What are the scalability considerations? What is Horizontal Scalability and Vertical Scalability?

Scalability consideration are...

• Adding new applications or customers to the system.
  
• Horizontal Scalability -- AKA "rack density". Throw hardware at the problem. Typically easier and affordable than most forms of vertical scalability.
  
• Vertical Scalability -- The ability to increase the capacity of existing hardware or software by adding resources - for example, adding processing power to a server to make it faster. nHardware/software scalability metrics Here are some metrics to measure the levels of resource saturation:
  * CPU utilization — Number of users supported versus CPU utilization
  * Memory — Number of users supported versus available memory

* Disk I/O — Number of users supported versus number of disks

* Network I/O — Number of users supported versus wire speed of network