ACDN: A CDN for Applications Pradnya Karbhari Michael Rabinovich Zhen Xiao Fred Douglis AT&T Labs -- Research

2 Content Distribution Networks Comprise surrogate servers that serve content on behalf of origin servers Can serve static pages, streaming content, applications Static pages can be served by client-side caches as well [Gadde et al. 2000] Same for streaming content Unique CDN value: Distributing applications Retaining control over content

3 ACDN Components Replication framework Dynamically install and uninstall applications based on demand Maintain replica consistency Content placement algorithms Request distribution algorithms

4 Replication Framework Inspired by work on software distribution (e.g., Marimba) Metafile for each application containing: A list of time-stamped files (data and executable files) An initialization script (or a pointer to it) FILE /home/applications/mapping/query_engine.cgi 1999.apr.14.08:46:12 FILE /home/applications/mapping/map_database 2000.oct.15.13:15:59 FILE /home/applications/mapping/user_preferences 2001.jan.30.18:00:05 SCRIPT mkdir /home/applications/mapping/access_stats setenv ACCESS_DIRECTORY /home/applications/mapping/access_stats ENDSCRIPT

5 Application Metafile A metafile is a simple static Web page Having a metafile is sufficient to deploy the application Having the current metafile is sufficient to bring the application replica up-to-date. Consistency of application replicas => consistency of cached copies of the metafile

6 Framework Tasks Creating a replica Obtaining a metafile Obtaining a tar file with all files listed in the metafile Running the initialization script Updating a replica Obtaining the diff of metafiles Obtaining files that are new Deleting a replica Retaining the deleted replica for some time to process residual requests before physical deletion

7 Algorithms Request distribution algorithm Load balancing among servers that have the requested application Proximity of servers to requesting clients Content placement algorithm Distributed decision - at each CDN server Total load on the server Percentage of requests from other regions Prediction of load after replication or migration Placement costs: bytes transferred during replication vs. bytes transferred during responses

8 Algorithmic Challenges Convergence Moving replicas around Load oscillations Responsiveness and stability Distributed vs. Centralized Algorithms Interplay between request distribution and content placement

9 Load Balancing Algorithm Initial probabilities: Loop through the replicas in order of decreasing proximity if load(i) < LW prob(i) =1.0 else if LW <= load(i) < HW prob(i) = (HW – load(i)) / (HW – LW) else prob(i) = 0.0 Adjustments: remainder = 1.0 Loop through the replicas in order of decreasing proximity prob(i) = prob(i) * remainder remainder = remainder – prob(i) Final probabilities: prob(i) = prob(i) / sum of all

10 Overview of the Prototype Internet Client DNS Redirector Primary Server Server2 Server3 Central Replicator Replicator