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Sync Errors

On this page

  • Overview
  • Sync Protocol Errors
  • Flexible Sync Errors
  • MongoDB Translator Errors
  • MongoDB Connection Errors
  • Sync Client Errors
  • Handle Sync Errors
  • Set the Client Log Level

While you develop applications using Atlas Device Sync, you may run into errors. This section lists common Sync errors and describes how to handle them.


If you encounter an error not listed on this page, you can file a support ticket.

The following table describes Device Sync protocol errors and how to handle them. Atlas App Services reports errors in your Device Sync logs.

Error Name

This error occurs when the client is using a realm file that the server cannot access after terminating and re-enabling Device Sync.

This error triggers a client reset. To recover from this error, perform a client reset.


This error indicates that the client attempted to synchronize a realm file associated with an identity other than the specified user. This may occur if Device Sync is terminated and reenabled while the user is offline, which invalidates their previous identity.

To recover from this error, delete the local realm file and then re-open the realm.


This error indicates that the client attempted to synchronize a realm file that has a different sync history than the server realm. This may occur if Device Sync is terminated and reenabled while the user is offline, which invalidates their previous sync history.

This error triggers a client reset. To recover from this error, perform a client reset.


This error occurs when a user's data access permissions are not sufficient for a given request. This can occur if a user attempts to open a realm without read permission or modify data without write permission.

To troubleshoot this error, review your rules to make sure that users have proper data access permissions.

This error indicates an internal failure that is not covered by a more specific error. For example, this might occur when you hit the storage limit of a free tier Atlas cluster.

The following errors may occur when your App uses Flexible Sync.

Error Name

This error indicates the client query is invalid or malformed. This error includes a message that provides details about why the query is invalid.

To recover from this error, you may need to verify the query syntax is correct, and that you are using query operators that are supported on the server. Additionally, confirm you are querying a queryable field in your Flexible Sync configuration. If your query uses an indexed queryable field, ensure that it meets the requirements for valid client-side queries for indexed queryable fields.


This error indicates that server permissions for the file ident have changed since the last time it was used.

This error triggers a client reset. To recover from this error, perform a client reset.


This error indicates that the client tried to open a session before the initial sync was complete. This may occur when the app has just enabled Sync, and is still in the process of building the Sync history.

The client attempts to reconnect until this process is complete. Then, this error resolves and Sync begins working normally.


This non-fatal error occurs when a client attempts an "illegal" write. The following are considered illegal writes:

  • Creating an object before opening a subscription.

  • Creating an object that would be outside of the client's query view. "Query view" includes both the client's subscriptions and the client's read permissions.

  • Modifying an object that is outside of the client's query view.

  • Creating, deleting, or modifying an object or field that the client does not have write permissions to.

  • Modifying an object in such a way that client would no longer have write permissions on that object or field after the write.

  • Updating an existing object's indexed queryable field value.

Because the local realm has no concept of "illegal" writes, the write will always succeed locally. Upon sync, the server will notice the illegal write. The server then undoes the change. The undo operation, called a "compensating write", syncs back to the client so the client's realm no longer has the illegal write. The server also sends this error to let the client know what has happened.

Any local writes to a given object between an illegal write to that object and the corresponding compensating write will be lost.


Consider the following pseudo-code example:

obj1.fieldA = 10 // illegal due to field-level permissions
obj1.fieldB = 5 // legal
DELETE obj1 // legal
DELETE obj2 // legal

Here, the user does not have permission to write to fieldA but attempts to write to it anyway -- an illegal write. The user then performs two legal writes to the same object and another legal write to a different object. Upon receiving the compensating write for the illegal write to obj1.fieldA, the subsequent two legal writes to that object are lost. The end result is that obj1 still exists and its two field values are whatever they were before the attempted modifications. Meanwhile, the deletion of obj2 is unrelated to the illegal write that caused the compensating write, so it stands and obj2 stays deleted.

The illegal write appears in the applog as a non-fatal error. Illegal writes might indicate that your application code is doing something you didn't intend.

In the following cases, you'll see ErrorWriteNotAllowed instead of ErrorCompensatingWrite after an "illegal" write:

  • When using older SDK versions that are linked to a realm-core version before realm-core 12.1.0. In this case, the server will not undo the illegal write, and you will need to perform a manual client reset.

  • When modifying an object in a collection with Data Ingest enabled. In this case, the error is non-fatal and does not trigger a client reset. The server skips over the illegal change and does not apply it to the synced MongoDB cluster.

The following errors may occur in the translation process between Device Sync and MongoDB Atlas.

Error Name

When the MongoDB translator cannot integrate a changeset, it retries for a fixed number of times. This error occurs when the translator reaches the maximum number of retries and could not commit changes. This is usually caused by insufficient cluster size. This can be due to a very large transaction that exceeds the cluster's available resources. For example, a device is offline for a long time and tries to sync an atypical quantity of batched transactions. Or the cluster resources are generally insufficient for the app's needs.

You can resolve this error by upgrading the cluster tier.

To avoid this error, ensure the linked MongoDB cluster meets the needs of your app. Also, make sure your app uses best practices for reading and writing data. Refer to Atlas Cluster Sizing and Tier Selection for more information.


This error occurs when a MongoDB Atlas write (i.e. not a Sync client) modifies a document such that it no longer conforms to the app's schema. Documents that do not match the schema cannot be synced and any local updates to the object represented by such a document will not propagate.

For more information, see Unsynced Documents.

This error occurs when a synced MongoDB cluster rejects the write operation for a propagated Device Sync change. This is usually caused by a duplicate key exception, which means that two objects use the same primary key. To avoid this error, use an ObjectId or UUID as the primary key value. Alternatively, ensure that every synced object has a unique primary key, even across partitions.
TranslatorFatalError - ChangeStreamHistoryLost

This error occurs when old entries in the oplog have expired before the server-side "translator" process could read them. Without these entries, the translator cannot bring the MongoDB cluster and the Realm object server to an equivalent state.

This can happen when:

  • Sync is paused for so long that entries fall off the oplog.

  • You drop a collection that the translator was using.

  • The MongoDB cluster is unreachable for too long.

Because the free tier has a shared oplog, it is more vulnerable to this error.

To resolve this error, terminate and re-enable Sync.

If you enable Device Sync on a newly-created cluster, the operation may fail to parse the cluster URI. This happens because an SRV record for the cluster hasn't propagated yet. There are two possible workarounds, either:

The sync protocol returns an ERROR message when an error appears to have been caused by a connected client. Each message contains a code number and a description of the error.

To see the full list of sync errors, refer to the error code list in the Realm Database Core GitHub repository.

Every application that uses Sync needs a sync error handler. To learn more about sync error handling, see your preferred SDK:

You can specify the client log level. Setting the log level to trace or debug can help diagnose issues while your application is is in development. You can log general information or details about all sync events, or log only warnings or errors.


Verbose logging negatively impacts performance. For production deployment, reduce the log level.

For more information about available log levels, including how to set the client log level, see your preferred SDK.

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