Question

I got a sample Script which looks nearly like this:

$memcache = new Memcache;
$memcache->addServer('memcache_host', 11211);
$memcache->addServer('memcache_host2', 11211);

If i use now

$memcache->set('var_key', 'some really big variable');

The key get/value get stored (only on one server), of course. But when i reorder the Memcache Serverlist, its recreating the cache. I noticed it always use the second Server… But why?

I just want to know and understand, how the hash for storage gets generated.
And which factors are relevant for the hash and server selection?

I could not find anything about.

Regards!

Answer 1

The Answer is:

crc32($KEY) % Server.length

So in my case, i got 2 Servers with the Sample Key “var_key”.

crc32(‘var_key’) % 2

The result is 0. So the first Server is the lucky one!

crc32(‘var_key_bit_longer_and_longer’) % 2

In this case the second server (Result is 1) gets chosen.

As we can see, the following factors are relevant:

• The Key
• Server count
• Order of Server

That’s the way how the server gets chosen, for standard hash strategy.