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Manage Sync Sessions - Swift SDK

On this page

  • Sync Connection Behavior
  • Check the Network Connection
  • Suspend or Resume a Sync Session
  • When to Pause a Sync Session
  • Wait for Changes to Upload or Download
  • Check Upload & Download Progress for a Sync Session
  • Manually Reconnect All Sync Sessions

Opening a synced realm starts a SyncSession for that realm. The Realm Swift SDK provides methods to manually pause and resume a sync session.

New in version 10.41.0.

In Realm Swift SDK version 10.41.0 and later, App Services defaults to sharing a single connection to the server for all opened synced realms. This is a change from earlier versions where opening more than one synced realm opened additional connections to the server. The connection to the server is independent of the SyncSession, and is based on the App Services user.

You can change this behavior from the App client configuration.


Realm's offline-first design means that you generally don't need to check the current network connection state. That said, the connectionState property is available if your app calls for some indication of connection state.

You can suspend and resume the sync session on the realm. Pausing a sync session only suspends that realm's sync session. If you have more than one open realm, suspend does not affect the sync sessions for other realms.

For most applications, there is no need to manually pause and resume a sync session. However, there are a few circumstances under which you may want to pause or suspend a sync session:

  • You only want to sync after the user takes a specific action

  • You only want to sync during a certain time of the day

  • You don't want to attempt to sync when there is poor network connectivity

  • You want to explicitly force a sync session to connect

In the case of poor network connectivity, continually trying to establish a network connection can drain the user's device battery.

The case of explicitly forcing a sync session to connect is most commonly related to being offline for some time. The sync client attempts to connect, and upon failure, goes into exponential backoff. After being offline for a long time, the client may not immediately reconnect. Pausing and resuming the sync session explicitly forces the connection.

When you do pause a sync session, keep these things in mind:

  • If the client may be offline longer than the client maximum offline time, the client will be unable to resume syncing and must perform a client reset.

  • Pausing a sync session pauses it in both directions. Changes that your app makes on the device do not sync with the backend, and changes to the data in the backend or on other devices do not sync to the device. There is no way to pause only uploads or pause only downloads.

  • Do not pause a sync session if you want a client to permanently stop syncing with the backend. To permanently stop syncing, copy the contents of the synced realm into a non-synced realm, and use the non-synced realm in the client.

Do not pause sync to stop syncing for indefinite time periods or time ranges in months and years. The functionality is not designed or tested for these use cases. You could encounter a range of issues when using it this way.

New in version 10.45.0.

To wait for all changes to upload or download from your synced realm, call realm.syncSession?.wait(for: ).

This method takes a ProgressDirection argument to specify whether to track upload or download progress.

You can use these methods with Swift's async/await syntax, or with the callback syntax. The callback version, realm.syncSession?.wait(for:queue:block:), can take a queue to dispatch the callback onto, and a block to invoke when waiting is complete.

Changed in version 10.50.0: transferredBytes and transferrableBytes deprecated in favor of progressEstimate

You can check upload and download progress by registering a token that provides a progressEstimate for a given upload or download direction and work scope. You can set a ProgressMode to determine the work scope: either observe indefinitely or unregister the block after the current work item has completed.

The progressEstimate value provided by the token is a double whose value ranges from 0.0 to 1.0. At 1.0, the upload or download is complete. You can use this progressEstimate to display a progress indicator or estimated data transfer percentage.

New in version 10.44.0.

Realm automatically detects when a device regains connectivity after being offline and attempts to reconnect using an incremental backoff strategy. For example, on Apple platforms, Realm listens for network change notifications and automatically triggers a reconnect immediately after receiving one.

In Swift SDK version 10.44.0 and later, you can choose to manually trigger a reconnect attempt with SyncSession.reconnect() instead of waiting for the duration of the incremental backoff. This is useful if you have a more accurate understanding of the network conditions and don't want to rely on Realm's automatic reconnect detection.

let syncSession = realm.syncSession!
// Work with the realm. When you need to force the sync session to reconnect...

When you call this method, the SDK forces all sync sessions to attempt to reconnect immediately. This resets any timers used for incremental backoff.

Calling this method does not guarantee the device can reconnect. If the SDK gets a fatal error, or if the device is already connected or is trying to connect, calling this method has no effect.


Cannot Reconnect Within Socket Read Timeout Duration

Realm has an internal default socket read timeout of 2 minutes, where Realm will time out if a read operation does not receive any data within a 2-minute window. If you call SyncSession.reconnect() within that window, the Swift SDK does not attempt to reconnect.

← Sync Data in the Background