Design Flash Sale System
Last Updated: December 29, 2025
What is a Flash Sale System?
A flash sale system enables e-commerce platforms to sell limited inventory at discounted prices during a short time window, typically lasting minutes to hours.
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The core challenge is handling extreme traffic spikes where millions of users compete for a few thousand items simultaneously. Unlike regular e-commerce, flash sales create a thundering herd problem where traffic can spike 100x or more within seconds of the sale starting.
Popular Examples: Amazon Lightning Deals, Flipkart Big Billion Days, Alibaba Singles' Day
What makes flash sales particularly interesting is the asymmetry of outcomes. Out of 5 million purchase attempts, only 10,000 might succeed. That means 99.8% of requests result in failure, either due to sold-out inventory or system protection mechanisms.
The architecture must be optimized for this reality: rejecting requests cheaply and quickly is just as important as processing successful orders.
This system design problem tests your ability to handle extreme concurrency, prevent overselling, ensure fairness, and maintain system stability under load.
In this chapter, we will explore the high-level design of a flash sale system.
Lets start by clarifying the requirements: