRL_STANDBY	nop, reply, hello, join, shutdown, + all replication opcodes
CTRL_INIT / ELECTING	requestVote, probeConfigTag, heartbeat, hello, nop, replConfigPush, walCatchupEntry, snapshotTransfer

This ensures a campaigning node never rejects state push from the current primary — rejecting during an election window would leave that node permanently behind.

Raft-to-Code Mapping

TLA+ Variable	XCE Code
currentTerm[i]	ha->term (_Atomic uint64_t)
state[i]	ha->role (INIT, ELECTING, PRIMARY, STANDBY)
votedFor[i]	ha->voted_for_name[]
log[i]	WAL segments in persist->wal_path
commitIndex[i]	persist->committed_lsn (_Atomic uint64_t)
lastApplied[i]	ha->applied_lsn (_Atomic uint64_t)