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asuth opened this issue Apr 11, 2011 · 2 comments
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How do I weight servers? #7

asuth opened this issue Apr 11, 2011 · 2 comments

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@asuth
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asuth commented Apr 11, 2011

Hi,

I can see you can weight servers in the php session config settings, but how do you weight servers yourself? Sorry if I'm missing something, I don't see this documented.

It would be nice to do something like this:

    $redis = new Redis();
    foreach ($connection as $ip => $weight)
        $redis->pconnect($ip, $port, $timeout, $weight);
@nicolasff
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Hello Andrew,

There is no way to do this other than by hand, but it isn't that complicated.

The session handler creates a "connection pool", where each server has a weight. Let's say that server A has weight 1, B has weight 2, and C weight 3. The sum of these weights is 6, so when a session is created, the session ID is transformed into a number (you can use crc32 for instance), and this number is reduced modulo 6 (the sum of our weights).
With $pos = crc32($sessid) % 6, if $pos == 0 then the session is stored on server A, if $pos = 1 or 2 on B, if $pos =3,4, or 5 then it's on C.

The reason why phpredis doesn't implement this for the key/value interface is that some of Redis' commands can map several keys (see sUnion for example, or sort). These wouldn't work as well in a multi-server environment, while the GET/SET interface for sessions doesn't have any ambiguity.

There will be a way to do it directly from phpredis in the future when part of the key is reserved for this specific purpose, but that's not the case at the moment.

@asuth
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asuth commented Apr 12, 2011

Got it. I'll have to think about the best way to abstract this out. It seems it would be pretty easy to implement the server-key matching, even in a lightweight php script.

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