Integration

Combining parts of a software product to form a whole is called integration. It is also one of the most troublesome tasks and it rarely goes smoothly.

In terms of timing and frequency, there are two general approaches to integration: late and one-time, early and frequent.

Late and one-time: wait till all components are completed and integrate all finished components near the end of the project.

Early and frequent: integrate early and evolve each part in parallel, in small steps, re-integrating frequently.

Big-bang integration: integrate all components at the same time.

Incremental integration: integrate a few components at a time. This approach is better than big-bang integration because it surfaces integration problems in a more manageable way.

Based on the order in which components are integrated, incremental integration can be done in three ways.

Top-down integration: higher-level components are integrated before bringing in the lower-level components. One advantage of this approach is that higher-level problems can be discovered early. One disadvantage is that this requires the use of stubs in place of lower level components until the real lower-level components are integrated into the system. Otherwise, higher-level components cannot function as they depend on lower level ones.

Bottom-up integration: the reverse of top-down integration. Note that when integrating lower level components, may be needed to test the integrated components because the UI may not be integrated yet, just like how top-down integration needs stubs.

Sandwich integration: a mix of the top-down and bottom-up approaches. The idea is to do both top-down and bottom-up so as to 'meet' in the middle.

Build automation tools automate the steps of the build process, usually by means of build scripts.

In a non-trivial project, building a product from its source code can be a complex multi-step process. For example, it can include steps such as: pull code from the revision control system, compile, link, run automated tests, automatically update release documents (e.g. build number), package into a distributable, push to repo, deploy to a server, delete temporary files created during building/testing, email developers of the new build, and so on. Furthermore, this build process can be done ‘on demand’, it can be scheduled (e.g. every day at midnight) or it can be triggered by various events (e.g. triggered by a code push to the revision control system).

Some of these build steps such as compiling, linking and packaging, are already automated in most modern IDEs. For example, several steps happen automatically when the ‘build’ button of the IDE is clicked. Some IDEs even allow customization of this build process to some extent.

However, most big projects use specialized build tools to automate complex build processes.

Some build tools also serve as dependency management tools. Modern software projects often depend on third party libraries that evolve constantly. That means developers need to download the correct version of the required libraries and update them regularly. Therefore, dependency management is an important part of build automation. Dependency management tools can automate that aspect of a project.

An extreme application of build automation is called continuous integration (CI) in which integration, building, and testing happens automatically after each code change.

A natural extension of CI is Continuous Deployment (CD) where the changes are not only integrated continuously, but also deployed to end-users at the same time.