Validation phase - System Testing - Performance Testing - Response time

Response time is the delay experienced when the request is made to the server and the server's response to the client is received. It is usually measured in units of time, such as seconds or milliseconds. Generally speaking, response time increases as the inverse of un-utilized capacity. It increases slowly at low levels of user load but increases rapidly as capacity is utilized. The sudden increase in response time is often caused by the maximum utilization of one or more system resources. Any time spent in a queue naturally adds extra wait time to the overall response time.
Response time in a typical web farm, response time can be divided into many segments and categorize these segments into two major types: network response time and application response time. Network response time refers to the time it takes for data to travel from one server to another. Application response time is the time required for data to be processed within a server.

Total Response Time = (N1+N2+N3+N4) + (A1+A2+A3)
where Nx represents the network response time and Ax represents the application response time.

In general, the response time is mainly constrained by N1 and N4. This response time represents the method your clients are using to access the Internet. To reduce these network response time, one common solution is to move the servers and/or web contents closer to the clients. This can be achieved by hosting your farm of servers or replicating your web contents with major Internet hosting providers who have high speed connections to major public and private Internet exchange points.

Reducing application response time is an art form unto itself because the complexity of server applications can make analyzing performance data and performance tuning quite challenging. Typically, multiple software components interact on the server to service a given request. Response time can be introduced by any of the components. This problem can be approached by:
- The application design should minimize round trips wherever possible.
- Optimize server components to improve performance for your configuration.
- Look for connection among threads or components competing for common resources.
- Finally, to increase capacity, you amy want to upgrade the server hardware.