SERVER-88198 Document how to classify variants that exist in evergreen configurations (#21085)

GitOrigin-RevId: 96666fcd50d572645e503bae5aeb4b56a441d529

docs/evergreen-testing/README.md

@@ -9,5 +9,6 @@ Documentation about how MongoDB is tested in Evergreen.
 - [Multiversion Testing](multiversion.md)
 - YAML configuration
   - [Configuration for Evergreen Integration](yaml_configuration/configuration.md)
+  - [Build Variants](yaml_configuration/buildvariants.md)
   - [Task ownership tags](yaml_configuration/task_ownership_tags.md)
   - [Task selection tags](yaml_configuration/task_selection_tags.md)

docs/evergreen-testing/yaml_configuration/buildvariants.md

@@ -0,0 +1,68 @@
+# Build Variants
+
+This document describes build variants (a.k.a. variants, or builds, or buildvariants) that are used in `mongodb-mongo-*` projects.
+To know more about build variants, please refer to the [Build Variants](https://docs.devprod.prod.corp.mongodb.com/evergreen/Project-Configuration/Project-Configuration-Files#build-variants) section of the Evergreen wiki.
+
+## YAML files structure
+
+Build variant configuration files are in `etc/evergreen_yml_components/variants` directory.
+They are merged into `etc/evergreen.yml` and `etc/evergreen_nightly.yml` with Evergreen's [include](https://docs.devprod.prod.corp.mongodb.com/evergreen/Project-Configuration/Project-Configuration-Files#include) feature.
+
+Inside `etc/evergreen_yml_components/variants` directory there are more directories,
+which are in most cases platform names (e.g. amazon, rhel etc.) or build variant group names (e.g. sanitizer etc.).
+
+Be aware that some of these files could be also used or re-used to be merged into `etc/system_perf.yml` which is used for `sys-perf` project.
+
+## Build Variants in `mongodb-mongo-master` and `mongodb-mongo-master-nightly`
+
+`mongodb-mongo-master` evergreen project uses `etc/evergreen.yml` and contains all build variants for development, including all feature-specific, patch build required, and suggested variants.
+
+`mongodb-mongo-master-nightly` evergreen project uses `etc/evergreen_nightly.yml` and contains build variants for public nightly builds.
+
+## Required and Suggested Build Variants
+
+"Required" build variants are defined as any build variant with a `!` at the front of its display name in Evergreen.
+These build variants also have `required` tag.
+
+[Required Patch Builds Policy](https://wiki.corp.mongodb.com/display/KERNEL/Required+Patch+Builds+Policy)
+
+"Suggested" build variants are defined as any build variant with a `*` at the front of its display name in Evergreen.
+These build variants also have `suggested` tag.
+
+## Build Variants after branching
+
+In each of platform or build variant group directory there can be these files:
+
+- `test_dev.yml`
+
+  - these files are merged into `etc/evergreen.yml` which is used for `mongodb-mongo-master` project on master branch
+  - after branching on all new branches these files are merged into `etc/evergreen_nightly.yml` which is used for a new branch `mognodb-mongo-vX.Y` project
+
+- `test_dev_master_and_lts_branches_only.yml`
+
+  - these files are merged into `etc/evergreen.yml` which is used for `mongodb-mongo-master` project on master branch
+  - after branching for LTS release (v7.0, v8.0 etc.) on a new branch these files are merged into `etc/evergreen_nightly.yml` which is used for a new branch `mognodb-mongo-vX.Y` project
+  - **important**: all tests that are running on these build variants will NOT run on a new Rapid release (v7.1, v7.2, v7.3, v8.1, v8.2, v8.3 etc.) branch projects
+
+- `test_dev_master_branch_only.yml`
+
+  - these files are merged into `etc/evergreen.yml` which is used for `mongodb-mongo-master` project on master branch
+  - after branching on all new branches these files are NOT used
+  - **important**: all tests that are running on these build variants will NOT run on a new branch projects
+
+- `test_release.yml`
+
+  - these files are merged into `etc/evergreen_nightly.yml` which is used for `mongodb-mongo-master-nightly` project on master branch
+  - after branching on all new branches these files are merged into `etc/evergreen_nightly.yml` which is used for a new branch `mognodb-mongo-vX.Y` project
+
+- `test_release_master_and_lts_branches_only.yml`
+
+  - these files are merged into `etc/evergreen_nightly.yml` which is used for `mongodb-mongo-master-nightly` project on master branch
+  - after branching for LTS release (v7.0, v8.0 etc.) on a new branch these files are merged into `etc/evergreen_nightly.yml` which is used for a new branch `mognodb-mongo-vX.Y` project
+  - **important**: all tests that are running on these build variants will NOT run on a new Rapid release (v7.1, v7.2, v7.3, v8.1, v8.2, v8.3 etc.) branch projects
+
+- `test_release_master_branch_only.yml`
+
+  - these files are merged into `etc/evergreen_nightly.yml` which is used for `mongodb-mongo-master-nightly` project on master branch
+  - after branching on all new branches these files are NOT used
+  - **important**: all tests that are running on these build variants will NOT run on a new branch projects