Lamport Timestamps

The algorithm of Lamport timestamps is a simple algorithm used to determine the order of events in a distributed computer system. As different nodes or processes will typically not be perfectly synchronized, this algorithm is used to provide a partial ordering of events with minimal overhead, and conceptually provide a starting point for the more advanced vector clock method.

Distributed algorithms such as resource synchronization often depend on some method of ordering events to function. For example, consider a system with two processes and a disk. The processes send messages to each other, and also send messages to the disk requesting access. The disk grants access in the order the messages were sent. Now, imagine process 1 sends a message to the disk asking for access to write, and then sends a message to process 2 asking it to read. Process 2 receives the message, and as a result sends its own message to the disk. Now, due to some timing delay, the disk receives both messages at the same time: how does it determine which message happened-before the other? ( happens-before if one can get from to by a sequence of moves of two types: moving forward while remaining in the same process, and following a message from its sending to its reception.) A logical clock algorithm provides a mechanism to determine facts about the order of such events.

Leslie Lamport invented a simple mechanism by which the happened-before ordering can be captured numerically. A Lamport logical clock is a incrementing software counter maintained in each process.

It follows some simple rules:

1. A process increments its counter before each event in that process;
2. When a process sends a message, it includes its counter value with the message;
3. On receiving a message, the receiver process sets its counter to be greater than the maximum of its own value and the received value before it considers the message received.

Conceptually, this logical clock can be thought of as a clock that only has meaning in relation to messages moving between processes. When a process receives a message, it resynchronizes its logical clock with that sender.