Memcached newbie questions

[Brian Moon]

> - Does php's memcached client API supports (consistent?) key hashing?

I believe verion 3(alpha) does.

> - Is memcache's php session handler a good way to store information
> about a user? Or would it be better to manually store user info to
> memcache? I'm trying to figure out if memcache's session handler saves
> variables as one big chunk (so it cannot overpass the 1MB limitation)
> or as different chunks (that transparently integrates them into the
> php $_SESSION array).

If your session data is not volitale and can be lost at any moment, then 
it is ok to store it ONLY in memcached.  But, most use memcached as a 
cache and not as storage.  Having said that, I have not used a memcached 
session handler for PHP.  It is not supported natively that I know of. 
So, you would have to look at the handler you are using.

> - The most intriguing issue I've been buffled about is the following:
> How to store SQL results. For example, say I store SELECTs as md5 into
> memcache. Then say at some point in the code, an UPDATE is executed.
> How can I determine which memcache keys (that is, which SELECTs)
> should be invalidated?? Is there a framework or some way that do this
> gracefully? Or should I use memcache only for queries that do not need
> be updated immediately, but can update after they expire?

You can really go two ways.

1. Cache raw sql queries and just let them expire.  Lacks granularity, 
but is easy to implement.  If you start removing cache just when the 
table changes, you are not doing much good.  The MySQL query cache can 
do that on a MySQL server.  But, it is not nearly as good as a smart 
memcached implementation.

2. Cache data at the object level instead of the query level.  This will 
require more code and more


For more information and ideas, you may want to read this:

http://www.socialtext.net/memcached/index.cgi?faq#cache_things_other_than_sql_data

And I have posted some high level cache concepts in this blog post:
http://doughboy.wordpress.com/2007/06/23/caching-and-patience/

-- 

Brian Moon
Senior Developer
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