Description

Backing up an xConnect site content in the Azure App Services might only be partially successful. According to Microsoft's official documentation, the backup process skips the locked files and backs up the rest of the site. This means the locked files are not included in the backup if the backup attempt happens at the moment when the file is in use.
The following messages might appear when experiencing this:

• File skipped - \data\jobs\continuous\IndexWorker\singleton.job.lock: Sharing violation 
  to ...\package\fs\data\jobs\continuous\IndexWorker\singleton.job.lock Uploading metadata to the blob. Backup completed...

  singleton.job.lock is created by KUDU to ensure that there is only one web job instance present. It is not a Sitecore XP related file and it should not be backed up. The Azure message can be safely ignored. You can check the code in the KUDU repository for details.

• File skipped - \site\wwwroot\App_Data\Logs\xconnect-log-XXXXXX-cortex-processing-20200721.txt: 
  Sharing violation to ...\package\fs\site\wwwroot\App_Data\Logs\xconnect-log-XXXXXX-cortex-processing-20200721.txt
  Uploading metadata to the blob. Backup completed...

  xConnect log files are not critical data and you can exclude them from the backup. Note that if a log file is locked today since being in use, it will not be locked tomorrow since new files are created daily. 

Solution

The Azure messages about backup failures can be safely ignored. 
To avoid such messages, you can exclude specific files or folders, so they are never added to the backup. For more details see Configure Partial Backups.