server down, but its back up.. what should be done ?

[Mikael Johansson]

The pecl/memcache extension will retry failed servers after a user
specified interval (default 15 seconds,) for more info see

 http://www.php.net/manual/en/function.memcache-addserver.php

When a server is down only keys that hashed to that specific server is
redistributed to other servers in the pool, other keys aren't affected
as long as the failed server is kept in the pool. The order in which
Memcache::addServer() is called and its weight affects the selection
process, removing or changing the weight of a single server might well
result in all keys hashing to different servers.

The ini setting "memcache.allow_failover" controls whether a failed
servers keys will be redistributed to the other servers or if requests
for those key will fail immediately.

If you have many servers the loss of one won't affect performance much
and failover might result in stale data (if the failed server goes up
and down before the redistributed keys have expired); however if you
have fewer servers the need for failover is greater as the loss of one
would affect the performance much more.

//Mikael

Evert|Rooftop wrote:
> Dear List,
> 
> One of our servers went down and it was marked by the other servers as
> 'invalid', so it won't use it anymore..
> 
> However, it's back up now.. will memcache (PHP PECL) automatically retry
> after a while or should apache be started on all those machines..
> 
> It also made me wonder about something else, when a server is down..
> will the hash thats used to determine the target server be adjusted to
> just use the other servers? Or will the hashes that would have targeted
> the server thats down be re-distributed?
> 
> Thanks,
> Evert
> 
> 
>