Creating “slim” stream mirrors of modular RPM packages enables the SIMP ISO to support modularity while still mix/matching specific RPMs from various sources. For instance, it permits adding a few epel-modular packages without distributing epel-modular's entire collection of modules x streams x packages.

See https://simp-project.atlassian.net/wiki/pages/resumedraft.action?draftId=2193326084 for a summary of motivating challenges and requirements .

Process

The “slim” mirroring process must happen during/before the ISO build process.

1. At the same time each modular RPM is acquired, save its source repo’s modulemd metadata.

2. Use unique N:S:V:C:A combinations from the resulting modular RPMs to determine which “slim” module streams to reconstruct.

3. For each unique “slim” modular stream: generate modulemd metadata for all relevant RPMs

4. Combine all “slim” modules' modulemd data into a single data structure and write it to modules.yaml

5. Rebuild the modular repository using createrepo_c (or createrepo_mod) with the new modules.yaml file

Implementing slim modular repos

Modular RPM data/metadata to get/record/cache

At a minimum, a new field (only required for modular RPMs) that specifies the N:S (module:stream) for modular packages should be added to the build’s packages.yaml.

Problems that are probably solved

yumdownloader can’t see RPMs in modules/streams that aren’t enabled

1. Add an optional field to packages.yaml entries to specify the N:S: for each modular RPM

2. Identify and enable all unique N: from packages.yaml (fail if there are conflicting S:)

3. dnf module enable each N:S: before beginning to use yumdownloader

4. Individual yumdownloader runs can change repository mirrors, which may be out of sync with each other and have different modulemd data.

   1. (When using the yumdownloader) the modulemd metadata must be fetched at the same time as the RPM is downloaded, in order to preserve the precise state of that RPM’s modular metadata.
      

A single RPM could be part of multiple streams in an upstream repository

Nothing in the modulemd data prevents this, so we need a way to determine the correct stream.

This isn’t a problem for External packages, because We already need to add a field to explicitly set N:S: to packages.yaml.

However: there is no way to hint streams in *pkglist.txt files for minimal BaseOS packages (unless we do something elaborate, like add comment keywords and a parser)

1. Most BaseOS EL8 modules have a default stream; use that if it exists

2. We could also default to the only stream.
   This is hacky, but it will work for EL8.3—Base OS (i.e., AppStream) modules without a default stream are currently very rare, and at the moment all of them have a single stream:

   # dnf module --disablerepo=\* --enablerepo=appstream list | grep -v '\[d\]' CentOS Linux 8 - AppStream Name Stream Profiles Summary 389-ds 1.4 389 Directory Server (base) libselinux-python 2.8 common Python 2 bindings for libselinux mod_auth_openidc 2.3 Apache module suporting OpenID Connect authentication parfait 0.5 common Parfait Module pki-core 10.6 PKI Core module for PKI 10.6 or later pki-deps 10.6 PKI Dependencies module for PKI 10.6 or later

3. This leaves open a potential edge-case: if in the future, we require an RPM from a Base OS modules without a default stream but ships with multiple streams (again: current population: 0), it will fail and there is no way to hint

   • We should probably have a way of formally declaring N:S for *pkglist.txt Base OS RPMs in the future. Some possibilities:

     • A separate *pkglist.modularity.txt file

     • N:S-declaring directives in the comments of *pkglist.txt

     • Could this be combined with packages.yaml? (not without a major rewrite)

Unsolved problems

What are the “Fetch RPM” flow differences between Base OS (prune_packages) & External (yumdownloader) packages?

How can we know the URL/path to an RPM’s source repo’s repomd.xml file?

This is simple enough to do by hand for an individual package, but I’m not sure how to automate it yet. Here are some ideas:

1. Option 1: see if yumdownloader can be convinced to display the repo root’s URL, like --urls does with the RPM

   1. haven’t found an option that does this

2. Option 2: walk up the dir tree until we find metadata (hacky, expensive)

3. Option 3: (somehow) find/define the DNF cache that was used to download the RPM and (somehow) fish out the modulemd data that was used for that specific package

Separate yumdownloader runs may result in RPMs for the same N:S having different N:S:V:C:A

Different RPMs could be sourced from different versions (V:) of the same module stream if yumdownloader pulls them from different repo mirrors that are out of sync with each other. Using the heuristic of a “slim” module stream per unique N:S:V:C:A , this would result in multiple module streams instead of one.

This is a rare edge case that V: is specifically intended to catch, and it seems correct to fail instead of rebuild a modular stream using RPMs from a different (stream) versions. However, I can’t really demonstrate that the potential impact of this is worth prioritizing its implementation.

The strongest impacts I came up with so far rely on the fact that there’s a good chance that, between two stream versions, the combined set of RPMs won’t match either stream exactly. But unless your mirrors were really out of sync, this probably wouldn’t matter much. The stream version is a snapshot in time of all the modulemd metadata for the stream; it doesn’t actually affect the RPM’s resolution.

(I honestly don’t know many details of how/when V: is used other than “highest wins”, but it might lead to weird edge cases:)

1. There’s a (staggeringly) remote change that the newer stream version dropped package(s) or one of its packages has a new dependency

2. The slim repo will use one N:S:V or the other, but neither upstream precisely matches its RPMs. After re-integrating with the full upstream repo or mirror, DNF might miss an update by deciding it already know the stream version resolve using the wrong stream version for some of the packages, to the wrong versions, miss updates

3. There may be other reasons to do with inter-modular dependencies.

4. TL;DR: Not sure if failing is the best way forward—input welcome.

Are there conditions where streams don’t provide C:A information when packages are noarch?

No. By the time they are built, they will have a context and arch.

Undecided

[Should/how to] persist cached modulemd metadata for already-downloaded RPMs between builds?

The current yumdownloader process

True or false: “Any mirrored “slim” module MUST NOT have multiple streams”

1. This sounds reasonable, but is it actually true?

2. It’s impossible to install multiple streams on a single SIMP server, but do we think we’d need to package multiple “slim” module streams for agents?

   1. My current inclination is to assume “no.” Given our approach toward modularity in general, that seems like a really edgy edge case.