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) and then the message piggybacks a copy of its own vector. Each time a process receives a message, 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 message (for every element). For example, if process receives a message from , it updates by setting . History Without using the specific name "vector clock", the concept of a vector clock was first mentioned in a 1986 paper by Rivka Ladin and Barbara Liskov where they use the term "multipart timestamp". To quote from page 31 of the Liskov/Ladin paper: We solve this problem by using multipart timestamps, where there is one part for each replica. Thus, if there are n replicas, a timestamp t is t = <t1, …, tn> where each part is a positive integer. Since there will typically be a small number of replicas (e.g., 3 to 7), using such a timestamp is practical. The term "vector clock" was first used independently by Colin Fidge and Friedemann Mattern in 1988. 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 ,