In high availability, there are several configurations that we can use. Our primary server is called the hot server, as it can process each and every request coming in. Our secondary server can be in any of the following states:

• Cold
• Warm
• Hot

A secondary cold server is a server that is there just in case the primary server goes offline, without any expectation of it holding the data and state that the primary server had.

A secondary warm server receives periodic updates of data from the primary server, but typically, it is not entirely up to date with the primary server. It can be used for some non-real-time analytics reporting to offload the main server, but typically, it will not be able to pick up the transactional load of the primary server if it goes down.

A secondary hot server always keeps an up-to-date copy of the data and state from the primary server. It usually waits in a hot standby state, ready to take over when the primary server goes down.

MongoDB has both the hot and warm server types of functionality, as we will explore in the following sections.