Back of the Envelope

Feed: Capacity Planning System Design

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The following is the back-of-the-envelope calculation for a URL shortener.

Traffic

The URL shortener is a read-heavy service. The Daily Active Users (DAU) for writes is 100 million. The Query Per Second (QPS) of reads is approximately 100 thousand.

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Storage

The shortened URL persists by default for 5 years in the data store. Each character size is assumed to be 1 byte. A URL record is approximately 2.5 KB in size. Set the replication factor of the storage to a value of at least three for improved durability and disaster recovery.

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Bandwidth

Ingress is the network traffic that enters the server (client requests).

Egress is the network traffic that exits the servers (server responses).

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Memory

The URL redirection traffic (egress) is cached to improve the latency. Following the 80/20 rule, 80% of egress is served by 20% of URL data stored on the cache servers. The remaining 20% of the egress is served by the data store to improve the latency. A Time-to-live (TTL) of 1 day is reasonable.

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