When you request multiple keys (pass an array of keys to the
get() function)
the PHP extension fetches the items from different servers
(depending on
the server pool added before)..
There are just few things to note. You have to maintain the
same pool when
you set the cache item and when you want to retrieve it.
Lets say if you have php code:
<?
$mc= new Memcache;
$mc->addServer('host', 11211);
$mc->addServer('host2', 11211);
$mc->set('somekey','somevalue');
?>
And in case 'host' or 'host2' goes down (or you change the
configuration
(switch the server for example or add a third server)) next
time you do <?
$mc->get('somekey'); ?> you will probably miss the
item, cause the
calculation which server has to be used to store/retrieve
the item is done
by algorithm (the one used in php pecl extension was
mentioned before in
mailling list, if you want I can search for it) which takes
the keys (hash)
and server pool as arguments.
But if you use it as general cache it doesnt really matter
and there is no
need even for 4 servers..
Just one Memcached server can handle huge amounts of load
(million keys,
thousands req per sec and pretty insane traffic (one of our
server writes
out as cache like nearly ~1.6-2Tb per day which is kinda
20-30Mb/s )).
Just from my own experience I like better to seperate the
different
memcached servers (you can even run few instances on one box
on different
ports or interfaces) content wise. That is you always know
on which server
some key or content type actually resides (you can write
your own
distribution code or course). There are also some concerns
where you dont
want to actually mix the cached data - for example we use
one memcached just
for php sessions for bunch of webfrontends because
memcached throws the
data (in case the allowed memory is full) out by expunging
least used /
oldest items, but as sessions are generated/read/written
each page request
you could loose some actually more usefull cached data.
rr
----- Original Message -----
From: "Jan Pfeifer" <pfjan yahoo.com.br>
To: "memcached list" <memcachedlists.danga.com>
Sent: Wednesday, January 10, 2007 9:26 PM
Subject: parallelized requests
hi all,
I'm setting up a cache layer here at work that will
probably be composed by 4 servers (or maybe 2 dual core
ones). For our
use case, we might need to get on average 10 keys to build a
web page,
but possibly up to 200 small key/values, or even more in
very rare
cases.
My question is: do the C or the PHP libraries parallelize
the requests for the keys to the different cache servers
when using the
multi-key request ? Or does it fetch the keys one server at
a time ?
thx in advance for any answers or pointers 
jan
ps.:
sorry
if this have been asked before (seems a basic question to
me), it's the
first time I'm using memcached, and I couldn't find anything
by
searching in the list.
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