In my systems everyone is on the same scope, so I haven't run into this
problem (and why I would prefer that any added mechanism would be disabled
by default). I'm a little curious as to how often multiple scopes are
actually used, and are used in the same manner as your system. What kind of
time period do you need to detect invalid DAs in?
You are correct that the issue here is not just a FindScopes one, it's the
fact that DAs don't expire in slpd. I ran into the same issue when moving
slpd unicast to UDP, which is why I added the timeout on the service
registration (following the protocol, of course :). If FindScopes were a
protocol-level command, I'd suggest a similar solution, but FindScopes just
queries the internal database as given to libslp by the connected slpd.
As an alternate to either option, the app could register a fake
registration on each scope it knows about through a previous SLPFindScopes,
which should help keep the knownDAs in sync. And openslp is not changed.
If multiple scopes aren't widely used, or used in the way you use it, this
may be the preferred option. Or it could act as a quick proof of concept.
I'm a little worried about removing the answer suppression in option A. You
are never guaranteed to receive a response from a DA in a particular
FindSrvs, and if there are a lot of DAs on the system the likelihood of
seeing that DA could decrease because you are processing all the DA adverts
each request. And I'm assuming you have some sort of list of potential
drops and not just drop if a DA doesn't respond to one request.
Option B has potential. Slpd could periodically do a unicast request and
time out in the same way that registration requests currently time out. If
this period wasn't too small there wouldn't be that much of an impact on the
system (and if disabled by default would be even less :). I think of the
two options I prefer this one.
Just my two cents.
--Nick
On 9/19/07, Morrell Richard <[EMAIL PROTECTED]> wrote:
>
> I have a problem with uncontrolled loss of DAs ie. where DAs can drop off
> the network without sending out a corresponding DA advert, such as power
> loss, or network device failure.
>
> All the DAs in our system have unique scopes, and we perform unicast
> searches of each scope (I have a patch to the 1.2.1 library that does
> parallel unicast to multiple DAs, which I haven't yet had time to port to
> the latest trunk for submission).
>
> We get the list of scopes using the SLPFindScopes call, which queries the
> local daemon. The problem is that when a DA goes down in an uncontrolled
> fashion, its scope never seems to get removed from the list of scopes, so
> we
> get timeouts for all subsequent searches until the DA comes back up again,
> which is unacceptable in our application (we can cope with a short period
> where this occurs, provided the situation is not permanent).
>
> We have tried setting the active DA discovery parameters to their most
> aggressive, in the hope that this would flag up the lost DAs, but this
> makes
> no difference.
>
> I have looked at the code, both version 1.2.1 which we are using, and the
> latest trunk, and I believe the problem is in both, and arises because the
> active DA discovery only adds new DAs to the DA cache, and does not remove
> them. DAs ARE removed from the cache if a unicast request to a DA fails,
> but these seem to be related only to service registration and
> deregistration
> and, since each DA has a unique scope, there is no requirement to perform
> these operations between DAs.
>
> The two approaches I was considering were
>
> a) Change the active discovery mechanism to query for all DAs (use an
> empty
> previous responders list), and construct a list of those known DAs that
> don't reply, removing them from the cache after a time. This behaviour
> could be enabled/disabled using a new property.
>
> b) Perform a regular unicast (DA request?) to each of the known DAs eg. on
> a
> round robin basis so that all DAs are polled within a time period
> controlled
> by a new property (could be set to zero to disable this behaviour)
>
> Obviously, I would like to feed any changes back into the project, so I am
> looking for feedback as to which approach would be preferable, or if there
> is another approach that would be better, or if someone else was working
> on
> the problem already.
>
> Thanks.
>
> Richard Morrell
>
>
>
> Software Architecture & Technologies
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