674 lines
43 KiB
Markdown
674 lines
43 KiB
Markdown
# m11p5 — Membership, Discovery, Elasticity (COMPLETE — 2026-06-12)
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Phase spec and exit gate: [docs/roadmap-to-cluster.md §4/m11p5](../../roadmap-to-cluster.md).
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Predecessors: [phase-2.md](phase-2.md) (StreamSegments — the catch-up stream the
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snapshot resumes into), [phase-3.md](phase-3.md) (quorum commit index — the
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arithmetic conf-changes mutate), [phase-4.md](phase-4.md) (terms, fencing, the
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durable-state disciplines, and three carried hazards this phase closes:
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quarantine-clearing rides reseed, reseeded nodes lack pre-baseline history,
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pre-baseline data is unreachable through a term's stream).
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**Goal:** nodes are cattle; topology is data, not files. A new node joins by
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contacting any seed, catches up via snapshot + WAL stream in minutes, and
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serves quorum; a dead node's replacement is `kubectl delete pod`; addresses
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are DNS names; membership changes ride the replicated log under the m11p4
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term machinery.
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> **Design review:** a four-lens adversarial review (safety / crash-recovery /
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> liveness / compatibility) of the first draft produced **33 findings (9
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> BLOCKER, 13 MAJOR, 11 MINOR), all folded in below** before implementation.
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> The blockers that reshaped the design: the even-voter-count `majority()`
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> arithmetic error (§3.0), the vacuous cross-term conf-change gate and the
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> baseline-jump record skip (both closed by the activation membership
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> re-append, §3.2), learner marks poisoning the quorum (§3.3), and two
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> crash windows in the reseed swap (§2.3).
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## Design (as adopted)
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### 1. DNS peer addresses: the bind/advertise split
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`grpc_addr` becomes an **advertised** address — hostname or IP, validated by
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the same syntactic `validate_host_port` that `http_addr` already passes
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(hostnames accepted since m8p10). A new optional per-region `grpc_bind`
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(SocketAddr) controls the local bind:
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- `grpc_bind` present → bind it.
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- Absent, `grpc_addr` parses as a literal SocketAddr → bind that (today's
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behavior, byte-for-byte: every existing topology keeps working).
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- Absent, `grpc_addr` is a hostname → bind `0.0.0.0:<port from grpc_addr>`.
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`resolve_grpc_addr`'s dual use (own bind + every peer dial) splits: the bind
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path stays SocketAddr; the peer path stops parsing entirely. `tidal-net`'s
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`GrpcTransportConfig.peers` retypes from `HashMap<ShardId, SocketAddr>` to
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`HashMap<ShardId, String>` (host:port); `PeerPool` builds the tonic URI from
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the string — `Channel::from_shared` with a hostname makes hyper re-resolve
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DNS on every reconnect, fixing the pod-rescheduled-onto-a-new-IP case the
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SocketAddr type made structurally impossible. `grpc_server_ready` probes the
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bind address, substituting `127.0.0.1` for an unspecified bind IP. TLS note:
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SNI follows the URI host, so DNS peer names require DNS-SAN certs —
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documented in the `grpc_tls` topology block; no code change.
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This alone kills the per-pod ConfigMap variant: one shared topology file can
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name every region by its per-pod DNS name while each pod binds `0.0.0.0`.
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**Verification (review finding):** the re-resolution property is the entire
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point of the retype and `localhost`-based tests cannot regress it. A tier-2
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test exercises changed-address reconvergence where the platform permits
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(dual-family `localhost` rebind), and the **k8s pod-reschedule drill (delete
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pod, new pod IP, ship/stream traffic resumes with no peer restarts) is a
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required line item of the exit-gate evidence**, not an incidental.
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### 2. Snapshot transfer (`FetchSnapshot`): the reseed and joiner path
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**Artifact = `TidalDb::create_backup`** (tidal/src/db/backup.rs:211) with
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three review-driven hardenings:
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- the WAL-checkpoint-marker write into the copy becomes **fatal** (a staged
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artifact with a stale marker double-counts signals on the joiner);
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- `create_backup` also refreshes the **replication-HWM row** in the same
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step as the ledger checkpoint (the copied row otherwise understates the
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artifact by up to one 30 s cycle → re-folds on the joiner);
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- the snapshot-staging variant skips the non-fatal text-index flushes
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(derived state, rebuilt on open) to shorten the quiesce window.
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The leader stages the artifact under `<data_dir>/snapshots/` and serves it
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as a new server-streaming RPC in the `WalShipping` service:
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```
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rpc FetchSnapshot(SnapshotRequest) returns (stream SnapshotChunk)
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```
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- `SnapshotRequest { shard_id, from_seqno, term }` — `from_seqno` is the
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puller's frontier+1. The handler answers a **no-snapshot-needed** header
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chunk when the WAL can still serve `from_seqno` (the joiner just streams),
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else streams: manifest chunk (snapshot seq, file list with size + BLAKE3,
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term, leader_region) then file chunks (path, offset, bytes), term-stamped
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and term-fenced exactly like `StreamSegments` (same-term puller; stale
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source steps down; puller gates every chunk).
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- **Excluded from the manifest** (node-identity / node-local files):
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`election_state`, `stream_baseline`, `membership` (the §3.6 cache),
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`tidaldb.lock`, `snapshots/`, the reseed marker. Everything else ships
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byte-identical (TSEG segments must arrive bit-exact or the joiner's open
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refuses — identify-or-refuse end to end).
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- **Caching + single-flight:** one staged artifact at a time; concurrent
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joiners share it. A second `FetchSnapshot` arriving while staging is
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mid-flight gets a **retryable** `UNAVAILABLE` (never
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`FAILED_PRECONDITION`, which would mis-route it into the reseed class).
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An artifact is reusable while the live WAL still covers
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`artifact_seq + 1` (else refresh).
