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Q.I have 2 servers. So i ll have 2 FD(FD0, FD1) & 2 UD(UD0, UD1). What if UD0 is down & at the same time the FD1 goes down due some reason. So what will happen ?

Nilesh Deokar
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Pratik ranjan pal
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2 Answers2

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If I co-relate the actual question and diagram in the Ashok's answer,

There are two scenarios here,

1) Update Domains will be down only if there is any updates going on(it can be planned or unplanned). So in case if FD1 goes down there won't be any updates happening in UD0 as because there are no other servers to take the load. Till FD1 comes online UD0 will have to wait to do the update.

2) In case any updates going on in UD1 or UD2 definitely UD0 will be running and serving the load/handling the traffic. At that time, if FD0 goes down, then your app will be down. To overcome this scenario you should have 3 FDs.

karthikeayan
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  • in your second example, having an update on UD1 or UD2 means that both UD0 and the other UD are still available. Let's say that UD1 is the one undergoing the outage. In that case UD0 and UD2 are available, and in that case FD0 going down only means that UD0 is out, so you're still left with UD2, and your application won't be down – amine Dec 15 '20 at 14:34
  • I would say that what shows the added values of having 3 FDs is if there's an update on UD0, and FD1 goes down, that means that both UD1 and UD2 are out too, so you're left with nothing up, and your application is not available anymore – amine Dec 15 '20 at 14:39
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Very simple: both of your servers would be out.

It's not even related to Azure here, even if you have 2 machines, hosted in two locations, by 2 different providers, and the first is down for maintenance, and the second one crashes, you'll end up with everything down. So, fault domains and update domains will not protect you from a full outage in such an event.

This is how FDs and UDs are useful in the case of two machines:

  • Having each machine in its own FD and its own UD allows you to avoid a full outage in the event of an unexpected outage in one FD and avoid full outage in the event of an update
  • Having both machines in the same FD but in different UDs allows you to avoid full outage during update operations, but does not prevent full outage in the event of an unexpected FD outage
  • Having both machines in the same UD, but in different FDs (yes it's possible) allows you to avoid full outage in the event of an unexpected outage in one FD, but you'll have full outage for each update operation
  • Having both machines in the same FD and in the same UD would not protect you from anything, you'll have a full outage for both unexpected FD outages and update outages
amine
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