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The document discusses Memcache and its implementations. Memcache is a fast, dumb server that sits between application servers and databases to improve performance. There are several Memcache implementations including the original Memcached by Danga, Tugela which adds Berkeley DB support, and Memcachedb which adds replication. Memcached generally performs well under normal conditions with enough memory, while Tugela can outperform Memcached on memory-constrained servers or with many simultaneous threads. Tugela lacks expiration policy management so items may expire without purging. The PHP implementation using UDP and consistent hashing from PECL is recommended.