Payment System: Capacity Estimation

Step 3: Back-of-the-Envelope Capacity Estimation

Step 2 set the requirements. Now we turn them into a few rough numbers, enough to size the system.

Scale. More than 100,000 merchants use the platform. Peak load reaches 5,000 TPS during major sales.

Storage. A transaction record is about 300 bytes. At 1 billion transactions a year, that adds up to roughly 300 GB of new storage per year.

Read versus write load. About 70 percent of the load is writes (transaction inserts and updates), and 30 percent is reads (queries). Read replicas or a cache can serve the frequent queries.