Large Scale Continuous Integration

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time. A change in one file often requires a corresponding change in another file. The reason that a single individual works on those files is because the changes are tightly coupled and don't lend themselves to a multi-person effort.

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As an individual developer, there are two things that you do to shield yourself and others from instability. Although you make frequent changes to your workspace which means its stability goes up and down rapidly, you only check-in your changes when you feel like you've done enough testing of those changes that you've sufficiently reduced the risk of disrupting everybody else that is depending on the mainline. When you check in your changes, you are essentially making an assertion that your change has reached a higher level of maturity.

Conversely, you only update your workspace when you are at a point that you feel you are ready to absorb other people's changes. Because other people only check-in when they feel the changes are ready and you only update when you feel you are ready, you are mostly shielded from the constant change that is going on all around you.

These two simple practices act as buffers which shield other people from the chaos of your workspace while you prepare to check-in and shield you from problems that other people may have introduced since you last updated your workspace.
While it may seem like a bit of a trivial case, you can think of this process as self-integration. This is the basis of Multi-stage Continuous Integration: if individual isolation is a good idea, then isolation for features, teams, team integration, staging, QA and release is an even better idea.

Moving From Known Good to Known Good
This integration pattern is also found in the interactions of customers and software suppliers and the interactions of software producers and third party software suppliers. Your customers don't want random builds that you create during development and you don't want random builds from third parties that you depend on. The reasons are simple and obvious. The level of quality of interim builds is unknown and the feature set is unknown. Your customers want something that has a high level of quality that has been verified to have that level of quality. Likewise, you want the same from your suppliers. Your suppliers include things like third party software libraries or third party off the shelf software that your application depends on such as databases and web servers.

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As with your own software, each third party that you rely on produces hundreds if not thousands of versions of their software, but they only release a small subset of them. If you took each of these as they were produced, it would be incredibly disruptive and you would have a hard time making progress on your own work. Instead, you move from a known good build to a known good build, their externally released versions. Your customers do the same.

This simple principle should be applied throughout your development process. Think of each individual developer as both a consumer and producer of product versions. Also think of them as a third party. Think of each team as well as each stage in your development process this way. That is, as a developer think of your teammates as customers of the work that you produce. Think of yourself as a customer of the work that they do. You want to move from known good version to known good version of everything you depend on.

It's All for One and One for All
The next level of coupling is at the

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