Vector clock

A vector clock is a <a href="/facts/Data_structure/yNsNvL1I">data structure</a> used for determining the <a href="/facts/Partial_ordering/qb4a3FAX">partial ordering</a> of events in a <a href="/facts/Distributed_system/o5nyhqTF">distributed system</a> and detecting <a href="/facts/Causality/Swh496X7">causality</a> violations. Just as in <a href="/facts/Lamport_timestamp/Dpxw6Ia4">Lamport timestamps</a>, inter-process messages contain the state of the sending process's <a href="/facts/Logical_clock/szv0m0lk">logical clock</a>. A vector clock of a system of N processes is an <a href="/facts/Array_data_structure/BKGvF8AM">array</a>/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 
 
 
 
 V
 
 C
 
 i
 
 
 
 
 {\displaystyle VC_{i}}
 
 as the vector clock maintained by process 
 
 
 
 i
 
 
 {\displaystyle i}
 
, the clock updates proceed as follows:

<ul><li>Initially all clocks are zero.</li>
<li>Each time a process experiences an internal event, it increments its own <a href="/facts/Logical_clock/szv0m0lk">logical clock</a> in the vector by one. For instance, upon an event at process 
 
 
 
 i
 
 
 {\displaystyle i}
 
, it updates 
 
 
 
 V
 
 C
 
 i
 
 
 [
 i
 ]
 ←
 V
 
 C
 
 i
 
 
 [
 i
 ]
 +
 1
 
 
 {\displaystyle VC_{i}[i]\leftarrow VC_{i}[i]+1}
 
.</li>
<li>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.</li>
<li>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 
 
 
 
 
 P
 
 i
 
 
 
 
 {\displaystyle P_{i}}
 
 receives a message 
 
 
 
 (
 m
 ,
 V
 
 C
 
 j
 
 
 )
 
 
 {\displaystyle (m,VC_{j})}
 
 from 
 
 
 
 
 P
 
 j
 
 
 
 
 {\displaystyle P_{j}}
 
, it first increments its own logical clock in the vector by one 
 
 
 
 V
 
 C
 
 i
 
 
 [
 i
 ]
 ←
 V
 
 C
 
 i
 
 
 [
 i
 ]
 +
 1
 
 
 {\displaystyle VC_{i}[i]\leftarrow VC_{i}[i]+1}
 
 and then updates its entire vector by setting 
 
 
 
 V
 
 C
 
 i
 
 
 [
 k
 ]
 ←
 max
 (
 V
 
 C
 
 i
 
 
 [
 k
 ]
 ,
 V
 
 C
 
 j
 
 
 [
 k
 ]
 )
 ,
 ∀
 k
 
 
 {\displaystyle VC_{i}[k]\leftarrow \max(VC_{i}[k],VC_{j}[k]),\forall k}
 
.</li></ul>

Vector clock open-in-new

Vector clock