CoAP
CoAP, constrained application protocol, IoT messaging, resource APIs, Observe, UDP
Your guide: Broker Bex
“A message with no subscriber is a tree falling in an empty forest — design the topic before the payload.”
CoAP Module Guide
CoAP is the application protocol many constrained IoT systems use when small devices expose resources, exchange compact UDP messages, and need REST-like behavior without HTTP overhead. This module groups the CoAP chapters into a focused book so the protocol can be built, searched, and reviewed as its own route.
Use this book when you need to reason about CoAP as a system design choice for constrained networks. The chapters move from fundamentals and message exchanges through methods, resource API design, Observe, advanced features, security, and implementation labs.
Start With the Device Story
Picture a battery sensor that needs to expose one temperature value, accept one configuration change, and stay asleep as much as possible. CoAP is the route through that design problem: name the resources, choose the message type, decide what must be reliable, then prove the packets, security, cache behavior, and implementation traces.
Use the module as a build sequence, not a glossary. Each chapter answers one question that the tiny device forces on you before the next question makes sense.
Recommended Route
- Start with CoAP Fundamentals to understand REST resources, compact UDP messages, tokens, discovery, and protocol-fit evidence.
- Use CoAP Message Format and CoAP Message Exchanges before reasoning about confirmable delivery, retries, acknowledgments, reset behavior, and parser evidence.
- Read CoAP Methods and Multicast and CoAP Resource API Design before designing resource paths, payloads, response codes, and gateway boundaries.
- Review CoAP Observe Server Push when polling reduction, notification thresholds, and rate limiting matter.
- Move into CoAP Advanced Features and CoAP Security and Applications before evaluating block-wise transfer, proxies, caching, DTLS, OSCORE, credentials, and deployment boundaries.
- Finish with CoAP Implementation Labs when you are ready to build, test, and compare constrained-network behavior.
Design Questions
- Which resources should devices expose, and which methods should each resource support?
- Which exchanges need confirmable delivery, and which can tolerate loss or delay?
- Does the design need Observe notifications, multicast discovery, block-wise transfer, caching, or proxies?
- Where is security enforced: DTLS, OSCORE, credentials, gateway policy, or application authorization?
- What implementation evidence proves the CoAP server, client, and constrained link behave as expected?