If I'm running multiple web server instances can a client application (like a user using a web browser) be using the different instances or would they be routed to the same instance every time? Let's say they duplicate a tab or open a new tab are those tabs still using the same instance too?
This would be in Azure with IIS/ASP.NET.
When you are using load balance in any environment, you almost always have the option to set session affinity. It means basically, a client who is directed to server 1 on his first request will always be routed to the same server. Azure does provide this flexibility too without question. Here is the documentation with some details on how to do that configuration.
There are a couple of ways how you could configure session affinity. One prominent way is by source IP. So, using a different tab or a different browser instance will not make any difference. Requests from a client machine will always carry the same IP address and hence will go to the same server.
Here is the Powershell sample to set source IP based affinity:
Set-AzureLoadBalancedEndpoint -ServiceName MyService -LBSetName LBSet1 -Protocol TCP -LocalPort 80 -ProbeProtocolTCP -ProbePort 8080 –LoadBalancerDistribution sourceIP
Here is some detail on a more specific scenario that happens when users access a load balanced site from behind a company;s firewall.
I am using ASP state server for session management in my asp website. In one of my server I have two asp website. For one website i used tcpip=loopback:42424 and it works. For another website i wanted to use some other ports. I tried different ports in the range 30000 to 50000 and it does not work and get error like
Unable to make the session state request to the session state server. Please ensure that the ASP.NET State service is started and that the client and server ports are the same. If the server is on a remote machine, please ensure that it accepts remote requests by checking the value of HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\aspnet_state\Parameters\AllowRemoteConnection. If the server is on the local machine, and if the before mentioned registry value does not exist or is set to 0, then the state server connection string must use either 'localhost' or '127.0.0.1' as the server name
But in my case asp state server is uses port of local machine only. So now how can i configure asp website to use different port for asp state server?
I'm writing a website that is going to start using a load balancer and I'm trying to wrap my head around it.
Does IIS just do all the balancing for you?
Do you have a separate web layer that sits on the distributed server that does some work before sending to the sub server, like auth or other work?
It seems like a lot of the articles I keep reading don't really give me a straight answer, or I'm just not understanding them correctly, I'd like to get my head around how true load balancing works from a techincal side, and if anyone has any code to share that would also be nice.
I understand caching is gonna be a problem but that's a different topic, session as well.
IIS do not have a load balancer by default but you can use at least two Microsoft technologies:
Application Request Routing that integrates with IIS, there you should ideally have a separate web layer to do routing work,
Network Load Balancing that is integrated with Microsoft Windows Server, there you can join existing servers into NLB cluster.
Both of those technologies do not require any code per se, it a matter of the infrastructure. But you must of course remember about load balanced environment during development. For example, to make a web sites truly balanced, they should be stateless. Otherwise you will have to provide so called stickiness between client and the server, so the same client will be connecting always to the same server.
To make service stateless, do not persist any state (Session, for example, in case of ASP.NET website) on the server but on external server shared between all servers in the farm. So it is common for example to use external ASP.NET Session server (StateServer or SQLServer modes) for all sites in the cluster.
EDIT:
Just to clarify a few things, a few words about both mentioned technologies:
NLB works on network level (as a networking driver in fact), so without any knowledge about applications used. You create so called clusters consisting of a few machines/servers and expose them as a single IP address. Then another machine can use this IP as any other IP, but connections will be routed to the one of the cluster's machines automatically. A cluster is configured on each server, there is no external, additional routing machine. Depending on the clusters settings, as we have already mentioned, a stickiness can be enabled or disabled (called here a Single or None Affinity). There is also a Load weight parameter, so you can set weighed load distribution, sending more connections to the fastest machine for example. But this parameter is static, it can't be dynamically based on network, CPU or any other usage. In fact NLB does not care if target application is even running, it just route network traffic to the selected machine. But it notices servers went offline, so there will be no routing there. The advantages of NLB is that it is quite lightweight and requires no additional machines.
ARR is much more sophisticated, it is built as a module on top of IIS and is designed to make the routing decisions at application level. Network load balancing is only one of its features as it is a more complete, routing solution. It has "rule-based routing, client and host name affinity, load balancing of HTTP server requests, and distributed disk caching" as Microsoft states. You create there Server Farms with many options like load balance algorithm, load distribution and client stickiness. You can define health tests and routing rules to forward request to other servers. Disadvantage of all of it is that there should be a dedicated machine where ARR is installed, so it takes more resources (and costs).
NLB & ARR - as using a single ARR machine can be the single point of failure, Microsoft states that it is worth consideration to create a NLB cluster of ARR machines.
Does IIS just do all the balancing for you?
Yes,if you configure Application Request Routing:
Do you have a separate web layer that sits on the distributed server
Yes.
that does some work before sending to the sub server, like auth or other work?
No, ARR is pretty 'dumb':
IIS ARR doesn't provide any pre-authentication. If pre-auth is a requirement then you can look at Web Application Proxy (WAP) which is available in Windows Server 2012 R2.
It just acts as a transparent proxy that accepts and forwards requests, while adding some caching when configured.
For authentication you can look at Windows Server 2012's Web Application Proxy.
Some tips, and perhaps items to get yourself fully acquainted with:
ARR as all the above answers above state is a "proxy" that handles the traffic from your users to your servers.
You can handle State as Konrad points out, or you can have ARR do "sticky" sessions (ensure that a client always goes to "this server" - presumably the server that maintains state for that specific client). See the discussion/comments on that answer - it's great.
I haven't worn an IT/server hat for so long and frankly haven't touched clustering hands on (always "handled for me automagically" by some provider), so I did ask this question from our host, "what/how is replication among our cluster/farm" done?" - The question covers things like
I'm only working/setting things on 1 server, does that get replicated across X VMs in our cluster/farm? How long?
What about dynamically generated,code and/or user generated files (file system)? If it's on VM1's file system, and I have 10 load balanced VMs, and the client can hit any one of them at any time, then...?
What about encryption? e.g. if you use DPAPI to encrypt web.config stuff (e.g.db conn strings/sections), what is the impact of that (because it's based on machine key, and well, the obvious thing is now you have machine(s) or VM(s). RSA re-write....?
SSL: ARR can handle this for you as well, and that's great! But as with all power, comes a "con" - if you check/validate in your code, e.g. HttpRequest.IsSecureConnection, well, it'll always be false. Your servers/VMs don't have the cert, ARR does. The encrypted conn is between client and ARR. ARR to your servers/VMs isn't. As the link explains, if you prefer it the other way around (no offloading), you can...but that means all your servers/VMs should then have a cert (and how that pertains to "replication" above starts popping in your head).
Not meant to be comprehensive, just listing things out from memory...Hth
Say web server A server clock is set to PST, web server B server clock is set to EST, and the state server clock is set to PST. Does this matter? I know it's critical for AppFabric cluster servers to have clock alignment, but what about client web servers to the ASP.NET State Server?
Reviewing the documentation out there I'm not finding anything that confirms or denies this in the Microsoft recommended configuration. This would be something you could test quite easily with your configuration, but more than likely, the time on the state server is the one that will be used.
This is a fairly basic question about state server but assume there are 2 servers behind a load balancer. How do I configure the session state server?
So, I have machine1 and machine2. I would assume that I would need to install the state server on 1 machine only and then use the internal IP to refer to that machine. Is this correct? As oppose too, I would not install state server on both machines.
In your scenario (and most webfarm scenarios), a single state server is right.
You could refer to it by the internal IP or setup a DNS entry for the IP on the internal network and refer to it using that.
Single state server is mad as you have no fault tolerance, if it does down its game over. You need a distibuted state server stored on both servers.