---
status: draft
last_updated: "2026-10-10 (ADR-024 — the mem engine joins the matrix:
host row, connection budget, durability posture, wasm32 toolchain note)"
---

# Deployment

What a deployer must know to size, run, and reason about alkstore
engines: host semantics, connection budgets, durability knobs, and
where engine differences may honestly surface in the contract. The
capability-surface *decision* is resolved
([ADR-016](decisions/016-deployment-honesty.md), 2026-10-06): no
runtime capability surface — this document's matrix is (with the
engine crates' own docs) where the honest boundary lives; this
document holds the facts.

## Host semantics

| Engine | Host posture | Notes |
|---|---|---|
| SQLite | **single-machine**, file-backed | NFS two-writers unsupported (honker's honesty posture, inherited, [ADR-003](decisions/003-sqlite-driver.md)). Cross-process *on one host* is verified POC ground (`data_version` is cross-process by nature). |
| Postgres | **multi-host native** | Nothing assumes a shared host; POC #2 ran all-through-network (docker bridge) with the same properties ([ADR-004](decisions/004-postgres-driver.md)). |
| Mem | **single-process, ephemeral** ([ADR-024](decisions/024-mem-engine.md) §2) | In-memory, per-instance; no cross-process story at all — two engine instances share nothing by design. No durability, by design: loss on process exit is the documented posture, and the dependency on `alkstore-mem` *is* the ephemerality declaration (honest for workloads that accept it). Never fleet-valid — the shared-pool predicate ([ADR-010](decisions/010-queue-semantics-depth.md) §3) fails structurally; there is no pool. Compile-clean on `wasm32-unknown-unknown` ([ADR-024](decisions/024-mem-engine.md) §5) — the family's member that runs in the sandbox. |

The unified trait must not pretend SQLite is multi-host — and it does
not: [ADR-016](decisions/016-deployment-honesty.md) resolves that
honesty to compile-time engine identity (the engine crate a binary
depends on *is* the deployment statement) plus this documented matrix.
There is no `Store::capabilities()` — the trait's shape constrains
where capability differences can surface
([ADR-006](decisions/006-wake-and-delivery-contract.md)), and the
resolution is: nowhere at runtime.

Options for OQ-08, with their outcome
([ADR-016](decisions/016-deployment-honesty.md)):

1. **Compile-time only** — a consumer chooses an engine crate at
   dependency time; the engine's docs carry its deployment facts.
   Smallest contract; nothing runtime to match on. **Adopted** —
   together with (3); the two compose (engine docs serve the consumer
   choosing the dependency, the matrix serves the operator choosing
   the topology).
2. **`Store::capabilities()`** — a runtime description
   (payload limits, wake cadence knobs, host semantics). Lets a
   consumer adapt (e.g., chunk large notify payloads) but adds a
   contract surface all engines must keep honest. **Rejected** —
   field-by-field under ADR-008 §5's act-differently rule, and no
   consumer-inventory row names a runtime-adapt need
   ([ADR-016](decisions/016-deployment-honesty.md) §2).
3. **Deployment matrix only** (this document) — no API surface. The
   honest-middle choice; matches the ecosystem's doc-first posture
   but provides no programmatic guard. **Adopted** (with (1)) — the
   "programmatic guard" gap is closed where it can honestly be:
   the engine-crate dependency edge cannot drift out of sync with
   the truth it states; the misconfiguration case (SQLite as shared
   network storage) follows the family's
   deployment-asserts-truth posture — documented detection symptom,
   no fabricated runtime machinery
   ([ADR-016](decisions/016-deployment-honesty.md) §3).

## Connection budgets

### SQLite engine

- Connections are in-process (writer + reader pool + watcher thread
  owning a connection). No external budget lines; the file lock is
  the OS-level resource.

### Postgres engine

| Connection class | Count | Notes |
|---|---|---|
| Pool | `max_size` per process | claims/queries via deadpool |
| Listener | **+1** per LISTEN-ing process | non-pooled, dedicated; pooled connections cannot carry LISTEN (deadpool#360, test-pinned — [ADR-004](decisions/004-postgres-driver.md)) |
| — | — | Sizing rule: `max_size + 1` per process; verify end-to-end accounting (POC #2's pgdiag-st-4: pool 6 + 1 listener + probe conns = exact server-side count) |

Shared-server co-tenancy (the alkblobs ADR-008 precedent —
`/workspace/@alkdev/alkblobs/docs/architecture/decisions/` — consumer
tables co-tenant the pg instance) is supported and expected — the
[naming / reserved namespace contract](core-contract.md#naming--reserved-namespace)
protects reserved names; queue/stream/lock/schedule tables live in one
engine-owned PostgreSQL schema (default `alkstore`, per-engine option)
— layout per [ADR-010](decisions/010-queue-semantics-depth.md) §8
(resolved from [queues.md](queues.md)'s namespace bullet).

