# Transports

Transports connect **module streams** across **process boundaries** and/or **networks**.

* **Module**: a running component (e.g., camera, mapping, nav).
* **Stream**: a unidirectional flow of messages owned by a module (one broadcaster → many receivers).
* **Topic**: the name/identifier used by a transport or pubsub backend.
* **Message**: payload carried on a stream (often `dimos.msgs.*`, but can be bytes / images / pointclouds / etc.).

Each edge in the graph is a **transported stream** (potentially different protocols). Each node is a **module**:

![go2_nav](../assets/go2_nav.svg)

## What the transport layer guarantees (and what it doesn’t)

Modules **don’t** know or care *how* data moves. They just:

* emit messages (broadcast)
* subscribe to messages (receive)

A transport is responsible for the mechanics of delivery (IPC, sockets, Redis, ROS 2, etc.).

**Important:** delivery semantics depend on the backend:

* Some are **best-effort** (e.g., UDP multicast / LCM): loss can happen.
* Some can be **reliable** (e.g., TCP-backed, Redis, some DDS configs) but may add latency/backpressure.

So: treat the API as uniform, but pick a backend whose semantics match the task.

## Choosing a backend

* `zenoh`: is our default. Reliable delivery and the same typed message model through `LCM` binary encoding of messages
* `lcm`: the legacy path, opt-in. Fast and simple, but UDP multicast is best-effort.

At the CLI level, you can select the stream transport globally with:

```bash
dimos --transport=lcm run unitree-go2
dimos --transport=zenoh run unitree-go2
```

Generally LCM is legacy and we suggest using zenoh (the default) going forward

## Zenoh

### What the default talks to

A stock zenoh session is pinned to localhost. It listens on `tcp/127.0.0.1:0` and
scouts for peers over loopback only, so sibling dimOS processes on this machine
find each other and nothing on the LAN can link to them.

Peers on this machine carry their data through shared memory, not the socket.
Zenoh negotiates it per link at handshake, a remote peer keeps getting the
payload over TCP.

Reaching off the machine is opt-in

| You want                                    | Pass                          |
|---------------------------------------------|-------------------------------|
| A robot, dialed directly                    | `--robot-ip 192.168.1.42`     |
| Any other peer or a router, dialed directly | `ZENOH_CONNECT=tcp/host:7447` |
| Peers discovered across the LAN             | `ZENOH_SCOUTING=1`            |
| Scouting on one named interface             | `ZENOH_INTERFACE=wlan0`       |

Every one of these unpins the listener back to zenoh's all-interfaces default.

**Two ways to override for one run or for your shell:**

1. **CLI:** `dimos --transport=zenoh ...` or `dimos --transport=lcm ...` (see [CLI](../cli.md) for precedence with `.env` and blueprints).
2. **Environment:** `DIMOS_TRANSPORT=zenoh` or `DIMOS_TRANSPORT=lcm`.


Architecture notes (Rerun bridge) live under [Zenoh](#zenoh) in PubSub transports below.

### Per-topic QoS

Zenoh publisher QoS lives on the Zenoh `Topic` object (see [`zenohpubsub.py`](https://github.com/dimensionalOS/dimos/blob/main/dimos/protocol/pubsub/impl/zenohpubsub.py#L27)):

```python
from dimos.core.transport import ZenohTransport
from dimos.protocol.pubsub.impl.zenohpubsub import Topic, ZenohQoS

blueprint = blueprint.transports(
    {("image", CameraModule): ZenohTransport(Topic("dimos/image", Image, qos=ZenohQoS(reliability="best_effort", congestion_control="drop")))}
)
```

When the factory builds transports from the global switch, it applies defaults (`default_zenoh_qos` in [`transport_factory.py`](https://github.com/dimensionalOS/dimos/blob/main/dimos/core/transport_factory.py#L65)):

* The agent channels (`human_input`, `agent`, `agent_idle`): reliable, block under congestion (never drop).
* `Image`/`PointCloud2` streams: best-effort, drop under congestion (latest wins).
* Everything else: zenoh defaults (reliable, drop under congestion).

## Benchmarks

Quick view on performance of our pubsub backends:

```sh
python -m pytest -sv -k "not bytes" dimos/protocol/pubsub/benchmark/tool_benchmark.py
```

![Benchmark results](../assets/pubsub_benchmark.png)

## Abstraction layers

<details>
<summary>Pikchr</summary>

```pikchr
color = white
fill = none
linewid = 0.5in
boxwid = 1.0in
boxht = 0.4in

# Boxes with labels
B: box "Blueprints" rad 10px
arrow
M: box "Modules" rad 5px
arrow
T: box "Transports" rad 5px
arrow
P: box "PubSub" rad 5px

# Descriptions below
text "robot configs" at B.s + (0.1, -0.2in)
text "camera, nav" at M.s + (0, -0.2in)
text "LCM, SHM, ROS" at T.s + (0, -0.2in)
text "pub/sub API" at P.s + (0, -0.2in)
```

</details>

![output](../assets/abstraction_layers.svg)

We’ll go through these layers top-down.