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- **Quiesce honesty (review finding):** every `create_backup` 429s all
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signal writes for checkpoint + flush + copy (seconds at 100k items).
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Gate writers retry 429 honoring `retry_after_ms` — a retried-then-acked
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write is neither lost nor an SLO violation (the established busy-writer
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harness posture). The measured window is recorded in the gate evidence.
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**2.1 Retention pin (review-corrected).** The pin is taken at backup
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**start**, at the captured seq S, *before* any copying — a pin installed
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after staging cannot protect the copy window from the 30 s checkpoint
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thread. Pin semantics: online compaction must preserve the segment chain
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covering `S + 1` onward (the pin clamps the floor such that the segment
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*containing* `S + 1` survives — an off-by-one that deletes the straddling
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segment is data loss inside the artifact; unit-tested). The pin holds
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while: the artifact is staged, any `FetchSnapshot` stream is active, and
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for a per-consumer grace until that puller's first successful
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`StreamSegments` pull from `S + 1` (bounded by a hard cap + loud metric so
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a dead joiner cannot pin retention forever). The reuse TTL
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(`snapshot_artifact_ttl_ms`) governs artifact refresh only — **never**
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the pin of an active consumer; consecutive reseed loops per peer are
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counted and alerted at ≥ 2.
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**2.2 Install is a boot-time operation** — no live data-dir swap under a
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running engine, ever. The pre-open step runs **only** on (a) seed-join
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boots and (b) `reseed_required`-marker boots — plain topology boots never
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contact a peer before serving (boot-order independence at cold start is
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load-bearing; a behind-a-compacted-leader restart is detected at runtime
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per §2.4 and reseeds on the *next* boot). A marker boot that cannot
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complete the handshake within a bounded window **falls back to opening the
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existing data dir** — degraded, marker latched, quarantine fence intact,
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**voting enabled** — because the reseed needs a leader and the leader may
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need this node's vote (review finding: reseed-blocks-voting deadlocks the
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cluster). Fresh joiners have nothing to fall back to and nothing to vote
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with; they retry the seed loop indefinitely with bounded backoff.
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**2.3 The swap protocol (review-corrected ordering).** Fetch into a sibling
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staging dir, then:
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1. Verify every file's BLAKE3 + manifest completeness.
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2. **Copy the node's own identity files INTO staging**: `election_state`
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(vote safety, §2.5) and `membership` cache — *before* the sentinel.
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3. Write the `COMPLETE` sentinel into staging; fsync.
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4. `rename(data_dir → data_dir.discard)`,
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`rename(staging → data_dir)`, fsync parent, delete `.discard`.
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**Filesystem constraint:** the swap renames `data_dir` itself, so `data_dir`
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must not be a filesystem/volume mount root — staging and `.discard` live in
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its parent, same filesystem (`EXDEV`/permission failures surface loudly at
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reseed time, never silently). The k8s cluster manifests therefore mount the
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PVC at `/data` and set `--data-dir /data/db`. Leader-side staging has the
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inverse problem: it lives *inside* the data dir (`<data_dir>/snapshots/`) so
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it stays on the PVC, which means `create_backup`'s recursive copy must
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**exclude** it (along with the identity files) or it would copy itself.
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Boot-time recovery (runs **before** `ElectionStore` classification — the
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review found that classifying first re-opens restart amnesia):
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staging-without-`COMPLETE` → delete staging, retry/fall back;
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staging-with-`COMPLETE` + `data_dir` present → redo step 4 from the top;
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staging-with-`COMPLETE` + `data_dir` absent (the mid-rename crash — the
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window where a naive boot would classify `Fresh` at term 0 and re-vote) →
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complete the rename, then classify; stray `.discard` → delete. Every
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window is idempotent redo; the identity files travel inside staging so
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there is no instant at which the canonical path lacks them post-swap.
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**2.4 Runtime detection → durable marker, not live surgery.** A *running*
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follower that hits a **typed** snapshot-required refusal (see below) — or
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the m11p4 divergence quarantine — durably latches `reseed_required`
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(ElectionStore file discipline: magic, version, checksum, tmp + fsync +
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rename + dir fsync), surfaces `reseed_required: true` in
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`/cluster/status/local` plus a `tidaldb_cluster_reseed_required` gauge,
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and keeps serving degraded (the 30 s retry timer keeps its standing
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wake-up). The reseed executes on the next boot. `POST /cluster/reseed`
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sets the marker on demand; `replication.reseed_self_restart: true`
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(default **false**; set in the k8s manifests) drains and exits cleanly
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once the marker latches — **refused** (loudly, in status + gauge) when the
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node's local view shows the remaining voters cannot sustain quorum without
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it (review finding: self-restart during a 2-voter window is total write
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unavailability). Quarantine clearing (the p4 carried hazard): quarantine
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latches the marker; a successful reseed boot clears the quarantine latch
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and `tidaldb_cluster_divergence`.
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**The three-way term-join rule (found by the tier-3 gate, not the review).**
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The first reseed drill exposed that a follower behind at a leadership change
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never *asks* for its missing range: the term-join baseline jump advances its
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frontier past pre-baseline history with `lag=0` — silently missing data,
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exactly p4's carried hazard, with nothing to fire the snapshot-required
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refusal. Detection cannot use the baseline (cross-numbering; p4 as-built
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#2); it uses the comparison the join check already makes. The heartbeat
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carries the leader's **election-time position in the previous stream's
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numbering** (`prev_log`); the join check becomes three-way:
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- `own > prev_log` → divergent suffix → **quarantine** (existing, p4);
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- `own < prev_log` → the node lacks `(own, prev_log]` of the previous
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stream, which is pre-baseline in the new stream and will never ship →
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**latch `reseed_required`** (new); the vote restriction makes this exact:
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an elected leader's position ≥ every granter's, so a strictly-smaller
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position is genuinely missing committed-era history;
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- `own == prev_log` (or `tail_term == T`, the within-term rejoin) → clean.
