Add global config reasearch

Author: majamassarini

research/global-config/index.md

@@ -0,0 +1,281 @@
+---
+title: Global configuration
+authors:
+  - mmassari
+sidebar_position: 1
+---
+
+# Global configuration styles
+
+I will analyze 3 approaches for managing configuration relationships:
+
+- templating
+- global configuration + overlays
+- inheritance
+
+# 1. Templating
+
+## https://github.com/packit/templeates/config/simple-pull-from-upstream.yaml.j2
+
+```yml
+# Packit pull-from-upstream config
+specfile_path: { { specfile_path } }
+
+upstream_package_name: { { upstream_package_name } }
+downstream_package_name: { { downstream_package_name } }
+upstream_project_url: { { upstream_package_url } }
+upstream_tag_template: v{version}
+
+jobs:
+  - job: pull_from_upstream
+    trigger: release
+    dist_git_branches:
+      - fedora-rawhide
+  - job: koji_build
+    trigger: commit
+    allowed_pr_authors: ["packit", { { allowed_pr_authors } }]
+    dist_git_branches:
+      - fedora-rawhide
+```
+
+## https://gitlab.gnome.org/packit/templates/configs/gnome-tests.yaml.j2
+
+```yml
+# Gnome default tests config
+jobs:
+  - job: tests
+    trigger: pull_request
+    packages: [{{ downstream_package_name }}]
+    tmt_plan: "smoke|full|packit-integration|{{ tmt_other_plans }}"
+    targets:
+      - fedora-rawhide
+  {% if tests_on_commit %}
+  - job: tests
+    trigger: commit
+    packages: [{{ downstream_package_name }}]
+    tmt_plan: "smoke|full|packit-integration|{{ tmt_other_plans }}"
+    targets:
+      - fedora-rawhide
+  {% endif %}
+```
+
+## https://gitlab.gnome.org/package/packit.yaml
+
+```yml
+# A gnome package packit config
+templates:
+  - https://github.com/packit/templeates/config/simple-pull-from-upstream.yaml.j2
+    vars:
+      specfile_path: specfile_path
+      upstream_package_name: upstream_package_name
+      downstream_package_name: downstream_package_name
+      upstream_project_url: upstream_project_url
+      allowed_pr_authors: allowed_pr_authors
+  - https://gitlab.gnome.org/packit/templates/configs/gnome-tests.yaml.j2
+      downstream_package_name: downstream_package_name
+      tmt_other_plans: tmt_other_plans
+      tests_on_commit: false
+```
+
+# 2. Global config + overlay
+
+## https://github.com/packit/templates/configs/standard-pull-from-upstream.yaml.j2
+
+```yml
+# Packit pull-from-upstream config
+specfile_path: { { specfile_path } }
+
+upstream_package_name: { { upstream_package_name } }
+downstream_package_name: { { downstream_package_name } }
+upstream_project_url: { { upstream_package_url } }
+upstream_tag_template: v{version}
+
+jobs:
+  - job: pull_from_upstream
+    trigger: release
+    dist_git_branches:
+      - fedora-rawhide
+  - job: koji_build
+    trigger: commit
+    allowed_pr_authors: ["packit", { { allowed_pr_authors } }]
+    dist_git_branches:
+      - fedora-rawhide
+```
+
+## https://gitlab.gnome.org/packit/templates/configs/default_packit.yaml.j2
+
+```yml
+# Gnome default packit config
+config:
+  base: https://github.com/packit/templates/configs/standard-pull-from-upstream.yaml.j2
+  values:
+    allowed_pr_authors: gnome-admins
+
+jobs:
+  - job: tests
+    trigger: pull_request
+    packages: [{{ downstream_package_name }}]
+    tmt_plan: "smoke|full|packit-integration|{{ tmt_other_plans }}"
+    targets:
+      - fedora-rawhide
+  {% if tests_on_commit %}
+  - job: tests
+    trigger: commit
+    packages: [{{ downstream_package_name }}]
+    tmt_plan: "smoke|full|packit-integration|{{ tmt_other_plans }}"
+    targets:
+      - fedora-rawhide
+  {% endif %}
+```
+
+## https://gitlab.gnome.org/package/packit.yaml
+
+```yml
+# A gnome package packit config
+config:
+  base: https://gitlab.gnome.org/packit/templates/configs/default_packit.yaml.j2
+  values:
+    specfile_path: specfile_path
+    upstream_package_name: upstream_package_name
+    downstream_package_name: downstream_package_name
+    upstream_project_url: upstream_package_url
+    tmt_other_plans: package-tests
+    tests_on_commit: false
+```
+
+# 3. Inheritance
+
+## https://github.com/packit/templates/configs/standard-pull-from-upstream.yaml
+
+```yml
+# Packit pull-from-upstream config
+specfile_path: -OVERRIDE ME-
+
+upstream_package_name: -OVERRIDE ME-
+downstream_package_name: -OVERRIDE ME-
+upstream_project_url: -OVERRIDE ME-
+
+upstream_tag_template: v{version}
+
+jobs:
+  - job: pull_from_upstream
+    trigger: release
+    dist_git_branches:
+      - fedora-rawhide
+  - job: koji_build
+    trigger: commit
+    allowed_pr_authors: ["packit"]
+    dist_git_branches:
+      - fedora-rawhide
+```
+
+## https://gitlab.gnome.org/packit/templates/configs/default_packit.yaml
