45502875
DotSlash - Creating Content Distribution Networks on Demand
Traditional content distribution networks, such as Akamai, are well-suited for static web services that routinely experience large traffic volumes. They are unsuited for active content, i.e., content generated by scripts from databases, and web sites that are unlikely to receive significant number of requests. However, a few such sites will invariably experience their "fifteen minutes of fame", typically by being mentioned on a high-volume news site such as SlashDot or CNN. Such flashcrowds or "slashdot effect" will routinely cause single-server websites to collapse. We have designed and prototyped an autonomic web replication system, called DotSlash, that drafts rescue servers fully automatically, without user intervention. The system discovers suitable rescue servers via wide-area service location, either among peer servers or from a dedicated pool of rescue servers, allocates them for temporary use and redirects requests to them. The system is completely transparent to clients and does not require URL rewriting or other client modifications. We have designed two versions. The first, an Apache extension, deals only with static content, e.g., HTML pages or media objects. The second version can also replicate and execute scripts remotely. We have prototyped the system for the common LAMP (Linux, Apache, MySQL and PHP) configuration and shown that a common benchmark for bulletin boards can be replicated without code changes, yielding capacity increases bounded only by the database server. Since many such systems, including most blogs, are bottlenecked by the web server, our system can significantly increase capacity and works even for extremely rapid load increases. We are currently investigating how such systems can be further extended by increasing the database capacity of read-mostly systems with loose consistency constraints.
9589474
DotSlash: A Self-Configuring and Scalable Rescue System for Handling Web Hotspots Effectively
DotSlash allows different web sites to form a mutual-aid community, and use spare capacity in the community to relieve web hotspots experienced by any individual site. As a rescue system, DotSlash intervenes when a web site becomes heavily loaded, and is phased out once the workload returns to normal. It aims to complement the existing web server infrastructure to handle short-term load spikes effectively. DotSlash is self-configuring, scalable, cost-effective, easy to use, and transparent to clients. It targets small web sites, although large web sites can also benefit from it. We have implemented a prototype of DotSlash on top of Apache. Experiments show that using DotSlash a web server can increase the request rate it supported and the data rate it delivered to clients by an order of magnitude, even if only HTTP redirect is used. Parts of this work may be applicable to other services such as Grid computational services.
30153007
Selective anti-entropy
Anti-entropy [1] is an important mechanism to achieve eventual consistency among a set of replicas. We refer to traditional antientropy [2, 3] as complete anti-entropy, where two replicas reconcile all inconsistent states between them in a two-way session or two one-way sessions. Complete anti-entropy works well in full replications, but may cause a summary problem (a replica cannot summarize its received updates using a summary vector) in partial replications (where the state set is divided into subsets referred to as scopes). To avoid the summary problem, we propose to use selective anti-entropy, which is to selectively reconcile inconsistent states between two replicas in a session, i.e., a replica specifies the subsets of updates it solicits, and thus controls what it receives from another replica. In selective anti-entropy, all updates that are initially received by the same replica R= belong to the same s u b s e t / ~ . The subset of a state is determined by the last update applied to it. A selective anti-entropy session where R¢ requests new updates in -P~i, Rx2, .... R~ k from R~ is denoted as O(R~, {/:/=~, Rx2, ..., R=k }, R~). We distinguish four types of selective anti-entropy sessions (listed in Figure 1) based on the number of subsets requested and whether the session is direct or indirect. We assume that a replica R¢ does not request updates in ~ from other replicas. A session is direct if it only requests updates in R--~ from R= (e.g., O___(R~, { ~ } , R~)), otherwise the session is indirect (e.g., O(R=, {1-~u}, R~)) or mixed (e.g., O(_R=, {R~, R~}, R~)). A select-multiple or select-all session can be viewed as comprising k (k >_ 2) select-one sessions, in which at most one is select-one-direct session, and the rest are select-one-indirect sessions. A select-all session is equivalent to a complete anti-entropy session. An anti-entropy session is safe if it will not cause the summary problem. A replica can always choose safe sessions by using selective anti-entropy. Any select-one-direct anti-entropy session, such as O(R=, {R~}, R~), is safe. A select-one-indirect anti-entropy session O(R=, {R-~y}, R~) is safe if S(R=) A S(Ru) C_ S(R~), where S(R=), S(Ru) and S(Rz) denote the scopes of Re, Rv and R~, respectively. In other words, if S(R=) n S(Ru) D S(R=), O(R=, {R-~}, Rz) may cause the summary problem. In Figure 2, for example, O(Ra, {R1}, R6) is safe because S(Ra) N S(R1) = {$1, $2} = S(R6) , but O(R3, {R6}, R2) may not be safe since
