Some issues with Continuous Integration:

a) How can you integrate continuously if the build process takes hours or even days?

''Get better hardware.  Profile and optimize the build.  Split up build into short and long runs.''

b) What if the unit testing or acceptance testing process takes too long as well?

''Get better hardware.  Profile and optimize the tests.  MockObject''''''s and/or faster substitutes to external products.  Look at distributed test execution, http://cs.allegheny.edu:8080/gkapfham/6.  Split up tests into short and long runs.''

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See also ContinuousIntegration, ExtremeProgrammingImplementationIssues, MockObject''''''s