503 Service Unavailable

This error is also commonly served when a site has more traffic than it can handle - a problem with poor website scalability.

Website scalability is closely linked to these factors:

Effective use of caching : Having a properly installed and configured caching solution is key to a scalable website. Magento, a leading ecommerce platform, now strongly recommends merchants utilize Varnish Cache, an open-source caching solution that can be installed on any website. Varnish Cache or another similar tool will reduce the number of requests that go back to your web server by serving users with copies of your website directly from the cache server. This both increases scalability and reduces your server costs at the same time, what we like to consider a win-win situation in the land of ecommerce sites!
Effective use of a CDN : Utilizing a Content Delivery Network that includes a caching solution will further improve your website scalability, by hosting a caching solution on a network of globally distributed servers. This means that an increase in website traffic can be absorbed by a combination of cached content and servers that are closer to your customers.