One response on “Mikel King: High Performance WordPress”

1. khromov
   November 12, 2015 at 1:27 pm | Reply

   Thanks for an interesting presentation.

   I’d like to point out that Memcached is not a real cluster. Whenever you add multiple Memcached servers to a “pool”, the Memcache(d) extension in PHP simply divides up the cache into multiple, equally-sized portions. But the different Memcached servers themselves are completely unaware of each other.

   This imitates a cluster when accessing it, but it has some severe implications – if you have 3 machines in a pool and one goes down, suddenly 1/3 of your cache is gone and once the timeout is reached on the still active servers the cache will be redistributed amongst the remaining nodes. Now that the remaining servers rebalance there will be a redistribution in the storage and suddenly all keys are invalidated.

   In practice a cache flush is hardly noticeable but on large-scale production this is a huge headache, and scaling Memcached over too many machines can be a death sentence for your stability.