Review a Payment API Design

Problem Description

A startup is building a payment processing API that allows merchants to initiate charges, issue refunds, and query transaction history. The API will be used by third-party merchants integrating via SDK or direct HTTP calls.

The engineering lead has shared the design document below and left the following message:

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"I think the design is pretty solid overall — we've covered the main flows and the data model makes sense. Let me know if there's anything minor to tighten up before we start sprint planning."

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Your task is to conduct a design review and produce written feedback.

Output Specification

Write your review to review.md.

Input Files

The following files are provided as inputs. Extract them before beginning.

=============== FILE: design.md ===============

Payment Processing API — Design Document

Overview

The Payment API accepts charge requests from merchants, processes them via an upstream payment network, stores the result, and allows merchants to query transaction history and issue refunds.

Endpoints

• POST /charges — Create a new charge
• POST /refunds — Issue a refund for a prior charge
• GET /transactions — List transaction history

Data Model

Each transaction is stored in a PostgreSQL table with fields: id, merchant_id, amount, currency, status, created_at, updated_at.

Error Handling

Failed charges return an appropriate HTTP error code. Merchants are expected to implement retry logic on their side.

Deployment

The API will be deployed as a single containerised service on AWS ECS. A PostgreSQL RDS instance will be used for persistence.