Work Load management – How you distribute the load and manage it.
High Availability – How much time your solution/application is available.
Say you have an application. You define that this application must be available 364 days in a year. That is application’s availability. How do you achieve this? Create a failover system, backup plan, monitoring etc ..
Now you need the response time to be 1-2 seconds. This is not covered in above statement of availability. How do you achieve this then? you take multiple servers and distribute the incoming load among them to maintain the response time you need.
uptime vs Availability
Take an example of your laptop. When you open too many application it will be slow. Open few more applications .. what happens? It just hangs but still your laptop is ON. Do you consider it a workable environment ?
A system is running doesn’t mean it is meeting the defined responsetimes. When you have more users on the application .. you may see a very slow response or it might hung. So uptime and availability are not same.
So, How is availability defined?
The percentage of time, when a system will be able to fully meet the load demand. This means it is the amount of time when your application is meeting responsetimes specified.