Quorum

Meaning of the word

It's important to keep in mind that this word in Latin isn't even a noun.

"I appoint these 20 people to oversee the trial, of whom (quorum) I require at least 5 to be present to make it official." Here, of whom which is a relative pronoun is used in English to represent the minium required people or votes.

Just the entire statement is now referred to by the word "quorum" in English. We "wait" for the quorum, we "require" the quorum, we "need" the quorum, we "have" the quorum, we "don't have" the quorum.

Quorum is used in distributed systems in a few ways.

1. We've many replicas of the same data. We must read or write to a minimum number of them for consistency.
2. We've many nodes in a cluster. We must make sure a minimum number exist.
3. We need a minimum number of votes to elect a leader in a cluster.

Reading from minimum number of replicas

For a read, the data comes from a set number of replicas. The client then compares them. It finds which one has the latest data and uses it.

Application need not make multiple calls

Usually the app doesn't worry about quorum. The driver or the coordinator on the server handles it. The client just sends a regular request to the coordinator.