System Design Notes

1. How to approach? 5 S

   Scenario

   ​ Users; Use case; inputs and outputs;

   Numbers

   ​ DAU; Request per second; Read heavy/Write heavy

   Service

   ​ Implement high level components.

   ​ Front-end; Middle layer; Back-end

   ​ Serial or Parallel

   Store

   ​ SQL/No-SQL;

   ​ Schema;

   ​ OOD;

   ​ How does one operation look like

   Scale

   ​ Address Bottlenecks

   ​ Horizontal Scaling

   ​ DB sharding

   ​ Caching

   ​ Load Balancer

2. Back-of-envelope calculations

   This is to handle the number part and give you an overview of the basic numbers and speed.

   1) Basic numbers:

​ CPU 3Ghz –> 0.3 ns –> 0.1 ms –> 1 ms –> 10 ms –> 20 ms

Handy conversion guide:

• 2.5 million seconds per month
• 1 request per second = 2.5 million requests per month
• 40 requests per second = 100 million requests per month
• 400 requests per second = 1 billion requests per month

​ 2) popular websites’ DAU/MAU:

​ Twitter: 300m MAU

​ 3) Database performance

​ How many reads/writes does MySQL support?

​ How many reads/writes does Casandra support?

To address the 40 average read requests per second (higher at peak), traffic for popular content should be handled by the Memory Cache instead of the database. The Memory Cache is also useful for handling the unevenly distributed traffic and traffic spikes. The SQL Read Replicas should be able to handle the cache misses, as long as the replicas are not bogged down with replicating writes.

​ 4) Load Balancer Performance

​ Nginx can distribute how many requests?

3. System Design Concepts