We need High Availability to ensure that our mission-critical infrastructure components continue to run in case of a failure or unavailability due to maintenance or other operations. The main target of having High Availability is to minimize the impact and downtime of a service; in this case, the Orchestrator service, which you'll be delegating and supporting your operations on, will become more and more critical over time.
For the Orchestrator service to be highly available and able to keep the automation layer of your infrastructure running, you'll need to consider the following factors:
Orchestration Database: You can use either Failover Clustering (SQL Server 2008) or AlwaysOn (SQL Server 2012 and above).
Runbook server: To deploy multiple Runbook servers, you should have more than one deployed in case one of your Runbook servers fail; the Runbooks will be able to run on the next Runbook server, making that way available the execution of tasks.
Orchestrator web service...