synchronous service discovery

On Thu, May 04, 2006 at 11:13:59PM +0200, Lennart Poettering
wrote:
> On Wed, 03.05.06 13:03, Mihai Ibanescu (misa+avahiredhat.com) wrote:
> 
> Hi!
> 
> > I am trying to use zeroconf with unicast dns
(using SRV and PTR records for
> > describing the services that are available in the
network).
> > 
> > Although I think I can see how I can use the
python bindings to do the service
> > discovery asynchronously, I don't think I was
able to find anything for
> > synchronous calls. I am looking for the equivalent
of doing a PTR lookup (both
> > for browsing the domain and for browsing instances
for each service type).
> > 
> > Is there such a thing in the API?
> 
> Nope. Avahi doesn't provide a synchronous DBUS
interface for service
> browsing. Only resolving can be done synchronously. The
reason is that
> mDNS/DNS-SD is a "living" system where
services appear and disappear
> every now and then. Avahi doesn't maintain a list of
all currently
> available services all the time. Instead, only if a
local client
> browses for something a list is assembled from the
responses recieved
> from the LAN. However, we never know when this list is
complete. (Just
> think of a crowd of a few thousands of people. Tell
them to shout
> their names, all at the same time. You'll see that you
can never make
> sure that everyone of them has really followed your
order or even that
> everyone understood your order in the first place)
> 
> Hence there is no clean way to implement a synchronous
browsing API:
> we just never know when the right time to return has
come.
> 
> However, the asynchronous browser API has a facility
that can emulate
> the behaviour for you: an AVAHI_BROWSER_ALL_FOR_NOW
event is emitted
> if Avahi thinks that it is unlikely that more responses
will turn up
> in the the near future. (This is used by
"avahi-browse -t") 
> 
> AVAHI_BROWSER_ALL_FOR_NOW is generated like this:
> 
>   1. On mDNS, if there are already local clients
querying the same
>      record type, return it immediately if the first of
the clients
>      began to query it more than a second ago. If they
began less than
>      a second ago, wait until the second elapsed.
> 
>   2. On mDNS, if there are no other local clients
querying the same
>      record type, just wait a second
> 
>   3. On unicast DNS, just wait until the server
responded, since we
>      don't need to aggregate multiple responses in
this case.
> 
> Keep in mind that this event is unreliable. It might
definietly happen
> that another service is found just msecs after this
event was
> generated.
> 
> I hope this helps,

Hi Lennart,

Thanks for the detailed response. Since (for now) I am only
using unicast DNS,
I'll probably just follow your advice. I didn't want to
add the complexity of
(and dependency upon) an event loop when simple DNS lookups
for PTR records
were enough.

As I found out, there are no good DNS libaries for python
that are in
widespread use, as far as I can tell, and it's yet another
dependency
- that's why I was looking for something within avahi.

Thanks again!
Misa
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synchronous service discovery

Thread: synchronous service discovery