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2.2 The Pub/Sub Ontology

Stage II: The Graph

"Do not speak to the listener. Speak to the ether. If the listener is worthy, they will hear you."

1. The Middleware (DDS)​

ROS 2 is built on top of DDS (Data Distribution Service). DDS is an industrial standard used in Battleships, Spacecraft, and Finance. It is a Data-Centric architecture.

  • Topic: A named channel (e.g., /camera/image_raw).
  • Type: The schema (e.g., sensor_msgs/Image).
  • Domain: The virtual network (ID 0-100). Nodes on Domain 0 cannot see nodes on Domain 1.

The IDL (Interface Definition Language)​

Before we can talk, we must agree on grammar. .msg files are language-agnostic. int32 x becomes int32_t in C++ and int in Python. The build system (colcon) generates these bindings.

2. Theoretical Quality of Service (QoS)​

In TCP/IP (Web), we demand reliability. If a packet is lost, we retry. In Robotics, Reliability is sometimes failure.

Scenario: You are driving at 100km/h.

  • Reliable: You miss a LIDAR packet. The system pauses to re-request it. By the time it arrives, it is 50ms old. You are now driving blind based on history.
  • Best Effort: You miss a packet. You ignore it. You process the next packet which is fresh.

The QoS Policies:

  1. Reliability: Reliable vs. Best Effort.
  2. Durability: Transient Local (History for late-joiners) vs. Volatile (Live only).
  3. History: Keep Last N.

Golden Rule:

"Use Reliable for Commands (Stop!). Use Best Effort for Sensors (High Bandwidth)."

3. Serialization (The Alchemy)​

How do you send a C++ Object over a wire? You can't. You must Serialize it into a byte stream (CDR - Common Data Representation).

  1. Struct Memory Layout: Padding and alignment.
  2. Endianness: Little vs. Big Endian.
  3. Zero Copy: If both nodes are on the same shared memory, we just pass the pointer. This bypasses serialization entirely.

Deep FAQ​

Q: Why not use JSON? A: JSON is slow. Parsing text takes CPU. CDR is binary and tightly packed. In a 1000Hz loop, JSON parsing would kill the latency budget.

Q: Can I subscribe to my own topic? A: Yes, but be careful of infinite loops. If your callback publishes to the same topic it listens to, you create feedback screeches (like a microphone near a speaker).