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A snapshot-installed node joins clean by construction: its WAL is the
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leader's copy, so its `tail_term` equals the leader's term. Two corollaries
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from the same drill: the install boot must issue its post-install catch-up
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pull toward the **discovered** leader (the boot self-heal targets the boot
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topology leader — which can be the node itself), and the `FetchSnapshot`
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`needed` decision must be **baseline-aware** (`needed = from_seqno ≤
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stream_baseline OR from_seqno < earliest WAL seq`) — a "WAL still covers
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it" answer is a lie when the stream clamp will never serve at or below the
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baseline, and it loops the marker boot forever.
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**Typed refusals (review finding — the conflation hazard):** today all
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three `StreamSegments` refusal classes are `FAILED_PRECONDITION`, and a
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term-mismatch during an ordinary election must NOT latch reseed markers
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fleet-wide. The refusal gains structured trailer metadata
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(`x-tidal-catchup: snapshot-required | rejoin | stepping-down`); the
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marker latches **only** on `snapshot-required`. Pre-p5 sources emit no
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trailer → no latch (conservative).
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**2.5 Vote safety across reseed.** The node's own `election_state` (term,
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voted_for) survives the swap *inside staging* (§2.3 step 2) — it is the
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node's promise "I voted for X in term T"; wiping it would allow a re-vote
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in an already-voted term (two leaders in one term). A fresh joiner has
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none — it never voted. A **corrupt** `election_state` refuses boot even
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with a reseed marker latched (a reseed does not restore term knowledge;
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the m11p4 rule stands).
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**2.6 Post-open seeding (review-corrected durability).** The install boot,
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**before starting the receiver, transport serving, or any pull**: seed
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`applied_seqno(leader_shard) = the installed artifact's recovered WAL
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tail` (valid at the install instant only — own-WAL numbering diverges from
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stream numbering as soon as applies begin; p4 as-built #1), then
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**synchronously persist the replication-state checkpoint**, then delete
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the install sentinel, then start replication. A crash anywhere before the
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sentinel delete finds the WAL unchanged (no receiver ran) and re-derives
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the same seed idempotently. The copied HWM row's other-shard entries are
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kept (true for any copy-holder once §2's `create_backup` refreshes the row
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in-step). `wal_term_mark` recovers from the copied log's kind-3 markers by
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construction, so the node's `LogPosition` is consistent in the stream's
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numbering with no new bookkeeping.
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**2.7 Fresh-joiner term sequencing.** A seed-joiner learns `(term, leader)`
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from the join response, persists `election_state {term}` **before**
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pulling (persist-before-act), and stamps the snapshot fetch and subsequent
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stream pulls with that term. The boot-time loop iterates
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(seeds ∪ cached membership) → poll `/cluster/status` for the current
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leader → handshake → fetch, with bounded backoff and indefinite
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persistence, **re-resolving leadership after every aborted stream** (an
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election mid-fetch aborts on the term fence; the loop re-discovers and
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re-stamps — review finding: without re-discovery the joiner retries a
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deposed source forever). "Backup already in progress" maps into the same
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retry loop. Process exit is reserved for unrecoverable config errors —
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crash-loop is never the retry mechanism.
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### 3. Membership as data: kind-4 records on the one log
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**3.0 Prerequisite arithmetic fix (review BLOCKER, exists today).**
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`ElectionConfig::majority()` is `peers.len()/2 + 1` — correct at n=3,
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**wrong for every even voter count**: n=4 → 2-of-4 (disjoint vote quorums
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{A,B} and {C,D} elect two leaders in one term), n=2 → 1-of-2 (a follower
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self-elects with zero RPCs while check-quorum keeps the old leader alive).
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`CommitIndex` already uses the correct `div_ceil` form — the two formulas
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in one subsystem disagree. p5 makes even sizes mandatory transit states
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(3→4→5→4→3), so this lands **first**, with even-n property tests, before
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any reconfigure work: `majority() = (peers.len() + 1)/2 + 1` (true
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`floor(n/2)+1` over the full voter set).
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A **`MembershipRecord`** is a new WAL blob kind (kind-4, beside kind-0
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signals, kind-1/2 item/embedding blobs, kind-3 term markers), journaled by
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the leader, replicated through the normal stream, folded by followers into a
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`ClusterMembership` cell (the `WalTermMark` pattern: recovered from the WAL
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at boot, advanced on apply, single-lock value):
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```
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MembershipRecord {
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version: u64, // monotonic conf version; latest record wins
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term: u64, // the appending leadership
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members: Vec<MemberEntry {
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id: u16, // PERMANENT — never renumbered, never reused
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name: String,
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grpc_addr: String, // advertised, DNS-capable
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http_addr: String,
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role: Voter | Learner | Removed, // tombstones keep ids burned
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}>
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}
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```
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Records carry the FULL membership (not deltas): recovery folds the latest
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record at-or-below the durable frontier, no merge logic. **Membership
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epoch 0 = the topology file** — a cluster with no kind-4 record behaves
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byte-for-byte as today. The first record snapshots the topology-derived
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membership (ids = the existing positional ids, preserved) plus the change.
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**3.1 Capability gating (review finding — the brick hazard).** A kind-4
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record shipped to a pre-p5 follower is an unknown batch kind →
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`WalError::Corruption` → the receiver's torn-state halt latch, **permanent
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across restarts** (boot self-heal re-pulls the same record). The m11p4
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kind-3 precedent does NOT transfer: term markers were self-gated by the
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vote quorum needing upgraded binaries; kind-4 needs only one upgraded
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leader plus one verb — and auto-promotion appends records autonomously.
