RE: CA question (caRepeater)

[johill@lanl.gov (Jeff Hill)]

Ralph,

> 
> I have a machine with six network interfaces to six different subnets.
> 
> I'm running six CA Proxy Gateways on that machine --- one for each
> subnet. All these Gateways differ on the client side, i.e. each Gateway
> has a different EPICS_CA_ADDR_LIST, but these settings aren't disjunct
> (each Gateway mirrors channels from an arbitrary number of the five
> other subnets).
> 
> How should I start caRepeater(s) to handle this situation correctly?
> 

If all of these servers are on the same port then there will be only one
CA repeater per client host.

The CA repeater is a client side beast. It is started automatically by the
client and it normally requires no special configuration if it can be found 
by the client in its path.

However, there is a known problem with the new ca server library which I have 
mentioned earlier on tech-talk. The problem is that if you run multiple ca servers 
on the same port on the same host then CA clients will see beacons from all of 
these servers as one beacon source. This causes the clients to perceive this 
beacon source's period to be irregular and therefore they are always concluding 
that there is a new server on the network. The result is that all clients with 
unresolved channels will, in unison, periodically reset their search interval 
to 0.5 seconds and then exponentially backoff to a longer interval. 

*** This will occur only on the subnets that the multiple servers ***
*** on the same host and port are attached to.                    ***

A fix for this problem is on my task list.

On a related topic, it is possible for one CA server to
attach to a list of network interface cards specified by IP address
in an environment variable (see the doc in CONFIG_ENV), but the gateway
appears to have limitations which prevent this.
It should be in principal possible to run one gateway which
attaches its server to some set of network interface cards and its client
to some other non-intersecting set of network interface cards and therefore
there would be no need for multiple gateways per host. 

Note also that due to limitations in most IP kernels all of the multiple 
servers on one host will receive all broadcast messages, but only one
of the servers will receive unicast messages. This implies that if
EPICS_CA_ADDR_LIST has a unicast address (the IP address of a single host)
then only one server will be contacted at this address.

Jeff