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transport

A simple middleware like ros to communicate between nodes, using zeromq and protobuf. This project is modified from orsf's disc_zmq.

CAUTION:

The project currently build and complied in window.

ref:

ros_with_zeromq

disc_zmq

Simple library (with implementations in languages of interest) to do discovery on top of zeromq messaging. The intent is that this library would be incorporated into other projects, where things like message serialization would be added.

Raw message definitions:

  • Header (HDR):

    • VERSION: 2 bytes
    • GUID: 16 bytes; ID that is unique to the process, generated according to RFC 4122
    • TOPICLENGTH: 1 byte; length, in bytes, of TOPIC
    • TOPIC: string; max 192 bytes
    • TYPE: 1 byte
    • FLAGS: 16 bytes (unused for now)
    • (max total: 2+16+1+192+1+16 = 228)

  • advertisement (ADV) or advertisement of service (ADV_SVC):

    • HDR (TYPE = 1)
    • ADDRESSLENGTH: 2 bytes; length, in bytes, of ADDRESSES
    • ADDRESS: one valid ZeroMQ address (e.g., "tcp://10.0.0.1:6000"); max length 6+255+6=267 bytes
    • (max total: 228+267=495)

  • subscription (SUB) or subscription to service (SUB_SVC):

    • HDR (TYPE = 2)
    • (null body)

ZeroMQ message definitions (for which we will let zeromq handle framing):

  • publication (PUB) or service request (REQ) or service reply (REP_OK or REP_ERR) are multipart messages, with the following parts:
    • TOPIC (placed first to facilitate filtering)
    • HDR (TYPE = ?)
    • BODY: opaque bytes

Defaults and conventions:

  • Default port for SUB and ADV messages: 11312
  • By convention, SUB and ADV messages are sent to a broadcast address.
  • In raw messages, all integers are sent little-endian
  • Every topic ADV should be accompanied by an ADV_SVC that advertises a service by the same name as the topic. This service can be used for polled topic REQ/REP interactions.

API sketch:

  • (un)advertise(topic)
  • (un)subscribe(topic, cb)
    • cb(topic, msg)
  • publish(topic, msg)
  • (un)advertise_service(topic, cb)
    • rep <- cb(topic, req)
  • init_service(topic)
  • wait_for_service(topic, timeout)
  • rep <- call_service(topic, req, timeout)
  • call_service_async(topic, req, cb)
    • cb(topic, rep)