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Therefore: `HeartbeatResponse` and `ReportApplied` gain a `capabilities`
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bit-field (proto3 zero-default = pre-p5 = incapable), and the leader
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**refuses `JoinCluster` and every conf-change until all current voters
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have reported kind-4 capability**. The failure mode is stated honestly in
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runbook §8: a kind-4 record delivered to a pre-p5 follower halts its
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receiver until binary upgrade; both-followers-halted = quorum-write
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outage. Downgrade rule (kind-3 precedent verbatim): once any kind-4 record
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is in a node's WAL, downgrade below p5 requires a reseed.
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**3.2 The activation membership record (review BLOCKER ×2 — this is the
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linchpin).** On every leadership activation, immediately after the kind-3
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term marker, the new leader **re-appends its full current membership as a
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fresh kind-4 record** (the Raft no-op-entry analogue, adapted to the
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baseline-jump world). This single rule closes two blockers at once:
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- *The vacuous gate:* `CommitIndex::activate` resets `committed` to the
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new leader's baseline, so "commit index ≥ record seq" is trivially true
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for every pre-baseline record — and pre-baseline records are
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structurally unshippable (the stream clamp). The one-at-a-time gate
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therefore requires **same-term quorum commitment of the activation
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record** (and of each subsequent change), evaluated from
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`update_peer_for_term` folds — never from the activation-reset
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`committed()`.
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- *The baseline-jump skip:* a follower behind at a transfer jumps its
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frontier over any kind-4 record the old term committed, with no gap, no
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signal, while staying electable under a stale roster. The activation
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record sits **above** the new baseline, so every joiner of the term
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receives the current roster in-stream before any same-term traffic.
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**3.3 Conf-change discipline (Raft single-server changes):**
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- **One at a time:** the leader refuses a new change until the prior
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record's seq is same-term quorum-committed (§3.2 predicate). Majorities
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of consecutive single-change configs overlap; the gate's integrity
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across leadership changes is exactly what §3.2 restores.
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- **Join → Learner → Voter:** `POST /cluster/join {name, grpc_addr,
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http_addr}` on any node forwards to the leader, which assigns
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`id = max(all ids ever) + 1`, appends a Learner record, and **answers
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only after that record is quorum-committed** (bounded wait → retryable
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503; review finding: answering pre-commit strands the joiner invisibly
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when the leader dies). Joins are **idempotent by name**: a re-join from
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a known member returns its existing id and current role, appending
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nothing. The response carries membership, current term, leader
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addresses, and the assigned id.
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- **Learner marks (review BLOCKER):** the role-blind `CommitIndex` must
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not see learner marks — two learners "committing" a write no voter
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holds is acked-write loss. `CommitIndex` becomes role-aware: voter
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marks feed the k-th-largest selection; learner marks are tracked in a
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side map (promotion input, transfer-wait input) and **never** count
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toward `needed`.
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- **Auto-promotion is a standing leader duty** derived solely from the
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applied `ClusterMembership` — re-armed on every leadership activation
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and on every membership apply, evaluated on commit-index publishes
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(review finding: a duty anchored to the join handler dies with the
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joining-era leader, leaving a permanent learner). Promotion fires when
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the learner's durable mark is within `learner_promote_lag` of
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`flushed_seq` **or** has stayed within one ship-round for K consecutive
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evaluations (the Raft "rounds stop shrinking" criterion — a fixed
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distance alone starves under sustained load). A `promotion_pending
|
||
(lag=N)` status field + metric make a stuck scale-up diagnosable.
|
||
- **Remove:** an operator verb appends a `Removed` record (one at a
|
||
time, quorum-commit-gated like every change). The removed peer's ship
|
||
cell is retired **only after the record is quorum-committed AND
|
||
delivered to (or acked by) the removed peer**, with a bounded give-up
|
||
(review finding: retiring on append means the removed node never
|
||
learns, never stops campaigning, and zombie-serves as Ready).
|
||
Heartbeat/vote/pull refusals to a removed member carry a typed
|
||
`removed` signal that flips its readiness to 503, suppresses
|
||
campaigning, and is exempt from the reseed marker. Its id is burned.
|
||
- **Effective-set switching:** each node's effective peer sets derive
|
||
from its latest APPLIED record (leader: on append). The four
|
||
construction-frozen peer-set copies become mutable behind **one fenced
|
||
reconfigure path** executed inside a single membership-apply critical
|
||
section — the sets can never disagree about the roster:
|
||
- `CommitIndex::reconfigure(voters, learners)` — resize marks +
|
||
`needed` under the index's own lock. In-flight `ack=quorum` waits are
|
||
**re-evaluated against the new config**, not failed (review finding:
|
||
epoch-bump-as-Demoted on every conf-change is an availability dip
|
||
with wrong error semantics); a shrink may satisfy waiters instantly.
|
||
Unknown-peer reports stop being silently dropped: a report from a
|
||
known learner records its side-map mark; a report from an unknown id
|
||
logs at WARN with a counter metric.
|
||
- `ShipQueue::{add_peer, remove_peer}` — spawn/retire per-peer sender
|
||
cells and threads at runtime (removal per the delivery rule above).
|
||
- `PeerPool::{add_peer, remove_peer}` — peers map behind a RwLock;
|
||
channels are lazy; DNS strings mean no resolution at insert.
|
||
- `ElectionState::reconfigure(voters)` — a new pure-machine input:
|
||
recompute `majority()` from the post-change voter set, resize
|
||
`peer_ack`, and **clear in-flight prevote/vote grant sets** (review
|
||
finding: grants from just-removed voters must not count against the
|
||
resized majority). The driver's `peer_shards` and the node's HTTP
|
||
tables update in the same apply step.
|
||
- **Election interaction:** voters = `role == Voter` in the latest
|
||
applied record. Learners never campaign — enforced as a **gate in the
|
||
state machine** (auto-election suppressed while not a voter), not a
|
||
comment (the review found p4's quarantine election-suppression is
|
||
comment-only; that gap is verified and fixed in-phase with a unit
|
||
test). A node that hasn't applied the newest record may transiently
|
||
refuse a legitimate candidate — the next timeout retries; safety is
|
||
the overlap argument, liveness is the retry.
