pos-reconciliation-loop

POS Reconciliation Loop

For storefronts whose in-store POS is the source of truth and whose website must mirror it. This is the Square track's distinctive capability (posReconciliation: true). Shopify inverts the model (Shopify is the truth); Stripe has no catalog to reconcile.

The core rule: one-way, POS-as-truth

Square explicitly warns that bidirectional sync is unsafe (concurrency, merge, duplicate, and delete hazards). So:

• Pick one master — the POS (Square) — and sync one way to the online surface.
• Never push online edits back into the POS ledger as an authoritative write.

The loop

1. Subscribe to catalog.version.updated and inventory webhooks.
2. Verify the webhook (constant-time HMAC — see webhook-hardening).
3. De-dupe by event_id (retries + out-of-order delivery are expected) BEFORE applying any change.
4. Pull only what changed — SearchCatalogObjects with begin_time, or the changed objects the event names — not a full re-sync.
5. Apply to the online catalog/inventory projection, storing cross-reference IDs as custom attributes.
6. Surface sold-out via the inventory count (e.g. location_override.sold_out).

The correctness tests (the track must pass these)

• A duplicate inventory webhook does NOT double-decrement stock (idempotency by event_id).
• Convergence holds under out-of-order delivery — applying events in a shuffled order lands on the same final stock as in-order.

Anti-patterns

• Bidirectional sync (Square warns against it).
• Full catalog re-sync on every webhook (use begin_time / changed-object deltas).
• Treating the website as a second source of truth (it is a projection of the POS).