Hello fellow OpenStackers and Kollars (or Koalas) in particular, today is the day when I finally sit down to write up my thoughts on Kolla, and I mean *Kolla*, the container image building project for OpenStack and friends, nothing more (nor less). *** Some background on Kolla to get everyone on the same page (or close enough): Kolla builds images for production use. Kolla is upstream for TripleO containers and Kolla-Ansible deployment. These are container images, think OCI and Docker in particular since Kolla actually relies on Dockerfile format to specify build recipes and Docker runs building (TripleO runs buildah on the same recipes). Kolla supports three distributions as bases: CentOS (TripleO does override this for RHEL as well), Debian and Ubuntu. In the interim periods Kolla supports two distribution releases to ease/smooth the transition process for operators (like currently CentOS 7 and 8 in Train, while Ussuri is 8 only). What is more, each distro has two flavours (or 'types' as they are called for now): binary and source. This 'type' applies only to OpenStack software. Binary means that Kolla uses downstream packages (rpm/deb) while source means to use pip and install from official tarballs (or repos if it is master) and PyPI for deps, utilising venv to keep it separate from distro stuff. Finally, there is particular target architecture: x86_64, aarch64 and ppc64le. All of above affect the support matrix which is based on 3 or 4 dimensions (distro, flavour, arch and sometimes distro version). [1] Kolla offers a high level of customisability via various overrides levels. See docs for details [2] Kolla engine offers hierarchical approach to image building, under the assumption that more than one image is often deployed on the same machine so layer sharing is beneficial. Kolla helps collect sources for OpenStack projects to build desired versions of them. All recipes are templated using Jinja2 syntax. Images contain both run-time and, mostly in the case of 'source' flavour, build-time tools and libraries. *** That's it for the background. :-) So where is the problem you might ask? Oh, there are plenty. The general is that Kolla has lots of logic in templates. Thankfully, Kolla has macros for most stuff but still Jinja2 limitations make it hard to document exceptions (no inline comments in arrays anyone?). We lose visibility into real dependency graphs and may be easily reinstalling same stuff, any optimisations are limited and would require parsing of both Jinja2 and Dockerfile syntax. In general the current approach is ugly and becomes unwieldy (e.g. getting warnings about empty continuation lines). There is layering hell: dependency on long &&ed commands which are templated out to avoid useless layers. Support matrix is great for an overview. There is a hidden layer to it though, different combinations may support different features. This is not documented so far and not so easy to follow from sources. This stems from the fact that different extra components have different availability in distros, but this is not easily apparent from sources. Also, there is no way to turn feature on/off. You might do an override, but then again, which to keep, which not to? Which is which? Oh! Finally, the images tend to be heavy due to inclusion of build-time deps. *** That's it for issues (hopefully!) My idea is to reuse Kolla engine where it shines: sources collection, system of plugins, hierarchical building; but replace the part that smells - Jinja2 templating. Giving up on Jinja2 might encompass giving up on Dockerfile syntax, but that is optional and depends on what makes it more amenable to avoid further pitfalls. With giving up on Jinja2, the idea is to generate building recipes fully programmatically from Python. It would be possible to introduce "Features" - sets of packages to install based on the (distro, arch) tuple. This would result in more flexibility - turning them on/off (some could be optional, some not). There could be more than one optimization strategy regarding when packages get installed: you want only standalone blah-blah? Then Kolla won't be installing XYZ and ABC just because ugma-ugma and tickle-tickle require them and you "could save some space" (TM). In the same vein, features could declare which components are build-time and which are run-time and this would make it straightforward to separate the sides. The above effort could well be coordinated with different projects to reuse bindep contents. So far Kolla does not use bindep because it often installs too much and not enough at the same time. Do note it would still be bindep-less for external services. There would still need to exist a general mechanism for providing custom command executions required by some images. For contributors and cores this new approach would bring more sanity as to the scope of proposed changes. Also, it would be possible to get quick insight into feature support and autogenerate docs for that as well. Similarly, current concept of unbuildable images would no longer be required because unbuildability would be dictated by lack of support for a required feature. *** Looking forward to your opinions/thoughts. *** [1] https://docs.openstack.org/kolla/train/support_matrix.html [2] https://docs.openstack.org/kolla/train/admin/image-building.html -yoctozepto