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From "Musall, Maik" <m...@selbstdenker.ag>
Subject Re: A way to refreshObject()
Date Wed, 22 Feb 2017 15:55:49 GMT
Hi Andrus,

ok, setting PersistenceState.HOLLOW works. So easy :-)

I'm subscribing to your argument to generally refreshing entire collections for consistency.
But this isn't about a simple CMS with a few blog posts. I have cases where I have spent considerable
resources to fetch large object graphs into memory, and then I'm sending off a few threads
doing middle-duration asynchronous processing on a few objects, have them update an computed
attribute and come back. When I'm receiving the message that object x is finished updating,
it wouldn't make sense to refresh the entire object graph, let alone repeat any expensive
db roundtrips.

However, I do have cases where I want to refresh the entire graph of objects in one ObjectContext,
but I still want to make use of the snapshots to save the fetches if they tend to be expensive.
This is how I'm doing it now, is this correct?

	for( Object obj : context.getGraphManager().registeredNodes() ) {
		((DataObject)obj).setPersistenceState( PersistenceState.HOLLOW );
	}

The assumption below all this of course is that I can only have one thread writing in one
ObjectContext, although I can read on one ObjectContexts with several threads at once (like
when using .parallelStream() on collections). Of course everything would be easier if the
asynchronous threads could just all operate on the same ObjectContext and there would be no
need to refresh anything in the first place.

And no, I don't want auto-synchronization. I do need the isolation of changes in the general
case.

Maik


> Am 22.02.2017 um 14:33 schrieb Andrus Adamchik <andrus@objectstyle.org>:
> 
> Hi Maik,
> 
> Like I said, I personally hate chasing individual object changes, and rather deal with
refreshing entire collections. Refreshing individual objects quickly leads to cases that are
nearly impossible to handle in the code, as modified objects may no longer match the criteria
for being in a given collection. 
> 
> Say in a CMS you have a list of published articles. Another context changes published
status of an article from true to false. You refreshed your object, but it is still in the
published list. So now you need to filter that list in-memory. But the list still doesn't
see any new articles that got published, so you need to track insert events, and so on. 
> 
> So instead of micro-optimizations with shared snapshot cache, you should use queries
with cache groups and query cache. They get refetched when a cache group is flushed in response
to an object commit. So technically you are not reusing the snapshot cache, but it really
doesn't matter. The benefit you get in code simplicity, consistency, and often performance,
always outweighs that. Besides this approach is cluster-friendly.
> 
>> However, this page says "By default when a single ObjectContext commits its changes,
all other contexts in the same runtime receive an event that contains all the committed changes",
so perhaps there may be a way to send objectcontexts this event regardless?
> 
> Not sure if you are convinced by the argument above :). But yeah, you can turn context
syncing back on, and get auto-synchronization. So when you set Constants.SERVER_CONTEXTS_SYNC_PROPERTY
to true (or rather don't set it to false), does it handle the refreshing the way that you
want?
> 
> Andrus
> 
> 
>> On Feb 22, 2017, at 4:02 PM, Musall, Maik <maik@selbstdenker.ag> wrote:
>> 
>> Hi Andrus,
>> 
>> this works, but requires using the newly localObject()ed object instance to be used
after refreshing. Any other references of that object in the original context will still hold
the unchanged values.
>> 
>> Btw, I found the page I mentioned: 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>
>> I used the mentioned property setting in there to turn of synchronization of ObjectContexts,
for the listed reasons. However, this page says "By default when a single ObjectContext commits
its changes, all other contexts in the same runtime receive an event that contains all the
committed changes", so perhaps there may be a way to send objectcontexts this event regardless?
>> 
>> Maik
>> 
>> 
>>> Am 22.02.2017 um 07:37 schrieb Andrus Adamchik <andrus@objectstyle.org>:
>>> 
>>> Since we mostly focused on policy- and event-based *query* caches, the API for
managing caching of individual objects is not as streamlined, but here it is FWIW:
>>> 
>>> T myObject = ..;
>>> ObjectContext context = myObject.getObjectContext();
>>> ObjectId id = myObject.getObjectId();
>>> 
>>> // kick it out, unset ObjectContext (side effect - sets ObjectId to null)
>>> context.getGraphManager().unregisterNode(id);
>>> 
>>> // restore ObjectId.. it was set to null in the call above (which we probably
should change)
>>> myObject.setObjectId(id);
>>> 
>>> // get a fresh fault
>>> myObject = context.localObject(myObject);
>>> 
>>> Andrus
>>> 
>>> 
>>>> On Feb 21, 2017, at 11:54 PM, Musall, Maik <maik@selbstdenker.ag> wrote:
>>>> 
>>>> Hi all,
>>>> 
>>>> I have an application using a big shared snapshot cache. Objects freshly
instantiated in an ObjectContext get their attributes populated based on the snapshot cache
at the time, and keep them during the context's lifetime, which is also what I want. (Last
week I found a page on cayenne.apache.org <http://cayenne.apache.org/> explaining this
behaviour, but I can't find it again. Seems not to live within the linked Guide hierarchy.)
>>>> 
>>>> In some cases though, I have object instances in ObjectContext 1 which I
know are getting modified in ObjectContext 2, and I want them to be updated after commit in
context 1. In EOF, I could call oc1.refreshObject(obj), or even oc1.refreshAllObjects(), which
will merge the new values in from the snapshot cache. Is there any way to do this in Cayenne,
short of refetching which would cost a roundtrip to the database even though I know I already
have the current values in the snapshot cache?
>>>> 
>>>> (I had a question posted recently about "creating a fault for an ObjectId",
but this is different because I need the commit in context 2 to go through to the database.)
>>>> 
>>>> Maik
> 


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