GA.JPH@ISUMVS.BITNET ("John Hascall") (09/23/87)
*N 01:30:52 09/23/87 FROM UCSDHUB!JACK!MAN!CRASH!JEH@SDCSVAX.UCSD.EDU: R
e: interprocess communication without privs
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From: Jamie Hanrahan <UCSDHUB!JACK!MAN!CRASH!JEH@SDCSVAX.UCSD.EDU>
Subject: Re: interprocess communication without privs
Comments: To: info-vax@kl.sri.com
To: John Hascall <GA.JPH@ISUMVS>
A previously-posted summary of the standard interproc. comm. techniques
(can't seem to find the article, or I'd run this as a followup thereto)
was quite good. I can only add two things: First, global sections
alone are not enough -- you also need an interproc. synchronization
technique to control access to the section. This can be common event
flags (limited to procs within a UIC group), hiber/wake, or the lock
manager. Which is more suitable depends on the function you want.
Second, references to "shared memory" only apply to multiple 780s
with MA780 multiport memory. These are 780s running individual copies
of VMS, each with its own private memory, but with a shared memory area
for fast sharing of data between systems. Global sections, common event
flag clusters, and mailboxes can all be created in the shared memory. I
mention this because many people think that "shared memory" is synonymous
with "global section" -- it isn't. 780s with MA780s are rare beasts
these days and I wouldn't spend a minute writing code to accomodate
them.
BUT, all is not lost. If you can ask that the users of the program have
NETMBX, you can do generalized (multi-group-UIC) interprocess comm.
with no others privs, by using DECnet task-to-task communication. This
will naturally be a bit slow (I've measured it at 10 msec per $QIO
call on a 1-MIP VAX; this is for reader and writer processes on the
same node -- naturally it gets worse when there's a real internode
link involved), but it will work, and it's also very general -- ie
processes running on remote nodes use exactly the same code to talk
to the "master" node as those running on the "master".
The best way to use DECnet this way is by running a "server" process
that keeps track of a database that's private to itself. All other
processes connect to the server and send it messages to request info
from, or to write into, the database. This neatly sidesteps all
synchronization issues (since the requests to the server process will
be single-threaded).
Some cooperation from the system manager on the node that will run
the server is required. But, the server process (and the account
under which it runs) needs only normal privs.
If you can get PRMMBX and SYSNAM privs, you can speed things up on
the node that runs the server by letting them talk to the server
through mailboxes. Only the server need have these -- once it starts
up, it creates the mailbox from which it will read commands; other
procs on the same node create a temporary mailbox for reading
responses from the server, and send the temp mailbox's physical
device name (or something that can be mapped thereto) in all
requests to the server. ("here's a request; send the reply *here*.")
Note to system analysts: This is a good model for any application
that needs a common database accessed by multiple "clients". The
clients need know nothing about how the database is set up; they
only need know where and how to send messages. Transaction logging
is simple -- just copy all the incoming request messages to a file.
(Then if the database is munged, just restore it from the most recent
backup and play all the request messages that came in since that
backup into the server's mailbox again...) You can change the database
implementation without changing the clients, too. Students of VAX/VMS will
recognize this model in the job controller, among other places.
Good luck!
--- Jamie Hanrahan
Simpact Associates, San Diego, CA
pnet01!jeh@crash.CTS.COM or jeh@pnet01.CTS.COM
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