understanding load balancing in asp.net

Question

I'm writing a website that is going to start using a load balancer and I'm trying to wrap my head around it.

1. Does IIS just do all the balancing for you?
2. 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.

Answer 1

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.