Data Sync Testing Checklist (2026)

April 13, 2026 · 20 min read · Testing Checklists

Data Sync Testing Checklist (2026) provides a concrete, actionable list of items to validate when verifying that data synchronization works correctly across devices, services, and offline modes. Modern applications frequently rely on background sync to keep user data consistent, whether the data originates from a local SQLite store, a cloud Firestore instance, or a proprietary enterprise backend. A missing or flaky sync path can lead to data loss, confusing UI states, or even compliance violations. This guide walks you through a detailed, check‑able matrix that covers happy‑path flows, error handling, edge and boundary conditions, accessibility, security/privacy, performance, and release‑readiness concerns. Each item includes a clear pass criterion, a tester might look‑where applicable—how an autonomous explorer can exercise in a single run.

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Happy Path Verification for Data Sync Testing Checklist (2026)

Core Sync Flow Validation

The most basic verification ensures that a change made on one endpoint appears on the other within an expected latency window and without corruption.

Test ID	Description	Pass Criteria	Automation Approach	Example
HP‑01	Create a new record on Device A while online; verify it appears on Device B after sync interval.	Record exists on B with identical field values; no duplicate entries.	Autonomous explorer (SUSA) can generate a tap‑type‑save sequence and then poll the remote store via API.	Add a contact named “Ada Lovelace” on phone A; after 5 s, contact list on phone B shows the same entry.
HP‑02	Update an existing record on Device B; confirm the update propagates to Device A and overwrites stale data.	Field values on A match B; version/timestamp increments correctly.	Scripted Appium test that edits a field, triggers sync, then reads back via UI or DB query.	Change the phone number of contact “Ada Lovelace” from 555‑0100 to 555‑0200; verify the number changes on A.
HP‑03	Delete a record on Device A; ensure deletion propagates to Device B (soft‑delete or hard‑delete as per spec).	Record no longer queryable on B; tombstone marker present if soft‑delete used.	Autonomous flow that long‑presses delete, confirms, then checks remote store for absence.	Delete a note titled “Meeting notes”; confirm note disappears from B’s list.
HP‑04	Perform a batch of mixed creates/updates/deletes (≥10 operations) in a single session; verify eventual consistency.	All operations reflected correctly; no lost updates; final state matches deterministic merge function.	SUSA can record a macro of rapid UI interactions and then validate via backend checksum.	User edits 5 contacts, adds 3 new ones, removes 2; after sync, contact count and data of the source device.
HP‑05	Sync initiation from background (e.g., OS‑triggered periodic sync) without user interaction.	Sync completes successfully; UI reflects latest state when app is foregrounded.	Use Android AlarmManager or iOS BackgroundTasks in test harness; assert UI update after background completion.	Device A sits idle for 15 min; background sync runs; when user opens app on B, latest changes are visible.

Conflict‑Free Convergent Data Types

When the system uses CRDTs or operation‑based merging, the happy path also includes verifying that concurrent edits converge without manual intervention.

Test ID	Description	Pass Criteria	Automation Approach	Example
HP‑06	Two devices concurrently edit different fields of the same record.	Final record contains both field updates; no data loss.	Autonomous explorer spawns two parallel sessions (Device A & B) that edit distinct fields, then waits for sync.	Device A changes first name to “Ada”, Device B changes last name to “King”; final record shows “Ada King”.
HP‑07	Two devices concurrently edit the same field with different values; system applies last‑write‑wins (LWW) rule.	The value with the higher timestamp (or vector clock) prevails; loser value is discarded.	Scripted test that sets device clocks offset, edits same field, then checks outcome.	Device A clock +2 s, edits email to a@example.com; Device B clock 0, edits email to b@example.com; after sync, a@example.com wins.
HP‑08	Concurrent create of identical record (same primary key) on two devices.	Only one record exists; duplicate is merged or rejected per policy.	Autonomous flow that creates same contact on both devices simultaneously, then verifies count.	Both devices create contact “Ada Lovelace” with same phone; final DB shows single entry.

Offline‑First Queue Persistence

Happy path also validates that actions performed while offline are queued and later transmitted correctly.

