Rajat Mukherjee

A scalable and highly available web server

We describe a prototype scalable and highly available web server, built on an IBM SP-2 system, and analyze its scalability. The system architecture consists of a set of logical front-end or network nodes and a set of back-end or data nodes connected by a switch, and a load balancing component. A combination of TCP routing and Domain Name Server (DNS) techniques are used to balance the load across the Front-end nodes that run the Web (httpd) daemons. The scalability achieved is quantified and compared with that of the known DNS technique. The load on the back-end nodes is balanced by striping the data objects across the back-end nodes and disks. High availability is provided by detecting node or daemon failures and reconfiguring the system appropriately. The scalable and highly available web server is combined with parallel databases, and other back-end servers, to provide integrated scalable and highly available solutions.

Network dispatcher: a connection router for scalable Internet services

Abstract Network Dispatcher (ND) is a TCP connection router that supports load sharing across several TCP servers. Prototypes of Network Dispatcher were used to support several large scale high-load Web sites. Network Dispatcher provides a fast IP packet-forwarding kernel-extension to the TCP IP stack. Load sharing is supported by a user-level manager process that monitors the load on the servers and controls the connection allocation algorithm in the kernel extension. This paper describes the design of Network Dispatcher, outlines Network Dispatchers performance in the context of http traffic, and presents several of its features including high-availability, support for WANs, and client affinity.

Buffering and caching in large-scale video servers

Video-on-demand servers are characterized by stringent real-time constraints, as each stream requires isochronous data playout. The capacity of the system depends on the acceptable jitter per stream (the number of data blocks that do not meet their real-time constraints). Per-stream read-ahead buffering avoids the disruption in playback caused by variations in disk access time and queuing delays. With heavily skewed access patterns to the stored video data, the system is often disk arm-bound. In such cases, serving video streams from a memory cache can result in a substantial reduction in server cost. In this paper, we study the cost-performance trade-offs of various buffering and caching strategies that can be used in a large-scale video server. We first study the cost impact of varying the buffer size, disk utilization and the disk characteristics on the overall capacity of the system. Subsequently, we study the cost-effectiveness of a technique for memory caching across streams that exploits temporal locality and workload fluctuations.

Design and performance tradeoffs in clustered video servers

We investigate the suitability of clustered architectures for designing scalable multimedia servers. Specifically, we evaluate the effects of: (i) architectural design of the cluster; (ii) the size of the unit of data interleaving; and (iii) read ahead buffering and scheduling on the real time performance guarantees provided by the server. To analyze the effects of these parameters, we develop an analytical model of clustered multimedia servers, and then validate it through extensive simulations, The results of our analysis have formed the basis of our prototype implementation based on an RS/6000 Scalable Parallel (SP) machine. We briefly describe the prototype and discuss some implementation details.