+
+```yml
+# Gnome default packit config
+inherit: https://github.com/packit/templates/configs/standard-pull-from-upstream.yaml
+
+jobs:
+  - job: koji_build
+    allowed_pr_authors: ["packit", "gnome-admins"]
+
+  - job: tests
+    trigger: pull_request
+    tmt_plan: "smoke|full|packit-integration"
+    targets:
+      - fedora-rawhide
+```
+
+## https://gitlab.gnome.org/package/packit.yaml
+
+```yml
+# A gnome package packit config
+inherit: https://gitlab.gnome.org/packit/templates/configs/default_packit.yaml
+
+specfile_path: specfile_path
+upstream_package_name: upstream_package_name
+downstream_package_name: downstream_package_name
+upstream_project_url: upstream_package_url
+
+jobs:
+  - job: tests
+    packages: ["downstream_package_name"]
+    tmt_plan: "smoke|full|packit-integration|package-tests"
+```
+
+# PROs and CONs
+
+## Templating
+
+### Pros
+
+Flexible, probably the most flexible implementation that allows to freely mix configuration snippets for creating a customized final configuration.
+
+### Cons
+
+The "pure" templating mechanism, in the above example, requires the package maintainer to know that koji builds, in the gnome ecosystem, should be allowed for any **gnome-admin**, instead _inheritance_ and _global config + overlays_ encapsulate well the knowledge in the middle layer packit config.
+
+Templating is flexible but on the other end it is more error prone; there is no _base configuration_ and a packager can list templates in the wrong order.
+
+Probably, in the end, the packager will use smaller config snippets, decreasing performance and readability.
+
+## Global config + overlays
+
+### Pros
+
+Good knowledge encapsulation in middle layers (see the **gnome-admin** for allowed_pr_authors in the above example).
+
+Explicit and thus easily readable, since the use of templating.
+
+Flexible, config snippets can easily be removed using template conditional functionalities (as in the above example for the test job with trigger commit).
+
+### Cons
+
+The templating syntax can be more error prone if compared with inheritance.
+
+## Inheritance
+
+### Pros
+
+Concise, it's the most concise syntax we could use and probably the least error prone.
+
+### Cons
+
+Poor flexibility, I don't see an easy way to disable the above test job with trigger commit.
+
+Not really explicit, even though we use a placeholder it is harder to recognize the keys that need overriding.
+
+# Implementation
+
+Personally I find the _global config + overlays_ approach the best and in this case we would need to:
+
+- add the following keys to the `PackageConfig` class:
+
+```yml
+config:
+  base: https://gitlab.gnome.org/packit/templates/configs/default_packit.yaml.j2
+  values: ...
+```
+
+- load the packit.yaml file and search for the `config` key in it. If a `config` key is found we need to **recursively** look for the _base config_ and start processing all the templates in the chain, creating a new temporary packit.yml that will be used instead of the original one.
+  I see this code tied with the `LocalProject` class but I can be wrong.
+  We should make the new code work both for the packit CLI and the packit-service. Thinking at packit CLI, we should probably stay flexible and let the `base: URI` be also a local url (like `file:///`).
+
+- let the user know what the final configuration looks like (both for CLI and service).
+
+## Jinja2 vs Ansible library
+
+For template management I would probably just use the **jinja2 template library**, even though we can also think about the ansible library.
+The ansible library could let us use `built-in filters and functions` but I don't see use cases for them and as a cons it has a heavier dependency footprint.
+
+# Performances
+
+Splitting the configuration in multiple configuration files will lead obviously to worst performance. Personally I don't see a way to prevent it.
+
+We can limit the number of recursion steps; 3/4 steps are, from my point of view, more than enough. Having a recursion limit will avoid an infinite recursion for malformed configurations.
+
+# packit-service defaults
+
+It could happen that the packit-service config defaults for the "Fedora CI instance" and those for the "Usual instance" (as an example) diverge.
+
+If it happens we could create two _hidden_, _inner_ packit config bases, one per instance, which will always be used for merging any packit config we process; in this way the differences will be grouped explicitly in a single place and we could, probably, enable and disable jobs for one instance or the other (as an example `pull-from-upstream` should not appear in fedora ci packit configuration) just using templating.