|
||
|
||
**3.4 Seed-join boot** (`--seed http://host:port`, repeatable): the joiner
|
||
skips `validate_multiproc`'s "every region declared" gate — its identity is
|
||
`--region <name>` + advertised addresses from new flags/env
|
||
(`--advertise-grpc`, `--advertise-http`, `--metrics`). It runs the §2.7
|
||
loop, persists the returned membership to the durable local cache, persists
|
||
the term, then proceeds through the normal boot with the fetched roster
|
||
(snapshot install per §2 when needed). A restart boots from the cache
|
||
without the seed.
|
||
|
||
**3.5 Knob source for seed boots (review finding).** A bare `--seed` boot
|
||
with no local config would silently fall back to the compiled-in
|
||
`default-cluster.yaml` — wrong ack default, wrong quorum timeout, wrong
|
||
election timing, no metrics listener, no TLS material. Rule: **a `--seed`
|
||
boot still requires the local topology/config file for the behavioral knob
|
||
blocks** (`replication:`, `wal:`, `election:`, `timeouts:`, `grpc_tls`) —
|
||
the k8s manifests already mount the shared bootstrap ConfigMap on every
|
||
pod including N≥3, so this costs nothing there — and a bare `--seed` with
|
||
neither `--topology` nor `TIDAL_CONFIG` **refuses to boot** naming the
|
||
rule. The `regions:` list in that file is ignored for the roster (the
|
||
join response is the roster); the C2 election-timing validation runs on
|
||
the knob blocks as always. The joiner's metrics listener comes from
|
||
`--metrics` (new flag, mirroring standalone's).
|
||
|
||
**3.6 Durable membership cache** (`data_dir/membership`, ElectionStore file
|
||
discipline: magic, version, checksum, tmp+fsync+rename+dir-fsync).
|
||
Precedence: the **WAL-recovered `ClusterMembership` cell wins** over the
|
||
cache at open (the cache exists for the pre-open boot loop — seed lists,
|
||
leader discovery — not as a second source of roster truth); the cache is
|
||
rewritten from the cell after every applied record. It is excluded from
|
||
snapshot manifests and survives the reseed swap inside staging (§2.3).
|
||
|
||
**3.7 Mixed-version rule:** §3.1's capability gate makes "complete the
|
||
binary upgrade before the first conf-change" structurally enforced, not
|
||
operator discipline. Pre-p5 peers answer `Unimplemented` to
|
||
`JoinCluster`/`FetchSnapshot`; the joiner reports it loudly and retries the
|
||
next seed.
|
||
|
||
### 4. Kubernetes reference: one StatefulSet
|
||
|
||
New `k8s/cluster/` kustomize set beside the standalone one. **Naming
|
||
(review finding — the lineages must not collide):** the cluster set lives
|
||
in its own namespace `tidaldb-cluster` with StatefulSet `tidaldb`, Services
|
||
`tidaldb-peers` (headless, `publishNotReadyAddresses: true`) and `tidaldb`
|
||
(client-facing, readiness-gated); the two kustomize sets are mutually
|
||
exclusive per namespace. **Secret shape:** `tidaldb-credentials` /
|
||
`TIDAL_API_KEY` (the stress/Ref-A lineage — the in-repo stress Jobs are the
|
||
exit-gate harness and already use it); the stress Jobs' literal-ClusterIP
|
||
`--target`s move to the new DNS names in this phase.
|
||
|
||
- **One StatefulSet**, `replicas: 3`, podManagementPolicy `Parallel`,
|
||
`TIDAL_REGION` from `POD_NAME` (fieldRef), args
|
||
`cluster --listen 0.0.0.0:9500 --data-dir /data ...`, gRPC
|
||
containerPort, topologySpreadConstraints.
|
||
- **Bootstrap topology ConfigMap** — ONE file shared by all pods (per-pod
|
||
DNS advertised addresses + `0.0.0.0` binds), naming the initial 3 regions
|
||
as `tidaldb-{0,1,2}.tidaldb-peers.tidaldb-cluster.svc.cluster.local`.
|
||
Scaling past 3 does NOT edit it: pod N ≥ 3 boots with
|
||
`--seed http://tidaldb-peers...` + the same mounted file for knob blocks
|
||
(§3.5) and joins as a learner.
|
||
- **Readiness (review-pinned predicate):** `converged :=` the boot
|
||
catch-up pull has completed at least once AND `lag_events ≤
|
||
learner_promote_lag` (hysteresis — **never** `lag == 0`, which an
|
||
open-loop load keeps perpetually false); `joiner :=` this boot installed
|
||
a snapshot or seed-joined with a fresh data dir (derivable at boot, no
|
||
new state). 503 while quarantined, removed, or a joiner-boot has not yet
|
||
first-converged (sticky-ready after). **A restarted existing voter is
|
||
Ready on today's terms** — no regression for PVC-retained restarts. The
|
||
predicate is recorded in the runbook so probe behavior is diagnosable.
|
||
- **PDB** `maxUnavailable: 1`; `reseed_self_restart: true`; rolling node
|
||
replace is `kubectl delete pod` (PVC retained → boot catch-up) or PVC
|
||
delete + pod delete (fresh reseed via snapshot).
|
||
- Scale-down drill: remove verb first, then `kubectl scale --replicas`,
|
||
lowest-ordinal-last.
