Mailing-List: contact dev-help@cayenne.apache.org; run by ezmlm
Precedence: bulk
Reply-To: dev@cayenne.apache.org
Content-Type: text/plain; charset=windows-1252
Mime-Version: 1.0 (Mac OS X Mail 9.2 \(3112\))
Subject: Re: Syncing DataContexts
From: Andrus Adamchik <andrus@objectstyle.org>
In-Reply-To: <110BA5B6-5954-4D07-B230-A6CF533E0839@swarmbox.com>
Date: Wed, 16 Dec 2015 21:09:38 +0300
Content-Transfer-Encoding: quoted-printable
Message-Id: <86484CB6-C1DD-4664-8E42-40C31C219323@objectstyle.org>
References: <5663008D.4020903@posteo.de> <5663FA71.4080505@maniatis.org>
 <5664B18B.3030304@posteo.de>
 <F376F698-9592-4080-A7C5-EF0C266ABB7A@objectstyle.org>
 <566D54D0.3000107@posteo.de>
 <110BA5B6-5954-4D07-B230-A6CF533E0839@swarmbox.com>
To: dev@cayenne.apache.org

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.=20

> 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 =
=93receivedQuantity=94 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 =93receivedQuantity=94 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=85=94

This can be related ... if this happens, IIRC Cayenne does not generate =
a snapshot event.=20

> 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