Design a Payment Processing System

Commerce, Payments & LedgersHarddesign-payment-system

Design money movement where a duplicate charge is a headline: idempotent charge requests, asynchronous provider webhooks, and daily reconciliation against the processor's ledger.

Idempotency KeysSagaReconciliationOutboxPCI Tokenization

Traffic & Capacity Estimates:

10k payments/second peak · 4 payment providers · zero tolerance for double charges

Functional Requirements

•Accept a payment intent, authorize with a provider, and capture funds, with full state tracking.
•Deduplicate retried charge requests by idempotency key so a client timeout never double-charges.
•Ingest asynchronous provider webhooks that may arrive out of order, duplicated, or late.
•Reconcile internal records against provider settlement files daily and flag every discrepancy.

Non-Functional Requirements

•Correctness beats availability: refuse a payment rather than risk charging twice.
•Card data must never touch your systems unencrypted — tokenize at the edge.
•Every state transition must be auditable and immutable after the fact.

Back-of-the-Envelope Math

10k payments/second * 6 state transitions = 60k ledger writes/second at peak.
Reconciliation over 500M daily transactions is a batch job whose runtime must fit inside the settlement window.

Key Architectural Trade-offs

Synchronous capture gives the user an immediate answer and couples you to provider latency; asynchronous capture is resilient and forces a 'pending' state into the product.
Provider failover improves availability and breaks idempotency unless keys are scoped per provider — a retry on a second provider is a second charge.
Storing the provider's response verbatim makes reconciliation and disputes tractable, and makes your PCI scope and retention policy a real design constraint.

Click or drag a component onto the canvas, then connect the handles to draw the data flow.

3 nodes · 2 edges

Components · 35

Client & Edge4

Compute & Gateway7

Storage & Caching11

Messaging & Streaming6

Coordination & Ops5

Intelligence2

React Flow