If a client loses its network connection, your app will continue functioning correctly.
Every client connected to a Firebase database maintains its own internal version of any active data. When data is written, it's written to this local version first. The Firebase client then synchronizes that data with the remote database servers and with other clients on a "best-effort" basis.
As a result, all writes to the database trigger local events immediately, before any data is written to the server. This means your app remains responsive regardless of network latency or connectivity.
Once connectivity is reestablished, your app receives the appropriate set of events so that the client syncs with the current server state, without having to write any custom code.
In addition, Firebase provides tools for persisting data locally, managing presence, and handling latency.
Firebase apps automatically handle temporary network interruptions. Cached data is available while offline and Firebase resends any writes when network connectivity is restored.
When you enable disk persistence, your app writes the data locally to the device so your app can maintain state while offline, even if the user or operating system restarts the app.
You can enable disk persistence with just one line of code.
By enabling persistence, any data that the Firebase Realtime Database client would sync while online persists to disk and is available offline, even when the user or operating system restarts the app. This means your app works as it would online by using the local data stored in the cache. Listener callbacks will continue to fire for local updates.
The Firebase Realtime Database client automatically keeps a queue of all write operations that are performed while your app is offline. When persistence is enabled, this queue is also persisted to disk so all of your writes are available when the user or operating system restarts the app. When the app regains connectivity, all of the operations are sent to the Firebase Realtime Database server.
If your app uses Firebase Authentication, the Firebase Realtime Database client persists the user's authentication token across app restarts. If the auth token expires while your app is offline, the client pauses write operations until your app re-authenticates the user, otherwise the write operations might fail due to security rules.
The Firebase Realtime Database synchronizes and stores a local copy of the data for active listeners. In addition, you can keep specific locations in sync.
The Firebase Realtime Database client automatically downloads the data at these locations and keeps it in sync even if the reference has no active listeners. You can turn synchronization back off with the following line of code.
By default, 10MB of previously synced data is cached. This should be enough for most applications. If the cache outgrows its configured size, the Firebase Realtime Database purges data that has been used least recently. Data that is kept in sync is not purged from the cache.