# Stochastic Reliability of a Server under a Random Workload

##### For the full paper, which includes all proofs, click here if you dare.

## Abstract

*Editor’s note: This paper is the first chapter from a PhD thesis published in 2016 by Rachel Traylor*. We generalize a 2011 model from Cha and Lee that gave a closed form for the survival function of a server under a random workload, and with random service times. In this work, the constant stress assumption of Cha and Lee is relaxed and allowed to be random; that is, every request to the server brings a random stress or workload to the server for the duration of its stay until completion. The efficiency measure of a random stress server is defined as the long run average number of jobs completed per unit time, and provides a way to measure server performance.

## Introduction

There are many types of systems which can be dubbed servers, such as a retail checkout counter, a shipping company, a web server, or a customer service hotline. All of these systems have common general behavior. Requests or customers arrive via a stochastic process, the service times vary randomly, and each request stresses the server if only temporarily. A general stochastic model that describes the reliability of such a server can provide the necessary information for optimal resource allocation and efficient task scheduling, leading to significant cost savings for businesses and improved performance metrics [6]. Such topics have been studied in literature for several decades [1, 2, 3, 20].

Much attention was devoted to reliability principles that model software failures and bug fixes, starting with Jelinski and Moranda in 1972 [11]. The hazard function under this model shows the time between the* i-*th failure and the (*i *+ 1)st failure. Littlewood (1980) [16] extended this initial reliability model for software by assuming differences in error size [12].

These models have been extended into software testing applications [4, 5, 19] and optimal software release times [7, 8, 17, 22]. The explosion of e-commerce and the resulting increase in internet traffic have led to the development of reliability models for Web applications. Heavy traffic can overload and crash a server; thus, various control policies for refreshing content and admission of page requests were created [10, 13, 15, 18, 23].

In particular, Cha and Lee (2011) [9] proposed a stochastic breakdown model for an unreliable web server whose requests arrive at random times according to a nonhomogenous Poisson process and bring a constant stress factor to the system that dissipates upon service completion. The authors provide a fairly general survival function under any service distribution g_{W}(w), define server efficiency to measure performance, and illustrate a possible admission control policy due to an observed property of the server efficiency under a specific numerical example.

Thus far, no extensions of [9] have been proposed. This work generalizes the model put forth by Cha and Lee in a variety of ways. First, the assumption of constant job stress is relaxed and replaced by a random variable, and a new survival function and efficiency equation are derived. This work, while suitable for IT applications, is general enough for use in almost any industry, including logistics, retail, manufacturing, and engineering systems.