Experimental Physics and
Industrial Control System

Andrew Johnson <[email protected]> · Tue, 28 Jun 2005 09:42:33 -0500

Benjamin Franksen wrote:

Event scanning using raw numbers is much too fragile.

That I accept; in V4 we should consider changing the EVNT field to

become something like a string or better yet, an extensible menu (I've

just added a note about such a requirement to the DBD Statement Syntax

wiki page on menu definitions).

In contrast, an input link for the PROC field never accidentally 'leaks'

to the outside, at least if the 'scanning' record instance is part of

the same database file.

What does a "Backward" link really mean, how do you implement it, and is

it really worth the overhead (which would be more than just one

additional link in every record)? For CA links the meaning is probably

relatively clear: "process me whenever you get an event from that

channel", and the CA client library already provides the necessary

infrastructure. However to implement it for a DB link we would have to:

* Add the "Backward" link to dbCommon

* Add a linked list to dbCommon, containing pointers to all the other

records that have a "Backward" link to this record

* When initializing the database, construct the linked lists for all records

* After (or before?) processing the FLNK of a record, also scan that

linked list and process all the records on it

* Perform special handling for "Backward" links whenever they get

changed, deleting the link owner from its target's old list and adding

it to the new target's list (this processins requirement is unique, we

never have to do anything like it for any other link fields)

IMHO this is unnecessary, and adding it would be using a sledgehammer to

crack a nut (as would using an empty tagged union to implement enum/menu).

- Andrew
--
Podiabombastic: The tendency to shoot oneself in the foot.

Experimental Physics and Industrial Control System

Experimental Physics and
Industrial Control System