[Monetdb-developers] MapiClient asynchronous interface?

Johann Borck johann.borck at densedata.com
Sat Nov 18 17:31:52 CET 2006


Fabian Groffen wrote:
> On 18-11-2006 16:22:48 +0100, Martin Kersten wrote:
>   
>> Hi Fabian, Johann
>>
>> I can see the need for an asynchronous call. It works for applications
>> that that don't care about the outcome of a transaction. It also seems
>> relatively easy to implement in the Mapi server side. You have to
>> trap the async query call and handle it in the context of a new
>> server thread that discards the output (>/dev/null).
>> The 'asynquery' can immediately return to the client with an ACK
>> and terminate the session.
>>     
>
> The what are the effects of a long running query?  I don't see any
> advantage here.  You can just pool between sockets with one "main"
> thread, and try to read every once in a while by checking if there is
> data available to be read in the (TCP) buffers.  This is, however, with
> many connections a deadly situation.
>
> Since Johann is working with a webserver, it sounds quite weird not to
> use threads in the first place to me anyway.
>
>   
Well, that's a big discussion :)  but using an event-driven state
machine with single- or few-threaded servers is not really an uncommon
thing, and has big advantages, e.g. the avoidance of context-switches.
Lighttpd is a prominent example, which performs extremely well compared
to, say, Apache. For really big amounts of concurrent connections the
one-thread (or even one-process)-per-connection model really is a
problem. Using epoll (or the new kevent) one can deal with thousands of
simultaneous connections without problems,  especially without having to
waste lots of memory (one object/struct vs one thread) and cpu-time
(context-switches + dealing with locking issues).

When I have some time, I'll very likely give it a try in case of the
MapiClient. Changing the io-model should not affect transactions, the
asynchronous version will of course have to follow the same protocol
that the synchronous version does.  I just thought maybe monetdb
supports this already, because in the TCP/IP module there are
asynchronous methods.

Anyway, thanks for this great product and regards,
Johann
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