26211954
Real-time communication in FDDI networks
This paper provides a comprehensive study of the use of FDDI networks to support real-time communication in mission-critical systems. Due to the potentially catastrophic results of failure in such systems, it is important to avoid message loss whenever possible. We develop efficient tests to check whether message loss may occur, either due to missing deadlines or due to buffer overflow. These tests are extremely useful in the design and operation of mission-critical systems using FDDI networks. Two types of tests are developed: message-parameter-based tests, which use detailed system knowledge to determine whether loss may occur, and utilization-based tests, which only require knowledge of the overall system load. In order to examine the system performance over a wide range of network loads, we also examine the probability that a randomly chosen message set will not suffer any loss. The work reported in this paper is compatible with the FDDI and SAFENET standards, and can be applied by engineers building real-time systems using these standards.
2637438
Modeling and regulation of host traffic in ATM networks
For any connection admission control (CAC) algorithm to work correctly and efficiently, accurate information of the traffic flow out of the host systems is required. We develop several approximation approaches for modeling the traffic flows of hard real-time connections. We show that without accurate traffic characterization, the CAC algorithm may do either of the following: (1) admit connections that may cause network congestions which results in violating connection deadline requirements; or (2) pessimistically reject many connections whose QoS can be guaranteed. We propose a traffic approximation model that can characterize the traffic correctly and efficiently, achieving a higher admission probability. From our experimental data we observe that the source traffic from a typical host is bursty. This burstiness may cause congestion within the network. To overcome this problem, we propose and analyze a simple traffic regulation mechanism at the application layer. The performance evaluation data shows that in the regulated system the traffic burstiness is lower and the probability of a connection being admitted is higher than the unregulated system.
12365152
A multi-access window protocol for transmission of time constrained messages
A novel window protocol for transmitting time-constrained messages in a multiaccess network is proposed that explicitly takes time constraints into account. The window is formed on the basis of latest time to send a message (LS). A newly arriving message is immediately considered for transmission if its LS is less than those of the all pending messages in the system. As a result, the protocol closely approximates the optimal minimum-laxity-first policy. A performance evaluation by simulation shows that the protocol performs well in a wide range of environments, even under overloaded conditions.<<ETX>>
12760716
Making Web services dependable
Web services offer great promise for integrating and automating software applications within and between enterprises over the Internet. However, ensuring that Web services are dependable, and can satisfy their clients' requests when the clients need them is a real challenge because, typically, a business activity involves multiple Web services and a Web service involves multiple components, each of which must be dependable. In this paper, we describe fault tolerance techniques, including replication, checkpointing, and message logging, in addition to reliable messaging and transaction management for which Web services specifications exist. We discuss how those techniques can be applied to the components of the Web services involved in the business activities to render them dependable.
1063355
Performance evaluation of admission policies in ATM based embedded real-time systems
We study the effect of the output link scheduling discipline of an ATM switch on the ability of an ATM LAN to admit real-time connections. Three output link scheduling policies are studied: first come first served (FCFS), round robin (RR), and packet-by-packet generalized processor sharing (PGPS). We derive connection admission criteria for the three scheduling policies. To evaluate the performance of the three scheduling policies, we introduce the metric of admission probability. The admission probability gives the probability that a randomly chosen set of real-time connections will be admitted into the network. The admission probability allows system designers to study the performance of different scheduling policies over a wide range of network loads. We observe that the performance of the three scheduling policies is sensitive to message deadlines. When the deadlines are small, PGPS outperforms both RR and FCFS, and RR outperforms FCFS. When the deadlines are large, all three scheduling policies perform the same. We also note that although PGPS is better than RR and FCFS most of the time, its improved performance is achieved at the cost of high implementation complexity and run time overheads.<<ETX>>