Control-theoretic Analysis of Admission Control Mechanisms for Web Server Systems

Web sites are exposed to high rates of incoming requests. The servers may become overloaded during temporary traffic peaks when more requests arrive than the server is designed for. An admission control mechanism rejects some requests whenever the arriving traffic is too high and thereby maintains an acceptable load in the system. This paper presents how admission control mechanisms can be designed with a combination of queueing theory and control theory. In this paper we model an Apache web server as a GI/G/1-system and then design a PI-controller, commonly used in automatic control, for the server. The controller has been implemented as a module inside the Apache source code. Measurements from the laboratory setup show how robust the implemented controller is, and how it corresponds to the results from the theoretical analysis

Planning and communicating prototype tests for the Nano Membrane Toilet: A critical review and proposed strategy

Urban sanitation in growing cities of the Global South presents particular challenges. This led to the Bill & Melinda Gates Foundation’s Reinvent The Toilet Challenge, which sparked the development of various non-sewered sanitation technologies like the Nano Membrane Toilet. Complex disruptive technologies like this entail an extensive product development process, including various types of prototype tests. While there is an abundance of literature discussing how to build prototypes, and the optimal number of tests, there has been little focus on how to plan and conduct tests, especially in a development endeavour of this complexity. Four approaches to testing are reviewed, and their strengths and weaknesses compared. A visualised testing strategy is proposed that encompasses the entire product development process and can be used to plan and communicate prototype tests for the Nano Membrane Toilet to ultimately achieve compliance with international standards

Prototype testing in product development: the case of the Nano Membrane Toilet.

The provision of safely managed sanitation in densely populated urban settlements of cities in low- and middle-income countries poses complex challenges. To address these challenges, Cranfield University is developing the Nano Membrane Toilet, a novel standalone household-level sanitation technology that operates off the grid and safely treats human waste while having an aspirational design. Developing such a complex novel product requires testing a multitude of prototypes in numerous ways. Since designing, building and testing the prototypes is often done by separate teams of various disciplines, the entire testing process requires a considerable amount of planning and communication. This thesis not only reports on field trials and laboratory testing of two prototypes of the Nano Membrane Toilet, but also investigates the under-explored field of planning and communicating prototype tests for complex product development processes. In a first trial, a prototype mechanical toilet flush is assessed in user tests and lab tests. It is shown to be liked by users of Urine Diversion Dehydration Toilets, and it appears to perform best when lubricated and with a silicone rubber with oil-bleed-effect for its swipe. A second round of tests explores settling and displacement as means of solid-liquid separation in the Toilets collection tank. Toilet paper is shown to inhibit settling of faeces, while a conical tank geometry promotes it. From a literature review and subsequent interviews with experts in prototype testing, a visual tool for planning and communication prototype tests for the Nano Membrane Toilet is developed, validated and refined into a generally applicable, modular version. This modular tool can be used to produce customised visual testing strategies for a variety of product development processes, to facilitate planning and communicating testing activities across interdisciplinary teams.PhD in Water, including Desig