As we saw in the previous lesson, primary-backup replication is a technique that is easy to implement and operate. It can easily support transactions and hide the distributed nature of the underlying system, i.e., when using synchronous replication.

However, primary-backup replication has some limitations in terms of performance, scalability, and availability.

As we’ve already discussed, there are many applications where availability and performance are much more important than data consistency or transactional semantics.

A frequently cited example is that of an e-commerce shopping cart, where the most important thing is for customers to be able to access their cart at all times and add items quickly and easily. It is acceptable to compromise consistency to achieve this, as long as there is data reconciliation at some point. For instance, if two replicas diverge because of intermittent failures, the customer can still resolve conflicts during the checkout process.

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