ChapterFlow
Guided reading for non-fiction: over 130 books in 12-minute chapters, each a summary, examples and a quiz, then spaced review. Free and Pro plans on Stripe.
Built and run by me. Live at chapterflow.ca, with Free and Pro plans.
Next.js, TypeScript, DynamoDB, S3, Lambda, Cognito, Stripe, CDK
What it does
ChapterFlow is a live, paid reading product for non-fiction. Over 130 books are cut into chapters of about 12 minutes, and every chapter runs the same loop: a summary, worked examples and a quiz, then spaced review days later, scheduled with FSRS-5. Free covers two complete books with no card. Pro removes the limit and adds a deeper reading mode, text-to-speech audio and reading analytics; at the time of writing it costs $7.99 CAD a month or $59.99 CAD a year, with a 14-day trial.
How it works
- One Next.js App Router app (React 19, TypeScript, Tailwind 4) serves the marketing site, the library, the reader and the dashboard.
- Cognito sign-in through OAuth route handlers; the admin console is limited to a Cognito admin group.
- Books, progress and plans live in DynamoDB; book packages live in S3 and arrive through a validated admin upload path.
- Stripe Checkout handles upgrades, and a signature-checked webhook updates the reader's plan.
- Spaced review is scheduled with FSRS-5, an open-source spaced-repetition scheduler.
- Also on AWS: Lambda, SSM parameters for configuration and SES for transactional email; infrastructure in CDK across dev, staging and prod.
- A push to main deploys dev; staging and prod are manual, and prod needs an approval.
- Text a reader submits for their own scenario goes to Anthropic's API for an automated quality and safety review before it can be published.
Decisions and tradeoffs
A short loop per chapter
Summary, examples, quiz, then spaced review days later. The product rests on two findings it cites: recalling an idea fixes it better than rereading it (Karpicke and Roediger, 2008), and spaced study beats cramming (Cepeda et al., 2006). The cost is content: every chapter needs examples and a quiz, not just a summary.
FSRS-5 instead of fixed intervals
The scheduler estimates when each idea is about to fade and brings it back then, so a reader reviews what is due and nothing more.
Books as validated packages
Each book is a structured package that is checked on upload, so a malformed book is rejected before any reader sees it.
Plan limits on the server
Starting a book reserves a slot with a conditional DynamoDB write, so the free limit holds across tabs and devices. Stripe events are verified by signature and applied once, even when Stripe sends one twice.
Security
- Cognito sign-in with session tokens in secure, httpOnly cookies; the admin console is limited to an admin group.
- Secrets come from per-environment deploy secrets and SSM parameters, never from the repo.
- Card details stay with Stripe; ChapterFlow keeps only the customer ID and subscription status.
- A hashed device ID protects free book slots from multi-account abuse.
- Readers can export their data as JSON, CSV or Markdown and delete their account.
What can go wrong
- A bad deploy: prod deploys need an approval, and each environment has a documented rollback.
- A malformed book: package validation rejects it before it reaches readers.
- A duplicate Stripe event: each event is recorded first and applied once.
- Two requests for the last free book slot: the conditional write lets only one through.
Observability
- Health checks, a runbook and a rollback procedure for each environment.
- Product analytics are opt-in and off by default.
- Operational logs are kept for about 18 months, then deleted.
Cost
- Serverless data plane (DynamoDB, S3, Lambda): cost follows usage.
What's next
- A native iOS app, in development: Swift 6 and SwiftUI, 19 Swift packages plus share, action, widget and notification extensions. Not yet on TestFlight or the App Store.
- More titles: readers request books from the home page, and Pro requests get priority.
Need something like this built or reviewed?
Open chapterflow.ca (opens in a new tab)will@soltani.orgSource on GitHub (opens in a new tab)