It's also important for both the servers to know which one is which – if both start behaving as primary instances, you'll likely be in trouble. Choosing the primary server is called the leader election pattern. There are a few ways to do so, for example, by introducing a third-party arbiter, by racing to take exclusive ownership of a shared resource, by choosing the instance with the lowest rank, or by using algorithms such as bully election or token ring election.
Leader election is also an essential part of the next related concept: achieving consensus.
