unikernel/duniverse/dune_/doc/explanation/tour/engine.rst

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+The Engine
+==========
+
+The engine is the core, reusable part of Dune. It contains all the composable
+primitives that make it a build system.
+
+The fact that it is split from the :doc:`rules part <rule-generation>` makes it
+possible to create a different build system using this library. For example,
+Jane Street internally uses a build system with this engine as a backend, but a
+different frontend and CLI.
+
+In the context of Dune, the engine keeps track of the various directories and
+the rules in them and is able to build files using them. In addition, it takes
+care of the various caches that Dune uses, such as the one present in the
+``_build`` directory, the :doc:`shared cache </caching>`, etc.