Test ID	Description	Pass Criteria	Automation Approach	Example
HP‑09	Disable network, perform 5 creates/updates/deletes, re‑enable network; verify all queued ops are sent.	No operation lost; server receives exactly the 5 ops in correct order.	Use Android Emulator network off/on toggles via ADB; SUSA can script the toggle and then audit server logs.	Turn off Wi‑Fi, add three tasks, edit two, turn Wi‑Fi back on; server logs show three POSTs, two PUTs.
HP‑10	While offline, modify a record that was deleted on the server; upon reconnect, system resolves per tombstone policy.	Record stays deleted (or is resurrected per spec) and no conflict error appears.	Autonomous test that deletes record on server via API, then edits locally offline, then reconnects and checks final state.	Server deletes contact “Ada”; offline user changes phone number; after sync contact remains deleted.
HP‑11	Queue persistence survives app kill/reboot while offline.	After restart, queued operations are still present and sent upon network restore.	Kill app process via ADB shell `am force-stop`, reboot device, then re‑enable network; verify ops sent.	User adds a note, force‑stops app, reboots, enables Wi‑Fi; note appears on server.

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Error Handling and Failure Scenarios in Data Sync Testing Checklist (2026)

Transient Network Errors

Sync must survive temporary blips without corrupting state or leaking resources.

Test ID	Description	Pass Criteria	Automation Approach	Example
EH‑01	Simulate packet loss (e.g., 30% loss) during an ongoing upload; verify retry mechanism delivers payload.	After loss period, upload completes successfully; no duplicate records.	Use `tc` netem on Linux testbed or Android’s `adb shell netcfg` to inject loss; monitor logs for retries.	Upload a 2 MB image; loss occurs for 2 s, then upload finishes with single server receipt.
EH‑02	Introduce high latency (500 ms RTT) for the duration of a sync cycle; ensure timeout settings do not cause premature abort.	Sync completes within adjusted timeout; no false failure reported.	Use `netem delay 500ms`; observe client retry count and final success.	Sync of 10 KB JSON takes ~1 s instead of 200 ms but still ends with success.
EH‑03	Simulate DNS failure; client should fallback to cached configuration or show appropriate offline UI.	App does not crash; shows “Unable to sync – check connection” banner; local queue retains ops.	Disable DNS via `/etc/hosts` manipulation; verify UI message and queue size.	User attempts sync; banner appears; after fixing DNS, sync resumes and processes queued ops.

Server‑Side Errors

The client must gracefully handle non‑2xx responses, malformed payloads, and authentication challenges.

Test ID	Description	Pass Criteria	Automation Approach	Example
EH‑04	Server returns 500 Internal Server Error on a PATCH request; client implements exponential backoff and eventually succeeds after server recovers.	After a series of retries with increasing delay, request succeeds; no infinite loop.	Mock server (e.g., WireMock) to return 500 for first 3 attempts then 200; assert retry count and eventual success.	Sync of settings fails three times, then succeeds on fourth attempt after 2 s, 4 s, 8 s delays.
EH‑05	Server returns 401 Unauthorized due to expired token; client refreshes token and retries the failed operation.	Token refresh flow executes; original operation retried with new token; final state correct.	Intercept network calls; inject 401 response; verify token endpoint called and subsequent request uses new token.	Upload fails with 401; client calls `/refresh`, gets new JWT, retries upload successfully.
EH‑06	Server returns malformed JSON (missing required field); client logs error and does not corrupt local state.	Local data unchanged; error visible in dev console or crash‑free log; optional user‑visible toast.	Provide malformed payload via mock; assert no changes to local DB and presence of error log.	Sync receives `{ "id": 12 }` missing `"name"`; client discards payload, shows “Sync error – invalid server response”.
EH‑07	Server enforces rate limit (429 Too Many Requests); client backs off and resumes after Retry‑After header.	Requests pause for indicated duration; subsequent sync succeeds without data loss.	Mock server to respond 429 with `Retry-After: 5`; verify client waits ~5 s before next attempt.	Batch of 20 updates throttled; after 5 s pause, remaining updates sent successfully.

Edge Cases in Conflict Resolution

When concurrent updates produce conflicts beyond simple LWW, the system must have a deterministic, testable resolution strategy.

