Distributed MLton
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README.md

README.md

Distributed MLton

================================

MLton support for distribution through for ZeroMQ (http://www.zeromq.org/). A key feature of this implementation being automatic marshalling of arbitrary ML datatypes, including functions (with caveats, of course). As a result, the basic send and receive functions are polymorphic, and are simply:

val send : socket * 'a -> unit
val recv : socket -> 'a

Check out MLton.ZMQ module @ src/basis-library/mlton/zmq.sig for more information.

Dependencies

===============

Along with MLton's dependencies, libzmq needs to be installed.

Notes + FAQ

========

1. Serialization support and caveats

====================================

Serialization support is achieved through two new functions under MLton structure:

val serialize   : 'a -> Word8.word vector
val deserialize : Word8.word -> 'a

The serilization support is very usable, but it has its caveats:

  • Serialization and deserialization can only be done by the same exact program. In other words, if the serialized object is sent over the network, the receiver program must be the same as the one which serialized the object. Since MLton object headers are unique for each program, this restriction is in place.
  • Use of serialization breaks static type-checking guarantees. Deserialize function's return type is determined by the type to which the result is unified. The onus is on the programmer to get this type the same as the original object type.
  • Serialization copies mutable ref cells and arrays.
  • Serialization of objects containing references to file handles, sockets are bound to break on the deserialization side. (Note to self, such instances must be statically prevented).

2. Why do you need all of MLton's distribution for ZeroMQ binding?

=================================================================

So as to take advantage of MLton's safe system call support, such that zeromq invocations play nicely with user-level threads with preemptive multithreading. Since MLton's system call library is not exposed to the end user, ZMQ binding is packaged as a module in MLton's basis library. As an added benefit, this also leads to a concise implementation since we take advantage of safe system call's error handling support.

3. PUB/SUB and Prefixes

===========================

In ZeroMQ, SUB sockets must set a subscription prefix using zmq_setsockopt() in order to receive any messages. The subscriptions are matched with the message prefix, and only those messages that match the subscription prefix. Since send and receive primitives in the MLton bindings operate over 'a messages, a send primitive with an explicit prefix argument is provided.

val sendWithPrefix : socket * 'a * Word8.word vector -> unit

MLton.ZMQ.recv primitive always drops the prefix, and only returns the 'a message. See tests/weather.sml for an example usage of pub-sub sockets.