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Re: Multi-User using Core Data?

On 10 Jun 2005, at 10:23, mmalcolm crawford wrote: 

On Jun 10, 2005, at 1:55 AM, Nicko van Someren wrote:

Apple obviously made a conscious decision not to support remote databases with Core Data - they already had an architecture in place to do it (EOF), so it clearly could have been done had they really wanted to. I have no reason if their reason not to include that support was technical, political, or what, but that being said, there are a couple of options give use a single data store for applications on separate machines.

I think that the main reason for not doing this is that CoreData is essentially free but you have to pay for the Enterprise Objects framework, so they felt it was necessary to "cripple" the freeware to a specification that is useful for desktop applications but left the corporates with deep pockets needing to buy EOF.

Core Data is not a "crippled" EOF. Its goal, and how it contrasts with EOF, is made clear in the FAQ:
<http://developer.apple.com/documentation/Cocoa/Conceptual/ CoreData/Articles/cdFAQ.html#//apple_ref/doc/uid/TP40001802-244739>

I appreciate that one is not a crippled copy of the other. What I was meaning is that the SQL database functionality of one is a subset of the other, e.g. a crippled specification. CoreData adds a bunch of extra features too; fetched properties, local stores and store aggregation being key, but the remote, shared database server support is missing.

Also note: WebObjects is bundled for free with the Developer Tools...

Really? WO is available as a download for paid-up commercial developers through ADC Select and Premier membership but I don't see any sign of it in the free developer tools.

1) You could make the SQLite data file location a preference value, and set it to a network drive location.

This is fine until you have two applications accessing the same data at the same time at which point the memory cache and the file will get woefully out of sync and "bad things" will happen.
This ("bad things" will happen) is not the case. Just like EOF, Core Data is designed to properly detect and deal with situations in which the persistent store is modified by another application, and there are well-specified patterns to follow if this occurs.

Really? On the page of the link you gave above, under the heading Change Management it says:

"There is an important behavioral difference between EOF and Core Data with respect to change propagation. In Core Data, peer managed object contexts are not "kept in sync" in the same was as are editing contexts in EOF. Given two managed object contexts connected to the same persistent store coordinator, and with the "same" managed object in both contexts, if you modify one of the managed objects then save, the other is not re-faulted (changes are not propagated from one context to another). If you modify then save the other managed object, then (at least if you use the default merge policy) you will get an optimistic locking failure."

Clearly then there is no change propagation even if we are talking through the same Persistent Store Coordinator, let alone if we are accessing the same file from two different applications on two different machines over a network. If we were using two application on the same machine perhaps we could watch the database files (using something like Uli's UKKQueue http://www.zathras.de/angelweb/ sourcecode.htm#UKKQueue ) and call refreshObject:mergeChanges: on anything we care about, but as far as I know file change notification does not work reliable over network mounts.

The section in the Core Data Programming Guide on Change Management <http://developer.apple.com/documentation/Cocoa/Conceptual/CoreData/ Articles/cdChangeManagement.html> makes no mention whatsoever regarding multiple applications accessing the same source (on the same machine or different ones), though it makes repeated remarks about the same application accessing the same data through multiple Managed Object Contexts (which all talk to the same Persistent Store Coordinator).

It would be very useful if Apple could make a clear statement (a Technical Note perhaps) about how to have multiple applications accessing the same data store through Core Data, and in particular how to do this with the applications on different machines. If it is genuinely the case that Apple did not deliberately dumb down the specification of Core Data then it would be useful to know how to achieve the same effect as WO/EOF through Core Data.

    Nicko

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References: 
 >Multi-User using Core Data? (From: "J. Scott Anderson" <email@hidden>)
 >Re: Multi-User using Core Data? (From: Jeff LaMarche <email@hidden>)
 >Re: Multi-User using Core Data? (From: Nicko van Someren <email@hidden>)
 >Re: Multi-User using Core Data? (From: mmalcolm crawford <email@hidden>)



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