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MMKV can keep local React Native state available between launches, but it does not sync edits to Supabase or resolve conflicts. To keep offline edits safe, store them with durable sync metadata, send pending changes through an authenticated server path when connectivity returns, and decide what happens when the server has a competing version.
What MMKV does—and what it does not
MMKV is a local key-value store with APIs for reading and updating values and reacting to key changes. Those capabilities can support local persistence and reactive UI updates. They do not provide a remote-write queue, retry policy, synchronization protocol, or conflict-resolution rules.
That boundary matters: a value saved locally is not necessarily saved on the server. Treat persistence and synchronization as separate responsibilities. Keep small local state in MMKV if that fits your app, but define explicitly how edits are tracked, transmitted, acknowledged, and reconciled.
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Give each offline edit a trackable lifecycle
Model synchronization status separately from the form or screen state the user edits. A practical design can associate each record with a stable identifier and metadata such as a pending, acknowledged, or conflict state. The exact schema is an application decision; neither a universal schema nor a required retention period is established by the cited guidance.
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When an edit is made offline, save the changed data locally and retain enough information to submit it later. On reconnection, send pending changes through a deliberate sync operation. Handle server errors and authorization failures rather than marking an edit complete just because a request was attempted. Once the server accepts a change, reconcile the local record with the authoritative server state and update its sync status.
This separation also makes failure handling clearer: the visible edit can remain available while its synchronization is pending, and the app can distinguish that situation from a rejected change or a conflict. Choose how long to retain pending data and what to show if it cannot be synchronized based on the data’s importance and your product requirements.
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Choose a policy for concurrent edits
Offline changes can become stale or conflict when the same data is changed in more than one place. Supabase’s offline-first tutorial, published on 8 October 2023 by Benedikt Müller, states: “Data can become stale, and if it is changed in multiple places while offline, data conflicts can occur.” Its example uses latest-change-wins, a simple policy that can overwrite a concurrent edit.
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| Policy | What it does | Trade-off |
|---|---|---|
| Latest-change-wins | Accepts whichever change the application treats as latest. | Simple, but a concurrent edit may be overwritten. This is the policy used in the tutorial’s example, not a universal recommendation. |
| Field-aware merge | Combines changes to different fields when the application can do so safely. | Requires rules for which fields can be merged and what happens when the same field changes. |
| Version check and reject | Accepts an edit only if the server record is still at the version the client based it on; otherwise it returns a conflict for reconciliation. | Preserves awareness of competing edits, but the client needs a path to refresh and resolve the conflict. |
| Human resolution | Shows the competing values and lets a user decide what to keep. | Useful when changes are consequential, but requires a clear review experience. |
These alternatives are design choices, not policies prescribed by the Supabase tutorial. A low-stakes preference may tolerate latest-change-wins; important collaborative content may need version checks, field-specific rules, or user review.
Understand the architecture in Supabase’s offline-first example
Supabase’s tutorial demonstrates a local-first database and an explicit sync path. It uses WatermelonDB backed by SQLite for local data, Supabase RPC for synchronization, and Realtime to trigger sync on other devices. This is the tutorial’s architecture—not an MMKV feature or a drop-in recipe for treating MMKV as a relational offline database.
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The distinction is useful when choosing storage:
| Option | Role supported by the documentation | What it means for your design |
|---|---|---|
| MMKV | Local key-value persistence, hooks, and listeners. | Useful for local state and reactive access; you still need to design remote synchronization and conflict handling. |
| WatermelonDB with SQLite in the tutorial | A local-first database used with an explicit Supabase sync operation. | Follow the tutorial’s database-and-sync architecture if it suits your app; do not describe it as MMKV working automatically with Supabase. |
If you choose MMKV, keep its synchronous local operations appropriately small. The cited MMKV documentation describes hooks, listeners, and persistence-adapter examples, but does not establish a safe maximum payload size or prescribe MMKV as a complete relational offline database.
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A Realtime notification can tell a device that it should check for newer server state. It should not be treated as a guaranteed, complete stream of every change: Supabase documents that Realtime does not guarantee delivery of every message. If a notification is missed, refetch authoritative state and reconcile it with local pending edits.
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Supabase recommends Broadcast for most Realtime use cases and notes that Postgres Changes does not scale as well. Whichever mechanism you choose, configure the relevant publication and access rules, and make the sync operation safe to retry or repeat without assuming an event arrives exactly once.
Keep authentication and sensitive data separate from the edit store
Persisting data makes it available across app launches, but persisted information can also be more exposed. React Native’s security guidance advises against using ordinary unencrypted Async Storage for tokens or secrets and recommends platform secure storage for sensitive credentials. Apply that security boundary to your design: do not put credentials in the general local edit store, and avoid persisting more user data than the app needs.
Authentication and session persistence are related to synchronization, but they are not the same as the queue of local edits. A sync request needs a valid user context and must still be authorized by the backend. Supabase’s Expo quickstart demonstrates client initialization and session persistence, while explicitly describing its path as optimized for getting started. Before relying on a client-facing table, review its row-level security (RLS) policies and keep privileged secrets out of the shipped client.
A practical sync flow
- Save the edit locally. Store the changed record and mark its synchronization state as pending. Keep that metadata distinct from the UI’s current editing state.
- Attempt synchronization when appropriate. Submit pending edits through your chosen authenticated sync operation. Do not assume a local write has reached Supabase.
- Handle the server response. Mark an edit acknowledged only after acceptance. Surface or retain failed edits when the request fails, including when authorization must be refreshed or the user no longer has access.
- Resolve competing versions. Apply the conflict policy appropriate to the record. If the policy requires a decision, preserve enough information to show the competing values rather than silently discarding one.
- Reconcile with authoritative state. Update local data from the server’s accepted state. Use Realtime as a reason to refresh, not as proof that the device received every change.
The exact conflict policy, schema, retention behavior, and offline authentication experience depend on the app’s data and requirements. The key design decision is to make each one explicit rather than assuming a storage library or notification channel supplies it.
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