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A Formal Semantics for the SmartFrog Configuration Language

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Abstract

System configuration languages are now widely used to drive the deployment and evolution of large computing infrastructures. Most such languages are highly informal, making it difficult to reason about configurations, and introducing an important source of failure. We claim that a more rigorous approach to the development and specification of these languages will help to avoid these difficulties and bring a number of additional benefits. In order to test this claim, we present a formal semantics for the core of the SmartFrog configuration language. We demonstrate how this can be used to prove important properties such as termination of the compilation process. To show that this also contributes to the practical development of clear and correct compilers, we present three independent implementations, and verify their equivalence with each other, and with the semantics. Supported by an extended example from a real configuration scenario, we demonstrate how the process of developing the semantics has improved understanding of the language, highlighted problem areas, and suggested alternative interpretations. This leads us to advocate this approach for the future development of practical configuration languages.

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Notes

  1. The concrete syntax of this formal representation is a:c.

  2. This is equivalent to a map data structure.

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Acknowledgments

This work has been partially supported by an HP Labs Innovation Research Program Award, and we would like to thank Patrick Goldsack for his help in clarifying the SmartFrog semantics and implementation. It is also partially sponsored by the Air Force Office of Scientific Research, Air Force Material Command, USAF, under Grant Number FA8655-13-1-3006. The U.S. Government and University of Edinburgh are authorized to reproduce and distribute reprints for their purposes notwithstanding any copyright notation thereon.

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Authors and Affiliations

  1. School of Informatics, University of Edinburgh, Edinburgh, EH8 9AB, UK

    Paul Anderson

  2. Hewlett-Packard Labs, Bristol, UK

    Herry Herry

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  1. Paul Anderson
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  2. Herry Herry
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Correspondence to Herry Herry.

Additional information

Herry Herry did part of the work for this paper while being a PhD student at School of Informatics, University of Edinburgh.

Appendices

Appendix 1: Concrete Syntax

$$\begin{aligned} \hbox {SF}&::= \hbox {B} \\ \hbox {B}&::= \hbox {A B} \,| \,\epsilon \\ \hbox {A}&::= \hbox {R V} \\ \hbox {P}&::= \hbox {R }|\, \texttt{\{ }\hbox { B }\texttt{\} } \\ \hbox {PS}&::= \hbox {P }(\texttt{,} \,\hbox {P})^* \,|\, \epsilon \\ \hbox {V}&::= \hbox {BV } \texttt{;} |\hbox { LR }\texttt{;} | \texttt{extends}\hbox { PS }\\ \hbox {R}&::= \hbox {I }(\texttt{:}\hbox { I})^* \\ \hbox {DR}&::= \texttt{DATA}\hbox { R} \\ \hbox {LR}&::= \hbox {R }\\ \hbox {Vec}&::= \texttt{[} (\hbox { BV }(\texttt{,}\hbox { BV})^* \,|\, \epsilon ) \texttt{]} \\ \hbox {Null}&::= \texttt{NULL} \\ \hbox {Bool}&::= \texttt{true} \,|\, \texttt{false} \\ \hbox {BV}&::= \hbox {Bool }|\hbox { Num }|\hbox { Str }|\hbox { DR }|\hbox { Vec }|\hbox { Null} \end{aligned}$$

Appendix 2: Manually Created Tests

The following examples show some of the manually-created test cases:

figure e

Appendix 3: Auto-Generated Tests

The following SF code is part of a randomly-generated, semantically-correct specification used for testing compatibility between the implementations:

figure f

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Anderson, P., Herry, H. A Formal Semantics for the SmartFrog Configuration Language. J Netw Syst Manage 24, 309–345 (2016). https://doi.org/10.1007/s10922-015-9351-y

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