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docs: Pretty up the yumrepofs doc, pending further expansion

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# yumrepofs
Yumrepofs is a virtual Yum repo server. Packages or metadata do not co-exist on disk, but is rather stored in blob storage (ex. S3)
`yumrepofs` is a technology that creates and manages a virtual yum repo server. Packages and/or metadata do not exist on disk together but is instead stored in a blob storage (for example, S3).
The goal is to be able to store RPMs and serve repositories without the need for NFS.
## Goals
## Serving
There are a few goals of `yumrepofs`. However, **the primary goal** is to be able to store and serve RPM packages and repositories without the need for a remote-backed storage solution like NFS or local server disks.
The lifecycle of a dnf call is as follows:
## Repository Serving
* dnf sends a request to {baseurl}/repodata/repomd.xml
* Server responds with metadata on where to find further metadata (ex. primary, filelists and other)
* dnf queries metadata listed in repomd.xml
Most of `dnf`'s common commands has calls with a (mostly) simple lifecycle:
The most important file in this case is definitely primary.xml. This is where dnf finds information about packages, and specifically where to fetch them from.
* `dnf` sends a request to a repository looking for `$BASEURL/repodata/repomd.xml`
* Server responds with the initial metadata and more information to find the rest (primary, filelists, modules, etc)
* `dnf` queries metadata listed in repomd.xml
Yumrepofs populates `href` fields with a path like this `Packages/{TASK_ID}/{KEY_ID}/{RPM_NAME}`. This in turn signes a S3 url and redirects the end user.
The most important metadata file in this case is `primary.xml`. This is where `dnf` finds information about packages, and specifically where to fetch them from.
## Updating the repo
And this is where `yumrepofs` comes in: It populates `href` fields with a path like `Packages/$TASK_ID/$KEY_ID/$RPM_NAME`. This in turn signs an S3 URL and thus will redirect the user's dnf client appropriately.
Let's start from the build step. Post-build, Peridot runs `createrepo_c` on all individual RPMs to create necessary metadata and stores that.
## Updating the repository/repositories
When an update request is sent to `yumrepofsupdater` containing a specific build, `GetBuildArtifacts` is called to fetch RPMs+Metadata.
After a build is completed in peridot, the following happens:
Using this method, packages wouldn't need to coexist on disk to create a new repo, because that metadata could then be swapped into the correct place
* Peridot runs `createrepo_c` on all individual RPMs - This creates necessary metadata and stores that information for use later
* When an update request is sent to `yumrepofsupdater` containing a specific build, `GetBuildArtifacts` is called - This fetches the RPMs *and* corresponding metadata
* `yumrepofsupdater` fetches the latest revision of that repository and architecture, unmarshalls it, and swaps the metadata from the `GetBuildArtifacts` call
Yumrepofsupdater first fetches the latest yumrepofs revision of that repo+arch, unmarshals it, then swaps in-place the metadata from the `GetBuildArtifacts` call.
Using this method, packages never need to coexist on disk to create a new repository in the traditional way, because the metadata could then be swapped into the correct place as needed.
Currently in-repo artifacts that are no longer to be found (based on NVRA), will be removed, just like with `createrepo_c`.
As of this writing, in-repo artifacts that are no longer found (based on its NEVRA) will be removed. This is just like with `createrepo_c` on a normal, on-disk repository.
## Comps/groups
## Comps, groups, and you
Using Peridot Catalogs, comps/groups can be declared that is later applied to every revision of target repo.
Using Peridot Catalogs, comps and groups can be declared that is then applied to a revision of the target repository. [comps2peridot](https://github.com/rocky-linux/peridot-releng/tree/main/comps2peridot) can be used to generate comps in a format Peridot accepts, for example.
[comps2peridot](https://github.com/rocky-linux/peridot-releng/tree/main/comps2peridot) can be used to generate comps in a format Peridot accepts.
## SQLite/Hashed hrefs
A configuration example can be found at [peridot-rocky](https://git.rockylinux.org/rocky/peridot-rocky/-/tree/r9)
`yumrepofs` has something called "hashed variants" that can be created on demand. Just like pungi's `hashed_directories` directive, this changes package href's to the format of `Packages/$INITIAL/$RPM_NAME`. For example: `Packages/b/bash-5.1.8-4.el9.x86_64.rpm`. If an end user decides to do a reposync from a hashed `yumrepofs` repository, they will get a sorted repository directory structure that has packages sorted by their first alphanumeric character, similar to the Rocky Linux repositories as well as the Fedora Project's repositories.
## Sqlite/Hashed hrefs
Yumrepofs has something called "hashed variants" that can be created on demand. This changes package hrefs to the format of `Packages/{INITIAL}/{RPM_NAME}` (ex. `Packages/b/bash-5.1.8-4.el9.x86_64.rpm`).
This variant also runs `sqliterepo_c` on metadata and stores the sqlite files in S3. This is also served the same way as any yumrepofs artifact.
This particular variant also runs `sqliterepo_c` on metadata and stores the sqlite files in S3, just like any other `yumrepofs` artifact. This is particularly useful for cases where applications that manage repositories (or "content") rely on sqlite metadata in a repository.