Write to a Synced Realm - .NET SDK

Overview

When writing data to a synced realm using Flexible Sync, you use the same APIs you use when writing to a local realm. However, there are some differences in behavior to keep in mind.

When you write to a synced realm, your write operations must match both of the following:

• The sync subscription query

• The permissions in your App Services App

If you try to write data that doesn't match the subscription query and the permissions expression, the write reverts and throws a CompensatingWriteException. You can look in the App Services logs to see details about why the write was denied.

Compensating Writes

When a client attempts an "illegal" write of an object, the following occurs:

1. Because the client realm has no concept of "illegal" writes, the write initially succeeds until realm resolves the changeset with the App Services backend.

2. Upon sync, the server applies the rules and permissions. The server determines that the user does not have authorization to perform the write.

3. The server sends a revert operation, called a "compensating write", back to the client.

4. The client's realm reverts the illegal write operation and throws a CompensatingWriteException.

Any client-side writes to an object between the illegal write and the corresponding compensating write will be lost.

In practice, this may look like an object being written to the realm, and then disappearing after the server sends the compensating write back to the client.

When a CompensatingWriteException is thrown, it includes an enumerable of CompensatingWriteInfo objects. Each CompensatingWriteInfo object contains properties that describe the object type, its primary key, and reason the server performed the compensating write.

Determining Which Data Syncs

The rule that controls which data you can write to a synced realm is the intersection of your Device Sync configuration, App Services permissions, and the Flexible Sync subscription query that you use when you open the realm.

Let's look at an example of how those components work together:

{
  "name": "owner-read-write",
  "apply_when": {},
  "document_filters": {
    "read": { "ownerId": "%%user.id" },
    "write": { "ownerId": "%%user.id" }
  },
  "read": true,
  "write": true
}

realm.Subscriptions.Update(() =>
{
    var completedItemsQuery = realm
        .All<MyTask>()
        .Where(i => i.Status == "completed");
    realm.Subscriptions
        .Add(completedItemsQuery,
            new SubscriptionOptions() { Name = "completedItems" });
});