Add early internals document draft

INTERNALS.md

@@ -0,0 +1,84 @@
+# Internals / architecture
+
+## The model
+
+Mainly in [Definitions.scala](https://github.com/alexarchambault/coursier/blob/master/core/shared/src/main/scala/coursier/core/Definitions.scala), [Resolution.scala](https://github.com/alexarchambault/coursier/blob/master/core/shared/src/main/scala/coursier/core/Resolution.scala), and [ResolutionProcess.scala](https://github.com/alexarchambault/coursier/blob/master/core/shared/src/main/scala/coursier/core/ResolutionProcess.scala).
+
+### Module
+
+[definition](https://github.com/alexarchambault/coursier/blob/462c16d6db98d35e180a25b0f87aa47083ad98aa/core/shared/src/main/scala/coursier/core/Definitions.scala#L12-L16)
+
+Uniquely designates a... module. Typically, just an organisation (`org.scala-lang`) and a name (`scala-library`). At times, can also contains so called attributes: (unordered) key-value pairs. E.g. the SBT plugins usually have some, like `scalaVersion` with value `2.10` and `sbtVersion` with value `0.13`.
+
+Two modules having different organisation / name / attributes are simply considered different.
+
+During resolution, all dependencies with the same module have their versions reconciled if possible. If not possible, we have a version conflict (conflicting versions of a module are needed), which makes the resolution fail.
+
+### Dependency
+
+[definition](https://github.com/alexarchambault/coursier/blob/462c16d6db98d35e180a25b0f87aa47083ad98aa/core/shared/src/main/scala/coursier/core/Definitions.scala#L43-L54)
+
+A dependency towards a given module, with a given version (can be a version interval too, like `[2.2,2.3)` - TODO add support for `last.revision`).
+
+TODO Add a word about the various fields of `Dependency`
+
+### Project
+
+[definition](https://github.com/alexarchambault/coursier/blob/462c16d6db98d35e180a25b0f87aa47083ad98aa/core/shared/src/main/scala/coursier/core/Definitions.scala#L69-L96)
+
+Metadata about a given version of a module.
+
+Usually comes from a POM file (Maven) or an `ivy.xml` file (Ivy). In the former case, the infos in `Project` may originate from several POMs, if the main one has a parent POM, or some import dependencies. Raw `Project`s are obtained from the various individual POMs, and merged back into one (done during resolution by `Resolution` - TODO give more details).
+
+TODO Describe the various fields of `Project`
+
+### Resolution
+
+[definition](https://github.com/alexarchambault/coursier/blob/462c16d6db98d35e180a25b0f87aa47083ad98aa/core/shared/src/main/scala/coursier/core/Resolution.scala#L477-L487)
+
+State of the resolution.
+
+At any given point during resolution, we have:
+- ... (TODO describe the various fields)
+
+Properties of Resolution: TODO describe the most important methods of `Resolution`
+
+### ResolutionProcess
+
+[definition](https://github.com/alexarchambault/coursier/blob/462c16d6db98d35e180a25b0f87aa47083ad98aa/core/shared/src/main/scala/coursier/core/ResolutionProcess.scala)
+
+Proceeds with the resolution per se, starting from an initial state (a `Resolution` instance), until the final one.
+
+Goes from one state to another by determining whether the current state:
+- needs some extra metadata about some versions of some modules (some IO is needed, with a cache and/or repositories), or
+- needs to take into account previously fed metadata (just calculations, no IO, giving the next state).
+
+So each step is either IO or calculations. Several IO steps can occur in a row, if some parent POMs or dependency imports are needed to get the full picture about a given module. A calculation step is either the last step, or followed by some IO. So the steps are like: IO, IO, IO, calculations, IO, IO, calculations, IO, calculations -> done. The last step is necessarily some calculations (IO can't end it - the newly fetched metadata of an IO step needs to be taken into account by a calculation step).
+
+TODO Describe ResolutionProcess a bit more...
+
+### Artifacts
+
+Once the resolution is done, we get the final `Resolution`. This `Resolution`can provide:
+- final dependency list,
+- artifact list,
+- dependency graph,
+- ...
+
+TODO Describe all of these a bit more
+
+## Cache / downloading
+
+### Fetching metadata
+
+What we need during resolution: fetching metadata, possibly several ones at once.
+
+TODO Describe the function to supply to `ResolutionProcess` to fetch metadata
+
+### Resolution result: artifact list
+
+TODO How we can fetch artifacts per se...
+
+### Cache structure on disk
+
+TODO More about the cache (structure on disk, error tracking, TTL tracking, locks)