Where does quorum collide?

The process by which bacteria track their own population density and switch on group behaviours only once enough cells are present: each cell steadily releases a small signalling molecule called an autoinducer, and when the surrounding concentration climbs past a threshold, meaning a large enough gathering of cells has built up, the population turns on shared genes together, controlling traits such as light production, biofilm formation, and the release of toxins. Miller and Bassler's 2001 review Quorum sensing in bacteria set out how these chemical circuits let a colony act as a single coordinated group. Here the referent is a density-dependent chemical signalling process among living cells, not a count of people needed to hold a vote or a number of computers needed to commit a change.[1]

Unit

One density threshold in a bacterial population, marked by autoinducer concentration

Recovery means

Not applicable

Measured by

Detected by tracking autoinducer concentration and the density-dependent switching of reporter genes such as bioluminescence

The minimum number of copies in a replicated computer system that must take part for an operation to be accepted, chosen so that any two operations are forced to overlap on at least one copy and so cannot silently disagree: by requiring, for example, a majority of copies to acknowledge every read and every write, the system stays consistent even when some machines fail or messages are lost. Gifford's 1979 paper Weighted voting for replicated data introduced this quorum approach, giving votes to copies and requiring read and write quorums whose sizes are set to overlap; majority quorums in the same spirit underpin later consensus protocols such as Paxos. Here the referent is a threshold of agreeing machines needed to commit safely, not a rule about people present at a meeting or a density of signalling bacteria.[2]

Unit

The minimum number of replica copies whose agreement commits an operation, often a majority

Recovery means

Not applicable

Measured by

Defined by the read and write quorum sizes, set so that every write quorum overlaps every read and write quorum