An Overview of Third-Party Payment Systems: History, Types, Architecture, and Core Concepts

This article provides a comprehensive technical overview of payment systems, focusing on the evolution of payment methods, the classification of first‑, second‑, third‑, and fourth‑party payments, and the internal architecture of third‑party payment platforms.

1. Background and Terminology

The author introduces key terms such as payment tools, users, merchants, user accounts, merchant accounts, reserve funds, system funds, and physical funds, explaining their roles within a payment ecosystem.

2. Development History of Payments

• First‑party payment : Direct cash transactions between buyer and seller, which become insufficient for large‑scale commerce.

• Second‑party payment : Bank‑mediated transfers that reduce cash handling but incur higher usage thresholds.

• Third‑party payment : Independent platforms (e.g., WeChat Pay, Alipay) that obtain a license, integrate with banks via APIs, and dominate everyday transactions.

• Fourth‑party payment : Aggregators that combine multiple third‑party channels to offer a one‑stop solution, though they currently lack regulatory qualification.

3. Third‑Party Payment System Overview

The system consists of three core account types: User Account (U‑account), Merchant Account (M‑account), and Bank Account (reserve fund). Each account records an ID, balance, and transaction log, serving as the fundamental data structure for the platform.

The article details typical operations:

Recharge: User transfers funds from a bank account to the reserve fund, which credits the user’s platform balance.

Withdrawal: Platform debits the user balance and transfers the amount from the reserve fund back to the user’s bank.

Transfer: Internal movement of funds between user accounts without affecting the reserve fund.

Payment: User pays a merchant, moving funds from the user’s account to the merchant’s account.

Refund: Reversal of a payment, returning funds from the merchant to the user.

Settlement: After deducting fees, the platform transfers merchant funds to the merchant’s external bank account.

These operations are illustrated with flow diagrams and balance‑change tables, highlighting that reserve‑fund changes always match platform‑balance changes for recharge/withdrawal, while internal transfers leave the reserve fund unchanged.

4. Interaction Model

The author presents a user‑centric view of a typical purchase (e.g., buying vegetables) and contrasts it with the system‑level flow, emphasizing the need for multiple backend services (payment, refund, settlement, etc.) to cooperate.

5. Domain Decomposition

The payment platform is split into four functional domains:

Payment (broad): Core transaction capabilities such as payment, transfer, and recharge.

Refund: Returning funds from merchant to user.

Withdrawal (Payment): Moving platform funds to external bank accounts.

Settlement: Disbursing merchant earnings after fee deduction.

Each domain provides foundational services that can be combined to support complex business scenarios.

The article concludes by summarizing the key takeaways and announces a forthcoming series that will dive deeper into each domain (transaction, refund, settlement, etc.).