Global Requirements for OpenStack Projects

Why Global Requirements?

During the Havana release cycle we kept running into coherency issues with trying to install all the OpenStack components into a single environment. The issue is that syncing of requirements.txt between projects was an eventually consistent problem. Some projects would update quickly, others would not. We'd never have the same versions specified as requirements between packages.

Because of the way that python package installation with pip works, this means that if you get lucky you'll end up with a working system. If you don't you can easily break all of OpenStack on a requirements update.

An example of how bad this had gotten is that python-keystoneclient would typically be installed / uninstalled 6 times during the course of a devstack gate run during Havana. If the last version of python keystoneclient happened to be incompatible with some piece of OpenStack a very hard to diagnose break occurs.

We also had an issue with projects adding dependencies of python libraries without thinking through the long term implications of those libraries. Is the library actively maintained? Is the library of a compatible license? Does the library duplicate the function of existing libraries that we already have in requirements? Is the library python 3 compatible? Is the library something that already exists in Linux Distros that we target (Ubuntu / Fedora). The answer to many of these questions was no.

Global requirements gives us a single place where we can evaluate these things so that we can make a global decision for OpenStack on the suitability of the library.

Since Havana we've also observed significant CI disruption occurring due to upstream releases of software that are are incompatible (whether in small or large ways) with OpenStack. So Global Requirements also serves as a control point to determine the precise versions of dependencies that will be used during CI.

Solution

The mechanics of the solution are relatively simple. We maintain a central list of all the requirements (global-requirements.txt) that are allowed in OpenStack projects. This is enforced for requirements.txt, test-requirements.txt and extras defined in setup.cfg. This is maintained by hand, with changes going through CI.

We also maintain a compiled list of the exact versions, including transitive dependencies, of packages that are known to work in the OpenStack CI system. This is maintained via an automated process that calculates the list and proposes a change back to this repository. A consequence of this is that new releases of OpenStack libraries are not immediately used: they have to pass through this automated process before we can benefit from (or be harmed by) them.

Format

global-requirements.txt supports a subset of pip requirement file contents. Distributions may only be referenced by name, not URL. Options (such as -e or -f) may not be used. Version specifiers, environment markers and comments are all permitted. A single distribution may be listed more than once if different specifiers are required with different markers - for instance, if a dependency has dropped Python 2.7 support.

upper-constraints.txt is machine generated and nothing more or less than an exact list of versions.

Enforcement for Test Runs

Devstack

When USE_CONSTRAINTS is set True, devstack uses the pip -c option to pin all the libraries to known good versions. edit-constraints can be used to unpin a single constraint, and this is done to install libraries from git. This is the recommended way to use devstack.

When USE_CONSTRAINTS is set False, devstack overwrites the requirements.txt and test-requirements.txt for all installed projects with the versions from global-requirements.txt. Projects that are not in projects.txt get 'soft' updates, ones that are get 'hard' updated. This attempts to ensure that we will get a deterministic set of requirements installed in the test system, and it won't be a guessing game based on the last piece of software to be installed. However due to the interactions with transitive dependencies this doesn't actually deliver what we need, and is not recommended.

Tox

We are working on the necessary changes to use upper-constraints.txt in tox jobs but it is not yet complete.

Enforcement in Projects

All projects that have accepted the requirements contract (as listed in projects.txt) are expected to run a requirements compatibility job. This job ensures that a project can not change any dependencies to versions not compatible with global-requirements.txt. It also ensures that those projects can not add a requirement that is not already in global-requirements.txt. This job should be proposed to infra at the same time as proposing the change to projects.txt in openstack/requirements.

Automatic Sync of Accepted Requirements

If an updated requirement is proposed to OpenStack and accepted to global-requirements.txt, the system then also automatically pushes a review request for the new requirements definition to the projects that include it.

For instance: if a review is accepted to global-requirements.txt that increases the minimum version of python-keystoneclient, the system will submit patches to all the OpenStack projects that list python-keystoneclient as a requirement or test requirement to match this new version definition.

This is intended as a time saving device for projects, as they can fast approve requirements syncs and not have to manually worry about whether or not they are up to date with the global definition.

Tools

All the tools require openstack_requirements to be installed (e.g. in a Python virtualenv). They all have help, which is the authoritative documentation.

update-requirements

This will update the requirements in a project from the global requirements file found in .. Alternatively, pass --source to use a different global requirements file:

update-requirements --source /opt/stack/requirements /opt/stack/nova

Entries in all requirements files will have their versions updated to match the entries listed in the global requirements. Excess entries will cause errors in hard mode (the default) or be ignored in soft mode.

generate-constraints

Compile a constraints file showing the versions resulting from installing all of global-requirements.txt:

generate-constraints -p /usr/bin/python2.7 -p /usr/bin/python3.4 \
  -b blacklist.txt -r global-requirements.txt > new-constraints.txt

edit-constraints

Replace all references to a package in a constraints file with a new specification. Used by devstack to enable git installations of libraries that are normally constrained:

edit-constraints oslo.db "-e file://opt/stack/oslo.db#egg=oslo.db"

Proposing changes

Look at the Review Guidelines and make sure your change meets them.

