How offline mode works in mobile applications

1) What is offline mode and why is it needed

Offline mode is the ability of an application to work without a network (or with an unstable Internet), and then synchronize when a connection appears. Is he:

Reduces failures and increases retention
Accelerates the initial screen (data is already local)
allows you to perform critical actions (drafts, viewing content, part of operations) "in the field."

2) Layers of offline architecture (on any stack)

1. Local data storage

Mobile native: SQLite/Room (Android), Core Data/SQLite (iOS), Realm, Key-Value (SharedPreferences/UserDefaults).

Web/PWA: IndexedDB (over - Dexie/LocalForage), Cache Storage for static.

2. Static Cache (App Shell)

Icons, fonts, CSS/JS, basic screen templates.

3. Operation queue (Outbox)

Write requests (create/modify/delete) are queued and sent to the server when a network appears.

4. Synchronization layer

Merge policies, versions, deduplication, retrays, backoff.

5. Network status signals

NetInfo/Reachability/Browser API for switching UI between online/offline/limbo.

3) How it looks on iOS/Android

Cache and DB - The data structure mirrors the main API responses (normalize entities).

Offline drafts: forms and actions are written to the local database with the flags' pending/sent/failed '.

Synchronization: the background task periodically reads the outbox and sends batches, marking the status.

Security: secrets/tokens - in Keychain (iOS )/Android Keystore. Data with PII/payments is encrypted (for example, AES-256 GCM) with a key from a secure container.

OS limitations: background tasks depend on power saving modes; plan for idempotency of requests and resumption after killing the process.

4) How it works in PWA (web)

Service Worker (SW) - proxy between network and application:

Precache (App Shell): the interface is available instantly.
Runtime cache: data/media by strategy below.
Background Sync/Periodic Sync (where available): sending a queue, updating the cache without user intervention.
IndexedDB for data and Cache Storage for statics.
Limitations: storage quotas, tight control of background tasks (especially iOS Safari).

5) Cache strategies (what and when to apply)

Cache First - for unchanging static (icons, fonts, JS versions).

Stale-While-Revalidate (SWR) - for lists/directories: instantly from the cache, pull up fresh data in the background.

Network First - for personal data when the network is there; backup - from the cache when offline.

Cache Only/Network Only - rare special cases (diagnostics, private resources).

Combine: statics - CF/SWR; dynamics - SWR/NF; records - through the queue.

6) Change queue and idempotency

Outbox model: Each action (POST/PUT/PATCH/DELETE) is serialized into a queue entry with a temporary ID, body, version, and deadline.

Sending in batches with exponential backoff in case of network/server errors.

Idempotent keys in headers/endpoints - resubmission will not create duplicates.

Database transactions - Queuing and updating the local state must be atomic.

7) Conflict resolution (server vs client)

Approaches:

Last Write Wins (LWW) - simple, but the risk of losing edits.
Versioning/ETag - the server rejects outdated versions → the client makes a merge/resave.
Operational Transformations/CRDT - for joint editing of complex entities.
Field rules - which fields are true on the server, which are on the client (for example, local labels/flags).

UX:

Show the "out of sync" badge, the "update" button, and the diff on conflict (to select the version).

8) Working with media and heavy resources

Deduplication and hashes (content-addressable) - do not load the same.

Placeholders/offline miniatures, full version - after the network.

Download queue with pause in case of bad mains/battery.

TTL policy for media cache - don't save gigabytes.

9) UX patterns to keep offline "human"

TOP rule: Never show "emptiness." App Shell + skeleton + the latest valid version of the content.

Clear statuses: Online/Offline/Synchronization .../Action required.

Undo/Retry: undo the last offline action; automatic and manual replay.

Local drafts: Visible Pending Submission lists.

Quiet mistakes: don't worry aggressively - unobtrusive indicators + magazine are enough.

10) Offline security and privacy

Encrypt sensitive data "on disk"; keys - in Keystore/Keychain.

Minimize PII collection/storage offline; Specify retention and auto-cleanup.

Never cache secrets/full PAN/CVV; payment provider tokens - only according to PCI rules.

Protect the SW/client from XSS (CSP, SRI), otherwise the attacker will be able to steal offline data the next time online.

11) Platform limitations

iOS: strict limits for background tasks in the browser; Web Push/periodic sync - with nuances; Keychain - reliable for secrets.

Android: flexible background services (WorkManager), but OEM optimizations can "kill" tasks - mark them as "important."

PWA: IndexedDB/Cache Storage quotas, system cleanup without warning when space is low.

12) Offline testing

Network profiles (Airplane, 2G/3G, packet loss, high RTT).

Kill/restore the process during a bruise.

Chaos tests: half the batch falls 429/503/timeout.

Conflicts-Concurrent edits from two devices.

Storage Quotas - Fill disk, check cache behavior.

13) Metrics and observability

Time To First Offline View (TTFOV): App Shell speed.

Offline coverage - the proportion of screens/operations available without a network.

Outbox health: queue length, average time to bruise, error rate.

Conflict ratio and proportion of hand merjas.

Storage quota/usage, OS purge rate.

User impact: sessions started without a network → conversion after a bruise.

14) Quick implementation plan (90 days)

1. Determine the offline scope: which screens are read from the cache, which operations can be postponed.

2. Select database and schema: normalized tables, indexes, versions.

3. Enable App Shell: PWA SW/static cache/icons/fonts.

4. Collect Outbox: queue, idempotence, backoff, batches.

5. Cache strategies: SWR for lists, Network First for personal data.

6. UX statuses + sinka log, retry/undo.

7. Security: disk encryption, CSP/SRI, PII minimization.

8. Bad network tests, chaos tests and metrics.

15) Frequent mistakes and how to avoid them

"Offline" is only for static. → You need drafts and outbox, otherwise the value is small.

No idempotency. → Duplicates of operations in retras. Enter the idempotent keys.

Hidden conflicts. → User loses edits. Show diff/reshalka.

Without TTL and cache cleaning. → The application swells, the OS cleans forcibly.

Sink blocks UI. → Synchronization is always in the background, UI is responsive.

Storing secrets in clear text. → Use Keychain/Keystore and encryption.

16) FAQ

Is it possible to make "full" offline for everything?

Often not: Payments, license checks and live data require a network. Make a hybrid: read from cache + deferred writes.

Which is faster: SWR or Network First?

SWR gives an instant response from the cache and a quiet update - the best UX for lists. Network First is needed where freshness (profile, balance) is important.

How to store big media?

Cache miniatures and short-lived TTL, originals - on request, with LRU cleaning.

Do I need to encrypt everything?

Encrypt PII/secrets and sensitive records. The rest is on risk policy and quotas.

Will offline worsen SEO/PWA?

No, with the right SW and SSR, on the contrary, it will improve speed and repeated visits.

Offline mode is not a "tick," but a system architecture: local database + static cache + change queue + reliable synk and thoughtful UX statuses. Add security (encryption, Keychain/Keystore), idempotency and metrics, test a bad network - and your application will remain useful even without the Internet, and when it appears, it will seamlessly catch up with the server without losing data and user trust.