[racket] Marketplace: A language for network-aware programming

We are happy to announce the release of Marketplace, a new programming
language for building functional network programs.  Marketplace combines
two fundamental ideas in a new way: nested virtual machines and
publish/subscribe messaging.  Nesting allows programs to isolate
processes and to delimit conversations. While publish/subscribe generalizes
point-to-point and broadcast messaging, it smoothly turns the appearance
and disappearance of participants and resources into _presence_ and
_absence_ messages. Such messages make it particularly easy to start and
stop services and to manage resources based on demand.

Here is a simple TCP echo server written in Marketplace:

#lang marketplace

(endpoint #:subscriber (tcp-channel ? (tcp-listener 5999) ?)
          #:conversation (tcp-channel from to _)
          #:on-presence (spawn #:child (echoer from to)))

;; echoer: TcpAddress TcpAddress -> Transition
(define (echoer from to)
  (transition stateless
    (endpoint #:subscriber (tcp-channel from to ?)
              #:on-absence (quit)
              [(tcp-channel _ _ data)
               (send-message (tcp-channel to from data))])))

The initial `endpoint` subscribes to TCP messages on port 5999. When a
conversational partner appears, the endpoint spawns a new process that runs an
`echoer` process.  The latter is stateless and subscribes to TCP
messages.  When it gets messages with payload `data`, it sends them back
out with the opposite addressing; when the TCP conversation disappears, it
quits.

Thus far, we have built several real systems using Marketplace: a
DNS server, a DNS proxy, and an SSH server.  The DNS proxy has handled
DNS traffic for ourselves and other members of our lab for the last
several months.

You can read an overview along with detailed documentation for
Marketplace at http://tonyg.github.io/marketplace/ .

To get the sources for Marketplace as well as the applications
point your browser to https://github.com/tonyg/marketplace .

Enjoy!

Tony Garnock-Jones
Sam Tobin-Hochstadt
Matthias Felleisen