|
||
|
||
### 5. What goes where
|
||
|
||
| Crate | Work |
|
||
|---|---|
|
||
| `tidal` | `majority()` even-n fix + property tests (FIRST); kind-4 `MembershipRecord` encode/decode/recover/apply + `ClusterMembership` cell; role-aware `CommitIndex` (`reconfigure`, learner side-map, waiter re-evaluation, WARN-counter for unknown reporters); `ShipQueue::{add_peer,remove_peer}`; `ElectionState::reconfigure` + learner campaign gate (+ verify/fix the p4 quarantine campaign gate); compaction retention pin (straddling-segment-safe, unit-tested); `create_backup` hardenings (fatal marker, HWM refresh in-step, staging variant skipping text flush) |
|
||
| `tidal-net` | proto: `FetchSnapshot`/`SnapshotChunk`, `JoinCluster`, `capabilities` on `HeartbeatResponse`/`AppliedReport`; typed catch-up refusal trailer (`x-tidal-catchup`); `SnapshotSource` trait (late-bound) + server handler (StreamSegments-shaped); standalone snapshot-fetch client + `PeerPool` verbs; `peers` retype to `String` + dynamic add/remove; changed-address reconvergence test |
|
||
| `tidal-server` | bind/advertise split; boot-time install (§2.2 scope, §2.3 swap + recovery, §2.5/§2.6 ordering); seed-join boot loop (§2.7) + flags (`--seed`, `--advertise-*`, `--metrics`) + §3.5 knob-source rule; membership runtime (one fenced apply path; activation record; capability gate; auto-promotion duty; removal delivery); `/cluster/join`, `/cluster/members` (+remove), `/cluster/reseed` verbs; reseed marker + quarantine wiring + self-restart quorum refusal; readiness predicate; metrics; status fields |
|
||
| `k8s/` | `k8s/cluster/` per §4; stress-Job retargeting |
|
||
| tests | unit: even-n majority properties, record/store round-trips, reconfigure paths (incl. grant-set clearing, learner marks, waiter re-evaluation), retention-pin straddle, swap-recovery windows, topology DNS derivation table; tidal-net sockets: snapshot stream (fake-source), join RPC, capability gate, typed refusal, changed-address; tier-3: `mp_seed_join_snapshot_catchup` (vs a compacted leader), reseed drill (quarantine → marker → restart → converged, gauges cleared, no `wipe_data_dir`), DNS-hostname topology + chaos, `mp_scale_3_5_3_under_load_zero_loss` (ledger invariants, 429-retrying writers, p99 probes) |
|
||
|
||
### 6. Configuration
|
||
|
||
| Knob | Default | Meaning |
|
||
|---|---|---|
|
||
| `regions[].grpc_bind` | derived (§1) | local gRPC bind when `grpc_addr` is a DNS name |
|
||
| `replication.reseed_self_restart` | false | drain + clean exit once `reseed_required` latches (k8s: true); refused when remaining voters can't sustain quorum |
|
||
| `replication.snapshot_artifact_ttl_ms` | 600000 | staged-artifact reuse window (never the active-consumer pin, §2.1) |
|
||
| `replication.learner_promote_lag` | 1024 | promotion distance; also the readiness hysteresis threshold (§4) |
|
||
| CLI `--seed <url>` (repeatable), `--advertise-grpc`, `--advertise-http`, `--metrics` | — | seed-join boot (requires the local knob file, §3.5) |
|
||
|
||
## Exit gate (from the roadmap, restated as tests)
|
||
|
||
1. **Seed join + snapshot catch-up:** a fresh node with an empty data dir
|
||
joins via `--seed` against a leader whose WAL has rotated AND compacted
|
||
past seq 1 → converges via snapshot + stream with zero operator verbs and
|
||
no full-log replay; tier-3 `mp_seed_join_snapshot_catchup`.
|
||
2. **Scale 3→5→3 online under load:** background `ack=quorum` writers
|
||
(WriterTally, lost==0, retrying 429s per §2) through both gateways while
|
||
two nodes join, promote to voters, then one is removed and
|
||
decommissioned → m11p3 ledger invariants (frontier + content) hold
|
||
throughout; the joiner serves quorum (its reports advance the commit
|
||
index); p99 impact <2× for <60 s; 100k-item joiner catch-up ≤5 min
|
||
(env-scaled gate run recorded here).
|
||
3. **Reseed self-healing:** a quarantined divergent node (p4 drill) latches
|
||
`reseed_required`, restarts, reseeds via snapshot, rejoins clean, and the
|
||
divergence gauge clears — no `wipe_data_dir` in the test.
|
||
4. **DNS:** a tier-3 cluster whose topology names peers by hostname boots,
|
||
replicates, and survives the existing chaos drills; unit tests pin the
|
||
bind/advertise derivation table; the k8s pod-reschedule drill is recorded
|
||
as exit-gate evidence (§1).
|
||
|
||
## Status
|
||
|
||
- [x] Design adopted — 33 adversarial-review findings (9 BLOCKER) folded in pre-implementation
|
||
- [x] §3.0 majority() fix + DNS/bind-advertise split — `tidal`, `tidal-net`, `tidal-server`
|
||
- [x] Snapshot transfer + boot-time install + reseed marker — all crates
|
||
- [x] Membership records + conf-changes + seed join — all crates
|
||
- [x] Kubernetes reference (`k8s/cluster/`)
|
||
- [x] Exit-gate suites green; gate evidence recorded
|
||
- [x] Docs (runbook §§1b/3/6/8/9/11, kubernetes.md, CHANGELOG, roadmap, spec 01 §2.2, monitoring.md)
|
||
|
||
## As built (deltas vs the adopted design)
|
||
|
||
1. **`majority()` even-n fix landed `div_ceil`-spelled, not the `(n+1)/2+1`
|
||
form the design wrote.** `(peers.len() + 1).div_ceil(2) + 1` is the exact
|
||
same `floor(n/2)+1` over the full voter set (`n = peers.len()+1`), but
|
||
spelled with `div_ceil` so the two arithmetic sites in the subsystem —
|
||
`ElectionConfig::majority()` and `CommitIndex` — now read identically
|
||
(the design's whole point: the two formulas can no longer drift). Even-n
|
||
property tests landed first, as planned.
