Offline support is a data contract A mobile app may lose connectivity while a user edits a record. Saving locally is easy; reconciling changes with the ser...
Offline support is a data contract
A mobile app may lose connectivity while a user edits a record. Saving locally is easy; reconciling changes with the server after reconnection is the real design problem. Decide which actions can be queued, which need immediate confirmation, and how conflicts are resolved.
Start with the domain. A field note can often be saved offline, while a payment capture or seat reservation may require an online server decision. Never show an offline action as fully completed when the server has not accepted it.
Store pending operations
Maintain a local outbox with operation ID, record ID, payload, creation time, user identity, and sync state. Use a stable idempotency key so a retry cannot create duplicate server records. Encrypt sensitive local data where the threat model requires it and clear it when the user logs out.
The server should accept a client generated operation identifier only as a deduplication key, not as authorization proof. It must recheck current permissions and business rules when the operation arrives.
Detect conflicts
Include a server version or updated timestamp with each record. If the server changed since the app last loaded it, do not silently overwrite the newer value. For simple independent fields, a merge may be possible. For financial or shared records, show a conflict for review or use a domain specific rule.
Last write wins is simple but can erase legitimate work. Use it only when the product explicitly accepts that loss.
Recover from partial sync
Send operations in a defined order when later actions depend on earlier ones. Record server acknowledgement per operation. If a batch fails halfway through, resume from the last acknowledged item rather than replaying the whole batch without deduplication.
The UI should show pending, synced, and needs attention states. A generic spinner that disappears after a timeout hides important uncertainty.
Test under bad networks
Test switching accounts while offline, repeated reconnects, an expired token, two devices editing the same record, and a server rejection after local acceptance. Confirm pending data survives app restart and that the user can understand a conflict.
Conclusion
Offline sync is reliable when local intent, server authority, and conflict resolution are explicit. It is a product decision as much as a storage technique. See mobile app development for API connected application work.
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