Day 49 — State Snapshots

You will build a system that can crash, restart, and pick up exactly where it left off — without losing a single trade or a single cent of P&L.

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Table of Contents

1. What You’re Building

2. The “Just Pickle It” Trap

3. The Failure Mode — Corrupt State Re-entry

4. The Architecture — Atomic JSON Snapshots

5. Implementation Deep Dive

6. Production Readiness: Metrics to Watch

7. Full Source Code

8. Build, Test and Demo

9. Homework: Production Challenge

10. Success Criterion

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1. What You’re Building

Think of any program that keeps important data — a video game that saves your progress, a banking app that stores your balance, a chat app that remembers your messages. They all have the same problem: the program runs in memory (RAM), but RAM is wiped when the power goes off. To survive a crash, the program has to write its state to disk regularly.

Our MockBroker has the same problem. It tracks cash balances, open positions, and trade history — all in memory. If the process dies, that data is gone. Today we build the mechanism that saves it safely.

The technical name for this saved copy is a state snapshot.

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