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In counterpoint, some emerging software disciplines such as extreme programming and the agile software development movement, adhere to a “test-driven software development” model. In this process, unit tests are written first, by the software engineers (often with pair programming in the extreme programming methodology).
Of course these tests fail initially; as they are expected to. Then as code is written it passes incrementally larger portions of the test suites. The test suites are continuously updated as new failure conditions and corner cases are discovered, and they are integrated with any regression tests that are developed. Unit tests are maintained along with the rest of the software source code and generally integrated into the build process (with inherently interactive tests being relegated to a partially manual build acceptance process). The ultimate goal of this test process is to achieve continuous deployment where software updates can be published to the public frequently.
