DotSlash: Providing Dynamic Scalability to Web Applications Weibin Zhao and Henning Schulzrinne Department of Computer Science, Columbia University More information available from or by to The Problem Web hotspots, also known as flash crowds, or the Slashdot effect Short-term dramatic load spikes  “15 minutes of fame” Unacceptable to sacrifice availability by losing valuable users in the critical period, but uneconomic to invest on more powerful infrastructure which is idle most of time DotSlash Approach A self-configuring and scalable rescue system designed to counteract the Slashdot effect, which allows a web site to obtain resources dynamically and use them autonomically without any administrative intervention to set up a distributed web server system on the fly Providing dynamic scalability to web applications by addressing bottlenecks at access network bandwidth, web servers, application servers, and database servers Usage Models Focusing on small web sites Aiming at utilizing spare capacity in a mutual-aid community to handle short-term load spikes, also useful for commercial hosting companies Advantages Transparent to clients and applications: no need to change client browsers and application programs Low cost: based on existing systems, no additional hardware cost Basic Components Workload monitoring for network and CPU Rescue server discovery Request redirection via DNS round robin and HTTP redirect Dynamic virtual hosting for serving new content on the fly Adaptive rescue control Dynamic replication of application programs On-demand distributed query result caching DotSlash Implementation A prototype system for the LAMP (Linux, Apache, MySQL, and PHP) configuration A DotSlash Apache module (Mod_dots) for per request processing A DotSlash daemon (Dotsd) for rescue control A DotSlash PHP extension for dynamic script replication A cache-enhanced PHP data driver for MySQL databases for on-demand query result caching DNS domain dot-slash.net for dynamic DNS update Enhanced Service Location Protocol (SLP) for rescue server discovery BINDmSLP HTTP Shared Memory SLP DNS DotSlash Other Dotsd Apache Dotsd Mod_dots DotSlash Performance Benchmarks: httperf for static content and RUBBoS bulletin board for dynamic content Experimental setup: our local area network and PlanetLab Achieve a 10-fold improvement for static content when access network is bottleneck Completely remove the web/application server bottleneck via dynamic script replication Relieve the database server bottleneck via on- demand query result caching Improve the maximum request rate supported by a factor of 5 and 10 for the RUBBoS submission and read-only mix, respectively Basic Rescue origin.com rescue.com Client (1) (2) (3)(4) HTTP redirect alias1.rescue.com (8) Reverse proxy (5) alias1.rescue.com (6) (9) (10)(7) Dynamic DNS DNS RR origin.com DNS rescue.com DNS Cache Client2 (2) (1) (4) (3) I. HTTP redirect & cache miss II. DNS RR & cache hit Basic Architecture MySQL Apache (5) PHP(6) (1) (2) (3) Dynamic Script Replication (4) (7) Origin Server Rescue Server Client Database Server (8) Apache On-demand Query Result Caching Web/Application Server Query Result Cache Data Driver Client Database Server Database Server Web/Application Server Query Result Cache Data Driver Rescue Server Origin Server Client Rescue Protocol