System Design

A leader ships every write to its followers. What a client is promised depends on how many copies a write waits for and how many a read asks, and a lag or a partition shows the difference.

Hash the nodes onto a ring too, walk each key clockwise to the first node point, and a node joining or leaving moves only its neighbours' keys instead of nearly all of them.

Cut one table over many machines by range, hash, or directory; route each request to its shard, watch a hot spot form, and split it at the median key.

Two sites, one link that drops: a request on the far side can be answered and risk being wrong, or refused and be unavailable, and the theorem says there is no third way.

Machines have no shared clock, so order comes from the messages: a counter that ticks and travels puts every cause before its effect, and a vector tells when neither came first.

A coordinator asks every machine to prepare, commits only on a unanimous yes, and logs each step so a crash on either side still ends with every account changed or none.

Randomized timeouts pick a leader by majority vote, the leader replicates a log, a majority commits an entry, and a higher term ends any stale leader: one log on many machines.