Document the channel logic (#610)

We got a lot of questions about the channel logic including the priority
and channel specific logic. This document tries to explain this better.

docs/advanced/channel_priority.md

@@ -0,0 +1,88 @@
+All logic regarding the decision which dependencies can be installed from which channel is done by the instruction we give the solver.
+
+The actual code regarding this is in the [`rattler_solve`](https://github.com/mamba-org/rattler/blob/02e68c9539c6009cc1370fbf46dc69ca5361d12d/crates/rattler_solve/src/resolvo/mod.rs) crate.
+This might however be hard to read.
+Therefore, this document will continue with simplified flow charts.
+
+# Channel specific dependencies
+When a user defines a channel per dependency, the solver needs to know the other channels are unusable for this dependency.
+```toml
+[project]
+channels = ["conda-forge", "my-channel"]
+
+[dependencies]
+packgex = { version = "*", channel = "my-channel" }
+```
+In the `packagex` example, the solver will understand that the package is only available in `my-channel` and will not look for it in `conda-forge`.
+
+The flowchart of the logic that excludes all other channels:
+
+``` mermaid
+flowchart TD
+    A[Start] --> B[Given a Dependency]
+    B --> C{Channel Specific Dependency?}
+    C -->|Yes| D[Exclude All Other Channels for This Package]
+    C -->|No| E{Any Other Dependencies?}
+    E -->|Yes| B
+    E -->|No| F[End]
+    D --> E
+```
+
+# Channel priority
+Channel priority is dictated by the order in the `project.channels` array, where the first channel is the highest priority.
+For instance:
+```toml
+[project]
+channels = ["conda-forge", "my-channel", "your-channel"]
+```
+If the package is found in `conda-forge` the solver will not look for it in `my-channel` and `your-channel`, because it tells the solver they are excluded.
+If the package is not found in `conda-forge` the solver will look for it in `my-channel` and if it **is** found there it will tell the solver to exclude `your-channel` for this package.
+This diagram explains the logic:
+``` mermaid
+flowchart TD
+    A[Start] --> B[Given a Dependency]
+    B --> C{Loop Over Channels}
+    C --> D{Package in This Channel?}
+    D -->|No| C
+    D -->|Yes| E{"This the first channel
+     for this package?"}
+    E -->|Yes| F[Include Package in Candidates]
+    E -->|No| G[Exclude Package from Candidates]
+    F --> H{Any Other Channels?}
+    G --> H
+    H -->|Yes| C
+    H -->|No| I{Any Other Dependencies?}
+    I -->|No| J[End]
+    I -->|Yes| B
+```
+
+This method ensures the solver only adds a package to the candidates if it's found in the highest priority channel available.
+If you have 10 channels and the package is found in the 5th channel it will exclude the next 5 channels from the candidates if they also contain the package.
+
+# Use case: pytorch and nvidia with conda-forge
+A common use case is to use `pytorch` with `nvidia` drivers, while also needing the `conda-forge` channel for the main dependencies.
+```toml
+[project]
+channels = ["nvidia/label/cuda-11.8.0", "nvidia", "conda-forge", "pytorch"]
+platforms = ["linux-64"]
+
+[dependencies]
+cuda = {version = "*", channel="nvidia/label/cuda-11.8.0"}
+pytorch = {version = "2.0.1.*", channel="pytorch"}
+torchvision = {version = "0.15.2.*", channel="pytorch"}
+pytorch-cuda = {version = "11.8.*", channel="pytorch"}
+python = "3.10.*"
+```
+What this will do is get as much as possible from the `nvidia/label/cuda-11.8.0` channel, which is actually only the `cuda` package.
+
+Then it will get all packages from the `nvidia` channel, which is a little more and some packages overlap the `nvidia` and `conda-forge` channel.
+Like the `cuda-cudart` package, which will now only be retrieved from the `nvidia` channel because of the priority logic.
+
+Then it will get the packages from the `conda-forge` channel, which is the main channel for the dependencies.
+
+But the user only wants the pytorch packages from the `pytorch` channel, which is why `pytorch` is added last and the dependencies are added as channel specific dependencies.
+
+We don't define the `pytorch` channel before `conda-forge` because we want to get as much as possible from the `conda-forge` as the pytorch channel is not always shipping the best versions of all packages.
+
+For example, it also ships the `ffmpeg` package, but only an old version which doesn't work with the newer pytorch versions.
+Thus breaking the installation if we would skip the `conda-forge` channel for `ffmpeg` with the priority logic.

mkdocs.yml

@@ -48,7 +48,10 @@ markdown_extensions:
   - pymdownx.inlinehilite
   - pymdownx.snippets
   - pymdownx.details
-  - pymdownx.superfences
+  - pymdownx.superfences:
+      custom_fences:
+        - name: mermaid
+          class: mermaid
   - pymdownx.tabbed:
       alternate_style: true
   - toc:
@@ -77,6 +80,7 @@ nav:
     - Tasks: advanced/advanced_tasks.md
     - Multi Platform: advanced/multi_platform_configuration.md
     - Info command: advanced/explain_info_command.md
+    - Channel Logic: advanced/channel_priority.md
   - Examples:
       - C++/Cmake: examples/cpp-sdl.md
       - OpenCV: examples/opencv.md