Sebastian Doeweling wrote:
- Some memory cache named memcached that is able to cache database
queries and distribute writes across the database architecture. This is similar to MySQL query caching?
No, it's not. MySQL query caching is useless, as a table's query cache is invalidated on every write. But memcached (http://www.danga.com/memcached/) has nothing directly to do with databases; it's just a fault-tolerant distributed in-memory blob cache. You can stuff arbitrary key-value pairs in it, so one popular use of it turns out to be database record caching.
In addition to stock memcached, Wikimedia sites use Tugela cache, which is Domas' unholy hybrid of BDB and memcached (essentially replacing the memory backend in memcached with disk-backing via BDB). Tugela adheres to the memcached API, though it requires manual item expiration management.
Another issue are database updates (these occur only, if I edit and change a page, right?
Yes, generally.
(assuming parser cache is enabled)) - what influence does caching have on these?
A page edit invalidates that page's cache.