Logical time

• All events in one process are ordered.
• The sending of a message occurs before the receiving of the message.
• Events in a distributed system are partially ordered.
• The order is called happened before.
• Logical time gives us a tool to talk about ordering without having to synchronize clocks.

Lamport clock

One counter per process:

• initially set to 0
• each process increments only its clock
• sent messages are tagged with a timestamp

Receiving a message:

• set the clock to the greatest of the internal clock and

the time stamp of the message

Vector clock

A vector with one counter per process:

• initially set to <0,….>
• each process increments only its index
• sent messages are tagged with a vector

Receiving a message:

• merge the internal clock and the time stamp of the message