Part 10 · 1 chapters · ~8 min

Capstone: The Ledger API in Rails

The shared ledger capstone in Rails 8: models with constraints, a posting service with ordered locks, idempotent transfers, a payout state machine with Solid Queue jobs and enquiry, Minitest concurrency tests, Bullet and strict loading, rack-mini-profiler measurements, and a Kamal deployment.

The build

code

ledger-rails/
  app/models/account.rb, journal_entry.rb, posting.rb, payout.rb      validations + DB check constraints
  app/services/ledger/post_entry.rb      transaction, ordered lock, balanced check (Σ postings = 0)
  app/controllers/transfers_controller.rb   Idempotency-Key, problem responses
  app/jobs/submit_payout_job.rb, enquire_payout_job.rb   state transitions; unknown on timeout
  test/integration/idempotency_test.rb   threads posting the same key → one entry
  test/models/invariant_test.rb          random operation sequences keep Σ = 0
  config/deploy.yml                      Kamal to two servers
acceptance: identical to every language course (Go course part 10); compare lines of code and time to first working version

Rails usually wins on time to a working, secure first version and loses on raw throughput per server; for most products the first matters far more, and YJIT has narrowed the second.