Core Data doesn't support client server applications.
Some developers have successfully used it on the client for its data
management (undo, change tracking, notifications, etc) and Cocoa
Bindings support, with an in-memory store they manage themselves. A
few developers have done more sophisticated things with a Core Data
sqlite store that is effectively a cache of a subset of the server's
database.
But there's no back end support for MySQL or Postgres besides what
you do for yourself.
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