Choose the Communication Pattern

ROS 2 gives nodes several ways to interact.

The first question should not be "which command do I remember?"

Ask what kind of interaction the system needs.

Diagrams open at a readable shape-aware scale. Zoom or expand when you need more detail.

Concept flow

Choose a ROS 2 communication pattern from the interaction shape

Topics, services, parameters, and actions solve different coordination problems.

1. NEEDidentify stream, request, configuration, or long-running goal
   classify
2. TOPICongoing typed stream
   or
3. SERVICE / PARAMone request-response operation or node configuration
   or
4. ACTIONgoal with feedback, result, and cancellation
   verify
5. CONTRACTinteraction shape matches the system requirement

Topic

Use a topic when publishers send a continuing stream of typed messages and subscribers receive updates.

Examples: velocity commands, sensor data, pose updates.

Service

The official service tutorial describes a call-and-response model. A client sends a request to a service server and receives a response.

Examples: clear the Turtlesim screen, spawn a turtle, request one operation.

Parameter

The parameter tutorial describes parameters as node configuration values or settings.

Examples: Turtlesim background color, use_sim_time, scale values.

Action

The action tutorial describes long-running work using goal, feedback, and result. Actions are cancelable and are built using topics/services underneath.

Example: rotate Turtlesim toward a target heading while receiving progress and a final result.

Decision drill

Classify each need and explain why:

1. laser scan updates arriving many times per second;
2. ask a node to reset once;
3. change a node's background color setting;
4. tell a mobile robot to navigate toward a location and report progress.

Checkpoint

Do not answer with only four labels. For each one, name the interaction shape that made you choose it.