what are some good "load balancing issues" to know ?

Question

Hey there guys, I am a recent grad, and looking at a couple jobs I am applying for I see that I need to know things like runtime complexity (straight forward enough), caching (memcached!), and load balancing issues
 (no idea on this!!)

So, what kind of load balancing issues and solutions should I try to learn about, or at least be vaguely familiar with for .net or java jobs ?

Googling around gives me things like network load balancing, but wouldn't that usually not be adminstrated by a software developer?

Answer 1

One thing I can think of is session management. By default, whenever you get a session ID, that session ID points to some in-memory data on the server. However, when you use load-balacing, there are multiple servers. What happens when data is stored in the session on machine 1, but for the next request the user is redirected to machine 2? His session data would be lost.

So, you'll have to make sure that either the user gets back to the same machine for every concurrent request ('sticky connection') or you do not use in-proc session state, but out-of-proc session state, where session data is stored in, for example, a database.

Answer 2

There is a concept of load distribution where requests are sprayed across a number of servers (usually with session affinity). Here there is no feedback on how busy any particular server may be, we just rely on statistical sharing of the load. You could view the WebSphere Http plugin in WAS ND as doing this. It actually works pretty well even for substantial web sites

Load balancing tries to be cleverer than that. Where some feedback on the relative load of the servers determines where new requests go. (even then session affinity tends to be treated as higher priority than balancing load). The WebSphere On Demand Router that was originally delivered in XD does this. If you read this article you will see the kind of algorithms used.

You can achieve balancing with network spraying devices, they could consult "agents" running in the servers which give feedback to the sprayer to give a basis for decisions where request should go. Hence even this Hardware-based approach can have a Software element. See Dynamic Feedback Protocol

Answer 3

network combinatorics, max- flow min-cut theorems and their use