Built by Philadelphia-based Avencia, Walkshed combines 10 data sources chosen from the NYC Data Mine to compute and display personalized walkability maps based on a set of seventeen priorities such as proximity to grocery stores, farmer's markets, restaurants, bars, Zipcars, playgrounds, bookstores, and Wifi hotspots.
The Walkshed front end was implemented in ASP.NET and runs within Apache Tomcat. The tile serving API was written in Ruby on Rails and requests map tiles from Avencia's DecisionTree service, itself a .Net application written in C#. Here's what it looks like:
Hosting for Walkshed is handled on a dynamic hybrid model. A private VMWare server handles routine traffic. Three High-CPU Extra Large Amazon EC2 instances running Microsoft Windows are used to improve fault tolerance and to handle traffic bursts. The EC2 instances use the new boot from EBS feature to improve boot speed. They also make use of Amazon Cloudfront; an article on one of their previous successes with CloudFront can be found here.
The Varnish HTTP accelerator handles caching of frequently requested files and heatmap tiles and scaling (via load balancing). Varnish also improves reliability and availability by retrying failed requests and monitoring the health of the servers.
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