Tier 4 — Data-center swarm

The core fabric shipped at v0.6.3: the peer mesh, W-of-N quorum-writes, mTLS with fingerprint allowlist, federated governance, pending decisions, namespace metadata, capabilities introspection v2. The shared distributed store (Postgres + pgvector) SHIPPED at v0.7.0 via --features sal-postgres.

The diagram distinguishes layers: solid edges = always-on at v0.7.0; the Postgres + Apache AGE + pgvector layer ships at v0.7.0 via `--features sal-postgres`.

Multi-rack swarm · pgvector + Apache AGE backbone (shipped at v0.7.0).

Shipping today.

The big T4-relevant capability that's already wired:

• src/daemon_runtime.rs (ServeArgs) — --quorum-writes N enables W-of-N replication; --quorum-peers a,b,c lists targets.
• src/handlers/ — write path calls broadcast_store_quorum then finalise_quorum. Local commit + (W-1) peer acks within --quorum-timeout-ms (default 2000) → 200 OK with the achieved ack count returned in the response. Falling short → 503 quorum_not_met.
• Same wiring on link, consolidate, pending_decision, namespace_meta, and the clear_namespace_meta path.
• mTLS for the outbound side via --quorum-client-cert / --quorum-client-key / --quorum-ca-cert.

ADR-0001 is realised, not aspirational. Phase 1 (scaffold) and Phase 2 (wired into write path) shipped. Phase 3 (chaos harness) and Phase 4 (formal convergence-bound report) are still work — the convergence guarantees in production are exercised by the a2a-gate scenario suite (CHANGELOG #325/#326/#327).

Postgres + pgvector backbone.

The single-SQLite-per-node ceiling bites at T4 scale (>10⁶ memories per node, hot writers). The structural fix is a shared store.

Today (v0.6.x): sal-postgres Cargo feature flag exists; sqlx + pgvector deps wired. The Postgres adapter compiles and runs; correctness fixes shipped in v0.6.0 pre-tag (#294 SAL upsert key alignment, #295 metadata.agent_id immutability via jsonb_set, #296 tier-downgrade protection via SQL tier_rank(), #297 schema parity with 6 tables + generated scope_idx column). That work is the foundation.

v0.7.0 SHIPPED: Performance maturation complete, ai-memory migrate --from sqlite:// --to postgres:// migration tool wired, pgvector + Apache AGE recipes in docs/migration-v0.7.0-postgres.md, official deployment via --features sal-postgres.

What the shared store unlocks at GA:

• Horizontal read scaling (Postgres read replicas).
• Shared vector index across the fleet — one embedding stored once.
• Standard Postgres operational primitives — backups, PITR, monitoring.
• Network filesystems no longer dangerous (the SQLite fcntl lock issue goes away).

Every primitive used in the T3 diagram extends to T4.

The topology just gets denser.

• Rack-to-rack peer mesh — sync_push works the same whether you have 4 nodes or 40. The number of edges is n*(n-1)/2 for full mesh, which is a gossip cost the operator should plan for. Most T4 deployments will use a partial mesh with a few designated bridge nodes per rack.
• Federated governance plane — broadcast_namespace_meta_quorum and broadcast_pending_decision_quorum mean a tightening of policy on org/eu/finance propagates to every rack, every node. v0.6.3 capabilities introspection lets every agent and operator confirm that the policy version they're seeing matches the published version.
• Per-rack pending queues — Approval workflows can be scoped to a rack (rack-a/approvers) so on-call shifts work cleanly even at fleet scale.
• mTLS — rustls is already wired in the peer transport. Enforcement is opt-in today; it's the obvious default at T4.
• Capabilities introspection v2 — Operators can fleet-scan /api/v1/capabilities to confirm every node is on the same schema version and feature tier. Drift is an alerting condition, not a surprise.

Honest ceilings.

A 10-rack, 100-node fleet with 3-of-5 quorum is in scope today. The v0.7 piece that's still maturing is the Postgres backbone — the vector-index drift across nodes and the per-node SQLite operational footprint are the things shared-store fixes.

Today: quorum + mTLS allowlist.

# rack-a/node-2 — quorum 3-of-5 across the rack, mTLS enforced
ai-memory --db /var/lib/ai-memory/store.db serve \
  --host 0.0.0.0 --port 9077 \
  --quorum-writes 3 \
  --quorum-peers https://rack-a-1:9077,https://rack-a-3:9077,https://rack-b-1:9077,https://rack-b-2:9077,https://rack-c-1:9077 \
  --quorum-timeout-ms 1500 \
  --tls-cert /etc/ai-memory/tls.crt \
  --tls-key /etc/ai-memory/tls.key \
  --mtls-allowlist /etc/ai-memory/peer-fingerprints.txt \
  --quorum-client-cert /etc/ai-memory/client.crt \
  --quorum-client-key /etc/ai-memory/client.key \
  --quorum-ca-cert /etc/ai-memory/ca.crt \
  --catchup-interval-secs 15

v0.7.0 — same node, swap to Postgres-backed store:

ai-memory serve \
  --store-url postgres://ai-memory@pg-cluster.svc.cluster.local:5432/store \
  --host 0.0.0.0 --port 9077 \
  --quorum-writes 3 \
  --quorum-peers https://rack-a-1:9077,https://rack-a-3:9077,https://rack-b-1:9077

The peer-fingerprints.txt file lists trusted SHA-256 fingerprints (one per line, optional : separators, # comments). The --mtls-allowlist flag's presence is what makes mTLS enforcement required — if a peer's cert isn't on the list, the connection is refused at TLS handshake.

Governance, skills, and attestations at T4.

• Governance — Same machinery as T2/T3, just at scale. The federated control plane is the central nervous system: a single policy update on org/eu/finance reaches every node within seconds. Pending-action queues are visible per-rack for on-call routing.
• Skills — Skills register as agents and get the same scope visibility. At T4, skills are typically pinned to specific racks (latency, data-residency) but their outputs (tags, contradictions, consolidations) propagate fleet-wide via sync_push.
• Attestations — v0.7.0 attested identity (#626 Layer-3 Ed25519 store-surface signatures) is the gating capability for trustworthy multi-tenant T4. Unsigned writes still land attest_level = "claimed" unless AI_MEMORY_REQUIRE_AGENT_ATTESTATION=1 — without enforcement the cluster is "trust-the-network": operationally fine inside a controlled VPC, not fine for cross-tenant or cross-organization deployments.

Source-of-truth references.

• src/daemon_runtime.rs (ServeArgs) — quorum-write CLI surface
• src/daemon_runtime.rs (ServeArgs::store_url) — --store-url postgres://... URL syntax
• src/daemon_runtime.rs (ServeArgs) — TLS / mTLS allowlist enforcement
• src/handlers/ + src/federation/{sync,quorum}.rs — broadcast_store_quorum + ack counting
• src/replication.rs — QuorumPolicy + AckTracker (functional)
• Cargo.toml — sal-postgres feature, sqlx + pgvector deps
• CHANGELOG.md — v0.6.0 pre-tag SAL fixes (#294-#297), v0.6.2 fanout bugs (#325-#327)
• docs/ADR-0001-quorum-replication.md — realised design
• docs/ARCHITECTURAL_LIMITS.md — the honest ceiling on each dimension