Why do you need load balancing?
Simple answer: Scalability and Redundancy.
If your application becomes busy, resource limits, such as bandwidth, cpu, memory, disk space, disk I/O, and more may reach its limits. In order to remedy such problem, you have two options: scale up, or scale out. Load balancing is a scale out technique. Rather than increasing server resources, you add cost effective, commodity servers, creating a “cluster” of servers that perform the same task. Scaling out is more cost effective, because commodity level hardware provides the most bang for the buck. High end super computers come at a premium, and can be avoided in many cases.Redundancy
Servers crash, this is the rule, not the exception. Your architecture should be devised in a way to reduce or eliminate single points of failure (SPOF). Load balancing a cluster of servers that perform the same role provides room for a server to be taken out manually for maintenance tasks, without taking down the system. You can also withstand a server crashing. This is called High Availability, or HA for short. Load balancing is a tactic that assists with High Availability, but is not High Availability by itself. To achieve high availability, you need automated monitoring that checks the status of the applications in your cluster, and automates taking servers out of rotation, in response to failure detected. These tools are often bundled into Load Balancing software and appliances, but sometimes need to be programmed independently.How to perform load balancing?
There are 3 well known ways:
- DNS based
- Hardware based
- Software based
This is also known as round robin DNS. You can inject multiple A records for the same hostname. This creates a random distribution – requests for the hostname will receive the list in a random order. If you wish to weight it (say serverA can take 2x the number of requests that serverB can), you can simply add more A records for a particular IP.Hardware based
There are many commercial vendors out there selling appliances to perform load balancing.Software based
This is where it gets fun, if you’re a technology enthusiast. If your budget doesn’t allow a load balancing appliance, or if you just like doing things yourself, software based load balancing is for you. You can turn a Linux server into your own load balancing appliance. Presumably, you could also use a Windows server, maybe even a Mac, but this article doesn’t cover those. For RHEL based, the “piranha” package provides Linux Virtual Server (LVS) and piranha (an LVS management tool – web based gui). Just “yum install piranha” and you’ll have everything you need to get started. Other softwares include BalanceNG (commercial) and a basic freeware counterpart balance.
This was super simple to use. Just download, run the program. There are a few basic input parameters, and you can be load balancing in no time. This is a no frills binary program. There are no configuration files, no startup/shutdown programs, no logging or reporting. But it does have a nifty console that you can get runtime statistics from. You could create your own tools around “balance” to monitor and gather statistics.
LVS and piranha on RHEL or CentOS
piranha is a gui that makes configuring Linux Virtual Server (LVS) easy. Here are some of the virtual server scheduling features:
- Round robin
- Weighted least-connections
- Weighted round robin
- Locality-Based Least-Connection Scheduling
- Locality-Based Least-Connection Scheduling (R)
- Destination Hash Scheduling
- Source Hash Scheduling