Note: Persistence helpers and the retry demo are included in the runnable :::{pyodide-cell} below so the demo definitions and execution appear together in the proper order for browser runs. The notebook also shows a CSV-based persistence pattern intended for local runs; browser file persistence may vary in JupyterLite/Pyodide.

## Hero: Reliable Invoice Automation for Business

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<img src="media/images/invoice.png" width="90">

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### Why this matters

Billing mistakes cost money and reputation. This notebook shows patterns to generate invoices deterministically, deliver them safely with idempotency keys, and retry transient failures — all without network calls so learners can run the demos in Pyodide.

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## Learning objectives

- Generate deterministic invoices suitable for testing and demos.
- Implement idempotent delivery using idempotency keys and an in-memory store (show persistence pattern via CSV simulation).
- Add a retry wrapper for transient failures while preserving idempotency.

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Invoice Automation — safe invoice generation and delivery patterns¶

Business framing: Reliable billing automation reduces manual errors, speeds cash collection, and improves customer experience. The in-notebook demo below shows deterministic invoice generation, idempotent delivery simulation, and logging suitable for Pyodide runs.

Pyodide-safe invoice generator + idempotent send simulation¶

Visual intuition: Billing workflow¶

Caption: Invoice generation, persistence, delivery, and safe retry using idempotency keys.

MCQ¶

Q: What mechanism ensures a repeated delivery attempt does not charge or send duplicate invoices?
- A) Shorter retry intervals
- B) Idempotency keys and stored results
- C) Larger thread pools
- (Answer: B)

Exercises¶

Persist _id_store to a CSV-like list (print rows) and load it back to simulate restart persistence.
Add a status field and simulate transient failures where send_invoice_safe sometimes returns failed and needs a retry wrapper that respects idempotency.
(Stretch) Show how you would integrate a real email API (describe required steps in markdown) and what secrets/config you would avoid storing in the notebook.

Notes: This demo is deterministic and runs without external services. For production, persist invoices to a database and use provider SDKs with secure secret management.

Invoice Automation System¶