Sebastian Doeweling wrote:
3) 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.
--
Ivan Krstic <krstic(a)fas.harvard.edu> | GPG: 0x147C722D