There are always times when failure is permanent. For instance, at the moment if the asset service is full. Even if
store and forward were added, unrecoverable failure can still occur if both sides are full.
I have had to spend my scarce time dealing with many bugs where the investigation has led to a piece of code that simply
swallows its exception, where signalling or logging the error condition would have made the cause of complex problems
obvious. So I think that lower level components should always appropriately signal failure or other relevant
conditions. It's a different question as to whether the caller thinks it appropriate to notify the user, log the
problem or do nothing (though it should always be possible to tell somehow when there is an issue). But the caller
should be given the capability to deal with errors or other conditions as appropriate.
As for Oren's second issue (null being mistaken as indicating a valid asset), this sounds like a straightforward bug
that should be fixed.
The 3rd issue, where very old asset servers sent no reply, I agree sounds like something we can risk fixing, as nobody
is using (or could use) the old UGAIM pre 0.7 service processes any more.
Regarding message queueing, I agree that this would be a great approach. However, OpenSimulator would still need some
kind of sqlite equivalent so that it can work out of the box, whilst still being pluggable if someone wants to use a
more heavyweight system. The sqlite equivalent must have equivalent support by OpenSimulator, just as the sqlite
database plugin should have that today.
On 18/04/14 22:43, Melanie wrote:
The point is no NOT let it fail. Asset storing never fails
permanently if it's retried until successful. The upper layers (all
the way to the viewer) are not equipped to handle an asset storing
failure. Propagating the exception would just annoy the user with
needless messages. Since asset servers can be "gone" for a while,
for instance when there is a net failure, there is no way to give it
a timeout, either. A sim in OSGrid, if it gets disconnected, could
run on locally cached assets and manage to reconnect after 20
minutes and simply upload all new assets since then. Screaming
"failure" at the user is pointless in such a scenario.
- Melanie
On 18/04/2014 22:56, Oren Hurvitz wrote:
There seems to be a misunderstanding here. We're talking about a case where
the operation has FAILED. The only question is whether to pretend that it
succeeded, so that the user will find out that it failed later, to their
surprise, or to report failure immediately. Obviously it's better to report
failure immediately.
On Fri, Apr 18, 2014 at 10:40 PM, Melanie <[email protected]> wrote:
Name one valid use case where current OpenSim is able to handle such
an exception gracefully, e.g. without user-visible error.
- Melanie
On 18/04/2014 13:28, Mike Chase wrote:
I'm inclined to agree with Oren. Asset Writes could fail for a variety
of
reasons and there are lots of use cases where you need to know the asset
is
on disk. I think propagating exceptions is the more sound approach IMO.
I also agree re: the custom comms vs a persistent queue mechanism but I
don't want to derail this topic. That can wait for another day.
Mike
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Oren Hurvitz
Sent: Friday, April 18, 2014 7:06 AM
To: [email protected]
Subject: Re: [Opensim-dev] Error detection when storing an asset
Regarding the hiding of exceptions: to be clear, I was already bitten by
this behavior; that's why I started to investigate how assets are
stored. I
have therefore already changed Kitely's version of OpenSim to propagate
exceptions, and the question is whether other people would like me to
contribute this change. If anyone has an opinion then please reply.
Regarding your suggestion to save assets to local disk and retry them
later:
this is basically what a persistent message queue does. If you're going
to
go that route then it would be best to add a real message queue rather
than
a home-grown one. I would LOVE it if OpenSim used a message queue for
communications, as it would allow ripping out thousands of lines of
homemade
communications code, and would be faster and more reliable to boot. But
that's a bigger issue and I'll put it aside for now.
In this particular case, using a persistent message queue isn't be the
right
solution: the right solution is to report failures immediately. Otherwise
you'd get weird behavior such as a user who thinks they've successfully
worn
a piece of clothing, but when they teleport to another region it
disappears
because the other region can't load the asset (because it was never
saved).
To prevent these problems you need to fail-fast, and tell the user
immediately when a problem happens. This doesn't mean to crash the sim; I
strongly doubt any asset failure would cause that, it would just fail the
specific packet or message that is currently being handled, as it should.
--
View this message in context:
http://opensim-dev.2196679.n2.nabble.com/Error-detection-when-storing-an-ass
et-tp7579223p7579225.html
Sent from the opensim-dev mailing list archive at Nabble.com.
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--
Justin Clark-Casey (justincc)
OSVW Consulting
http://justincc.org
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