Posted by Jerry Stuckle on 09/21/07 20:45

The Natural Philosopher wrote:
> Jerry Stuckle wrote:
>> The Natural Philosopher wrote:
>>> Jerry Stuckle wrote:
>>>
>>>>> You cannot, for example, have multiple instances of Apache
>>>>> listening on
>>>>> port 80 of a single IP address, and I thought assigning 255 IP
>>>>> addresses
>>>>> to a single web server went out with browsers that don't understand
>>>>> HTTP/1.1 and the Host: header.
>>>>>
>>>>
>>>> I didn't say anything about multiple instances on port 80, did I?
>>>>
>>>
>>> Actually you can, and often do, have multiple instances of Apache
>>> (effectively) listening on port 80..
>>>
>>> That's how virtual hosting and load balancing works.
>>
>> No. You can only have one instance of Apache running on port 80 on a
>> single machine.
>>
>> Virtual hosting works because that one instance has several hosts
>> defined. But it is still only one instance.
>>
>> Load balancing works because the instances of Apache are on different
>> systems.
>>
>
> Semantics.
>

No, much more than semantics. If you have multiple instances of Apache
running on different ports, those processes can have different owners.
A single instance of Apache only has one owner.

> You can chain more listeners than one to a single port. At what point
> that becomes 'more than one instance tied to a port' as against 'one OS
> routine vectoring calls to multiple processes' is a moot one.
>
> My process list for apache shows this:-
>
> ps -eadf| grep apache
> root 9310 1 0 Jul16 ? 00:00:00 /usr/sbin/apache
> www-data 22302 9310 0 Sep16 ? 00:00:00 /usr/sbin/apache
> www-data 22303 9310 0 Sep16 ? 00:00:00 /usr/sbin/apache
> www-data 22304 9310 0 Sep16 ? 00:00:00 /usr/sbin/apache
> www-data 22305 9310 0 Sep16 ? 00:00:00 /usr/sbin/apache
> www-data 22306 9310 0 Sep16 ? 00:00:00 /usr/sbin/apache
> www-data 22807 9310 0 Sep16 ? 00:00:00 /usr/sbin/apache
> www-data 22892 9310 0 Sep16 ? 00:00:00 /usr/sbin/apache
> www-data 23339 9310 0 Sep16 ? 00:00:00 /usr/sbin/apache
> www-data 23417 9310 0 Sep17 ? 00:00:00 /usr/sbin/apache
> www-data 23418 9310 0 Sep17 ? 00:00:00 /usr/sbin/apache
>
> i.e 10 instances of apache on ONE server instance are running. Plus the
> original spawner. This is on an IDLE web server..the spare processes are
> there to add load capacity without the overhead of a fork().
>

Sure. They are different processes.

> It is perfectly possible for more than one process to bind() to the same
> socket, provided that the socket options are set up that way. The only
> proviso being that the processes understand each other and work
> co-operatively. In essence its no more complex than e.g sharing a
> hardaware interrupt: All processes that are attached to that interrupt
> wake up, and one of them decides 'its mine' and takes it whilst the
> others go back to sleep..
>
>
>
> I am fairly sure apache does this routinely as above:


Or does the main process get the request and hand it off to the
appropriate process? This would seem to be much lower overhead. I'm
not sure about it.

--
==================
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Jerry Stuckle
JDS Computer Training Corp.
jstucklex@attglobal.net
==================

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