[System Design] Payment System

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Overview

This is a note about design a payment system.

The content covers

• scope analysis, and
• architecture.
  • high level design
  • details

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Scope

According to the conversation, the payment system is for an ecommerce application like amazon.com, the system handles everything related to money movement, and interacts with a lot of internal services , such as accounting, analytics, etc.

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Architecture: High level design

Overview

At a high level, the payment flow is broken down into two steps to reflect how money flows

• pay-in flow
• pay-out flow

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Pay-in flow

• components in pay-in flow
  • payment event : triggered by end user
  • ledger: the ledger keeps a financial record of the payment transaction. for example, when a user pays the seller 1, werecorditasdebit 1 from a user and credit $1 to the seller.
  • wallet: the wallet keeps the account balance of the merchant.
  • Payment executor: The payment executor executes a single payment order via a Payment Service Provider (PSP). A payment event may contain several payment orders.

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Stripe Webhooks

• What is webhook ? When event triggered in stripe, stripe sends a request to your server to notify you that something happened with the event information.

• Tools: Stripe CLI

• Some frequently seen problems to solve

  • trigger different microservices on server side
  • how to avoid duplicated events , store all the events in database
  • how to check requests from attackers, use stripe sdk to validate hash