### Mem engine

- No connections — there is nothing to budget. The engine instance
  owns its state in-process; sharing nothing is the isolation design
  ([ADR-024](decisions/024-mem-engine.md) §4's instance rule; the
  two-opens-one-engine constructor is rejected, re-entry via a
  consumer-inventory row).

### TLS posture (v1)

The pg engine hardwires `NoTls` on **every connection path** in v1 —
the pooled connections, the dedicated listener connection, and every
reconnect attempt alike; the engine does not ride the consumer's
`Config` sslmode setting, and a DSN carrying `sslmode=require` (or any
TLS demand) fails at connect. The listener's `NoTls` posture is
test-pinned in the type it carries
([ADR-004](decisions/004-postgres-driver.md)); v1 TLS should therefore
be treated as **effectively unavailable** — network confidentiality
between the process and the Postgres server must come from the
deployment topology itself (private network, egress rules, a local
unix socket or sidecar), not from driver TLS. Encrypting traffic
between engine and server is a **post-v1 deployment concern** (wire a
TLS connector through the pool and listener construction); until then
the boundary above is the honest statement, per
[ADR-016](decisions/016-deployment-honesty.md)'s spirit — the matrix
states the limitation rather than having code pretend otherwise.

## Consumer-obligation notes on engine options

Consumer-*constructed* numeric option values are **trusted as given —
the engine does not validate them against domain extents**. The
engine would be a second normative home for semantics the contract
deliberately leaves to the caller's constants; the
[ADR-023](decisions/023-fourth-review-round.md) §2 domain table
covers the trait-surface arguments, not these. This applies to the
queue stamps (`QueueOpts`: `visibility_timeout_s`,
`dead_letter_retention_s`, `max_attempts`) on both engines:

- Negative or zero `visibility_timeout_s` stamps claims whose deadline
  is already past — every claim is instantly reclaimable (the
  dual-execution window is the consumer's documented budget
  [ADR-010](decisions/010-queue-semantics-depth.md) §2).
- Zero or negative `max_attempts` stamps rows the claim statement
  never hands out — each is dead-lettered with reason "max attempts
  exceeded" at the next claim call on that queue (the pre-claim
  sweep's `attempts >= max_attempts` arm), i.e. the queue silently
  discards its work.
- Negative `dead_letter_retention_s` deletes every dead row at the
  next `sweep_expired` call (retention is driver-free — nothing runs
  without a caller).
- The stamps ride *future enqueues only* (`QueueOpts`'s documented
  shape) — repair the opts before enqueueing rather than repairing
  rows afterwards.

The same trusted-as-given shape applies to `PgOpts::max_size`'s
*positive-integer* contract — 0 is typed-failed at open, the one knob
with an engine-side guard; everything above has none. The
consumer-obligation shape here is the visibility-budgeting precedent
([ADR-010](decisions/010-queue-semantics-depth.md) §2): the constant is
the consumer's, the behavior of a mis-set one is documented, nothing
runtime is fabricated.

## Durability knobs

| Engine | Knob | Shape |
|---|---|---|
| SQLite | `synchronous` | WAL + `NORMAL` shipped ([ADR-003](decisions/003-sqlite-driver.md)); FULL is available consumer-side for stricter durability; commit fsyncs land at WAL checkpoints (the ~1000-commit spike cadence, POC #1) |
| SQLite | `poll_interval` | the watcher's `data_version` poll cadence — 1 ms shipping default ([ADR-023](decisions/023-fourth-review-round.md) §4, the measured-wake-latency posture), carried on `SqliteOpts`; the idle cost is ~1000 poll reads/sec/instance; raising the interval trades wake latency (interval-bound) for idle CPU — the tuning recipe for latency-tolerant deployments |
| Postgres | `synchronous_commit` | per-session knob; `on` is ship config (p50 2.40 ms seam); `off` trades max-tail (40.9 ms) for slightly better p50 — measured, honest trade ([ADR-004](decisions/004-postgres-driver.md)); session-level SET mechanics POC-verified |
| Mem | — (none) | There is no durability knob and no durability: the honest row is the absence itself ([ADR-024](decisions/024-mem-engine.md) §2). Wake latency is sub-millisecond in-process; no tuning surface exists |

