Composition and silent reads

Copyright 2010-2011 © Gabriel Zs. K. Horvath

So far all the read operations performed in the atomic block were being recorded, so as to be re-executed at commit time. We will see in this post that there are circumstances where one does not want the reads to be recorded. I will call these silent reads.

Composition

One of the most important and powerful concept in software engineering is the one of composition. We want to be able to compose existing data structures together to build new ones. Or we want to add new methods to existing ones without having to perform open heart surgery on that component. So let’s look at the concrete example of trying to implement the Last method on top of the set data structure:

public IEnumerable<T> Last(this IEnumerable<T> that) {
    var enumerator = that.GetEnumerator()
    if (enumerator.MoveNext()) {
        T t = enumerator.Current;
        while (enumerator.MoveNext()) {
            t = enumerator.Current;
        }
        yield t;
    }
}

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Eliminating spurious conflicts

Copyright 2009-2011 © Gabriel Zs. K. Horvath

In my previous post I introduced software transactions with semi-mutable versioned variables. I also gave an example which created spurious conflicts in concurrent transactions. As promised I will demonstrate in this post how these conflicts can be eliminated.

Semantics

Let’s go back to the concurrent debit/credit example of our previous post:

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Software transactions with semi-mutable versioned variables

Copyright 2009-2011 © Gabriel Zs. K. Horvath

In this post I will introduce software transactions with semi-mutable versioned variables.

Introduction

Transactions have been around for a long time. They are typically associated with databases, but also commonly used in other areas such as source control systems and installers. Database transactions are the inspiration of memory transactions, either with hardware support or as software transactional memory. Every transactional system has its own variation; however they all share the fundamental concept of atomicity and provide some level of isolation. Transactions allow the concurrent execution of multiple execution threads while preserving the illusion of serial execution within each thread and preserving consistency.

Transactional systems tend to suffer from spurious conflicts which unnecessary fail transactions. These spurious conflicts are conflicts which have no valid semantic or logical origin. We will see how software transactions can help reduce or sometimes entirely remove these spurious conflicts.

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