We use Service Fabric to deploy stateless microservices. One of the microservices is designed as a singleton. This means it is designed to be deployed on a single node only:
InstanceCount = 1
Normally, if there is more than 1 instance and one fails, the others keep working.
But how does the single instance behave? I cannot find this scenario in the documentation. I only found out that when the node is updated and the parameter IsSingletonReplicaMoveAllowedDuringUpgrade is set to true then it can be moved to other node, but no source explicitly says what happens when the singleton fails during execution.
Does it restart automatically? And if so, then how long is the downtime?
Service Fabric will restart the service for you automatically. The time it takes to restart can depend on how loaded the machine is, how large the service is, and they type of failure, but is typically within a couple of seconds.
The amount of time it takes to restart can also depend on how the service failed. Process crashes are quicker to recover from. Machine failures or networking cuts can take longer to detect, but even in these cases SF will usually restart things within 10-30 seconds.