🔹 1. Publisher & Subscriber (Pub/Sub model)

• Publisher: A node that sends data on a topic (topic carries message)
  • msg = format of data
  • topic = channel that carries that data
• Subscriber: A node that listens to a topic and receives that data.

📌 Example:

• A camera node publishes images on /camera/image_raw.
• A display node subscribes to /camera/image_raw and shows the image.

Message type (.msg) defines what kind of data flows through a topic.

geometry_msgs/msg/Twist   → linear & angular velocity
std_msgs/msg/String       → simple string

🔹 2. Service (Request–Response)

• Different from pub/sub: communication is synchronous.
• One node requests something, another node responds.

📌 Example:

• A turtlesim client calls the /spawn service → “please create a new turtle at (x, y).”
• The server responds → “ok, turtle2 created.”

Service type (.srv) has two parts:

Request  → input data
Response → output data

Example std_srvs/srv/Empty: