What Network Load balancer will you use?
There are multiple ways to achieve NLB.
・Windows Network Load Balancing (Windows Role and features)
・Heard ware Network Load Balancer
・Cloud vendor's NLB
I believe it depends on which NLB will you use.
This KB explains HA options for AF.
KB00634 - High Availability (HA) options for PI Asset Framework (PI AF)
Microsoft documentation is also helpful.
>I mean what's going on when PI AF Application service going down or unavailable on one of AF servers and what Network Load Balancer does in this case?
By default, Windows Network Load Balancing does not monitor service level. It monitors NIC level availability.
So, if the one of the machine's AF Service is shutdown, it could not detect it and there are possibilities that the Windows Network Load Balancing passes the access to the machine that AF service is stopping.
If you want to avoid it, you need to implement it by yourself.
There should be Network Load Balancers vendors that provide NLB with monitoring service level health function. I am not familiar with it though.
what is then benefit of HA option for AF Application server on Load Balancer?
KB00634 shows Pros as "Allows for load balancing across multiple servers using Network Load Balancing (NLB) clusters or round-robin D"
NLB is obviously High Availability option. Since OSIsoft does not recommend to use AF Collective, NLB is a good option.
If you want to monitor service level availability, then you need to choose good vendors/ or implement service monitor architecture.
to be honest this still doesn't explain either mechanism of HA or benefit.
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NLB allows you to have multiple access points for AF Services.
If there are 2 AF service machines in the NLB, AF client tool goes to either one of the machine.
If one of the machine is shut down, NLB does not pass the access to the machine that is not running.
That is why AF client tool can connect to AF when one of the machine is down.
Also the other benefit is that if there are a lot of clients, load distribution can be achieved.
If you don't understand well, could you explain which part is unclear?
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As a very simple example, if Windows needs to be updated on one of the AF Servers, or one of the AF Servers needs to be rebooted for other reasons, having an NLB setup avoids having any downtime for your users. Of course there are other benefits such as Network load balancing as Kenji Hashimoto discussed above.
The mechanisms for NLB is not OSIsoft technology. Therefore, to understand the specific mechanism you would need to contact the specific vendor. Of course, if you have questions on how a specific NLB will interact with the AF Server let us know and we will explain in more details.
From my personal experience, I've seen most customers use an F5 Network Load Balancer.