Offline app: how to build one that works without signal

When do you need an app that works offline?

Not every app needs to work offline. An app where customers order or pay live always needs a connection. But for field work, offline is often no luxury. Think of:

• Filling in job sheets and checklists at a customer's home or on site.
• Capturing photos and signatures at a handover or repair.
• Inspections and measurements in buildings with poor signal.
• Logging hours and mileage on the road.
• Looking up a product catalogue or manual without wifi.

Ask yourself: what happens if the connection drops for five minutes? If the answer is 'then my employee can't carry on', an offline mode is often worth the investment. This mostly applies to an employee app for people who work out and about a lot.

How does an offline app work technically?

An offline app keeps data in two places: on the device and on the server. The trick is keeping the two neatly in step. That's called synchronising.

• Local storage: the app keeps data in a small database on the phone or tablet.
• Queue: changes the user makes offline are placed in a queue.
• Sync: as soon as there's a connection, the app sends the queue to the server and fetches new data.
• Status display: the user can see whether something has been sent or is still waiting.

You'll also hear the term 'offline first'. It means the app always works with the local data first and updates the server in the background. As a result the app feels fast even with a good connection, because it isn't constantly waiting for the server.

Working offline in a native app or a web app

A web app can work offline too. A progressive web app uses a service worker for this: a bit of code that keeps files and data in the browser. For simple uses, such as showing pages loaded earlier or filling in a form, that's often enough.

A native app, built for iOS and Android, gives you more control. You can store larger amounts of data, keep photos more reliably and sync better in the background. If it involves a lot of data or heavy files, native is usually the safer choice. Read more about the trade-off in native app or web app and about what a progressive web app can do.

The hardest part: sync conflicts

Say an engineer edits a job sheet offline, and at the same time the planner at the office changes the same job sheet. Which version wins? That's called a conflict, and you need to decide in advance how the app handles it.

• Last change wins: simple, but you can lose work.
• Merge per field: the engineer's change to one field and the office's change to another both stay.
• Ask the user: the app shows both versions and lets someone choose.
• Split responsibilities: some fields can only be changed by the engineer, others only by the office. Then conflicts hardly ever happen.

In practice a combination works best. Discuss this early with your developer. It's much harder to build in afterwards.

What to watch out for with an offline app

• Choose what needs to work offline. Not everything does. Often it's enough that today's tasks and the forms are available.
• Mind the storage space. Photos and videos quickly take up a lot of room. Shrink them before they're saved.
• Secure the local data. If a phone gets lost, customer data mustn't simply be readable. Use encryption and a login.
• Show the status clearly, for example 'not sent yet' with an icon. That way nobody assumes something has arrived while it's still waiting.
• Think about updates. If you change the structure of your data later, the app must still be able to handle old offline data.
• Test in real conditions: airplane mode on, patchy signal, a battery that dies halfway.

Especially if your app exchanges data with other software, such as a scheduling system or your accounts, the sync needs to fit well. Read more in our article on connecting your app to your systems.

How much does an app that works offline cost?

Working offline makes an app more expensive than the same app without an offline mode. That's because you build extra parts: local storage, the queue, the sync and the conflict rules. Testing takes more time too. How much more depends on how many features need to work offline and how complex the data is.

A tip to save money: start with one offline task, for example filling in job sheets, and only expand once that runs smoothly. That way you only pay for what you actually use.

Having an offline app built by Violexa

At Violexa we build apps and web apps that keep working when the signal drops. We agree upfront which tasks need to work offline, how your data is stored securely on the device and what happens in a conflict. You can read more about our approach on the websites and apps page. A custom app or web application starts at €1,799 excl. VAT (€2,177 incl. VAT) per project. For a larger project you get a fixed price upfront, in writing.

Want to know whether your app needs to work offline, and what that costs? Book a free quickscan. In a 30-minute conversation, online or on site, we look at your situation. Afterwards you get short written advice with a price indication, with no obligation.

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