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Idempotent payments — the same charge twice, charged once

Queues are at-least-once: a client retries, a broker redelivers, a proxy double-sends — and the same message arrives more than once. For a payment that means a double charge. BabelQueue ships an optional, dependency-free idempotency guard (ADR-0022) that dedupes a consumer on meta.id — the canonical per-message identity — so a message whose id was already processed is acked and skipped instead of run again.

This example proves it end-to-end, cross-language:

  1. A Python producer publishes the same payment envelope twice — the identical meta.id, trace_id and data, byte-for-byte (what a redelivery actually looks like on the wire).
  2. A Go consumer wraps its charge handler with idempotency.Wrap(store, …). The first delivery charges; the second is recognised as a replay of an already-processed id and skipped — no double charge.

The dedupe key is meta.id, so it works across SDKs: a payment a PHP or Python producer sent is de-duplicated by a Go, Java or Python consumer — same envelope, same identity, one charge. The guard is purely additive, so the wire envelope stays frozen at schema_version: 1.

Idempotency here is seen-set, post-success dedupe under at-least-once with an idempotent handler — not exactly-once and not an in-flight concurrency lock. It stops an accidental duplicate from re-running the side-effect. (A deliberate replay off a DLQ is the dlq-redrive/ replay-bypass story.)

Run it

# 1) start Redis
docker compose up -d            # or: docker run -d -p 6379:6379 redis:7
# 2) producer — Python   (sends the SAME payment twice, same meta.id)
cd producer-python
python -m venv .venv && . .venv/bin/activate
pip install -r requirements.txt
python charge.py
cd ..
[python] published attempt 1  meta.id=…  amount=4200 EUR
[python] published attempt 2  meta.id=…  amount=4200 EUR
[python] 2 deliveries of the SAME payment (meta.id=…) on the 'payments' Redis list — now run the consumer; it must charge exactly once.

# 3) consumer — Go   (needs babelqueue-go ^1.5, which ships the idempotency helper
#    in the core module — no extra `go get`; the Redis transport is …/redis)
cd consumer-go
go run .

Expected consumer output — two deliveries arrive, but the side-effect runs once. The second is skipped because its meta.id was already processed:

[go] charged       payment_id=pay_7f3a amount=4200 EUR  meta.id=…  (charged once)
[go] skipped       payment_id=pay_7f3a  meta.id=…  (duplicate — not charged again)
[go] handled 2 delivery(ies) — charged exactly once, no double charge.

How the guard works

idempotency.Wrap(store, handler) returns a handler that, on each delivery:

  • looks up the envelope's meta.id in a Store (a "seen-set");
  • if already seen → returns nil (the runtime acks it, so the broker stops redelivering) without calling your handler — the charge does not re-run;
  • if new → runs your handler, then records the id as processed only on success. A handler that raises leaves the id unrecorded, so retry / dead-letter still apply and a later delivery runs it again.

The reference InMemoryStore is for tests and single-process consumers. A production fleet uses a Redis- or database-backed Store (the same three methods — Seen / Remember / Forget) so dedupe is shared across workers.

Configuration

All scripts read these environment variables:

Variable Default Meaning
BROKER_URL redis://localhost:6379/0 Redis connection URL
QUEUE payments queue the payment envelopes flow over

Swap the ends

The dedupe key is the canonical meta.id, so any SDK can be on either side:

  • Python consumer: from babelqueue.idempotency import InMemoryStore, wrap, then app.register("urn:babel:payments:charge", wrap(store, handler)).
  • PHP consumer: the BabelQueue\Idempotency helper, same meta.id dedupe.
  • PHP / Go / Java / Node producer: publish the envelope; a redelivery (same meta.id) is what the guard collapses — the producer needs no special code.

See babelqueue.com for the per-SDK idempotency APIs.