Now you know how to set up a basic AD synchronization without considering the availability infrastructure, we'll look at how you can achieve Azure AD synchronization in a high availability environment.
The first thing you should know is that the Azure AD Connect tool cannot be clustered, so you need to use staged mode to implement it in passive mode. In the case of a failure, you have three days to recover the AD Connect Server. This can be either done through recovering from backup redeployment or switching to the staged, passive AD Connect Server.
So, for placement in either high availability or non-high availability infrastructures, it is recommended that you place systems that are involved in the synchronization of Azure VMs. This is so that you do not transfer as much data through the open internet, thus improves the performance of communication and identity token exchanges between Microsoft Cloud Services. More details...