Grokking the System Design Interview, Volume II
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Google Calendar: Capacity Estimation

Step 3: Back-of-the-Envelope Estimation

Before diving into design, we estimate the scale to guide our choices:

Summary:

  • About 100M users. 10–20M active daily. Peak use can reach tens of millions.
  • Up to 50M new events per day. Hundreds of writes per second on average. A few thousand at peak.
  • Reads dominate. Roughly 100–200M reads per day. Around ten times more reads than writes.
  • Events add ~50 GB per day and ~20 TB per year. Long term storage and pruning are needed.
  • Peak load of a few thousand writes/sec and 5–10k reads/sec, so we need sharding, caching, and load balancing.

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