Software system scalability: concepts and techniques

ABSTRACT

With physical limits approaching in hardware, with demands on large-scale software systems continually increasing, and with the use of resource-poor wireless devices rapidly growing, issues of scale are becoming paramount in software engineering.

Indeed, it is increasingly the case that software systems undergo a successful initial development, only to be found unfit for purpose in deployment, sometimes years after the initial development was completed. This happens because the systems are frequently designed without a precise understanding of, and accounting for, the evolving characteristics of the environment in which they will be deployed. This lack of understanding leads to problems of scalability and the need for radical re-design of the software to make it more scalable.

The technical literature is full of references to the term scalability but devoid of precise definitions and clear understandings of the term. Furthermore, software developers lack the means to construct suitable objective characterizations of system scalability and to analyze and compare the scalability of different designs.

In this talk I discuss the nature of scalability and the conflicting views that researchers and practitioners have of scalability, using several examples to illustrate the problem. I then present a precise definition of the term and a simple analysis technique that can be used to characterize, evaluate and compare the scalability of software systems. I conclude with a discussion of techniques for eliciting the scalability requirements of a software system, which are needed in order to instantiate the definition and analysis technique for a particular system design or set of candidate designs. I illustrate these ideas with a real-world example of a financial fraud detection system that already has undergone two complete re-designs due to scalability problems.

INDEX TERMS

Primary Classification:
  C. Computer Systems Organization
  C.4 PERFORMANCE OF SYSTEMS
      Subjects: Design studies

Additional Classification:
  C. Computer Systems Organization
  C.4 PERFORMANCE OF SYSTEMS
      Subjects: Measurement techniques; Modeling techniques; Performance attributes

  D. Software
  D.2 SOFTWARE ENGINEERING
        D.2.11 Software Architectures

General Terms:
Design, Economics, Measurement, Performance

Keywords:
execution environment, experimental program analysis, goal-oriented requirements engineering, microeconomics, scalability, software design