## Using transports with blueprints

See [Blueprints](../blueprints.md) for the blueprint API.

From [`unitree/go2/blueprints/smart/unitree_go2.py`](https://github.com/dimensionalOS/dimos/blob/main/dimos/robot/unitree/go2/blueprints/smart/unitree_go2.py).

Example: rebind a few streams from the default `LCMTransport` to `ROSTransport` (defined at [`transport.py`](https://github.com/dimensionalOS/dimos/blob/main/dimos/core/transport.py#L226)) so you can visualize in **rviz2**.

```python
nav = autoconnect(
    basic,
    voxel_mapper(voxel_size=0.1),
    cost_mapper(),
    replanning_a_star_planner(),
    wavefront_frontier_explorer(),
).global_config(n_workers=6, robot_model="unitree_go2")

ros = nav.transports(
    {
        ("lidar", PointCloud2): ROSTransport("lidar", PointCloud2),
        ("global_map", PointCloud2): ROSTransport("global_map", PointCloud2),
        ("odom", PoseStamped): ROSTransport("odom", PoseStamped),
        ("color_image", Image): ROSTransport("color_image", Image),
    }
)
```

## Using transports with modules

Each **stream** on a module can use a different transport. Set `.transport` on the stream **before starting** modules.

The runnable example below uses a tiny synthetic image publisher instead of `CameraModule` so it works without a webcam and in CI; the wiring is the same as with a real camera.

```python
import time

import numpy as np
import reactivex as rx

from dimos.core.core import rpc
from dimos.core.coordination.module_coordinator import ModuleCoordinator
from dimos.core.module import Module, ModuleConfig
from dimos.core.stream import In, Out
from dimos.core.transport import LCMTransport
from dimos.msgs.sensor_msgs.Image import Image, ImageFormat

class TickerCameraConfig(ModuleConfig):
    frequency_hz: float = 2.0

class TickerCameraModule(Module):
    """Publish synthetic frames so this example runs without a webcam."""

    config: TickerCameraConfig
    color_image: Out[Image]

    @rpc
    def start(self) -> None:
        super().start()

        def emit(_: int) -> None:
            img = Image.from_numpy(
                np.zeros((480, 640, 3), dtype=np.uint8),
                format=ImageFormat.RGB,
                frame_id="synthetic",
            )
            self.color_image.publish(img)

        period = 1.0 / max(self.config.frequency_hz, 0.1)
        self.register_disposable(rx.interval(period).subscribe(emit))

class ImageListener(Module):
    image: In[Image]

    async def handle_image(self, img: Image) -> None:
        print(f"Received: {img.shape}")

if __name__ == "__main__":
    # Start local cluster and deploy modules to separate processes
    dimos = ModuleCoordinator()
    dimos.start()

    camera = dimos.deploy(TickerCameraModule, frequency_hz=2.0)
    listener = dimos.deploy(ImageListener)

    # Choose a transport for the stream (example: LCM typed channel)
    camera.color_image.transport = LCMTransport("/camera/rgb", Image)

    # Connect listener input to camera output
    listener.image.connect(camera.color_image)

    dimos.start_all_modules()

    time.sleep(2)
    dimos.stop()
```

```results
12:36:05.648 [inf][ation/worker_manager_python.py] Worker pool started. n_workers=2
12:36:06.053 [inf][/coordination/python_worker.py] Deployed module. module=TickerCameraModule module_id=0 worker_id=0
12:36:06.159 [inf][/coordination/python_worker.py] Deployed module. module=ImageListener module_id=1 worker_id=1
12:36:08.283 [inf][dination/module_coordinator.py] Stopping module... module=imagelistener
12:36:08.284 [inf][dination/module_coordinator.py] Module stopped. module=imagelistener
12:36:08.285 [inf][dination/module_coordinator.py] Stopping module... module=tickercameramodule
12:36:08.334 [inf][dination/module_coordinator.py] Module stopped. module=tickercameramodule
12:36:08.335 [inf][ation/worker_manager_python.py] Shutting down all workers...
Received: (480, 640, 3)
Received: (480, 640, 3)
Received: (480, 640, 3)
Received: (480, 640, 3)
12:36:08.336 [inf][/coordination/python_worker.py] Worker stopping module... module=ImageListener module_id=1 worker_id=1
12:36:08.336 [inf][/coordination/python_worker.py] Worker module stopped. module=ImageListener module_id=1 worker_id=1
12:36:08.388 [inf][/coordination/python_worker.py] Worker stopping module... module=TickerCameraModule module_id=0 worker_id=0
12:36:08.388 [inf][/coordination/python_worker.py] Worker module stopped. module=TickerCameraModule module_id=0 worker_id=0
12:36:08.394 [inf][ation/worker_manager_python.py] All workers shut down
```

See [Modules](../modules.md) for more on module architecture.

