Real-Time Fraud Detection with Stateful Processing

Problem Description

A payment processing company needs to build a real-time fraud scoring system that goes beyond what standard Flink SQL can express. They need custom stateful operators that can:

1. Transaction Velocity Tracker — A stateful function that tracks per-user transaction patterns. For each incoming transaction, it should:

   • Maintain a count of transactions in the last hour using state
   • Track the running average transaction amount per user
   • Emit a fraud score (0.0-1.0) based on: velocity (>10 txns/hour = 0.5), amount spike (>3x average = 0.3), new merchant (first time = 0.2)
   • Use a 24-hour state expiration to clean up inactive users
   • The output should include: user_id, transaction_id, fraud_score, risk_factors (comma-separated), transaction_amount, avg_amount

2. Session Revenue Calculator — A stateful function that groups checkout events into sessions and emits session summaries when a session expires. Requirements:

   • Session timeout of 15 minutes (passed as a parameter)
   • Track session start time, event count, and total revenue
   • Use timers to detect session expiration and emit the session summary
   • Output: user_id, session_start (TIMESTAMP), session_end (TIMESTAMP), event_count (INT), total_revenue (DOUBLE)

3. Write the SQL queries that invoke both functions against input tables, with proper partitioning and time ordering.

The transaction data comes from a table transactions with fields: user_id (STRING), transaction_id (STRING), amount (DOUBLE), merchant_id (STRING), event_time (TIMESTAMP(3)) with a 1-second watermark.

The checkout data comes from a table checkouts with fields: user_id (STRING), item_id (STRING), price (DOUBLE), checkout_time (TIMESTAMP(3)) with a 2-second watermark.

Output Specification

• TransactionVelocityTracker.java — The fraud detection PTF implementation
• SessionRevenueCalculator.java — The session revenue PTF implementation
• queries.sql — SQL statements to invoke both PTFs with correct syntax