ZeroMQ Transport

The ZMQ transport allows agents to communicate over a network, including across different machines. It is based on the ZeroMQ library, which uses C/C++ internally and operates in the microsecond range on a local network.

PUB/SUB — Asynchronous

Each agent publishes on one address and subscribes on another. There is no blocking — if the consumer is behind, it reads the last available message.

from synapsys.transport import ZMQTransport
import numpy as np

# Plant (publisher)
pub = ZMQTransport("tcp://0.0.0.0:5555", mode="pub")
pub.write("y", np.array([3.14]))

# Controller (subscriber)
sub = ZMQTransport("tcp://192.168.1.10:5555", mode="sub")
y = sub.read("y")

Bidirectional topology

For two-way communication, use two PUB/SUB connections:

Plant:       PUB :5555  ->  SUB :5556
Controller:  PUB :5556  ->  SUB :5555

See examples/distributed/02_zmq/plant_zmq.py and examples/distributed/02_zmq/controller_zmq.py in the repository.

REQ/REP — Lock-step

Guarantees full synchronisation: the client blocks until it receives a reply, and the server processes one request at a time.

from synapsys.transport import ZMQReqRepTransport

# Plant (server)
server = ZMQReqRepTransport("tcp://0.0.0.0:5555", mode="server")
u = server.read("u")          # blocks until the controller sends
server.write("y", y_array)    # replies with new state

# Controller (client)
client = ZMQReqRepTransport("tcp://localhost:5555", mode="client")
client.write("u", u_array)    # sends control action
y = client.read("y")          # blocks until the plant replies

Order matters in REQ/REP

The server must call read() before write(), and the client must call write() before read(). Reversing the order causes deadlock.

Latency

Scenario	Typical latency
Loopback (localhost)	~50–100 µs
1 Gbps LAN	~100–300 µs
WAN / Internet	> 1 ms

For comparison, SharedMemoryTransport operates at < 1 µs.

API Reference

See the full reference at synapsys.transport — ZMQTransport and ZMQReqRepTransport.