Vector clock. A vector clock is a data structure used for determining the partial ordering of events in a distributed system and detecting causality violations. Just as in Lamport timestamps, inter-process messages contain the state of the sending process's logical clock. A vector clock of a system of N processes is an array/vector of N logical clocks, one clock per process; a local "largest possible values" copy of the global clock-array is kept in each process.

Denote  as the vector clock maintained by process , the clock updates proceed as follows:

 Initially all clocks are zero.
 Each time a process experiences an internal event, it increments its own logical clock in the vector by one. For instance, upon an event at process , it updates .
 Each time a process sends a message, it increments its own logical clock in the vector by one (as in the bullet above, but not twice for the same event) then it pairs the message with a copy of its own vector and finally sends the pair.
 Each time a process receives a message-vector clock pair, it increments its own logical clock in the vector by one and updates each element in its vector by taking the maximum of the value in its own vector clock and the value in the vector in the received pair (for every element). For example, if process  receives a message  from , it first increments its own logical clock in the vector by one  and then updates its entire vector by setting .

History

Lamport originated the idea of logical Lamport clocks in 1978. However, the logical clocks in that paper were scalars, not vectors. The generalization to vector time was developed several times, apparently independently, by different authors in the early 1980s. At least 6 papers contain the concept.
 The papers canonically cited in reference to vector clocks are Colin Fidge’s and Friedemann Mattern’s 1988 works,
 as they (independently) established the name "vector clock" and the mathematical properties of vector clocks.

Partial ordering property

Vector clocks allow for the partial causal ordering of events. Defining the following:
  denotes the vector clock of event , and  denotes the component of that clock for process .
 
 In English:  is less than , if and only if  is less than or equal to  for all process indices , and at least one of those relationships is strictly smaller (that is, ).
  denotes that event  happened before event . It is defined as: if , then 

Properties:

 Antisymmetry: if , then ¬
 Transitivity: if  and , then ; or, if  and , then 

Relation with other orders:

 Let  be the real time when event  occurs. If , then 
 Let  be the Lamport timestamp of event . If , then

Other mechanisms

 In 1999, Torres-Rojas and Ahamad developed Plausible Clocks, a mechanism that takes less space than vector clocks but that, in some cases, will totally order events that are causally concurrent.

 In 2005, Agarwal and Garg created Chain Clocks, a system that tracks dependencies using vectors with size smaller than the number of processes and that adapts automatically to systems with dynamic number of processes.
 In 2008, Almeida et al. introduced Interval Tree Clocks. This mechanism generalizes Vector Clocks and allows operation in dynamic environments when the identities and number of processes in the computation is not known in advance.

 In 2019, Lum Ramabaja proposed Bloom Clocks, a probabilistic data structure based on Bloom filters. Compared to a vector clock, the space used per node is fixed and does not depend on the number of nodes in a system. Comparing two clocks either produces a true negative (the clocks are not comparable), or else a suggestion that one clock precedes the other, with the possibility of a false positive where the two clocks are unrelated. The false positive rate decreases as more storage is allowed.

See also
Lamport timestamps
Matrix clocks
Version vector

References

External links 
 Why Logical Clocks are Easy (Compares Causal Histories, Vector Clocks and Version Vectors)
 Explanation of Vector clocks
 Timestamp-based vector clock implementation in Erlang
 Vector clock implementation in Objective-C
 Vector clock implementation in Erlang
 Why Vector Clocks are Hard
 Why Cassandra doesn’t need vector clocks

Logical clock algorithms