[freetds] Freetds API query

Lowden, James K LowdenJK at bernstein.com
Mon Jun 9 13:29:12 EDT 2003

> From: manoranjan_sahu at agilent.com [mailto:manoranjan_sahu at agilent.com]
> Sent: June 9, 2003 12:14 PM
> From my unix system, my application needs to be notified of 
> any change(insert, update, delete) to a table (say T1). Does 
> freetds has any api I can use to do this task?

This question crops up from time to time.  Basically, the answer is No.
There's nothing built-in, and there's no efficient way to create one.  

The first, completely unfixable (by you) problem is that there's no kind of
"tell me when this table changes" anywhere in SQL or the TDS protocol, nor
any way afaik to trick the system into sending you a message when a table's
state changes.  The TDS protocol is half-duplex, meaning you only hear from
the server when you've asked for something; you'll never receive a
spontaneous message.  And the one thing you'd like most to ask, but can't,
is "tell me when this table changes".  

You're therefore reduced to polling.  Polling is not only inefficient; it
also scales badly.  Because there's no IPC between server processes (spids)
and no analog to wall(1), each process is forced to do its own polling.  For
10 users, that's no problem.  For 10,000, it's infeasible.  

I'm oversimplifying a little bit: Sybase offers a facility called
"registered procedures" that do what you want.  It's part of OpenServer,
which FreeTDS doesn't support.  Both vendors also offer hooks into the
server's OS, where you could work some IPC and/or broadcast magic.  


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