Memcached "cluster"

Brad Fitzpatrick brad at
Wed Dec 21 18:33:06 UTC 2005

On Wed, 21 Dec 2005, Kevin Pratt wrote:

> Just looking for some clarification.
> It is up to the application to distribute the keys across a memcached "cluster"

No, it's up to the memcached client library.  Your application is
blissfully unaware of the details.

Memcached can be thought of as a two-layer hash.  The client library does
the first one (to pick the memcached instance), and the memcached instance
itself has a hash table internally.

> memcached instances do not pass information between each other.


> Please correct me if I am wrong.

Only wrong in that application code doesn't need to do it.

> So an example of how memcached could be used would be setting up one instance
> per web server and each web server uses its own memcached instance.

That's a bad idea.  Waste of memory having redundant cache.  And when your
app invalidates something in the database, you'd have to wipe it from 'n'
different instances.

> I was wondering if there was a way to have all the memcached instances act as
> one? So a key stored in one instance is available to all instances.

That's unnecessary --- just store it in one.  Let the client library
handle it.

- Brad

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