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Lesson plan 05: Application protocols and remote calls
Design a framed request protocol with bounded messages and explicit serialization choices.
Placement: Part 1: Foundations: understanding work, time, and resources.
Prerequisites: Lesson 04: Networking and transport, plus the earlier dependency chain.
Learning outcomes
- Design a framed request protocol with bounded messages and explicit serialization choices.
- Follow timeout, retry, cancellation, and lost-reply cases without confusing uncertainty with failure.
- State the atomicity and retention requirements of operation identifiers used for retry deduplication.
- Separate a general mechanism, a documented implementation choice, and an illustrative numerical assumption.
Teaching sequence
- Establish the motivating situation: A purchase succeeds but its reply disappears; a retry must not create a second purchase.
- Introduce the vocabulary and the correctness obligation before an optimization. Define framing, serialization, RPC.
- Develop the mechanism: Walk through request identity, transactionally recorded results, response framing, and bounded retry behavior.
- Follow the complete worked example: Four dependent exchanges at 2 ms RTT spend 8 ms on communication. Explain the assumptions, intermediate units, and limiting interpretation.
- Extend the scenario to changed load, skew, or failure, explaining HTTP, WebSockets, pooling, timeouts, retries, cancellation, idempotency.
- Connect the mechanism to a documented implementation and an alternative with a different workload fit. Close by stating the remaining cost and the next lesson dependency.
Required chapter material
Use a complete execution trace or worked calculation, supported by an annotated static diagram or table. Include language-neutral pseudocode or labeled SQL when it clarifies the mechanism. State preconditions, ownership, visibility, and failure boundaries for algorithms. Describe operational costs as well as the normal path. All questions raised in the narrative must be resolved in the text.
Visual explanation
A lost-reply sequence shows a committed operation, client timeout, retry, and retrieval of the stored result. The accompanying prose explains the arrows, state changes, or table entries so the lesson remains understandable in a plain Markdown reader.
Editorial checks
Confirm that the numerical example agrees with its units and assumptions. Check that a faster path preserves the stated contract, or explicitly identify the changed guarantee. Keep hypothetical figures separate from product measurements. Explain both an appropriate use and a limitation of the case design. Check terminology against the glossary and links against the course index.