## Inspecting traffic (CLI)

`dimos spy` is the universal transport spy: one live view of every topic moving on every
dimOS pubsub transport (names, message rates, bandwidth, sizes, and liveness), whether the
system runs on LCM, Zenoh, or both.

```bash
dimos spy                     # everything, all transports
dimos spy --transport zenoh   # filter to one transport (repeatable flag)
dimos lcmspy                  # deprecated alias for: dimos spy --transport lcm
```

![dimos spy](../assets/lcmspy.png)

`dimos topic echo /topic` listens on typed channels like `/topic#pkg.Msg` and decodes automatically:

```sh
Listening on /camera/rgb (inferring from typed LCM channels like '/camera/rgb#pkg.Msg')... (Ctrl+C to stop)
Image(shape=(480, 640, 3), format=RGB, dtype=uint8, dev=cpu, ts=2026-01-24 20:28:59)
```

## Implementing a transport

At the stream layer, a transport is implemented by subclassing `Transport` (see [`core/stream.py`](https://github.com/dimensionalOS/dimos/blob/main/dimos/core/stream.py#L83)) and implementing:

* `broadcast(...)`
* `subscribe(...)`

Your `Transport.__init__` args can be anything meaningful for your backend:

* `(ip, port)`
* a shared-memory segment name
* a filesystem path
* a Redis channel

Encoding is an implementation detail, but we encourage using LCM-compatible message types when possible.

### Encoding helpers

Many of our message types provide `lcm_encode` / `lcm_decode` for compact, language-agnostic binary encoding (often faster than pickle). For details, see [LCM](../lcm.md).

## PubSub transports

Even though transport can be anything (TCP connection, unix socket) for now all our transport backends implement the `PubSub` interface.

* `publish(topic, message)`
* `subscribe(topic, callback) -> unsubscribe`

```python
from dimos.protocol.pubsub.spec import PubSub
import inspect

print(inspect.getsource(PubSub.publish))
print(inspect.getsource(PubSub.subscribe))
```

```results
    @abstractmethod
    def publish(self, topic: TopicT, message: MsgT) -> None:
        """Publish a message to a topic."""
        ...

    @abstractmethod
    def subscribe(
        self, topic: TopicT, callback: Callable[[MsgT, TopicT], None]
    ) -> Callable[[], None]:
        """Subscribe to a topic with a callback. returns unsubscribe function

        The unsubscribe function must not block waiting for an in-flight
        callback (callers may hold an event loop the backend needs for
        progress), must be callable from within the callback itself, and once
        it returns no further deliveries start (a callback already executing
        may still complete).
        """
        ...
```

Topic/message types are flexible: bytes, JSON, or our ROS-compatible [LCM](../lcm.md) types. We also have pickle-based transports for arbitrary Python objects.

### LCM (UDP multicast)

LCM is UDP multicast. It’s very fast on a robot LAN, but it’s **best-effort** (packets can drop).
For local emission it autoconfigures system in a way in which it's more robust and faster then other more common protocols like ROS, DDS

```python
from dimos.msgs.geometry_msgs.Vector3 import Vector3
from dimos.protocol.pubsub.impl.lcmpubsub import LCM, Topic

lcm = LCM()
lcm.start()

received = []
topic = Topic("/robot/velocity", Vector3)

lcm.subscribe(topic, lambda msg, t: received.append(msg))
lcm.publish(topic, Vector3(1.0, 0.0, 0.5))

import time
time.sleep(0.1)

print(f"Received velocity: x={received[0].x}, y={received[0].y}, z={received[0].z}")
lcm.stop()
```

```results
Received velocity: x=1.0, y=0.0, z=0.5
```

### Zenoh

### Shared memory (IPC)

Shared memory is highest performance, but only works on the **same machine**.

```python
from dimos.protocol.pubsub.impl.shmpubsub import PickleSharedMemory

shm = PickleSharedMemory(prefer="cpu")
shm.start()

received = []
shm.subscribe("test/topic", lambda msg, topic: received.append(msg))
shm.publish("test/topic", {"data": [1, 2, 3]})

import time
time.sleep(0.1)

print(f"Received: {received}")
shm.stop()
```

```results
Received: [{'data': [1, 2, 3]}]
```

