How many users a single-small Windows azure web role can support?

Question

We are creating a simple website but with heave user logins (about 25000 concurrent users). How can I calculate no. of instances required to support it?

Answer 1

Load testing and performance testing are really the only way you're going to figure out the performance metrics and instance requirements of your app. You'll need to define "concurrent users" - does that mean 25,000 concurrent transactions, or does that simply mean 25,000 active sessions? And if the latter, how frequently does a user visit web pages (e.g. think-time between pages)? Then, there's all the other moving parts: databases, Azure storage, external web services, intra-role communication, etc. All these steps in your processing pipeline could be a bottleneck.

Don't forget SLA: Assuming you CAN support 25,000 concurrent sessions (not transactions per second), what's an acceptable round-trip time? Two seconds? Five?

When thinking about instance count, you also need to consider VM size in your equation. Depending again on your processing pipeline, you might need a medium or large VM to support specific memory requirements, for instance. You might get completely different results when testing different VM sizes.

You need to have a way of performing empirical tests that are repeatable and remove edge-case errors (for instance: running tests a minimum of 3 times to get an average; and methodically ramping up load in a well-defined way and observing results while under that load for a set amount of time to allow for the chaotic behavior of adding load to stabilize). This empirical testing includes well-crafted test plans (e.g. what pages the users will hit for given usage scenarios, including possible form data). And you'll need the proper tools for monitoring the systems under test to determine when a given load creates a "knee in the curve" (meaning you've hit a bottleneck and your performance plummets).

Final thought: Be sure your load-generation tool is not the bottleneck during the test! You might want to look into using Microsoft's load-testing solution with Visual Studio, or a cloud-based load-test solution such as Loadstorm (disclaimer: Loadstorm interviewed me about load/performance testing last year, but I don't work for them in any capacity).

EDIT June 21, 2013 Announced at TechEd 2013, Team Foundation Service will offer cloud-based load-testing, with the Preview launching June 26, coincident with the //build conference. The announcement is here.

Answer 2

No one can answer this question without a lot more information... like what technology you're using to build the website, what happens on a page load, what backend storage is used (if any), etc. It could be that for each user who logs on, you compute a million digits of pi, or it could be that for each user you serve up static content from a cache.

The best advice I have is to test your application (either in the cloud or equivalent hardware) and see how it performs.

Answer 3

It all depends on the architecture design, persistence technology and number of read/write operations you are performing per second (average/peak).

I would recommend to look into CQRS-based architectures for this kind of application. It fits cloud computing environments and allows for elastic scaling.

Answer 4

I was recently at a Cloud Summit and there were a few case studies. The one that sticks in my mind is an exam app. it has a burst load of about 80000 users over 2 hours, for which they spin up about 300 instances.

Without knowing your load profile it's hard to add more value, just keep in mind concurrent and continuous are not the same thing. Remember the Stack overflow versus Digg debacle "http://twitter.com/#!/spolsky/status/27244766467"?