Defend Your System

Overview

• Primary references: your own design documents from the course: ADRs (craft.02), C4 diagrams (craft.09), the threat model (craft.17), and the design reviews (review.01 to review.03)
• Supplementary: Kleppmann, Designing Data-Intensive Applications; the shared interview concepts and the company folders in this track for the question styles each company uses
• Prerequisites: the whole course through Pass 11, or a system of your own that you designed and operate
• Estimated time: 1 week in course Pass 11

Key Takeaways

• The strongest interview answer is about a system you built: you know where it breaks, why each choice was made, and what you would change.
• Answer with evidence: a load report, a benchmark, a drill postmortem, an ADR with the rejected options.
• Every choice has a cost: name the alternative you rejected and what it would have bought.

How to Study

Answer the question bank out loud against your own design, then write each answer down with the artifact that backs it. iv.01 is a proof module graded by rubric.

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Concepts & Techniques

Core Insight

System design interviews test judgment under questioning. Defending a system you built end to end turns that from rehearsal into recall: the trade-offs are ones you made, measured, and lived with through drills and migrations.

1. The question bank

Key ideas:

• Architecture: why a C ABI and HTTP between the languages; why the gateway owns keys and limits; why a durable engine instead of cron.
• Scale and failure: what breaks first at 10x load; what happens when a decode pod dies mid-stream; how resume stays exactly-once.
• Models and data: why this tokenizer and vocabulary size; how you know the eval numbers are not contaminated.

Course modules

Module	Topic	Kind	Pass
iv.01	Defend your system	proof	11

Chapters

#	Module	Chapter	Kind	Pass
1	iv.01	Defend your system	proof	11

Connections to Other Tracks

Track	Connection
System Design	the design reviews this module draws on
Documentation Writing	ADRs as the record of your choices
Interview Prep	company-specific formats