Hi!
Baron Schwartz wrote:
The situation is more complicated.
I have 2 servers and ring replication between them.
On 1 server there is a process which cashes some data from DB (this
decrease number of queries very match and increase performance).
But sometimes there are some modification of data on 2-d server. Changes
go to the 1-st server by replication.
And only then replication of this changes is done the cashes of data
must be updated
That is why I need the condition of changing some tables on 1-st table.
Hi Andrey.
Andrey Kotrekhov wrote:
SQL
Hello!
Is there any way to inform mysql client application about changing in
the some table.
One process puts periodically record into the table.
Second process waits new records.
It is very expensive way to do SELECT from this table each second.
Is there any other right way to inform second process?
I was waiting to see if anyone else had any ideas, because I don't
think my ideas are very good. I should say that I think this is
generally a hard problem to solve with generic SQL, but I think some
systems do support their own extensions to provide signalling or
notification.
In MySQL, the two things I can think of are:
1) make the polling more efficient. Instead of polling the table
you're watching, have the inserting process set a flag in another,
smaller table, and poll that table. Another thing you can do to make
polling more efficient, when it is necessary, is use a variable wait
time. Exponential or Fibonacci has worked well for me. For more on
this, see
http://www.xaprb.com/blog/2006/05/04/how-to-make-a-program-choose-an-optimal-polling-interval/
2) Take advantage of things that use semaphores under the covers. The
most obvious one for me is a string lock with GET_LOCK(). The waiting
process and the polling process could get and release the lock to
signal each other. Be careful though, as if you use more than one
lock, there is a potential for deadlock, and if you don't there is a
potential for race conditions.
I have not implemented this myself, and think I would probably write
something buggy on my first attempt, so these are just crude ideas.
Cheers
Baron
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Andrey Kotrekhov [EMAIL PROTECTED]
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