Core Data CloudKit stops syncing after incomprehensible archive error

This error message your post provided indicates that Core Data failed to unarchive the import request (NSCloudKitMirroringImportRequest). Archiving / unarchiving an import / export request is a Core Data internal logic, and so I agree your assessment that it is not about your code.

The feedback report you filed (FB10392936) doesn't mention the error message, and so I'd suggest that you file a new one to specifically report the error – If you do so, please share your report ID here for folks to track.

Regarding FB10392936, you most likely hit CloudKit throttles. For more information of this topic, see the following documentations:

• TN3162: Understanding CloudKit throttles
• Understand system throttles.

When uploading a big data set and hitting CloudKit throttles, the only thing you can do is to lower the CloudKit operation rate. When using Core Data + CloudKit, however, you don't control how NSPersistentCloudKitContainer synchronizes data. Based on today's API, the only thing you can do is to somehow control the upload pace in the following way:

• Discover the synchronization state by observing eventChangedNotification and looking into event details. For a sample that demonstrates how to handle the notifications, see Sharing Core Data objects between iCloud users.

• Upload more data (by saving the data to the Core Data store, which triggers an export) after the previous export is successful.

With this, the time to upload the whole dataset may still be long, but you will much less likely run into the system throttles.

Best,
——
Ziqiao Chen
 Worldwide Developer Relations.

as requested, I've created another feedback issue: FB15652576

on FB10392936, the error message is in the followup comments. here is the error from that issue - also an incomprehensible archive error.

on that issue yes I had to lower the CloudKit operation rate to complete the upload - it took a month or 2 - it would be great if there was some mechanism for bulk uploads that bypass the throttles. But throttles shouldn't also corrupt the CloudKit database. not only would it throttle, but it would corrupt the database - the only way to recover was to reset the database and reload the schema and start from scratch- not possible in production.

> [error] error: CoreData+CloudKit: -[NSCloudKitMirroringDelegate _importFinishedWithResult:importer:](1306): <PFCloudKitImporter: 0x2815fca80>: Import failed with error: Error Domain=NSCocoaErrorDomain Code=4864 "*** -[NSKeyedUnarchiver _initForReadingFromData:error:throwLegacyExceptions:]: incomprehensible archive (0x53, 0x6f, 0x6d, 0x65, 0x20, 0x65, 0x78, 0x61)" UserInfo={NSDebugDescription=*** -[NSKeyedUnarchiver _initForReadingFromData:error:throwLegacyExceptions:]: incomprehensible archive (0x53, 0x6f, 0x6d, 0x65, 0x20, 0x65, 0x78, 0x61)} 2022-09-08 14:04:43.136554+1000 Tupperty[1354:139717] [error] error: CoreData+CloudKit: -[NSCloudKitMirroringDelegate recoverFromError:](2165): <NSCloudKitMirroringDelegate: 0x28397c8c0> - Attempting recovery from error: Error Domain=NSCocoaErrorDomain Code=4864 "*** -[NSKeyedUnarchiver _initForReadingFromData:error:throwLegacyExceptions:]: incomprehensible archive (0x53, 0x6f, 0x6d, 0x65, 0x20, 0x65, 0x78, 0x61)" UserInfo={NSDebugDescription=*** -[NSKeyedUnarchiver _initForReadingFromData:error:throwLegacyExceptions:]: incomprehensible archive (0x53, 0x6f, 0x6d, 0x65, 0x20, 0x65, 0x78, 0x61)}

Thanks for filing the feedback report for us.

"on that issue yes I had to lower the CloudKit operation rate to complete the upload - it took a month or 2 - it would be great if there was some mechanism for bulk uploads that bypass the throttles."

"a month or 2" is ... too long. I'd suggest that you mention the above in your feedback report for the CloudKit team to consider.

"But throttles shouldn't also corrupt the CloudKit database."

The error message doesn't indicate that the CloudKit database is corrupted though. It can be that CoreData+CloudKit failed to unarchive the persisted request and then the synchronization process stopped. You might follow up with your feedback report to see what the Core Data team has to say.

Best,
——
Ziqiao Chen
 Worldwide Developer Relations.