ROS 2

ROS 2 organizes robotics software into communicating components. A useful design makes each component's responsibility, interface, configuration, and failure behavior explicit. Choose communication by the work: streams, bounded request/response operations, and tasks that need ongoing feedback have different contracts.

Treat discovery, compatible quality of service, delivered messages, and correct robot behavior as separate checkpoints. A visible node proves very little about a delayed sensor reading or an actuator's actual state. This guide uses Jazzy documentation as an explicit reference baseline; package availability and behavior must be checked against the distribution and middleware you deploy.

TL;DR

Quick Example

This synthetic counter publishes integers and has no actuator interface. It requires a sourced ROS 2 Jazzy environment with rclpy and std_msgs. Keep any later experiment in an isolated simulation or development graph, with no connection to operational hardware.

The intended behavior is an incrementing counter on the named topic, with a requested timer period of half a second. That period is a scheduling request, not a measured delivery rate or deadline guarantee. The sensor-data QoS profile is intentional: a subscriber must use compatible settings. A reliable subscription should not be assumed compatible with a best-effort publisher.

The code follows the official Jazzy publisher pattern. Run it in a sourced ROS 2 environment with a subscriber to see messages flow, then test discovery, QoS compatibility, and callback timing with your real middleware, simulation, or hardware. Official Jazzy publisher example, Jazzy QoS documentation.

Core Concepts

Nodes and processes are different boundaries

A node is a participant in the ROS graph and a logical unit of functionality. Several nodes can share a process, and a system can span processes or machines. Decide where process isolation is useful for fault containment and where composition is useful for your deployment constraints. Neither arrangement automatically makes the system reliable or fast. Jazzy node concepts.

Give each node a responsibility that can be described and tested. A component that receives a sensor reading, filters it, plans motion, manages permissions, and writes reports may hide several independent failure domains. Splitting it can clarify contracts, but excessive fragmentation can add configuration and diagnostic work. Use ownership and observable behavior to justify the boundary.

Interfaces describe different conversations

Messages define fields for topic communication. Service definitions have request and response parts. Action definitions include goal, result, and feedback. That structural distinction provides a starting point for selecting an interface; it does not specify your application's timeout, cancellation policy, units, or error semantics. Jazzy interface definitions.

This table is a design aid. A request sent twice can still produce two application effects unless your own contract handles duplication. A cancellation request also needs an observed outcome; sending it does not establish that machinery reached a safe state. Keep physical safety behavior in the system's reviewed control and safety design.

QoS is a compatibility contract

Publisher and subscription settings use an offered/requested model. For reliability alone, best effort offered to reliable requested is incompatible; reliable offered to best effort requested can be compatible. All relevant policies must agree, so passing this one comparison does not establish an overall connection. History, depth, durability, deadline, lifespan, and liveliness answer additional questions. Jazzy QoS policies and compatibility.

Ask whether older data has value. A visualization may prefer the newest observation over delayed backlog; a different consumer may need a complete application record. Those requirements can lead to different queueing and reliability choices. Measure the actual resulting behavior under loss and load rather than labeling one profile universally better. Middleware reliability is not a functional-safety certificate or a hard real-time guarantee.

Launch makes intended configuration reviewable

ROS 2 launch describes which programs run, their arguments and configuration, and process-state monitoring or reactions. Launch descriptions can use XML, YAML, or Python. The goal is a repeatable system description instead of a collection of remembered terminal commands. Jazzy launch concepts.

Review the resolved configuration as an artifact: package revisions, parameter files, remappings, namespaces, and deployment environment. Starting all expected processes is only the first acceptance condition. A process can remain alive while its inputs are absent or its outputs violate the task contract. Define readiness and degraded operation separately from process existence.

Best Practices

Diagnose from the graph to the consumer's decision

Start with the intended participants and interfaces. Check that the expected names and types are visible, then compare endpoints and QoS. Next inspect actual samples and their timing, and finally verify the consuming algorithm's interpretation. This order narrows the fault without confusing successful discovery with useful communication.

For a fictional perception pipeline, count received frames and rejected frames separately. Record observation age, processing completion time, and the reason a consumer discards a sample. A healthy average rate can conceal bursts, stale timestamps, or long pauses. Preserve the workload and clock assumptions when reporting a measurement so another engineer can interpret it.

Define observability before the first field fault

Specify which logs, counters, and traces establish each contract. Include configuration identity and meaningful reasons for degraded behavior. Treat diagnostic instrumentation as part of the workload: excessive logging can change timing and storage consumption. Compare behavior with the intended production diagnostic settings rather than a noisier development setup alone.

Recorded data can help reproduce an algorithmic failure. Keep its provenance, interface versions, and timing assumptions. A replay that produces the expected output is useful evidence for that scenario; it does not reproduce every live network, scheduling, sensor, or physical condition. Keep simulation, replay, bench testing, and operational evidence distinct in the review record.

Make loss and restart ordinary test cases

In an isolated environment, remove a publisher, restart a consumer, introduce stale data, and change one QoS requirement deliberately. State the expected failure signal and recovery behavior first. Verify whether reconnection restores useful output and whether stale state survives a restart. Keep any real actuation behind the project's qualified robotics and safety review.

Common Mistakes

Treating a matching topic name as delivery

Using a service for a long operation

Blaming every missing message on the network

Treating a successful launch as readiness

FAQ

Is one node the same as one process?

No. Nodes are logical graph participants; process placement is a deployment decision. Evaluate isolation, ownership, restart behavior, and observed resource use when deciding whether to compose components or separate them.

Does reliable QoS mean every operation succeeds?

No. Reliability is one policy within a broader communication contract. Compatibility, application handling, resource limits, timing, and task semantics still matter. Test the actual offered/requested policies and the consumer's behavior under failure.

Can a publisher example validate a robot?

No. Even a working publisher only shows that messages flow. Robot validation needs task-level tests of perception, control, hardware integration, and safety, usually in simulation first and then on hardware.

Related Topics

References