|
||
|
||
2. **The three-way term-join rule is implemented, AND `FetchSnapshot`'s
|
||
`needed` decision is baseline-aware** (§2.4 as adopted). Both were found by
|
||
the first tier-3 reseed drill, not the pre-implementation review, and both
|
||
are in the shipped design above — recorded here too because they were the
|
||
single largest correctness delta from the first draft: a follower behind at
|
||
a leadership change advances its frontier past pre-baseline history with
|
||
`lag=0` and never asks for the gap, so detection rides the heartbeat's
|
||
`prev_log` comparison (`own < prev_log` → latch `reseed_required`), and the
|
||
snapshot handler answers `needed` from `from_seqno ≤ stream_baseline OR
|
||
from_seqno < earliest WAL seq` rather than a naive "the WAL still covers it"
|
||
(which loops a marker boot forever at or below the baseline).
|
||
|
||
3. **Three product bugs surfaced and were fixed during the C3 build** (the
|
||
membership-runtime + verb stage), all in the as-built behavior above:
|
||
(a) the membership-apply path had to enter the async reactor context
|
||
correctly before driving the fenced reconfigure — a stray non-reactor enter
|
||
wedged the first conf-change; (b) a freshly-joined Learner did not receive
|
||
leader heartbeats until its first applied record, so its promotion clock
|
||
never started — heartbeats now flow to learners (they are in the ship set,
|
||
just not the quorum); (c) boot-time reconcile of the durable membership
|
||
cache against the WAL-recovered `ClusterMembership` cell had to prefer the
|
||
cell unconditionally (§3.6) — an early version let a stale cache shadow a
|
||
newer in-stream roster on restart.
|
||
|
||
4. **Auto-promotion landed in the C3 stage, alongside the membership runtime**
|
||
— not as a separate increment. It is the standing leader duty of §3.3
|
||
(re-armed on every leadership activation and every membership apply,
|
||
evaluated on commit-index publishes), driven solely from the applied
|
||
`ClusterMembership`, so it survives the joining-era leader's death.
|
||
|
||
5. **Removal grace mechanics (D1).** The `Removed` record retires the peer's
|
||
ship cell only after the record is quorum-committed AND delivered-to /
|
||
acked-by the removed peer, with a bounded give-up that increments
|
||
`tidaldb_cluster_remove_delivery_giveups_total` and a typed `removed`
|
||
refusal that flips the removed node's readiness to 503 and suppresses its
|
||
campaigning (§3.3 as adopted; D1 is where the delivery-then-retire ordering
|
||
and the give-up counter were wired end to end).
|
||
|
||
6. **`needed_peers` moved from a construction-frozen constant to a function**
|
||
of the latest applied record. The four peer-set copies (CommitIndex,
|
||
ShipQueue, PeerPool, ElectionState) were `const`-derived at boot in p4; the
|
||
fenced reconfigure path (§3.3) made them a function of the applied
|
||
`ClusterMembership`, evaluated inside the single membership-apply critical
|
||
section so they can never disagree about the roster.
|
||
|
||
7. **Re-appending the activation membership record is non-fatal on failure.**
|
||
The §3.2 activation re-append (the linchpin that closes the vacuous gate and
|
||
the baseline-jump skip) logs and continues if the append itself fails rather
|
||
than aborting the activation — a leader that cannot journal its roster is
|
||
still a leader for fencing purposes, and the next apply/activation retries
|
||
the record. (The conf-change *gate* still requires same-term quorum
|
||
commitment of the record before the NEXT change, so safety is unaffected;
|
||
only the cosmetic "every activation always has a record" property degrades
|
||
gracefully under a write fault.)
|
||
|
||
8. **The faster election machinery changed `/cluster/promote`'s topology-era
|
||
behavior — fixed by gating the transfer leg on the era.** m11p4's promote
|
||
handler relied on a behavioral accident: in the term-0 topology era,
|
||
`transfer_to(target)` never took (the target couldn't acknowledge a
|
||
`TimeoutNow` at a term it hadn't joined), so the handler always fell
|
||
through to the legacy fan-out and its documented `{ok, leader, baseline,
|
||
acked, failed}` response shape. p5's election work made that same transfer
|
||
*succeed*, returning the elected-era `{ok, leader, term, transfer}` shape
|
||
early and breaking the runbook's deserialization plus the non-leader
|
||
forward leg (`cluster_runbook` ×2 + `cluster_lifecycle` ×1, surfaced by
|
||
the D2 regression sweep). Fix: a topology-era leader does not
|
||
fence-transfer — it falls through to the legacy fan-out as documented —
|
||
while the self-campaign leg (the reseed drill's re-leadership path) and
|
||
the genuinely-elected era keep the fenced transfer. `term`/`transfer`
|
||
remain additive fields on the elected path.
|
||
|
||
## Exit-gate evidence
|
||
|
||
All gate suites are tier-3 (`--features cluster-e2e`), real OS processes over
|
||
localhost loopback, release test binary + debug-spawned servers. The k3s
|
||
(Ref-A) re-run is **pending infra access** (the standing M11 caveat, see
|
||
Carried hazards) — the StatefulSet reference (`k8s/cluster/`) is the
|
||
deliverable, but the under-load gate numbers below are localhost, stated
|
||
honestly as such.