Test ID	Description	Pass Criteria	Automation Approach	Example
EH‑08	Two devices concurrently update the same field to different values; system uses merge function (e.g., concatenate with separator).	Final value equals merge of both inputs; no data loss.	Autonomous explorer runs parallel edits, then checks merged result.	Device A sets note body to “Buy milk”, Device B sets to “Buy eggs”; final body “Buy milk; Buy eggs”.
EH‑09	Concurrent creation of records with identical natural key but different metadata; system preserves both via UUID generation.	Two distinct records exist, each with unique system‑generated ID; metadata intact.	Create same contact name on two devices, verify both appear with different internal IDs.	Contacts “John Doe” created on A and B; after sync both appear, each with different `uid`.
EH‑10	Tombstone collision: Device A deletes record X while Device B updates same record X after deletion.	Policy (e.g., delete wins) applied consistently; no resurrected record unless spec says otherwise.	Delete via API on A, edit locally on B offline, then sync; verify final state.	After sync, record X is deleted; B’s update is discarded.
EH‑11	Vector clock overflow (exceeds 64‑bit) after many updates; system handles wrap‑around or resets without losing ordering.	Sync continues correctly; no false conflict detection due to clock wrap.	Stress test: generate >10⁹ updates via automated script, observe clock handling.	After 1 billion increments, clock rolls over; subsequent sync still converges correctly.

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Edge/Boundary Cases in Data Sync Testing Checklist (2026)

Field Length and Type Limits

Validate that the sync layer respects schema constraints and does not truncate or corrupt data.

Test ID	Description	Pass Criteria	Automation Approach	Example
EB‑01	Insert a UTF‑8 string longer than the column’s max length (e.g., 200 chars in VARCHAR(100)).	Server rejects with 400; client does not store invalid value locally; error shown to user.	Send oversized payload via API; assert validation error and unchanged local DB.	Attempt to save a note with 250 char title; server returns “title too long”; local note unchanged.
EB‑02	Insert a numeric value exceeding the column’s precision (e.g., 123456789012345 in DECIMAL(10,2)).	Server returns validation error; client keeps original value; no silent truncation.	Similar to EB‑01 but with numeric overflow.	Attempt to store price 9999999999.99 in DECIMAL(8,2); error returned.
EB‑03	Send binary blob (image) larger than allowed max size (e.g., 15 MB when limit is 10 MB).	Upload fails with appropriate error; client retains file locally for retry.	Mock server size limit; verify client error handling.	User tries to attach a 12 MB photo; client shows “File exceeds limit”.
EB‑04	Empty or whitespace‑only values for required fields.	Server rejects; client displays field‑level validation message.	Send blank email; assert error.	Submitting signup with empty email yields “Email required”.
EB‑05	Unicode surrogate pairs and emojis in text fields.	Data stored and retrieved exactly as entered; no corruption or replacement characters.	Send string “😀👍🏽🌟”; verify round‑trip integrity.	After sync, note still shows the exact emoji sequence.

Timing and Clock Skew

Clock differences between client and server can affect LWW or timestamp‑based ordering.

Test ID	Description	Pass Criteria	Automation Approach	Example
EB‑06	Client clock ahead of server by 2 minutes; verify that timestamps are adjusted or ignored per spec.	Sync uses server‑provided timestamp for ordering; client does not rely solely on local clock.	Set device time via `adb shell date`, perform sync, inspect server‑stored timestamps.	Client sends update with timestamp T+2min; server stores with its own Tserver; later reads show correct order.
EB‑07	Client clock behind server by 5 minutes; ensure queued operations are not mistakenly deemed stale and dropped.	Operations are accepted; server treats them as newer based on logical version if used.	Same as EB‑06 but with negative offset.	Offline edit performed while clock lags; after sync, edit appears as latest.
EB‑08	Device experiences NTP sync jump (e.g., clock jumps forward 1 hour) during an ongoing sync session.	No duplicate or lost updates; system recovers using logical counters or version vectors.	Force time change via `adb shell date` mid‑test; monitor for anomalies.	Clock jumps while uploading a batch; after jump, remaining uploads succeed without conflict.

Data Volume and Throughput

Stress the sync pipeline with large batches to uncover buffering, memory, or thread‑pool limits.

