Why "offline-first" and not just "works offline sometimes"
Many point-of-sale systems are cloud-first: every sale is a request to a server. When Wi-Fi or mobile data is unreliable, the queue at the counter stops. Some add an "offline mode" as an afterthought, which often means a limited set of features and a risky merge later.
Offline-first turns this around. The local database on the till is the primary record, and the cloud is a copy that is kept up to date. Losing the connection is a normal, expected state, not an error.
How it works
- Every sale is written locally first. The till saves the sale to its own database the moment it is completed, so the cashier is never blocked.
- Sales are appended, never overwritten. Each sale is added to a local log as a new entry. Nothing is edited in place, which makes it impossible for a later sync to silently overwrite an earlier sale.
- A sync process sends the log to the cloud whenever a connection is available. Entries carry identifiers, so the back office can tell which ones it has already received and apply each one exactly once.
- The back office applies entries in order. An ingest worker on the server processes the log, so reports and stock levels catch up with what happened on the till, even after hours offline.
What we built
For a multi-campus cafeteria, Kryyvo built a desktop till with Tauri and Rust, a Vue interface and a PostgreSQL back office. Every sale is appended to a local log and never overwritten, so a sale cannot be lost or counted twice; an ingest worker applies the log to the cloud back office once the connection returns. In testing, the till kept selling for 72+ hours without internet, backed by 350+ automated tests. See the Campus Cafeteria POS case study.
When you need an offline-first POS
- Your stores, kiosks or canteens have unreliable internet.
- A stopped till costs you sales or long queues.
- You run several locations and need one accurate back office.
- You need to trust that every bill is recorded, even during outages.
FAQ
Can an offline-first POS still show live stock levels?
Each till knows its own sales instantly. Stock across locations is as current as the last sync, which is usually seconds when online. During an outage, each till keeps working with what it knows and catches up when it reconnects.
What happens if two tills sell the same item while offline?
Both sales are recorded, because they really happened. When the tills sync, the back office applies both and adjusts stock. Business rules decide what to do if stock goes negative, for example flagging it for review.
Is offline-first harder to build?
It needs more care in the data model and the sync logic than a simple cloud app, which is why it is worth designing in from the start rather than adding later.