Finagle is the RPC library used to build services at Twitter, it is widely used on thousands of servers, and it processes billions of messages every day. It has been designed to be fast but also reusable and composable. Finagle employs a series of algorithms to achieve efficient load balancing, fast error discovery, and more. It is protocol agnostic and there are implementations for HTTP, Thrift, Memcache, Redis, MySQL and more. This talk will describe the abstractions exposed by Finagle, as well as a brief overview of its internals and some of the algorithms used under the hood.