### DDS Transport

For network communication, DDS uses the Data Distribution Service (DDS) protocol:

```python
from dataclasses import dataclass
from cyclonedds.idl import IdlStruct

from dimos.protocol.pubsub.impl.ddspubsub import DDS, Topic

@dataclass
class SensorReading(IdlStruct):
    value: float

dds = DDS()
dds.start()

received = []
sensor_topic = Topic(name="sensors/temperature", data_type=SensorReading)

dds.subscribe(sensor_topic, lambda msg, t: received.append(msg))
dds.publish(sensor_topic, SensorReading(value=22.5))

import time
time.sleep(0.1)

print(f"Received: {received}")
dds.stop()
```

```results
Received: [SensorReading(value=22.5)]
```
## A minimal transport: `Memory`

The simplest toy backend is `Memory` (single process). Start from there when implementing a new pubsub backend.

```python
from dimos.protocol.pubsub.impl.memory import Memory

bus = Memory()
received = []

unsubscribe = bus.subscribe("sensor/data", lambda msg, topic: received.append(msg))

bus.publish("sensor/data", {"temperature": 22.5})
bus.publish("sensor/data", {"temperature": 23.0})

print(f"Received {len(received)} messages:")
for msg in received:
    print(f"  {msg}")

unsubscribe()
```

```results
Received 2 messages:
  {'temperature': 22.5}
  {'temperature': 23.0}
```

See [`pubsub/impl/memory.py`](https://github.com/dimensionalOS/dimos/blob/main/dimos/protocol/pubsub/impl/memory.py) for the complete source.

## Encode/decode mixins

Transports often need to serialize messages before sending and deserialize after receiving.

`PubSubEncoderMixin` at [`pubsub/encoders.py`](https://github.com/dimensionalOS/dimos/blob/main/dimos/protocol/pubsub/encoders.py#L39) provides a clean way to add encoding/decoding to any pubsub implementation.

### Available mixins

| Mixin                | Encoding        | Use case                           |
|----------------------|-----------------|------------------------------------|
| `PickleEncoderMixin` | Python pickle   | Any Python object, Python-only     |
| `LCMEncoderMixin`    | LCM binary      | Cross-language (C/C++/Python/Go/…) |
| `JpegEncoderMixin`   | JPEG compressed | Image data, reduces bandwidth      |

`LCMEncoderMixin` is especially useful: you can use LCM message definitions with *any* transport (not just UDP multicast). See [LCM](../lcm.md) for details.

### Creating a custom mixin

```python
import json

from dimos.protocol.pubsub.encoders import PubSubEncoderMixin

class JsonEncoderMixin(PubSubEncoderMixin[str, dict, bytes]):
    def encode(self, msg: dict, topic: str) -> bytes:
        return json.dumps(msg).encode("utf-8")

    def decode(self, msg: bytes, topic: str) -> dict:
        return json.loads(msg.decode("utf-8"))
```

Combine with a pubsub implementation via multiple inheritance:

```python
from dimos.protocol.pubsub.impl.memory import Memory

class MyJsonPubSub(JsonEncoderMixin, Memory):
    pass
```

Swap serialization by changing the mixin:

```python
from dimos.protocol.pubsub.encoders import PickleEncoderMixin
from dimos.protocol.pubsub.impl.memory import Memory

class MyPicklePubSub(PickleEncoderMixin, Memory):
    pass
```

## Testing and benchmarks

### Spec tests

See [`pubsub/test_spec.py`](https://github.com/dimensionalOS/dimos/blob/main/dimos/protocol/pubsub/test_spec.py) for the grid tests your new backend should pass.

### Benchmarks

Add your backend to benchmarks to compare in context:

```sh
python -m pytest -sv -k "not bytes" dimos/protocol/pubsub/benchmark/tool_benchmark.py
```

## Available transports

| Transport      | Use case                            | Cross-process | Network | Notes                                |
|----------------|-------------------------------------|---------------|---------|--------------------------------------|
| `Memory`       | Testing only, single process        | No            | No      | Minimal reference impl               |
| `SharedMemory` | Multi-process on same machine       | Yes           | No      | Highest throughput (IPC)             |
| `LCM`          | Robot LAN broadcast (UDP multicast) | Yes           | Yes     | Best-effort; can drop packets on LAN |
| `Zenoh`        | Reliable network stream transport   | Yes           | Yes     | Recommended on macOS for heavy replay |
| `Redis`        | Network pubsub via Redis server     | Yes           | Yes     | Central broker; adds hop             |
| `ROS`          | ROS 2 topic communication           | Yes           | Yes     | Integrates with RViz/ROS tools       |
| `DDS`          | Cyclone DDS without ROS (WIP)       | Yes           | Yes     | WIP                                  |