Gestion de code source

Introduction

La gestion de code source est l'une des pierres angulaires du développement d'applications. Lorsque vient le temps de gérer le code source d'un projet sur lequel vous travaillez, deux façons s'offre à vous. Vous pouvez faire des copies de sauvegarde multiples, envoyer le code source à vos collègues par courriel, et vous creuser la tête afin de savoir qui possède ou utilise quelle version du code et comment réconcilier les modifications apportées par chaque collaborateur. Vous pouvez au contraire adopter une approche beaucoup plus saine en utilisant des outils de contrôle des révisions développés dans le but de faciliter ce processus.

Toutes les applications et bibliothèques d'envergure sont développées en utilisant de tels outils. En recherche, ces outils sont encore davantage importants, car la traçabilité est essentielle afin de pouvoir reproduire des résultats donnés. Les outils de gestion de contrôle des révisions sont en quelque sorte le cahier de laboratoire d'un expérimentateur.

Advantages

Revision control tools offer you a great many advantages and these more than compensate for the occasional inconvenience. Firstly, they permit you to collaborate more easily. They eliminate the risk that a collaborator might delete your modifications or vice-versa without leaving a trace. These tools save the history of all the modifications made to a project and in that way function somewhat like a time machine, allowing you to re-initialize your project to an earlier version, in order to reproduce your results for example. They also make it easier to document these changes, so that all of the users of a project are notified of the changes made and the reasons they were made.

Basic Functionality

Source code management tools function using a basic principle of separating local modifications made by a user, in his or her local directory, and what is called the repository. The repository contains, in a structured manner, the history of all of the modifications made by all of a project's contributors. The development of a software project using a source code management tool is thus modified in comparison to the development of a purely local project. Rather than simply saving your modifications to the local disk drive, you as a contributor have to submit (commit) your modifications to the repository in order to make them available to other developers. Inversely, developers need to make sure they are using the latest version of a file by retrieving it from the repository (checkout, update) before making their own modifications. If two programmers modify the same source code file at the same time, the source code management tool may report a conflict during the submission of the two rival modifications or automatically resolve the conflict if possible.

Families of Revision Control Tools

The principal revision control tools can be divided into two families or generations. First generation tools, which include CVS and SVN, use a single central repository. All of the modifications are under the control of and retrieved from this authoritative repository. Second generation tools such as Git, Mercurial and Bazaar, use local repositories. The advantage of such a system is that development work can be done independent of any remote server. The commit and checkout operations can thus be much faster and perform more complex operations. As an example, Git and Mercurial offer advanced management of branched development. In a branched development model, each new feature corresponds to a branch of the development tree. The production version of the project then corresponds to the principal branch and additional features are developed in parallel until they are sufficiently mature to be fused with the principal branch or else abandoned and the branch left to die. This development model is particularly well-adapted to very large projects involving several programmers.

In exchange for this flexibility, with second generation tools all modifications that need to go to an external repository must do so in two steps: first, they are submitted to the local repository (commit), then they are pushed (push) to the external repository. Equally so, to retrieve the modifications from an external repository, you must first obtain them (pull or get) so they can be imported into the local repository, then update your working version (update or checkout).

Choosing a Tool

If you want to contribute to an existing project, you don't really have any choice - you will have to use the tool that has been chosen by the initial development team. If you are starting your own project, the choice will depend on the breadth of your project. If it's a project with only a few contributors, which will remain private and for which you would simply like to have a history of all the modifications, a first generation tool like SVN can be sufficient. If your project is larger, with external collaborators, you should consider a second generation tool like Git or Mercurial.

This page is derived from [1].