Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Mon, 08 Oct 2007 10:49:39 -0500

Heinrich du Toit wrote:

Why not put the aliasing in a PV gateway?
This way you don't have to change the IOC's
And you don't have to change and client sofware.
You simply need to change the software on one gateway.

The disadvantage of using a gateway for aliasing is that you now have
the relevant PV names listed in two places, so if you do eventually
change the name of the original PV (because everyone is now using the
alias) you have to remove it from the gateway configuration as well as
change it in the IOC, and these changes have to be synchronized.
You might also want to avoid running large arrays or quickly changing
signals through a PV gateway for performance reasons; there is no
performance effect of doing aliasing on the IOC (other than to the PV
name lookup process). Access Security could also be a problem through a
PV gateway.
On Mon, 2007-10-08 at 10:01 -0400, Lawrence T. Hoff wrote:

	I may have been the one who initiated the
discussion a few years ago. I encountered a situation
which I believed was best addressed using server-side
aliases. I created a patch to EPICS core (which could
be loaded into a VxWorks system as a separate module),
which implemented server-side aliases.

See http://www.aps.anl.gov/epics/tech-talk/2003/msg00707.php for Larry's
original posting, although I do not recommend using this code verbatim.
P.S. As near as I could tell, there was not universal
enthusiasm for server-side aliases at that time. Maybe
at next week's EPICS collaboration meeting the discussion
could begin anew?

PV Aliasing has been on my "To Do" list for Base for quite a while (my
boss wants it too), prompted by your demonstration that it's not hard to
do with the current data structures. The important question arises as
to whether we need the ability to add aliases after iocInit() â I would
guess not often, but I can see circumstances where it would be useful.
The only issues remaining that I can see would be a review of database
locking (if we allow addition of aliases to a running IOC) and Access
Security issues, and to decide how to specify the aliases to the IOC. I
would prefer to extend the DB file syntax rather than use something like
Larry's addPvAlias(name, alias) command, but adding alias support to
dbLoadRecords() might preclude runtime aliasing.
I hope we'll be able to make some progress on implementing aliasing
(amongst other things) at the EPICS Codeathon next April.
- Andrew
--
When a distinguished but elderly scientist states that something is
possible, he is almost certainly right.  When he states that something
is impossible, he is very probably wrong.  -- Arthur C. Clarke

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System