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From Andrus Adamchik <and...@objectstyle.org>
Subject Re: Syncing DataContexts
Date Wed, 16 Dec 2015 18:09:38 GMT
Hi Matt,

> On Dec 15, 2015, at 11:37 PM, Matt Watson <matt@swarmbox.com> wrote:
> It is my understanding that if more than one ObjectContext has the same DataObject, when
changes to that object are committed, the other context that has that same object will automatically
be updated with the changes. But this does not seem to be happening for me.

Within the same VM, yes, unless you turn it off explicitly. Try running in debugger with a
breakpoint in ObjectStore.snapshotsChanged(..) method and see if it is called. 

> The specific scenario, is that I have two different people receiving product to the same
PurchaseOrderLine. If they both bring in that PurchaseOrderLine before anything has been received,
the “receivedQuantity” on that POL starts out at 0 (ZERO). When person A scans a box,
the POL.receivedQuantity increments to 1, and then increments to 2 when he scans another.
Then if person B scans a box, their POL still thinks the “receivedQuantity” is ZERO and
it increments it to 1. So what I have in the database is a POL with receivedQuantity 1, but
there are actually InventoryTransactions showing that it should be at 3.

So I presume you commit after every scan? Again, this would mean the events should be propagated
and merged into objects.

> Not sure if this is related but we often see in the log during a commit -> "snapshot
version changed, don't know what to do…”

This can be related ... if this happens, IIRC Cayenne does not generate a snapshot event.


> I read the Cayenne docs here (https://cayenne.apache.org/docs/4.0/cayenne-guide/performance-tuning.html#turning-off-synchronization-of-objectcontexts
<https://cayenne.apache.org/docs/4.0/cayenne-guide/performance-tuning.html#turning-off-synchronization-of-objectcontexts>)
that it does not recommend using synchronization for a large number of users (peer Contexts),
however I tested this with only 2 and still experience the issue.  And now that I have just
reread that documentation, I may have misunderstood that the object is automatically updated,
and that instead the Context is notified of the changes via an event, but if I am not listening
that event (& handling), then that could be why the other Context does not have the updated
information on that object.

In addition to poor performance characteristics, snapshot syncing is prone to various race
conditions (e.g. an object property may get refreshed via an event, but then immediately overwritten
from the UI), so its use is rather limited. I'd personally turn it off and use some other
approach, e.g. optimistic locking in combination with query cache (query cache which was discussed
a few days ago here [1].

Andrus

[1] http://stackoverflow.com/questions/34244260/cayenne-cache-does-query-cache-replace-object-cache



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