Wednesday, May 05, 2010

Lessons from Designing a Two Billion Page View Mobile Application

In his guest post on High Scalability Jamie Hall, CTO MocoSpace detailed some key architectural lessons that can be a good guide in designing large scale enterprise and web applications. Below are my views on some of them.

1. Make your boxes/servers sweat.

In my experience most of the enterprises have their server resources under utilized. This is mostly due improper or no capacity planning, over-sizing, less emphasis on monitoring the utilization etc. Surprisingly, inspite very little utilization of resources, they never achieve the performance SLAs. This brings us to the second point that Jamie mentions...

2. Understand where your bottlenecks are in each tier

There is limited understanding of the application and technologies... for example, whether an application is CPU, memory or IO intensive? Enterprises going for COTS applications like SAP very rarely understand the application architecture and internals. Without this knowledge they have to blindly depend on the vendors to do the intial sizing for them and no way ready to understand the application behaviour themselves during the application life-cycle and resolve the associated issues. Also there is limited load and performance testing carried out in house... All this results in having extra processing power in machines that needs more memory and vice versa !

3. Profile the database religiously.

Database is normally the most critical component of any business application. All performance related issues can be generally traced back to the database.  While optimizing databases for performance apart from doing database profiling, focus should also be on caching of read-only data on app layer, database sharding and alternative data stores (NoSQL key value stores).

4. Design to disable.

Hot deployment and disabling of rolled out features through configuration are very critical for the application life-cycle management. That's where evolving languages like Erlang that provide hot deployment of code are very promising inspite of the fact that there is  still some way to go for their enterprise adoption.  

5. Communicate synchronously only when absolutely necessary

This is the key to identify failures and error conditions, and thereby easily manage distributed applications. Yet I see people find ways implement synchronous interfaces between applications.

6. Think about monitoring during design, not after.

Do have your applications designed for monitoring. Identify the KPIs that needs monitoring. Otherwise you would have no way to troubleshoot when you end up with issues in production.

7. Distributed sessions can be a lot of overhead.  

Inspite of the fact that distributed session management feature using technologies like application clustering are common with all server side applications and tools, this is always a bottleneck for scalability particularly when you want to scale-out. If you can design applications where you can have stateless sessions or the session info stored in the client to be passed with every request, life can become very easy. Jamie also advises to use sticky sessions that are now a days available with all loadbalancing appliances.

8. N+1 design.

Have N+1 design rather than clustering and local failover for the web and app servers.

Finally, a few other things that could be important are..

a. Keep it simple... Use the best tool/framework for your requirement that has a low CPU and memory footprint...  You can compromise some of the non-realistic non-functional requirements to keep it simple.

b. Design your application to manage failure rather than to avoid failure.

c. Try and leverage client side processing as much as possible keeping in mind the browser or other client capabilities and the client devices to be used... Ofcourse for mobile applications client side processing should be kept to minimum.