ROS 2 Architecture Overview

ROS 2 (Robot Operating System 2) is a sophisticated middleware framework designed for distributed robotics applications. Understanding its layered architecture is crucial for implementing effective bindings and tools like roc.

Layered Architecture

ROS 2 follows a layered architecture that separates concerns and provides modularity:

┌─────────────────────────────────────────┐
│           User Applications             │
├─────────────────────────────────────────┤
│              ROS 2 API                  │
│         (rclcpp, rclpy, etc.)           │
├─────────────────────────────────────────┤
│               RCL Layer                 │
│        (Robot Control Library)         │
├─────────────────────────────────────────┤
│               RMW Layer                 │
│       (ROS Middleware Interface)        │
├─────────────────────────────────────────┤
│          DDS Implementation             │
│    (Fast-DDS, Cyclone DX, etc.)        │
└─────────────────────────────────────────┘

Key Components

1. DDS Layer (Bottom)

• Purpose: Provides the actual networking and discovery mechanisms
• Examples: Fast-DDS, Cyclone DX, Connext DDS
• Responsibilities:
  • Network communication
  • Service discovery
  • Quality of Service (QoS) enforcement
  • Data serialization/deserialization

2. RMW Layer (ROS Middleware Interface)

• Purpose: Abstract interface that isolates ROS 2 from specific DDS implementations
• Location: /opt/ros/jazzy/include/rmw/
• Key Types:
  • rmw_context_t - Middleware context
  • rmw_node_t - Node representation
  • rmw_publisher_t / rmw_subscription_t - Topic endpoints
  • rmw_qos_profile_t - Quality of Service profiles
  • rmw_topic_endpoint_info_t - Detailed endpoint information

3. RCL Layer (Robot Control Library)

• Purpose: Provides a C API that manages the ROS 2 graph and lifecycle
• Location: /opt/ros/jazzy/include/rcl/
• Key Functions:
  • rcl_init() - Initialize RCL context
  • rcl_node_init() - Create nodes
  • rcl_get_topic_names_and_types() - Graph introspection
  • rcl_get_publishers_info_by_topic() - Detailed topic information

4. Language-Specific APIs

• rclcpp: C++ client library
• rclpy: Python client library
• rclrs: Rust client library (emerging)

Why This Architecture Matters for roc

The roc tool operates primarily at the RCL and RMW layers, bypassing the higher-level language APIs to:

1. Direct Access: Get raw, unfiltered information about the ROS 2 graph
2. Performance: Avoid overhead of higher-level abstractions
3. Completeness: Access all available metadata (QoS, GIDs, type hashes)
4. Compatibility: Work consistently across different ROS 2 distributions

Discovery and Communication Flow

sequenceDiagram
    participant App as roc Application
    participant RCL as RCL Layer
    participant RMW as RMW Layer
    participant DDS as DDS Implementation
    
    App->>RCL: rcl_init()
    RCL->>RMW: rmw_init()
    RMW->>DDS: Initialize DDS participant
    
    App->>RCL: rcl_get_publishers_info_by_topic()
    RCL->>RMW: rmw_get_publishers_info_by_topic()
    RMW->>DDS: Query DDS discovery database
    DDS-->>RMW: Publisher endpoint info
    RMW-->>RCL: rmw_topic_endpoint_info_t[]
    RCL-->>App: rcl_topic_endpoint_info_array_t

This architecture allows roc to access detailed information that is often abstracted away in higher-level tools, making it particularly powerful for debugging and system introspection.