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MMKV can keep local values available between launches and help your React Native UI react to local changes. It does not sync those changes to Supabase, retry failed requests, or resolve competing edits. To support offline editing safely, treat local persistence, pending-change tracking, server synchronization, conflict handling, and sensitive-data storage as separate responsibilities.
What MMKV does—and what it does not do
MMKV is a key-value persistence option for local app state. Its documented hooks and listeners can read and respond to changes in local keys. Those capabilities are useful for keeping small pieces of state available on a device, but they do not constitute a remote synchronization protocol.
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In particular, local persistence alone does not tell your app which edits have reached the server, how to retry after a failed request, whether a remote change is newer, or how to combine two edits to the same record. Those rules belong in an explicit sync design. Keep synchronous local operations appropriately small; the cited MMKV material does not establish a safe maximum payload size or make MMKV a relational offline database.
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Do not use the visible form value as your only record of an edit. Model the locally edited data and its synchronization state deliberately. A practical design has stable record identifiers and separate metadata that lets the app distinguish an edit awaiting upload from one acknowledged by the server or one that needs conflict handling.
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This is an architectural recommendation, not a universal schema prescribed by the documentation. The exact fields and retention period depend on the product. The key distinction is between what the user sees and the information the sync process needs to decide what to send and what to do with the response.
- Stable identity: identify the same record consistently on the device and server so a retry does not accidentally create a second logical record.
- Pending state: retain enough information to know which local changes have not yet been acknowledged.
- Conflict state: preserve a change that cannot be safely reconciled automatically instead of silently treating it as synced.
- Visible state: keep sync bookkeeping distinct from the form or screen state where practical, so presentation logic does not become the sync protocol.
If your offline dataset is relational or requires a substantial local sync engine, evaluate a local-first database rather than assuming a key-value store supplies those database and synchronization behaviors.
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How Supabase’s offline-first example is structured
Supabase’s offline-first tutorial, written by Benedikt Müller and dated 8 October 2023, demonstrates a specific architecture: WatermelonDB backed by SQLite stores local data, Supabase RPC is used for synchronization, and Realtime can trigger synchronization on other devices. It is an example of combining a local database with an explicit sync path—not an example of MMKV providing synchronization.
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The tutorial states: “Data can become stale, and if it is changed in multiple places while offline, data conflicts can occur.” Its example uses a latest-change-wins approach. That is one policy for that example, not a guarantee that the latest timestamp or arrival should always win in another app.
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Plan the sync path before relying on reconnection
When connectivity returns, the app needs a deliberate operation to submit pending edits, inspect the result, and reconcile local state. Reconnection is an opportunity to run that operation; it is not proof that every prior write succeeded.
- Identify work to send. Read the locally tracked edits that remain pending, using the application’s stable record identifiers and sync metadata.
- Submit through an authenticated, authorized path. Send the changes to the server through the app’s intended Supabase operation. A client-side session does not replace database authorization.
- Handle the response. Distinguish a successful acknowledgment from server errors and authorization failures. Do not mark an edit complete merely because a request was attempted.
- Reconcile local and server state. Apply the chosen conflict policy, update sync metadata, and refresh authoritative server state when needed.
- Retry deliberately. If a request fails, retain the pending work and make retries safe for the application’s data model. The cited materials do not prescribe a universal retry or idempotency schema.
Supabase’s Expo quickstart demonstrates client initialization and session persistence, but describes its path as optimized for getting started. Treat it as setup guidance, not a production security review. Review row-level security (RLS) policies for every client-accessible table, and never ship privileged secrets in the app.
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Choose conflict behavior for the data, not for convenience
A conflict occurs when separate edits made without a shared current state cannot both be applied as if nothing changed. Decide what the product should do before silently overwriting one version.
- Latest-change-wins: simple to explain and potentially suitable for low-stakes fields, but it can discard a concurrent edit. This is the policy shown in Supabase’s tutorial example.
- Version checks or rejection: detect that the server record changed since the client based its edit on it, then require a retry or another resolution step. This avoids pretending an outdated edit was applied cleanly, but asks the application to handle the rejected change.
- Field-aware merge: combine independent changes when the data model makes that safe. It requires explicit rules; two edits to the same field may still need a decision.
- Human resolution: preserve competing versions and ask the user to choose or edit a combined result. This is more appropriate when losing a change matters, at the cost of extra interface and workflow complexity.
No one policy is universally correct. A personal preference field and a collaborative document have different consequences if one edit replaces another. Keep enough information to implement the policy you choose, and communicate unresolved conflicts instead of presenting them as successful syncs.
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Use Realtime as a prompt to reconcile, not as the record of truth
A Realtime event can tell a device that it may need to synchronize, but Supabase says Realtime does not guarantee delivery of every message. An app therefore should not rely on receiving every event or assume an event contains a complete, authoritative history of changes. When state may have changed, fetch or reconcile against the authoritative database state.
Configure the relevant publication and access rules for subscriptions. Supabase recommends Broadcast for most use cases; its documentation notes that Postgres Changes is simpler but does not scale as well. Whichever mechanism you choose, keep the recovery path independent of receiving a particular notification.
Keep authentication and sensitive storage separate
Persisting app data helps it survive launches, but it also means information remains on the device. React Native’s security guidance distinguishes ordinary persistence from secure storage: it describes Async Storage as unencrypted and unsuitable for tokens or secrets, and recommends platform secure storage for sensitive credentials.
Do not treat the local edit store as the place for credentials simply because it persists reliably. Decide what user data the app actually needs offline, avoid retaining more than necessary, and use the platform’s secure-storage facilities for sensitive credentials. Session persistence and the data queue are related to sync, but they are different concerns.
Quick Recap
Compare the responsibilities before choosing a design
| Option or mechanism | Role supported by the cited documentation | What it does not establish |
|---|---|---|
| MMKV | Local key-value persistence, hooks, and listeners for local changes. | A remote sync protocol, retry queue, conflict policy, or relational offline database. |
| WatermelonDB with SQLite in Supabase’s tutorial | Local-first database in the tutorial’s architecture, paired with RPC-based synchronization. | A requirement that every React Native app use this stack, or that it is built into MMKV. |
| Supabase Realtime | Can signal that another device’s changes may warrant synchronization. | Guaranteed delivery of every event or a replacement for fetching authoritative state. |
| Broadcast | Supabase’s recommended choice for most subscription use cases. | A guarantee that every app should use it regardless of its needs. |
| Postgres Changes | A simpler change-subscription option. | The same scaling characteristics as Broadcast; Supabase says it does not scale as well. |
| Platform secure storage | Storage guidance for sensitive credentials. | A substitute for designing the app’s local edit model or sync behavior. |
A practical design checklist
- Can the app distinguish local edits that are pending from changes the server has acknowledged?
- Does every pending edit have an identity and a deliberate server operation?
- What happens when the server rejects an edit, the user lacks authorization, or another device changed the same data?
- Can the app recover by fetching current server state if a Realtime message is missed?
- Are RLS policies appropriate for every table the shipped client can access?
- Are credentials kept in platform secure storage rather than treated as ordinary persisted app state?
- Does the chosen local store fit the shape and size of the offline data, rather than being mistaken for a sync engine?
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