[Xastir] Multiple Servers

[Dan Brown]

On Wed, 25 Oct 2006, Ray Wells wrote:

> Gerry Creager wrote:
> 
> > But do you *need* to connect to each of the servers?  Connecting to 
> > the Core at one point, and using a rotate scheme, should accomplish 
> > what you would want, the ability to fail over another server when, 
> > albeit rarely, we have a server die.
> >
> > Multiple connections to multiple servers tends to lead to loops on the 
> > servers.  AE5PL's done a lot of work to stop that and the Core sysops 
> > have periodically gone thru and individually asked folks to help us 
> > stamp out redundant connections.
> >
> > gerry
> >
> Hi Jerry,
> 
> No, I don't need to connect to each server at the same time. All I'm 
> looking for is a redundancy scheme whereby if one server falls over, 
> Xastir will attempt to connect to an alternative server in a manner 
> similar to UIView.
> 
> Server failures are rare but, in accordance with Murphy's Law they will 
> happen when I'm not here, which is quite often, and I want to cover that 
> possibility.

I believe that this has been somewhat implemented by the APRS IS guys using
round robin DNS records with a 60 second (or so) ttl for "rotate.aprs.net"

Using "dig" on a command line: 

------------------------------------------------------

Myhost.foo$ dig rotate.aprs.net

; <<>> DiG 9.2.1 <<>> rotate.aprs.net
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6864
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 2, ADDITIONAL: 2

;; QUESTION SECTION:
;rotate.aprs.net.               IN      A

;; ANSWER SECTION:
rotate.aprs.net.        46      IN      A       165.91.140.28
rotate.aprs.net.        46      IN      A       64.58.200.20
rotate.aprs.net.        46      IN      A       134.173.254.38

;; AUTHORITY SECTION:
aprs.net.               5986    IN      NS      ns3.aprs.net.
aprs.net.               5986    IN      NS      ns7.aprs.net.

;; ADDITIONAL SECTION:
ns3.aprs.net.           5986    IN      A       24.123.66.139
ns7.aprs.net.           5986    IN      A       64.58.207.2

;; Query time: 47 msec
;; SERVER: 192.168.1.7#53(192.168.1.7)
;; WHEN: Tue Oct 24 19:56:43 2006
;; MSG SIZE  rcvd: 149

------------------------------------------------------



If you use dig a few times in a row, you'll see the 3 lines in the "Answer
section" change order, and the TTL (46 here) will decrease over time.  I've
not looked at the Xastir code, but, As long as users use hostnames and
xastir (and other stuff like nscd) doesn't cache the Hostname->Ip mapping
any longer than specified, you should be able fail over by using
"rotate.aprs.net" within 60 seconds, plus or minus timeouts to see if things
are truely down. 

Still, it would be nice functionality to have -- use one "interface"
specification and have multiple possible hostnames to connect to.  I
personally just have multiple interface specifications.  If one goes down,
I can start another manually, fairly easily. 


--
Dan Brown 
brown at brauhaus.org