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Linearizability is stronger than causal consistency

So what is the relationship between the causal order and linearizability? The answer is that linearizability implies causality: any system that is linearizable will preserve causality correctly [7]. In particular, if there are multiple communication channels in a system (such as the message queue and the file storage service in Figure 9-5), lineariz- ability ensures that causality is automatically preserved without the system having to do anything special (such as passing around timestamps between different components).

The fact that linearizability ensures causality is what makes linearizable systems simple to understand and appealing. However, as discussed in “The Cost of Linearizability” on page 335, making a system linearizable can harm its performance and availability, especially if the system has significant network delays (for example, if it’s geographically distributed). For this reason, some distributed data systems have abandoned linearizability, which allows them to achieve better performance but can make them difficult to work with.

The good news is that a middle ground is possible. Linearizability is not the only way of preserving causality—there are other ways too. A system can be causally consistent without incurring the performance hit of making it linearizable (in particular, the CAP theorem does not apply). In fact, causal consistency is the strongest possible consistency model that does not slow down due to network delays, and remains available in the face of network failures [2, 42].

In many cases, systems that appear to require linearizability in fact only really require causal consistency, which can be implemented more efficiently. Based on this observation, researchers are exploring new kinds of databases that preserve causality, with performance and availability characteristics that are similar to those of eventually consistent systems [49, 50, 51].

As this research is quite recent, not much of it has yet made its way into production systems, and there are still challenges to be overcome [52, 53]. However, it is a promising direction for future systems.

 
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