Part 10 · 1 chapters · ~8 min

Capstone: The Ledger API in Go

The shared capstone for every language course: the ledger API from Ledgers part 9 (accounts, balanced postings, idempotent transfers, a payout state machine against a simulated rail) built in Go with Gin, pgx, sqlc and errgroup, with tests, a benchmark and a profile.

The build

code

ledger-go/
  cmd/ledger/main.go           wiring, config, graceful shutdown
  internal/ledger/post.go      PostEntry: balanced check, ordered row locks, funds check, append (one transaction)
  internal/transfers/http.go   Gin handlers with Idempotency-Key, problem responses
  internal/payouts/machine.go  state machine; rail client with success / failure / timeout; enquiry worker
  internal/db/queries.sql      sqlc queries; migrations with goose
  internal/ledger/post_test.go table-driven tests; testcontainers-go Postgres
  bench/                        go test -bench: transfers per second; pprof the hot path

acceptance (identical in every language course, so implementations can be compared)
  - 100 concurrent identical requests with one key → one entry, one response
  - Σ postings = 0 per currency after a 10,000-operation fuzz run (go test -fuzz)
  - a timed-out payout never completes without rail confirmation
  - report: requests/s at p99 < 50 ms on this machine, binary size, image size, memory at idle