Test ID	Description	Pass Criteria	Automation Approach	Example
EB‑09	Sync a batch of 10 000 small records (≈1 KB each) in a single session.	All records transferred successfully; memory usage stays below defined threshold (e.g., 150 MB); no OOM crashes.	Use automated script to generate records locally, trigger sync, monitor via `adb shell dumpsys meminfo`.	After sync, server count matches 10 000; client memory peak 120 MB.
EB‑10	Sync a single large multimedia object (e.g., 100 MB video) in chunks; verify resumable upload works if interrupted.	Upload pauses on network loss and resumes from last successfully transmitted byte offset; final file matches source byte‑for‑byte.	Kill network mid‑upload using `adb shell svc wifi disable`; after restore, verify continuation.	Upload stops at 42 MB, resumes, completes at 100 MB with correct SHA‑256.
EB‑11	Perform rapid fire sync triggers (e.g., 5 sync requests per second) from multiple threads/apps on same device.	Server processes each request; no request is dropped or silently ignored; rate‑limit responses handled correctly.	Spawn 5 background Jobs via WorkManager each issuing a sync; collect server logs.	Server logs show 25 requests processed over 5 s; no 500 errors.
EB‑12	Simulate low‑memory condition (e.g., using `adb shell am kill -S SIGSTOP` on background services) while a sync queue is pending.	Sync queue persists; after memory pressure relieved, queued ops are still sent.	Kill sync service, then free memory, verify queue still present.	After killing sync service, reopen app; queued edits still transmit.

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Accessibility Checks in Data Sync Testing Checklist (2026)

Screen Reader Announcements

Users relying on TalkBack or VoiceOver must receive timely feedback about sync status.

Test ID	Description	Pass Criteria	Automation Approach	Example
AC‑01	When sync starts, an accessibility announcement (e.g., “Syncing…”) is spoken.	Announcement appears within 500 ms of sync initiation; does not repeat excessively.	Use Android Accessibility Test Framework (ATF) to capture spoken events; assert presence.	Tap sync button; TalkBack says “Syncing, please wait”.
AC‑02	Upon successful sync, announcement states “Sync completed, X items updated”.	Message includes correct count; spoken within 1 s of completion.	Same as AC‑01 but check for success message.	After adding 3 contacts, TalkBack says “Sync completed, 3 items updated”.
AC‑03	On sync failure, announcement conveys error and suggests action (e.g., “Sync failed. Check your connection.”).	Error message is specific, not generic; spoken promptly.	Simulate network off; trigger sync; verify announcement.	TalkBack says “Sync failed. No internet connection. Try again later.”
AC‑04	Queue size indicator (e.g., “3 items pending sync”) is accessible via a labeled element.	Element has appropriate `content‑description` or `aria‑label`; value updates in real time.	Inspect UI hierarchy for pending‑sync badge; assert label changes after each offline edit.	After two offline creates, badge reads “2 items pending”.

Touch Target Size and Contrast

Sync‑related controls must meet WCAG 2.2 AA minimums.

Test ID	Description	Pass Criteria	Automation Approach	Example
AC‑05	Manual sync button dimensions ≥ 48 dp × 48 dp (or 44 × 44 px).	Measured size passes automated accessibility scanner (e.g., Android Accessibility Test Library).	Run `axe-android` or `accessibility-test` on screen; verify no violations.	Button size reported as 56 dp × 56 dp.
AC‑06	Color contrast between sync button background and foreground text ≥ 4.5:1.	Contrast ratio verified by automated tool; no failures.	Same scan as AC‑05; check contrast metric.	Background #0066FF, text #FFFFFF → contrast 5.25:1.
AC‑07	Progress indicator (spinner or bar) is perceivable without relying on color alone (e.g., includes label or pattern).	Indicator has accompanying text or ARIA `aria‑valuetext`.	Inspect DOM for `aria-label` or visible text near spinner.	Spinner accompanied by “Syncing…” text.
AC‑08	Error toast or snackbar is dismissible via swipe and also announces dismissal.	Dismissal action accessible; announcement states “Message dismissed”.	Perform swipe gesture via accessibility service; check spoken feedback.	Swipe away snackbar; TalkBack says “Message dismissed”.

Keyboard and Navigation Order

Ensuring that users who navigate via keyboard or switch devices can reach sync controls.

Test ID	Description	Pass Criteria	Automation Approach	Example
AC‑09	Tab order places sync button after primary content but before footer navigation.	Sequential focus follows logical flow; no skipped or trapped focus.	Use keyboard emulator (e.g., `adb shell input keyevent TAB`) and record focus changes.	Focus moves: list item → sync button → settings icon.
AC‑10	Sync button is activatable via Enter/Space key when focused.	Pressing Enter or Space triggers sync identical to tap.	Simulate keypress; verify network request issued.	Press Enter on focused button → sync starts.
AC‑11	In a list view with sync‑per‑item controls (e.g., “Sync this folder”), each control is reachable and labeled.	Every item’s sync control has distinct accessible name (e.g., “Sync folder Photos”).	Iterate through list items via accessibility service; capture names.	Item “Photos” yields name “Sync folder Photos”.
AC‑12	Sync status region is marked as a live region (`android:accessibilityLiveRegion="polite"` or `aria-live`) so changes are announced automatically.	Live region present; updates such as “Syncing…” or “Sync completed” are spoken without user focus.	Observe speech request.	When sync finishes, TalkBack announces “Sync completed”.

