Designing a High‑Concurrency Red Packet System with Atomic CAS Operations

Red Packet Generation

When a red packet is created, its ID and quantity are stored in the database and also placed in a cache.

User Interaction

Grabbing a red packet consists of two actions: the "grab" step determines whether the user obtains eligibility, and if successful, the "open" step calculates the exact amount, records the transaction flow, and credits the user's account.

Amount Calculation Method

The amount is random, ranging from 0.01 to twice the remaining average value. For example, if 100 CNY is split into 10 packets, the average is 10 CNY; each packet’s amount varies between 0.01 CNY and 20 CNY. If the first three packets have already taken 40 CNY, the remaining 60 CNY for 7 packets will be between 0.01 CNY and (60/7 × 2) ≈ 17.14 CNY.

Ensuring Safety Under High Concurrency

The "grab" action reduces the remaining packet count by one. This decrement operation is atomic, implemented with a cache CAS (compare‑and‑swap) mechanism to guarantee uniqueness.

CAS works by allowing a write only if the value read earlier has not been modified by another process. For instance, if the remaining count is 10 and many users attempt to grab simultaneously, CAS ensures that only one request successfully decrements the count at a time.