World Wide Web Caching: Trends and Technology

1. World Wide Web Caching: Trends and Technology Greg Barish and Katia Obraczka USC Information Science Institute IEEE Communications Magazine, May 2000 Presented By: Ossama M. Younis Ph.D. Student, Purdue University

2. Presentation Outline • Introduction • Limitations and problems • Desirable properties of a Web Caching system • Cache deployment options • Design techniques • Caching architectures • Load balancing and Content-based routing • Conclusion

3. Introduction • What is Web Caching ? • Introducing proxy servers at certain points in the network that serve in caching Web documents for faster client access. • Comparable to the cache memory in a computer system. • Why is it needed ? • Rapid growth in HTTP traffic to form the largest part of the Internet traffic which causes more network congestion and server unavailability. • The number of Web static pages almost doubles every year.

4. The Expected gains: • Bandwidth saving • Improving content availability. • Improving web server availability. • Reducing network latency. • Server load balancing. • Improving user’s perception about network’s performance.

5. Main Issues and Problems • Caching system architecture • Proxy placement • Caching contents • Proxy cooperation/Data sharing • Cache resolution/Routing • Prefetching • Cache placement/Replacement • Cache coherency • Security and legal ethics of caching • Control information distribution

6. Desirable Features: • Fast access • Robustness • Transparency • Scalability • Efficiency • Adaptivity • Stability • Load balancing • Dealing with heterogeneity • Simplicity

7. Cache Deployment options • Near the content consumer • Better response time • Local service of requests • Near the content provider • Improves access to logical sets of data • Problem: critical to delay sensitive content • Problem: security constraints • At strategic points in the network • Problem with administrative control • Caching on a per-user basis • Uses the user’s local file system

8. Design Techniques • Main Concerns: • Speed • Reliability • Scalability • Main design techniques: • Hierarchical caching • Intercache communication • Other design issues

9. Hierarchical Caching • Caches are arranged in a tree-like structure • A child cache can query parent caches and other siblings • A parent cache can never query children • This maintains information gradually filtering down to the leaves • To avoid swamping parents with information, clustering may be applied to hierarchies.

10. Intercache Communication • Multiple Distributed Caches in meshes • Improves scalability, availability, and physical locality • Protocols: • ICP (Internet Cache Protocol) [Squid]: Caches issue queries to other caches to determine the best location of object retrieval. Main problem is the message overhead • CRP (Content Routing Protocol): ICP with multicast feature to query cache meshes • Cache digests [Squid]: summarizes cache objects

11. Intercache Communication • Protocols (cont.) • WCCP (Web Cache Communication Protocol) [Cisco]: Enables transparent redirection of HTTP traffic to Cisco Cache Engine • CARP (Cache Array Routing Protocol) [Microsoft]: Uses Hashing Schemes for location determination of the required proxy having the requested information

12. Other Design Issues • Hash-Based request routing • Maps a key (such as the url) to a cache within a cluster • Reduces (eliminates) the need of caches to query each other • Optimized disk I/O • Improves spatial locality • Use in-memory data structures to avoid disk I/O • Microkernel O.S. • Designed to optimize cache performance (resource alloction, task execution, disk access, and transfer time)

13. More Design Issues • Content prefetching • Local based • Server-hint based • Implementation: • Between clients and servers • Between clients and proxies • Between proxies and servers • Improvements: • Less latency (from 26% improvement to 57%) • Improved access time

14. More Design Issues • Cache coherency (consistency) • Strong consistency techniques: • Client polling (If-Modified Since) • Invalidation callbacks • Weak consistency techniques • TTL and Adaptive TTL • Piggyback Invalidation

15. Caching Architectures • Proxy Caching • Deployed at the edges of the network • Unavailable cache  Unavailable network • Single point of failure • User browser manual reconfiguration in times of failure

16. Caching Architectures • Reverse Proxy Caching • Placing proxies near the content provider • Transparent Caching • Eliminates the needs to manually configure web browsers

17. Caching Architectures • Adaptive Web Caching • Uses distributed cache meshes to solve the hot spot problem • Caches dynamically join and leave the groups based on content demand • Administrative boundaries must be relaxed • Push Caching • Keep data close to those clients requesting this information • Assumption: we are able launch caches that may cross administrative boundaries • Incurs cost (storage and transmission)

18. Caching Architectures • Active Caching • Applies caching to dynamic documents • 30 % of client HTTP requests contains cookies • The servers provides the cache with the objects and any associated cache applets

19. Load Balancing and Content Based Routing • Main goal: Improving performance and scalability • Local load balancing: Incoming requests are intercepted and redirected to one member of a group of servers or caches, all of which exist in the same geographic area. This can be achieved by using L4 or L5 switches. • Global Load balancing: Instead of distributing requests among members of a local group, requests are distributed to servers or caches which are near the client, to achieve lower network latencies. • Examples: Cisco’s WCCP, Distributed Director, DFP, etc.

20. Conclusions • In this paper, the main web caching techniques, design issues, and architectures are discussed. • The paper doesn’t compare any of these techniques in terms of scalability, availability, or performance. • Some major issues were not addressed: • Content security • Handling more complex objects and real-time data