From:	MERC::"uunet!CRVAX.SRI.COM!RELAY-INFO-VAX" 14-FEB-1993 03:00 14-FEB-1993 03:00:00.00
To:	Info-VAX@KL.SRI.COM
CC:	
Subj:	Re: Sending commands to another process?

> 	I'd like to be able to send "commands" of many different types to
> (across campus).  Both are on the same node of the VaxCluster.    Did I leave

The best way to send 'commands' (not data) is via locks.
It is very quick and reliable, works across clusters (unlike mailboxes).
The big restriction is a limitation of 16 bytes per message.
The service is SYS$ENQ, with a lock status block.

The program to receive the data (the slave) creates a EXC mode lock with a LKSB
and a blocking AST.
Any one wishing to send a command tries to get the lock at PW.
The slave's blocking AST, goes off, handler lowers the mode to NL, tries to get
it back to EXC.
Writer gets the lock at PW, writes the data, converts it to NL.
The slave gets the lock back at EXC and reads the data.
Reverse to send replies back.

I have this coded, but its in PL/1 which I don't think is a widely available
language.

In cluster land, avoid global sections and mailboxes (in-memory things), use
files instead of global sections, and decnet instead of mailboxes, add to your
repetrar (can't spell) locks for fast commands.

--
Daiajo Tibdixious MACS BSc(hons,math/comp sci)	System Programmer
BHP Information Technology Wollongong Facilities Management DEC System Support
Phone: +61 42 757522 x 6968	FAX: +61 42 757801	Voicelink: 851
Computer Centre (34) PO Box 261 Warrawong NSW 2502 Australia
"DEVAX1::WETIBD"@itwol.bhp.com.au | Daiajo.D.Tibdixious@bhp.bhpmel04.telememo.au
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