Linux and Unix Books page5 Posted on: October 6, 2010 at 12:00 AM
This paper presents how the Strategy pattern has been used to build BAST, an extensible object-oriented framework for programming reliable distributed systems.
Linux and Unix Books page5
Using the Strategy Design Pattern
This paper presents how the Strategy pattern has been used to build BAST, an extensible object-oriented framework for programming reliable distributed systems. Protocol composition plays a central role in BAST and relies on the notion of protocol class. In this paper, we focus on the recursive use of the Strategy pattern to overcome the limitations of inheritance, when trying to flexibly compose protocols. In a companion paper, we have presented how generic agreement protocol classes can be customized to solve atomic commitment
and total order multicast, which are central problems to transactional systems and to group-oriented systems respectively. In we also show how BAST allows distributed applications to be made fault-tolerant, by application programmers who are not necessarily skilled in reliability issues.
0080">Impeding Spam Across Multiple Servers This paper describes an Irish ISP's attempts to combat the abuse of resources caused by unsolicited commercial email. We describe the extension of a multicast system, used to implement POP-before-SMTP relaying, to share information about remote mail servers between multiple mail systems. The information may then be used to tarpit abusive servers - placing delays between SMTP protocol answers thus mitigating their impact on our systems. We then examine how effective this has been, and come up with some ideas for future development.
Unsolicited commercial email (spam) is a problem that, by now, needs no introduction. If you know about email, you know about spam. There are whole books on how to stop it, and it's even on the nightly news [1, 2]. The spam problem from an ISP perspective is also reasonably well known - spam is expensive in terms of time spent receiving it, space spent storing it, and staff months spent dealing with complaints and the technical aspects of cleaning up after it.
The building unobtrusive disk maintenance applications There are many disk maintenance activities that are required for robust system operation and, yet, have loose time constraints. Such ``background'' activities need to complete within a reasonable amount of time, but are generally intended to occur during otherwise idle time so as to not interfere with higher-priority application progress. Examples include write-back cache flushing, defragmentation, backup, integrity checking, virus scanning, report generation, tamper detection, and index generation.
Current systems use a variety of ad hoc approaches for such activities. Most trickle small amounts of work into the storage subsystem, either periodically or when an idle period is detected. When sufficient idle time is not available, these activities either compete with foreground requests or are not completed. More importantly, trickling work into a storage subsystem wastes significant disk scheduling
opportunities-it restricts the scheduler to only considering a small subset of externally-chosen requests at externally-chosen points in time.
A Public Internet Measurement Facility
The ability to measure the Internet is of widespread value for diagnosing connectivity problems and understanding Internet topology [20,53], routing [35,54] and performance [3,51].
Existing systems such as NIMI  provide much of the needed functionality, but not all. These research systems provide the advantages of dedicated hardware that can be used for a wide range of network measurements. In return, users must possess credentials or an account, which creates a barrier that limits access to a community of users trusted by the administrator. Thus these systems do not help unaffiliated users like a network operator trying to debug poor network performance.
The popularity of Web-accessible traceroute servers offers a different solution. Several hundred public traceroute servers are available, constituting the largest de facto Internet measurement facility. These servers are typically used to debug two-way connectivity problems, providing indirect benefit to the traceroute server host.
The energy efficient prefetching and caching Prefetching and caching are standard practice in modern file systems. They serve to improve performance--to increase throughput and decrease latency--by eliminating as many I/O requests as possible, and by spreading the requests that remain as smoothly as possible over time. This strategy results in relatively short intervals of inactivity. It ignores the goal of energy efficiency so important to mobile systems, and in fact can frustrate that goal. Magnetic disks, network interfaces, and similar devices provide low-power states that save energy only when idle intervals are relatively long. A smooth access pattern can eliminate opportunities to save energy even during such light workloads as MPEG and MP3 playback.
The aim of our work is to create bursty access patterns for devices with non-operational low-power states, increasing the average length of idle intervals and maximizing utilization when the device is active, without compromising performance. At present we are focusing on hard disks.