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Applications are often measured by two important parameters like performance and scalability.

When asked about what is scalability, many think it is about improving the performance. But

that is not the case. Performance is what an individual user experiences but scalability is the

number of users who happen to experience that performance.

Application performance depends on different parameters. For any application, the response

time is considered the most important performance measure. Apart from this, the elapsed time

for completing all of the activities determines the overall performance of any application.

Scalability refers to the number of concurrent users who have a positive (satisfactory)

experience. If the application can maintain its performance with growing number of concurrent

users then it is said that it scales well. Suppose, with 1000 concurrent users the average response

time of an application is 1 second but as the number of concurrent users grows to 2000, the average

response time becomes 2 seconds; then the application is said to be not scaling well. In an ideal

scalable system, the response time should not increase with the number of the concurrent users.

An application may scale well without performing well. This may happen when the

application handles many concurrent users with same performance but that performance is

poor irrespective of the number of concurrent users.

Again, the performance tends to degrade with growing number of concurrent users. So

there is always a clash between performance and scalability. This problem is settled by setting a

threshold value which indicates the limit of scalability. The limit is determined by the maximum

number of concurrent users for whom an application may perform consistently. Beyond this

limit, the performance would degrade.

Performance and scalability are different issues and both are important for a computing system.

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