A Finagle developer works on Twitter's open-source network stack for the JVM, known as Finagle. They develop and maintain asynchronous Remote Procedure Call (RPC) systems that support a host of protocols, such as HTTP, Thrift, and Mux. They focus on creating robust, fault-tolerant, and high-concurrency systems capable of handling numerous simultaneous connections. A Finagle developer improves system efficiency, troubleshoots issues, and collaborates to design new features. They also work on enhancing reliability, scalability, and performance of the system, while ensuring the codebase remains clear and maintainable.