These are engine-configuration concerns, *not* trait surface. What
part of engine config is contract-level shape vs engine-crate docs is
decided: constructors and option structs live in the engine crates;
the contract is the trait the constructor returns
([ADR-008](decisions/008-contract-v1-pinning.md) §6).

## Toolchain / platform notes

| Note | Engine | Affects |
|---|---|---|
| rusqlite 0.40.x needs rustc ≥ 1.99 | SQLite | any binary linking `alkstore-sqlite` ([ADR-003](decisions/003-sqlite-driver.md)) |
| `bundled-sqlite` adds a C build (~10 s dev, cacheable) | SQLite | build/CI time |
| `libsqlite3-sys` collision with sqlx today | any mixed-driver binary | structurally avoided ([ADR-001](decisions/001-crate-split.md) single-driver rule) |
| Listener `application_name` set for diagnosability (kill-targetable) | Postgres | ops runbooks ([ADR-004](decisions/004-postgres-driver.md)) |
| `alkstore-mem` compiles clean on `wasm32-unknown-unknown` (wave-6 acceptance, [ADR-024](decisions/024-mem-engine.md) §5); tokio features limited to the wasm-supported subset (sync + time); current-thread-runtime-clean internals (the suite's mem column runs under that flavor) | Mem | wasm targets, sandboxes, downstream ffi/napi/python adapters — the only engine that can compile to wasm (rusqlite/tokio-postgres are structurally out) |

## Consumer-facing latency profile (indicative, POC-measured)

From both POCs (single-box, relative shapes are the deliverable —
[ADR-003](decisions/003-sqlite-driver.md) and
[ADR-004](decisions/004-postgres-driver.md) carry the full tables):

- Tx seam: SQLite ~0.35 ms p50; Postgres ~2.4 ms p50 (ship config).
- Wake: ~1.1–2.2 ms p50 both engines at default cadence.
- Queue claim: LISTEN-driven 3–6 ms p50 (pg); poll-only interval-bound
  (32–50 ms at a 50 ms poll).
- Absolute numbers will differ per hardware/network; they set
  *expectations of order*, not SLAs — contract docs must not bake
  them in ([ADR-007](decisions/007-transactional-seam.md)'s pg-POC
  note).

## Design Decisions

| ADR | Decision | Summary |
|---|---|---|
| [001](decisions/001-crate-split.md) | Crate split | single-driver binaries shape the matrix |
| [003](decisions/003-sqlite-driver.md) | SQLite driver | bundling, toolchain floor |
| [004](decisions/004-postgres-driver.md) | Postgres driver | listener budget line, forwarder posture |
| [006](decisions/006-wake-and-delivery-contract.md) | Wake contract | where capability differences may surface |
| [008](decisions/008-contract-v1-pinning.md) | Contract v1 | constructor/options in engine crates; no capability surface in v1 (OQ-08; resolved by [ADR-016](decisions/016-deployment-honesty.md)) |
| [016](decisions/016-deployment-honesty.md) | Deployment honesty | no runtime capability surface — compile-time identity + this matrix; `PayloadTooLarge` is the one runtime asymmetry carriage |
| [021](decisions/021-tx-reads-and-value-shape-fixes.md) | Third review round | drop = rollback keeps the connection budgets exact under error paths (SQLite lease releases; pg client re-pools) |
| [024](decisions/024-mem-engine.md) | Mem engine | the matrix's third engine — honest-ephemeral: single-process, no durability, never fleet-valid, wasm32 compile-clean; no connection budget, no knobs |

## Open Questions

Open questions are tracked in
[open-questions.md](open-questions.md). Key
questions affecting this document:

- **OQ-08**: capability-surface shape — **resolved**
  (2026-10-06, [ADR-016](decisions/016-deployment-honesty.md)):
  no runtime capability surface; compile-time engine identity +
  this matrix; re-entry via a consumer-inventory row naming a
  runtime-adapt need.