Rethinking how we make ready-to-use operating system images.

Unpacking RPM: an introduction

(tl;dr version: let’s start mapping out the RPM ecosystem as a whole - from high-level abstractions and use cases down to the bits and bytes - so we can make improvements and build the software ecosystem we really want.)

Hi! I’m Will Woods, and today I want to talk about… RPM.

[crowd boos, throws tomatoes]

I know, I know, everyone who’s talked to me at all in the past five years is sick of hearing me talk about RPM. But bear with me for a moment, because I think this is important.

Nearly everything we do in Fedora (and, by extension, in RHEL and CentOS) revolves around RPM. We’ve built entire software ecosystems around it:

..and so on and so forth. There’s a lot.

Here’s the thing: RPM wasn’t designed for any of this.

RPM was designed in the the mid-1990s, a faraway time when dial-up internet, floppy disks, and single-speed CD-ROM drives were the norm and source code was released by putting a tarball on the project’s FTP server.

Red Hat Linux 4.2 for i386 consisted of 459 RPMs - a big jump from the 387 packages available in RHL 3.0.3. (Fedora 27 for x86_64 currently has 66,913 packages, including updates.)

There’s a lot more to talk about here, but the overall point is: typical use-cases for the RPM ecosystem today are very different - orders of magnitude different - than when the tools and file formats were originally designed. A lot has changed over the years, and we’ve done some extremely clever (and some extremely ugly!) things to keep everything up and running, and to get it to handle all the new use cases that keep coming up.

So. It all works, at least. But are we sure that it works well?

Most projects keep their sources in git repos, but we’re still building RPMs from hand-imported tarballs and patches - the same way we’ve done it since 1997. Nobody loves dealing with patches and tarballs, but since it seems too hard (or too “drastic”) to change RPM to work the way we’d like, we just do what we always do:

  1. Shrug and hope The Community will deal with it
  2. Write and maintain custom one-off tools to handle the hard parts (like texlive does)
  3. Build other tools that work around the problem by adding another layer of code and metadata (mock-scm, tito, gofed, and so on)

I think we can do much, much better. There’s a lot of room for improvement in nearly every aspect of the RPM ecosystem. I think the problems we’ve been having with Modularity and 32-hour “nightly” composes suggest that we may have pushed parts of this system to its limits - or at least to a point where we can’t fix the problems by just adding another layer of code and metadata.

If we want to make the system work better, we need to understand how it works. We need to look past what it’s doing to why: what is the intent? What are the problems that users are trying to solve? And are we giving them good tools to solve those problems?

I think we need to look at the entire system, inside and out, from upstream sources to built images, and document how it all works at an abstract level: What are the most important tasks and use cases? What are the inputs and outputs? How do all the pieces interact? How do people actually use this stuff, here in the year 2018?

Once we understand how each part works and how they interact with each other, we can start designing improvements - or replacements - that work the way we want our software build ecosystem to work. Reproduceable builds? Sure! Atomic updates with rollback? That too! New metadata, like TestRequires? Absolutely!

All we have to do, in my opinion, is keep asking ourselves three questions:

  1. What is this part of the system actually trying to do?
  2. If we designed something to do this today, what would it look like?
  3. How do we get there from here?

This is the basic framework for what we’ve been doing inside Project Weldr so far, and it seems to be working pretty well for us. I’ve given a couple of talks about one particular part of the system - RPM scriptlets - and rethinking that piece made us able to build bootable images 100x faster than the existing tools. And we’re just getting started.

Next post: RPM package names!

Written by Will Woods on March 15, 2018