System Design Interview - An insider's guide

If clients write data to server1, server1 cannot propagate this change to server2.

When partitions occur, you have to choose between consistency and availability.

Cyclic Redundancy Code (CRC32) is used as the hash function in our implementation. Using CRC32 is completely optional, and you can use any hash algorithms that have good distribution (For example, SHA-1 or MD5).

A master database receives data from applications.

Slaves are therefore read-only from the application's point of view while a master is read-write.

paper “Scaling Memcache at Facebook”

Dynamic content caching is a relatively new concept and beyond the scope of this book.

Celebrity problem. It’s also called hotkey problem. Excessive access to a specific shard could cause server overload. Imagine data for Katy Perry, Justin Bieber and Lady Gaga all end up on the same shard. For social applications, that shard would become overwhelmed with read operations.

In the strict quorum approach, the write operation could be blocked because the system does not receive acknowledgements from a pre-defined amount of servers, as illustrated in quorum consensus.