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Lamport timestamps

Although the three sequence number generators just described are inconsistent with causality, there is actually a simple method for generating sequence numbers that is consistent with causality. It is called a Lamport timestamp, proposed in 1978 by Leslie Lamport [56], in what is now one of the most-cited papers in the field of distributed systems.

The use of Lamport timestamps is illustrated in Figure 9-8. Each node has a unique identifier, and each node keeps a counter of the number of operations it has processed. The Lamport timestamp is then simply a pair of (counter, node ID). Two nodes may sometimes have the same counter value, but by including the node ID in the timestamp, each timestamp is made unique.

Lamport timestamps provide a total ordering consistent with causality

Figure 9-8. Lamport timestamps provide a total ordering consistent with causality.

A Lamport timestamp bears no relationship to a physical time-of-day clock, but it provides total ordering: if you have two timestamps, the one with a greater counter value is the greater timestamp; if the counter values are the same, the one with the greater node ID is the greater timestamp.

So far this description is essentially the same as the even/odd counters described in the last section. The key idea about Lamport timestamps, which makes them consistent with causality, is the following: every node and every client keeps track of the maximum counter value it has seen so far, and includes that maximum on every request. When a node receives a request or response with a maximum counter value greater than its own counter value, it immediately increases its own counter to that maximum.

This is shown in Figure 9-8, where client A receives a counter value of 5 from node 2, and then sends that maximum of 5 to node 1. At that time, node 1’s counter was only 1, but it was immediately moved forward to 5, so the next operation had an incremented counter value of 6.

As long as the maximum counter value is carried along with every operation, this scheme ensures that the ordering from the Lamport timestamps is consistent with causality, because every causal dependency results in an increased timestamp.

Lamport timestamps are sometimes confused with version vectors, which we saw in “Detecting Concurrent Writes” on page 184. Although there are some similarities, they have a different purpose: version vectors can distinguish whether two operations are concurrent or whether one is causally dependent on the other, whereas Lamport timestamps always enforce a total ordering. From the total ordering of Lamport time?stamps, you cannot tell whether two operations are concurrent or whether they are causally dependent. The advantage of Lamport timestamps over version vectors is that they are more compact.

 
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