Node

Key Terminologies

• A node is a process that performs computation
• It's a single executable program
• Nodes communicate via topics, services, or actions
• They can be written in almost any supported language
• Most common language is C++ and Python

Nodes can communicate with each other via Data Distribution Service (DDS). There are also Pre-built ROS 2 Nodes, which we will use for tracking the TF Tree, etc.

1) Messages

• It is the communication language of strings, ints, floats, arrays between nodes

2) Interfaces

• It is the blueprint or the format of the message
• It defines the programming Application Programming Interface (API) for data retrieval/sent via topics or services
• ROS already provides pre-defined interfaces, but we can also create custom ones

3) Topics

• These are named communication channels between nodes
• It uses a Publish-Subscribe Model
• A node publishes to this particular topic and all the other subscribing nodes recieves from the topic

4) Services

• These are also named communication channels between nodes
• It uses a Request-Response Model
• The client node sends a request to a server node, which then executes a task and then returns a response

5) Callbacks

• It is a function that ROS will execute when an event occur
• For example, message received or timer fired
• This ensures that the ROS architecture is modular

6) Timers

• This is a built-in mechanism that integrates with ROS executor that runs a specific callback function at a defined interval
• So example, run this callback function every 10ms

7) Parameters

• These are named configuration settings stored within a node
• This allows us to change the values of these parameters without altering the code and can be set via terminal, YAML or launch files

8) Logging

• This is structured node status output (DEBUG, INFO, WARN, ERROR, FATAL) that replaces raw printf() calls
• Logs can route to terminal, files, or /rosout for centralised filtering and monitoring

9) ROS 2 Packages

• This is the basic unit of file management or organisation in ROS
• it contains code (packages.xml, CMakeLists.txt, setup.py source files) needed to run something
• colcon build uses packages.xml (which is similar to a requirements.txt) to build the package

10) Coordinate Frames (tf2)

• Called tf2 that map the robot to the environment
• It is like the basis of ROS mapping and navigation