|
||
|
||
| Gate (roadmap, restated as test) | Suite / test | Result |
|
||
|---|---|---|
|
||
| 1. Seed join + snapshot catch-up (fresh node, leader compacted past seq 1, zero verbs, no full-log replay) | `cluster_reseed.rs` + `cluster_membership.rs::mp_seed_join_snapshot_catchup` | **ok** — converges via snapshot + stream; env-scaled: 320 items join→converged 5.3s / 44 MB, 2000 → 13.0s / 269 MB, 5000 → 26.4s / 655 MB, every probed item searchable. The catch-up metric is ~26s at 5000 heavy items, large headroom under the 5-min budget. |
|
||
| 2. Scale 3→5→3 online under load (ack=quorum writers retrying 429s, ledger invariants hold, joiner serves quorum, p99 <2× for <60s) | `cluster_membership.rs::mp_scale_3_5_3_under_load_zero_loss` | **ok** — `lost=0`, `max_acked_seq=1467`; INVARIANT A (max acked seq ≤ leader frontier) + INVARIANT B (sampled acked content searchable) held at every 3→4→5→4 transition; per-window write p99 before=54.6ms, join-A=105.5ms, join-B=68.6ms, after=102.6ms (all <2× steady, within the <2×-for-<60s Ref-A figure; recorded, not hard-asserted per the non-flaky posture). |
|
||
| 3. Reseed self-healing (quarantined divergent node latches `reseed_required`, restarts, reseeds via snapshot, gauge clears, no `wipe_data_dir`) | `cluster_reseed.rs` | **ok** — quarantine → marker → restart → converged; `tidaldb_cluster_divergence_quarantined` + `tidaldb_cluster_reseed_required` clear after a genuine reseed; no `wipe_data_dir` in the test. |
|
||
| 4. DNS (hostname-topology cluster boots, replicates, survives chaos; bind/advertise derivation table pinned) | `cluster_membership.rs::mp_dns_hostname_topology_replicates` + unit derivation tests | **ok** — a 3-node hostname topology (which a pre-m11p5 `SocketAddr::parse` would have refused) boots, replicates a leader write to a follower, and survives a SIGKILL+restart drill (the resolver re-dials on reconnect — the point of the String peer retype). The k8s pod-reschedule drill (changed pod IP) is the deliberate Ref-A line item, pending infra access. |
|
||
|
||
Full suite tail (2026-06-12): `cluster_membership` 5 passed / 0 failed (86.78s,
|
||
incl. "[scale] exit gate 2 met" and "[dns] exit gate 4 met"); `cluster_multiproc`
|
||
5 passed, `cluster_chaos` 4 passed, `cluster_quorum` 2 passed,
|
||
`cluster_election` 3 passed, `cluster_reseed` 2 passed. `cargo test -p tidaldb
|
||
--lib` 1896 passed / 0 failed; `cargo clippy --workspace --all-targets
|
||
--all-features -- -D warnings` clean; `cargo fmt --check -p tidal-server` exit 0.
|
||
|
||
> The D2 sweep surfaced a promote regression in `cluster_lifecycle` /
|
||
> `cluster_runbook` — **fixed post-D** (see As-built #8): all eight tier-3
|
||
> suites are green. Final post-fix sweep (2026-06-12): multiproc 5, chaos 4,
|
||
> lifecycle 2, runbook 9, quorum 2, election 3, reseed 2, membership 5 — all
|
||
> 0 failed when run per-suite; `tidaldb --lib` 1896 passed; tidal-net 10 +
|
||
> tidal-server 19 ok-binaries; workspace clippy `-D warnings` clean; fmt
|
||
> clean; `scripts/check-docs.sh` OK. The suites are load-marginal when run
|
||
> back-to-back on one saturated host (a colocated sweep flaked one runbook
|
||
> drill that passes 9/9 isolated) — the same documented sensitivity class as
|
||
> p4's `mp_rolling_upgrade` note; widen `TIDAL_TEST_CONVERGENCE_BUDGET_SECS`
|
||
> on slow runners.
|
||
|
||
## Carried hazards (tracked, not regressions)
|
||
|
||
1. **Ref-A (k3s) run pending infra access.** Every under-load and catch-up
|
||
number above is localhost loopback on a debug-spawned server, not Ref-A.
|
||
The k3s deployment has been unreachable from the local environment since
|
||
m11p1 (the standing caveat in p2/p3); p5's k8s-centric gate cannot fall back
|
||
to localhost the way p1–p3 did, so the StatefulSet reference (`k8s/cluster/`)
|
||
is the deliverable and the k8s pod-reschedule drill (gate 4, §1) +
|
||
100k-item Ref-A catch-up remain the named Ref-A line items. The 5000-item
|
||
localhost run is the largest that completed in reasonable wall time — the
|
||
ceiling was the ~13 items/sec HTTP **seed** rate against a debug server (the
|
||
seed cost, not the catch-up path the gate measures), not a catch-up limit.
|
||
|
||
2. **Tier-3 suites are load-marginal under colocation.** Each suite is green
|
||
in isolation (the way gate runs are recorded), but running all eight
|
||
back-to-back on one saturated host can flake a FAST_ELECTION-timed drill
|
||
(one runbook failure in a full-sweep run that passes 9/9 isolated, three
|
||
times over). Same class as p4's `mp_rolling_upgrade` note; the reseed
|
||
suite's content probes were widened from a fixed 15 s to the harness
|
||
`convergence_budget()` in-phase. m11p9's nightly CI should run suites
|
||
serially or on isolated runners.
|
||
|
||
3. **The quiesce window is recorded, not yet measured at 100k on Ref-A.**
|
||
§2's `create_backup` 429s signal writes for checkpoint + flush + copy; gate
|
||
writers retry 429 honoring `retry_after_ms` (neither lost nor an SLO
|
||
violation), and the localhost windows are within budget — but the
|
||
"seconds at 100k items" figure the design names is a Ref-A measurement
|
||
still owed once infra access returns.
|
||
|
||
4. **DNS changed-IP coverage is the k8s drill, not a localhost test.** The
|
||
re-resolution property (a pod rescheduled onto a new IP, ship/stream resumes
|
||
with no peer restarts) is the entire point of the `String`-peer retype and
|
||
cannot be regressed by a localhost test that never changes addresses. The
|
||
`mp_dns_hostname_topology_replicates` test proves the hostname-topology boot
|
||
+ reconnect re-dial; the changed-address reconvergence is the k8s
|
||
pod-reschedule line item under hazard 1.
|