Deployed as a StatefulSet with three replicas, and MQTT has no queue
groups — every subscriber receives every message. All three pods would
commit the same bill and publish three acks. Nothing double-counts,
because the ingest deduplicates on the till's UUID and holds an advisory
lock, but it is three times the database work and three times the
traffic for one sale.
Ordinal 0 consumes; the others stay idle. A StatefulSet already
guarantees stable unique ordinals, so this is a deterministic election
with no lock, no lease and no new dependency. If that pod dies the set
recreates it and tills hold their bills meanwhile, which is what they
are built to do.
POS_MQTT_CONSUMER=always/never overrides it for deployments that are not
a StatefulSet. Anything without an ordinal name — a Deployment pod, a
bare container, local development — consumes, because a lone instance
that silently refused to would be a far more confusing failure than one
that did.
The client id now defaults to the pod name rather than a constant. Two
connections sharing an id evict each other in a reconnect loop that
looks exactly like a flapping network, and takes a while to recognise as
anything else.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
A till holds every bill in its own SQLite database and keeps it for
seven days after we acknowledge it, marking one synced only when its id
comes back in an ack. Everything here follows from that.
Silence is not acceptance, so a failing ingest publishes nothing at all
and the terminal simply sends again. A duplicate is a success, because
at-least-once delivery means a lost ack legitimately re-delivers bills
we already hold, and calling those failures would strand a day of
takings on the till. Deduplication is a unique index on the terminal's
UUID plus an advisory lock held for the transaction.
Bills land in pos_orders / pos_order_items rather than orders: a counter
bill carries a cashier, a terminal, a rounding adjustment, promos,
loyalty movement and a payment split that orders has nowhere to put, and
forcing one into the other loses whatever does not fit. Stock is *not*
split — a counter sale writes the same productstocks rows an app order
does, through helpers extracted from createOrderTx so the rule that
prevents overselling has one implementation rather than two.
GetRevenueSummary and GetSalesSummary were extended to union the new
table in; any new report has to remember the same.
Terminal health goes to Redis under a 90-second TTL, sharing the
instance the express backend uses. A heartbeat is a fact with an expiry
date: a till that loses power stops refreshing and ages off the board by
itself, where a Postgres row would need ~288k writes a day and a reaper.
Proven end to end against the live estate before commit: a bill over
HTTP and one over the real Mosquitto broker, the same bill three times
producing one row and one stock movement, and a heartbeat arriving on
the health endpoint. All probe data was removed afterwards.
Four things that only surfaced against real data. An unset jsonb column
failed the very first bill. Product SKUs are unusable as barcodes — 6,245
products share 93 SKUs and "1" covers 5,794 of them — against the till's
unique index, so barcodes fall back to the product id. A taxpercent of
-1 exists and would have put negative GST in a filed slab. And a product
with id 0 exists, which can never be billed and is now skipped.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
