Automating system tests using declarative virtual machines

Abstract—Automated regression test suites are an essential software engineering practice: they provide developers with rapid feedback on the impact of changes to a system’s source code. The inclusion of a test case in an automated test suite requires that the system’s build process can automatically realise all the environmental dependencies of the test. These are external elements necessary for a test to succeed, such as shared libraries, running programs, and so on. For some tests (e.g., a compiler’s), these requirements are simple to meet. However, many kinds of tests, especially at the integration or system level, have complex dependencies that are hard to provide automatically, such as running database servers, administrative privileges, services on external machines or specific network topologies. As such dependencies make tests difficult to script, they are often performed only manually, if at all. This particularly affects distributed systems and system-level software. This paper shows how we can automatically instantiate the complex environments necessary for tests by creating (networks of) virtual machines on the fly from declarative specifications. Building on NixOS, a Linux distribution with a declarative configuration model, these specifications concisely model the required environmental dependencies. We also describe tech-niques that allow efficient instantiation of VMs. As a result, complex system tests become as easy to specify and execute as unit tests. We evaluate our approach using a number of representative problems, including automated regression testing of a Linux distribution. I