- Web front-end written in PHP. Facebook's HipHop  then converts it to C++ and compiles it using g++, thus providing a high performance templating and Web logic execution layer
- Business logic is exposed as services using Thrift . Some of these services are implemented in PHP, C++ or Java depending on service requirements (some other languages are probably used...)
- Services implemented in Java don't use any usual enterprise application server but rather use Facebook's custom application server. At first this can look as wheel reinvented but as these services are exposed and consumed only (or mostly) using Thrift, the overhead of Tomcat, or even Jetty was probably too high with no significant added value for their need.
- Persistence is done using MySQL, Memcached , Facebook's Cassandra , Hadoop's HBase . Memcached is used as a cache for MySQL as well as a general purpose cache. Facebook engineers admit that their use of Cassandra is currently decreasing as they now prefer HBase for its simpler consistency model and its MapReduce ability.
- Offline processing is done using Hadoop and Hive
- Data such as logging, clicks and feeds transit using Scribe  and are aggregating and stored in HDFS using Scribe-HDFS , thus allowing extended analysis using MapReduce
- BigPipe  is their custom technology to accelerate page rendering using a pipelining logic
- Varnish Cache  is used for HTTP proxying. They've prefered it for its high performance and efficiency .
- The storage of the billions of photos posted by the users is handled by Haystack, an ad-hoc storage solution developed by Facebook which brings low level optimizations and append-only writes .
- Facebook Messages is using its own architecture which is notably based on infrastructure sharding and dynamic cluster management. Business logic and persistence is encapsulated in so-called 'Cell'. Each Cell handles a part of users ; new Cells can be added as popularity grows . Persistence is achieved using HBase .
- Facebook Messages' search engine is built with an inverted index stored in HBase 
- Facebook Search Engine's implementation details are unknown as far as I know
- The typeahead search uses a custom storage and retrieval logic 
- Chat is based on an Epoll server developed in Erlang and accessed using Thrift 
Tuesday, June 28, 2011
What is Facebook's architecture? - Quora: "What is Facebook's architecture?"
at 7:46 PM