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Security and Privacy Checks in Data Sync Testing Checklist (2026)

Transport Security

All sync traffic must be encrypted and resist man‑in‑the‑middle attacks.

Test ID	Description	Pass Criteria	Automation Approach	Example
SE‑01	Verify that all sync endpoints use TLS 1.2 or higher; no fallback to plain HTTP.	Network capture shows TLS handshake; no HTTP requests to sync URLs.	Use `mitmproxy` or Wireshark filter `tcp.port == 443 && http` to confirm absence of HTTP.	Sync to `https://api.example.com/sync` shows TLS 1.3 handshake.
SE‑02	Certificate pinning (if implemented) blocks connections with invalid or self‑signed certs.	Connection fails with pinning error when presented with mismatched cert; app does not proceed.	Install a rogue CA cert on device; attempt sync; assert failure.	Sync attempt logs “Certificate pinning failed”.
SE‑03	Sensitive fields (passwords, tokens, PII) are never included in sync payloads in plaintext.	Payload inspection shows encryption or omission; no raw secrets visible.	Export sync request body; search for known secret patterns.	Sync JSON contains `"authToken": ""` not the raw token.
SE‑04	Perfect Forward Secrecy (PFS) is negotiated; session keys differ per connection.	Wireshark shows different ephemeral keys in each TLS handshake.	Capture two separate sync sessions; compare `tls.handshake.extensions_server_name` and key exchange values.	First session uses ECDHE_X25519, second uses ECDHE_SECP256R1.

Authentication and Authorization

Sync must respect user identity and scoped permissions.

Test ID	Description	Pass Criteria	Automation Approach	Example
SE‑05	Sync requests include a valid, short‑lived access token (e.g., JWT with ≤ 15 min expiry).	Token present in Authorization header; expired tokens trigger refresh flow.	Decode JWT from captured header; verify `exp` claim.	Token `exp` = now + 10 min.
SE‑06	Attempt to sync using a token belonging to another user (or a tampered token) results in 403 Forbidden.	Server rejects with 403; client does not retry indefinitely.	Replay captured request with altered `sub` claim; assert 403.	Sync fails, logs “Insufficient scope”.
SE‑07	Sync respects granular scopes: a user with read‑only scope cannot create or delete records via sync.	Write operations from read‑only token are rejected (403) and not applied locally.	Assign read‑only token; attempt to create a record via sync; verify failure.	Create request returns 403; local DB unchanged.
SE‑08	Refresh token rotation: after each successful token refresh, old refresh token is invalidated.	Reusing old refresh token yields 401 Invalid Grant; new token works.	Capture refresh response, store old token, attempt to reuse; assert failure.	Old refresh token returns 401; new token yields 200.

Data Minimization and Retention

Only necessary data should be synchronized and retained.

Test ID	Description	Pass Criteria	Automation Approach	Example
SE‑09	Sync payload excludes fields marked as `sensitive: true` in schema (e.g., social security number).	No SSN appears in request/response bodies; field either omitted or hashed.	Inspect JSON for SSN pattern; assert absence.	Payload contains `"ssn": null` or field omitted.
SE‑10	Deleted records are removed from sync queue after server acknowledgment; no tombstone persists longer than configured TTL (e.g., 30 days).	After TTL, tombstone entries are purged from local DB and not resent.	Insert tombstone with timestamp, fast‑forward clock via `adb shell date`, verify removal.	Tombstone older than 31 days not present in DB.
SE‑11	Sync logs do not contain full PII; at most hashed identifiers appear.	Log files scanned for email, phone, address patterns yield only masked versions.	Grep logs for `@example.com`; expect no matches.	Log shows `user_id: sha256(abc123)` instead of actual email.
SE‑12	Offline‑encrypted queue (if used) employs a strong, randomly generated key stored in Keystore/Keychain.	Key is not extractable via root; attempts to read return protected blob.	Try to export keystore entry via `keytool -list -v`; assert requires authentication.	Key extraction prompts for biometric; fails without.

