When talking about synchronous replication, there is one phenomenon that must not be left out. Imagine we have a two-node cluster replicating synchronously. What happens if the slave dies? The answer is that the master cannot distinguish between a slow and a dead slave easily, so it will start to wait for the slave to come back.

At first glance, this looks like nonsense, but if you think about it more deeply, you will figure out that it is actually the only correct thing to do. If somebody decides to go for synchronous replication, the data in the system must be worth something so it must not be at risk. It is better to refuse data and cry out to the end user than to risk data and silently ignore high durability requirements.

If you decide to use synchronous replication, you must consider using at least three nodes in your cluster. Otherwise, it will be very risky and you cannot afford losing a single node without facing significant downtime or risk of data loss.