More into the basics

We were talking about throughput in terms of the store. Taking that forward to the application, throughput will be the total number of bytes sent by the application in a second, and like the store, irrespective of how many requests are thrown at the server, this throughput will not increase beyond a certain limit.

Now, that brings us to the topic of number of users on an application. We see two different terms in load testing, concurrent users and active users. Are they the same? Taking the store example, we have some customers browsing the shelves, and others interacting with the shop personnel. It is evident that all these users are using up the shop floor, although only some of them are interacting with the resources, here the shop personnel. These number of users interacting with the shop personnel will be the active customers, and all the customers using the floor (that inlcudes the active customers as well) are termed concurrent customers.

Concurrent users: Concurrent users refer to users who have logged into the system irrespective of whether or not they are doing anything on the system.

Active users: Active users are a subset of concurrent users who are logged in and are performing some action at a specific point in time.

Note: These definitions vary across the world, and when a bunch of people get together to talk about these terms, it is prudent to agree on what the terms mean.

Let us now relate response times, throughput and users in layman terms. In the store, the person in front of the queue has a response time of 1 minute. For the second person in the queue, to complete his purchase, he has to wait for the customer in the front to complete his purchase (1 min) and then purchase his item (1 min). So that adds up to 2 minutes for the second person in the queue. Likewise, for the 5th person in the queue, it will take 5 minutes before his purchase is complete.

The same logic applies to a website as well. Let us assume that the website can process 100 requests per second (this will be the throughput of the website, in case you haven’t yet grasped the concept), the 101-st request will have to wait one of the earlier requests is processed, so the response time for this request will 2 seconds. That boils down to the conclusion that the response time of a transaction is the sum of waiting time and the request processing time.

This is too much of theory, so let’s see a few graphs in the next part that will illustrate how throughput looks and how response times look with increase in the number of users.