nearle_pos/docs/sync-contract.md

Terminal ↔ back office sync contract

What the till guarantees, and what the back office must do to hold up its end. Everything here is enforced by tests in test/unit/retention_test.dart, test/unit/transport_test.dart and test/unit/sync_engine_test.dart.

The shape

Customer pays
     │
     ▼
One SQLite transaction: bill + stock + loyalty        ← never blocked on network
     │
     ▼
orders row lands at sync_status = 0                    ← this table IS the outbox
     │
     ▼
SyncEngine drains on: sale committed · network back · 5-min poll · head office
     │                asked · cashier pressed Sync
     ▼
Transport publishes a batch
     │
     ▼
Back office commits and names the ids it took
     │
     ▼
Those rows → sync_status = 1, folded into day_archive, kept 7 days, then purged

Anything the back office does not name stays at 0 and goes again.

Non-negotiables

1. Only an application acknowledgement counts. A broker PUBACK means "I hold these bytes". It is not evidence the ledger accepted anything, and the terminal never treats it as such. The back office must answer on the ack topic naming the order ids it committed.

2. Silence is not acceptance. A 200 OK with an empty body, or an ack with no accepted array, marks zero bills synced. The terminal will send them again rather than guess.

3. Delivery is at-least-once, so the back office must be idempotent. QoS 1 re-delivers, and a lost ack makes the terminal re-send the whole batch. Every order.id is a UUID minted at the till. Put a unique index on it and upsert. Without this you will double-count a day's takings the first time a shop's line wobbles.

Invoice numbers are INV-2608-T4A9-00042 — the terminal code is in there because each till's sequence counter lives in its own database and starts at 1. Unique per terminal, not globally sequential. Do not assume gaps mean missing bills; a till that was replaced restarts its own series.

4. A refusal is final, a failure is not. Naming an id in rejected halts the terminal's drain — it will not retry the same bytes, and a person has to press Sync. Use it for "this bill is wrong" (unknown product, duplicate invoice). For "I am having a bad minute", drop the connection or return 5xx instead, and the terminal will back off and retry.

MQTT topics

Topic	Direction	QoS	Retained
pos/{store}/{terminal}/order	till → cloud	1	no
pos/{store}/{terminal}/ack	cloud → till	1	no
pos/{store}/{terminal}/status	till → cloud	1	yes
pos/{store}/{terminal}/command	cloud → till	1	no
pos/{store}/catalogue	cloud → all tills	1	yes

{store} and {terminal} come from the device's own identity, minted on first run and stored in its database. They are never literals — 100 tills sharing one id would collide on every topic and evict each other from the broker, since a second connection with the same client id kicks the first off.

status is also the Last Will. If a till loses power the broker publishes {"state":"offline"} on its behalf — that is what makes a "which tills are dark" board possible, and it is the only way to tell closed for the night from unplugged.

Running this on NATS

The MQTT gateway maps / to ., so the topics above arrive as subjects and a JetStream consumer binds to them directly:

Purpose	Subject
Every till's bills	pos.*.*.order
Every till's presence	pos.*.*.status
One store's bills	pos.store-01.*.order
Ack back to one till	pos.store-01.T4A9.ack

SyncConfig.asNatsSubject() does the translation, so a consumer's subject can be read off the terminal rather than guessed.

Two things to get right on the NATS side:

• The stream must be durable and file-backed. A memory stream loses a shop's bills on a server restart, and the till has already been told they landed.
• Publish the ack from the consumer, after the database commit — not from an ingest handler that has merely queued the work. The ack is the terminal's only evidence, and it deletes its copy seven days later on the strength of it.

Fleet presence

Every terminal publishes a retained record on its status topic on connect and once a minute. Retained matters: a dashboard connecting at noon gets all 100 terminals' last state immediately instead of a blank board.

{
  "schema": 1, "state": "online",
  "device_id": "…", "terminal_code": "T4A9", "terminal_name": "Counter 2",
  "store_id": "store-01", "app_version": "1.1.0",
  "reported_at": "2026-08-01T14:22:05Z",
  "pending_bills": 3, "last_upload_at": "…", "catalogue_revision": "rev-8821",
  "sync_halted": false, "sync_error": null, "consecutive_failures": 0,
  "transport": "mqtt"
}

The Last Will answers is it reachable. These fields answer is it healthy — a till can be connected and still be holding 200 unsent bills or running last month's price list, and only pending_bills and catalogue_revision will say so.

Payloads

Uplink — pos/{store}/{terminal}/order

{
  "schema": 1,
  "batch_id": "9f1c…",
  "store_id": "store-01",
  "terminal_id": "T4A9",
  "sent_at": "2026-08-01T14:22:05.123Z",
  "orders": [ { "id": "…", "invoice_number": "…", "items": [ … ] } ]
}

Ack — pos/{store}/{terminal}/ack. Must echo batch_id; anything else is ignored as belonging to a batch the terminal is no longer waiting on.

{
  "batch_id": "9f1c…",
  "accepted": ["order-uuid-a", "order-uuid-b"],
  "rejected": { "order-uuid-c": "duplicate invoice number" }
}

No ack within SyncConfig.ackTimeout (20s default) → the outcome is unknown, nothing is marked synced, and the batch goes again.

HTTP equivalent — POST {base}/orders, same body, ack shape as the 200 response. Carries an idempotency-key header that is stable across retries of the same bills.

Retention on the terminal

Accepted bills stay for 7 days (OrderDao.retentionWindow) so a batch the back office later loses can be re-sent in full. After that only the archived day totals survive, and a lost bill's line items are gone for good.

While a bill is retained it exists in two places — its own row and day_archive. forBusinessDate therefore reads pending rows only; without that filter every synced bill would be counted twice and the shift report would overstate the day.

What is deliberately not built

• Downlink beyond catalogue-changed and sync-requested. The plumbing routes unknown commands to the events log rather than dropping them, so adding one is a server change plus a case arm.
• Credentials survive a restart. The back-office dialog writes the broker host, port, TLS flag and credentials into syncConfigProvider, which is in-memory. Terminal name and store id persist (they live in the database); the credentials do not, and must be re-entered after a restart. Persisting them means encrypting them at rest, which is the next piece of work.
• Historical correction. Bills already synced by an older build went up with an overstated total. Nothing here fixes that; it needs a server-side reconciliation against bill_discount.