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Performance Checks in Data Sync Testing Checklist (2026)

Latency and Throughput

Measure the time from user action to visible remote effect and the data rate the sync channel can sustain.

Test ID	Description	Pass Criteria	Automation Approach	Example
PF‑01	End‑to‑end latency for a single field update (≤ 2 seconds on 4G, ≤ 5 seconds on 3G).	Timestamp from UI action to server receipt ≤ threshold.	Instrument code with `System.nanoTime()` around UI event and network completion; log diff.	Update name field; measured latency 1.3 s on LTE.
PF‑02	Throughput for bulk upload: ≥ 500 KB/s sustained over a 10 MB file on Wi‑Fi.	Average upload speed measured by client meets threshold.	Use `adb shell cat /proc/net/dev` to compute bytes transferred over interval; compute rate.	Upload 10 MB file in 18 s → 556 KB/s.
PF‑03	Battery impact: sync operation should not increase drain > 5 % per hour compared to idle baseline.	Measure battery capacity before/after a 10‑minute sync loop; delta ≤ 5 % of total.	Use `adb shell dumpsys batterystats` to capture charge; repeat with sync loop.	Idle drain 2 %/h; with sync 4.5 %/h → pass.
PF‑04	CPU usage during sync spike stays below 30 % of a single core on median device.	Profile via `adb shell top -m 10 -d 1`; average CPU % for sync process ≤ 30.	Run sync of 1 000 small records; capture CPU samples.	Average CPU 22 % → pass.
PF‑05	Network utilisation efficiency: ratio of useful payload bytes to total transmitted bytes ≥ 85 % (accounting for TLS overhead, retransmissions).	Calculate payload vs. total bytes from network capture; ratio meets target.	Use `mitmproxy` to record bytes; compute ratio.	Payload 8 MB, total 9.2 MB → ratio 86.9 % → pass.

Scalability and Concurrency

Ensure the sync service can handle many devices or many simultaneous sessions without degradation.

Test ID	Description	Pass Criteria	Automation Approach	Example
PF‑06	Simulate 50 concurrent devices each performing a sync of 1 KB record; server latency average ≤ 200 ms.	Use a device farm or Android emulator cluster; measure response times.	Deploy 50 emulators via `adb -s emulator- shell am start -n ...`; collect latencies.	Average latency 165 ms → pass.
PF‑07	Server-side sync worker pool does not exhaust threads under burst of 200 requests per second.	HTTP 503 or queue length stays bounded; no thread‑exhaustion errors in logs.	Use `wrk` or `k6` to generate burst; monitor server metrics.	Max concurrent threads 48 of 64 limit; queue depth ≤ 5.
PF‑08	Client-side sync queue processing does not block UI thread; frame drop rate < 2 % during sync.	Use `adb shell gfxinfo` to measure jank; ensure 95th percentile frame time < 16 ms.	Trigger sync while scrolling list; capture frametimes.	95th‑pct frame time 12 ms → pass.
PF‑09	Power‑optimized sync: when device is in Doze mode (Android) or App Nap (iOS), sync defers until maintenance window unless marked high‑priority.	Sync requests are deferred; no wake‑locks acquired unnecessarily.	Force Doze via `adb shell dumpsys deviceidle force-idle`; attempt sync; verify no alarm triggered.	Sync request logged as “deferred due to idle”.
PF‑10	Network type awareness: sync lowers payload size or defers when on metered/cellular connection per user setting.	On metered network, client sends compressed payload or skips non‑essential sync.	Toggle metered via `adb shell cmd connectivity set-metered falsewifi true`; inspect request headers for `Content-Encoding: gzip`.	Payload size drops from 1.2 MB to 0.4 MB with gzip.

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Release Readiness Checks in Data Sync Testing Checklist (2026)

Version Compatibility

Backward and forward compatibility ensures that rolling updates do not break sync.

Test ID	Description	Pass Criteria	Automation Approach	Example
RR‑01	Client vN‑1 can successfully sync with server vN (new schema additions are optional).	Older client ignores unknown fields; no crashes or data loss.	Deploy older APK to test device; point to staging server with new field; run sync.	New field `"preferences": {}` ignored; sync completes.
RR‑02	Client vN can sync with server vN‑1 (removed fields are handled gracefully).

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