What's the difference between an RPC System, like Twitter's Finagle, and an Enterprise Service Bus, like Mule? What kind of problems are each of them good at solving?
I will try to answer this as a soft explanation, rather than a technical breakdown of features:
One may say that Finagle is a asynchronous messaging library that allows services to connect to one another freely (not tightly tied to architectural system integration standards) while supporting multiple protocols.
From the Finagle website:
Finagle is a network stack for the JVM that you can use to build asynchronous Remote Procedure Call (RPC) clients and servers in Java, Scala, or any JVM-hosted language. Finagle provides a rich set of protocol-independent tools.
An enterprise service bus (ESB), on the other hand, is a asynchronous messaging architecture which typically adheres to industry standards and protocols. An ESB promotes a system where message flow is controlled and routed between systems, and where servers can register their service and clients can register what messages they are interested in. The services offered by servers can be registered and versioned.
You will typically find Finagle being used somewhere between a website and backend services. But, you will typically find an ESB inside a large corporate, where it's responsible to integrating systems like finance, support, sales, etc.
Both solutions offer asynchronous messaging and buffering to various extends, but are not designed to solve the same problem. For ESB, you would probably think 'strict, enterprise', but for Finagle you would probably think 'flexible, web'.
Hope this helps
Update:
Not quite related, but if you are exploring this space, I would look at Kafka these days.