Chapter 6

The Batteries

Redis, S3, Mail, Jobs, AI — what each one does, where it lives, and how to modify it.

~44 min total6 lessonsAssignment

By the end of this chapter you'll be able to

Identify each battery, what it does, and when you need it
Find the file you would edit to change each battery's behaviour
Wire a new use of a battery into your project (e.g., add a cache, send a transactional email, queue a job)

Lessons

Module: Caching & files

What are the Batteries?

The included-out-of-the-box services and what they save you.

5measy

Redis cache

Speed up reads, throttle, and store ephemeral state.

7mmedium

S3-compatible file storage

Upload files, store on R2/MinIO/AWS, serve with signed URLs.

8mmedium

Module: Async & integrations

Mail (Resend)

Transactional + marketing email with templates you can edit.

7measy

Background jobs (asynq)

Queue work, retry on failure, schedule with cron — without blocking requests.

9mmedium

AI (Claude + OpenAI)

Streaming chat, embeddings, and how the abstraction lets you swap providers.

8mmedium

Chapter assignment

Cache + email + job — a real use of three batteries

Build a small feature using three batteries: when a new Order is created, (1) cache the order total in Redis with a 5-min TTL, (2) enqueue a job that sends a confirmation email via Resend, (3) the job worker actually sends it. Test the whole loop with a real Resend test key.

See success criteria

Lesson 1 takes ~5 min.

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ch.5: Frameworks & Patterns ch.7: Architecture Modes