Activity:

Uploading Contacts in xDB through xConnect by a scheduled task. Number of contacts: 8000+

Environment: Sitecore 9.0.2 XP0 The scheduler is synchronous. The contacts are getting imported successfully.

Environment: Sitecore 9.0.2 XP scaled In the synchronus mode, the logs mention the scheduler as Not due. In the async mode, the entries are made only for around 3800 data

Is it necessary to have it as async? And why my application is breaking there?

Logs:

[Error] Sitecore.XConnect.Operations.AddContactOperation: Sitecore.XConnect.Operations.EntityOperationException: Operation #0, AlreadyExists, Contact
2019-08-08 06:58:56.249 +00:00 [Error] Sitecore.XConnect.Operations.SetFacetOperation`1[Sitecore.XConnect.Facet]: Sitecore.XConnect.Operations.FacetOperationException: Operation #1, ReferenceNotFound, Contact, Personal
2019-08-08 06:58:56.249 +00:00 [Error] Sitecore.XConnect.Operations.SetFacetOperation`1[Sitecore.XConnect.Facet]: Sitecore.XConnect.Operations.FacetOperationException: Operation #2, ReferenceNotFound, Contact, Emails
2019-08-08 06:58:56.265 +00:00 [Error] Sitecore.XConnect.Operations.SetFacetOperation`1[Sitecore.XConnect.Facet]: Sitecore.XConnect.Operations.FacetOperationException: Operation #3, ReferenceNotFound, Contact, Addresses
2019-08-08 06:58:56.265 +00:00 [Error] Sitecore.XConnect.Operations.SetFacetOperation`1[Sitecore.XConnect.Facet]: Sitecore.XConnect.Operations.FacetOperationException: Operation #4, ReferenceNotFound, Contact, UserAdditionalInformationFacet
2019-08-08 06:58:56.265 +00:00 [Error] ["XdbContextLoggingPlugin"] XdbContext Batch Execution Exception
Sitecore.XConnect.Operations.EntityOperationException: Operation #0, AlreadyExists, Contact
2019-08-08 06:58:56.296 +00:00 [Error] ["XdbContextLoggingPlugin"] XdbContext Batch Execution Exception
Sitecore.XConnect.Operations.FacetOperationException: Operation #1, ReferenceNotFound, Contact, Personal