Scheduled Integration is hard. I remember being involved in projects where developers would get a copy of the latest source code and a task, and race off like horses at the track while they spent a few weeks implementing their assigned feature. At the end of these many weeks was a scheduled integration, where all developers in the team would reconvene with their code modifications, and try to get their respective code to play well in a single sandbox. Project managers always seemed to expect that this would be a quick and painless process, but, of course, it never was. Reintegration of the code sometimes took just as long as the implementation, itself. Even if developers are all working in one big room throughout the project, the code remains isolated, never shared or reintegrated. Fortunately, Continuous Integration specifically focuses on this problem by reducing (and often eliminating) the end-of-project reintegration cycle through a continuous constant integration process.
For a moment, compare the process of software development with the process of learning new technologies in your field. In the past few years of Microsoft's .NET platform, we've seen .NET 2.0, Windows Communication Foundation, Windows Presentation Foundation, Workflow Foundation, Extension Methods, Generics, Linq, Dynamic Data, ASP.NET MVC, and more. For a developer that has not kept up with the latest trends, to suddenly and quickly catch up with the bleeding edge would be a major undertaking, and psychologically intimidating. This Scheduled Integration approach to learning can be overwhelming; the learning process is isolated, and only occasionally reconnects to the latest technologies. However, a developer that has kept up with the trends, learning through a continuous integration process, has been constantly updating, constantly learning the Next Big Thing over these past few years; this developer is already long familiar with .NET 2.0, WF, WCF, WPF, Generics, and Linq, and is now working on only Dynamic Data and MVC. The Continuous Integration process ensures that those involved are current with the latest developments in their project, and that the overwhelming burden of integration at the end of a project is instead simplified by distributing it throughout the course of the timeline.
At its core, Continuous Integration, or CI, is a development model where developers regularly commit and update against their source control repository. By committing (giving everyone else access to your code changes) and updating (getting code changes from everyone else), the scheduled, tedious integration process at the end of the project is eliminated. Added on top of this single fundamental, is ensuring the code works: Automating the build through scripting frameworks like Make, MSBuild, or NAnt helps developers validate their status quickly, by invoking a validation step that not only compiles, but executes a suite of unit tests against the code base. Validation results, along with the latest valid code, are then made easily accessible to anyone on the team.
The ten tenets of Continuous Integration are:
By continuously integrating all new code and feature sets, development teams can eliminate that long and tedious scheduled integration effort, which reduces overall effort, time line, and budget. Through self-testing code and building every mainline commit, code is continuously tested against a full suite of tests, allowing quick analysis and identification of breaking changes. Through the easy accessibility of the latest bits, the team can test early and often, allowing quick identification of broken functionality, and for early identification of features that don't quite align with what the client had in mind. And finally, the immediate and public feedback on the success or failure of the build provides incentives for developers to write code in smaller increments and perform more pre-commit testing, resulting in higher quality code.
I consider Continuous Integration to be an essential, required part of any development effort, every time. I started using CI in 2004, and I have since become dependent on it. I even have a Continuous Integration box at home, validating home projects for my development-team-of-one, and I am comforted by having a Continuous Integration server analyzing and validating every code change that I make. I break unit tests as often as anyone else does, and there still continues to be plenty of times that I even break the compile. At least once, every developer among us has checked in the project file while forgetting to add myNewClass.cs to Source Control. It will break the compile every time. Fortunately, Continuous Integration is always watching my code commits; it will let me know that it could not find myNewClass.cs, every time. And my application's quality is remarkably better for it. Every time.