The causal order is not a total order

A total order allows any two elements to be compared, so if you have two elements, you can always say which one is greater and which one is smaller. For example, natural numbers are totally ordered: if I give you any two numbers, say 5 and 13, you can tell me that 13 is greater than 5.

However, mathematical sets are not totally ordered: is {a, b} greater than {b, c}? Well, you can’t really compare them, because neither is a subset of the other. We say they are incomparable, and therefore mathematical sets are partially ordered: in some cases one set is greater than another (if one set contains all the elements of another), but in other cases they are incomparable.

The difference between a total order and a partial order is reflected in different database consistency models:

Linearizability

In a linearizable system, we have a total order of operations: if the system behaves as if there is only a single copy of the data, and every operation is atomic, this means that for any two operations we can always say which one happened first. This total ordering is illustrated as a timeline in Figure 9-4.

Causality

We said that two operations are concurrent if neither happened before the other (see “The “happens-before” relationship and concurrency” on page 186). Put another way, two events are ordered if they are causally related (one happened before the other), but they are incomparable if they are concurrent. This means that causality defines a partial order, not a total order: some operations are ordered with respect to each other, but some are incomparable.

Therefore, according to this definition, there are no concurrent operations in a line- arizable datastore: there must be a single timeline along which all operations are totally ordered. There might be several requests waiting to be handled, but the data- store ensures that every request is handled atomically at a single point in time, acting on a single copy of the data, along a single timeline, without any concurrency.

Concurrency would mean that the timeline branches and merges again—and in this case, operations on different branches are incomparable (i.e., concurrent). We saw this phenomenon in Chapter 5: for example, Figure 5-14 is not a straight-line total order, but rather a jumble of different operations going on concurrently. The arrows in the diagram indicate causal dependencies—the partial ordering of operations.

If you are familiar with distributed version control systems such as Git, their version histories are very much like the graph of causal dependencies. Often one commit happens after another, in a straight line, but sometimes you get branches (when several people concurrently work on a project), and merges are created when those concurrently created commits are combined.

 
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