[Asterisk-Users] Making asterisk distributed

[Stephen Hon]

Clustering in its purest definition (HA, numa, parallel processing, etc)
with asterisk is not really supported at the moment. However, you can do
some things to increase the availability and scalability of phone
services by using multiple pbx devices. I am by no means an expert in
this area, but I am also interested in this area, so I'll meekly offer
up my 2 cents.

In terms of scalability, you could potentially have many asterisk
'nodes' that have their own pstn connections. Getting creative with the
dial plans using the 'switch' command may achieve your scaling needs.
There is some information on this in the wiki.
http://www.voip-info.org/wiki-Asterisk+-+dual+servers

In terms of HA/failover capabilities, you're pretty much concerned with
the connectivity issues.

Regarding the connectivity to the PSTN, failover mechanisms could be
provided either through your PSTN carrier or through the use of a layer
1 switching devices. 

Most carriers provide, for a small charge, a failover capability across
multiple PRI circuits. For instance, if you have a block of DID numbers
pointed to a certain PRI, and that PRI were to go off-line for whatever
reason, they would then send the calls directed to those DIDs to another
PRI. 

There are also layer 1 switches that can detect when a circuit is down
and physically switch over to another port. This method can be used for
a more cost effective active/standby configuration, because now you
don't have to subscribe to two or more PRI circuits if you don't have
that kind of call volume. There is also some information regarding these
devices in the wiki.
http://www.voip-info.org/tiki-index.php?page=Failover%20switches

The other side of the equation is what to do with the PBX->phone
connectivity. So here, the problem is that a SIP phone will try to
register or is already registered at pbx1.domain.com to make and receive
calls, but if that pbx is down and the calls have been rerouted to
another PRI, where and when should the phone try to register next?  

There are a few ways to get around this, one is to use DNS-based
redirection based upon your PRI failover pattern, other people are using
layer 4-7 switched like Foundry's Serveriron products that employ
different virtualization techniques, and some phones like the Cisco 79x0
and the Polycom Soundpoints have the ability to register with a primary
and backup sip servers. Once you have the phones registered to the right
place, your users should be able to make and receive calls as usual.
Networked storage of certain data and a little creativity with the
configuration files should allow them continual access to their
voicemail and other user specific data. 

All in all, there are no standardized methods of achieving High
Availability with asterisk alone. Conversely there are several methods
of using asterisk with other technologies that can achieve your expected
level of availability. Just like many other open-source technologies,
you will have to closely evaluate what you need and determine what
'solution' will work best for you.


-----Original Message-----
From: asterisk-users-admin at lists.digium.com
[mailto:asterisk-users-admin at lists.digium.com] On Behalf Of Steven
Critchfield
Sent: Wednesday, August 04, 2004 5:31 AM
To: asterisk-users at lists.digium.com
Subject: Re: [Asterisk-Users] Making asterisk distributed

On Wed, 2004-08-04 at 09:30, Jeremy McNamara wrote:
> Trilogy India wrote:
> 
> > Hi,
> > 
> > I want to know, if someone has tried to use clustering
> > in asterisk to increase its scalability and make it
> > distributed??
> > 
> > If yes, how easy it is to cluster?
> > 
> > Can someone please ive me details about the same
> 
> 
> Asterisk will not benefit from clustering.

Not in the HPC sense, but it will in the high availability sense.
-- 
Steven Critchfield <critch at basesys.com>

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