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The "happens-before" relationship and concurrency

How do we decide whether two operations are concurrent or not? To develop an intuition, let’s look at some examples:

• In Figure 5-9, the two writes are not concurrent: A’s insert happens before B’s increment, because the value incremented by B is the value inserted by A. In other words, B’s operation builds upon A’s operation, so B’s operation must have happened later. We also say that B is causally dependent on A.

• On the other hand, the two writes in Figure 5-12 are concurrent: when each client starts the operation, it does not know that another client is also performing an operation on the same key. Thus, there is no causal dependency between the operations.

An operation A happens before another operation B if B knows about A, or depends on A, or builds upon A in some way. Whether one operation happens before another operation is the key to defining what concurrency means. In fact, we can simply say that two operations are concurrent if neither happens before the other (i.e., neither knows about the other) [54].

Thus, whenever you have two operations A and B, there are three possibilities: either A happened before B, or B happened before A, or A and B are concurrent. What we need is an algorithm to tell us whether two operations are concurrent or not. If one operation happened before another, the later operation should overwrite the earlier operation, but if the operations are concurrent, we have a conflict that needs to be resolved.

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