All changes to global-requirements.txt may dramatically alter the contents of upper-constraints.txt due to adding or removing transitive dependencies. As such you should always generate a diff against the current merged constraints, otherwise your change may fail if it is incompatible with the current tested constraints.

Regenerating involves five steps.

1. Install the dependencies needed to compile various Python packages:

   sudo apt-get install $(bindep -b)

2. Create a reference file (do this without your patch applied):

   generate-constraints -p /usr/bin/python2.7 -p /usr/bin/python3.4 \
     -b blacklist.txt -r global-requirements.txt > baseline

3. Apply your patch and generate a new reference file:

   generate-constraints -p /usr/bin/python2.7 -p /usr/bin/python3.4 \
     -b blacklist.txt -r global-requirements.txt > updated

4. Diff them:

   diff -p baseline updated

5. Apply the patch to upper-constraints.txt. This may require some fiddling. edit-constraint can do this for you when the change does not involve multiple lines for one package.

Review Guidelines

There are a set of questions that every reviewer should ask on any proposed requirements change. Proposers can make reviewing easier by including the answers to these questions in the commit message for their change.

General Review Criteria

• No specifications for library versions should contain version caps

  As a community we value early feedback of broken upstream requirements, so version caps should be avoided except when dealing with exceptionally unstable libraries.

  If a library is exceptionally unstable, we should also be considering whether we want to replace it over time with one that is stable, or to contribute to the upstream community to help stabilize it.

• Libraries should contain a sensible known working minimum version

  Bare library names are bad. If it's unknown what a working minimum is, look at the output of pip freeze at the end of a successful devstack/tempest run and use that version. At least that's known to be working now.

• Commit message should refer to consuming projects(s)

  Preferably, the comments should also identify which feature or blueprint requires the new specification. Ideally, changes should already be proposed, so that its use can be seen.

• The blacklist is for handling dependencies that cannot be constrained. For instance, linters which each project has at a different release level, and which make projects fail on every release (because they add rules) -those cannot be globally constrained unless we coordinate updating all of OpenStack to the new release at the same time - but given the volunteer and loosely coupled nature of the big tent that is infeasible. Dependencies that are only used in unconstrained places should not be blacklisted - they may be constrained in future, and there's no harm caused by constraining them today. Entries in the blacklist should have a comment explaining the reason for blacklisting.

For new Requirements

• Is the library actively maintained?

  We really want some indication that the library is something we can get support on if we or our users find a bug, and that we don't have to take over and fork the library.

  Pointers to recent activity upstream and a consistent release model are appreciated.

• Is the library good code?

  It's expected, before just telling everyone to download arbitrary 3rd party code from the internet, that the submitter has taken a deep dive into the code to get a feel on whether this code seems solid enough to depend on. That includes ensuring the upstream code has some reasonable testing baked in.

• Is the library python 3 compatible?

  OpenStack will eventually need to support python 3. At this point adding non python 3 compatible libraries should only be done under extreme need. It should be considered a very big exception.

• Is the library license compatible?

  The library should be licensed as described in Licensing requirements, and the license should be described in a comment on the same line as the added dependency. If you have doubts over licensing compatibility, like for example when adding a GPL test dependency, you can seek advice from Robert Collins (lifeless), Monty Taylor (mordred) or Jim Blair (jeblair).

• Is the library already packaged in the distros we target (Ubuntu latest / Fedora latest)?

  By adding something to OpenStack global-requirements.txt we are basically demanding that Linux Distros package this for the next release of OpenStack. If they already have, great. If not, we should be cautious of adding it. finding-distro-status

• Is the function of this library already covered by other libraries in global-requirements.txt?

  Everyone has their own pet libraries that they like to use, but we do not need three different request mocking libraries in OpenStack.

  If this new requirement is about replacing an existing library with one that's better suited for our needs, then we also need the transition plan to drop the old library in a reasonable amount of time.

For Upgrading Requirements Versions

• Why is it impossible to use the current version definition?

  Everyone likes everyone else to use the latest version of their code. However, deployers really don't like to be constantly updating things. Unless it's actually impossible to use the minimum version specified in global-requirements.txt, it should not be changed.

  Leave that decision to deployers and distros.

• Changes to update the minimum version of a library developed by the OpenStack community can be approved by one reviewer, as long as the constraints are correct and the tests pass.

Finding Distro Status

From the OpenStack distro support policy:

OpenStack will target its development efforts to latest Ubuntu/Fedora, but will not introduce any changes that would make it impossible to run on the latest Ubuntu LTS or latest RHEL.

As such we really need to know what the current state of packaging is on these platforms (and ideally Debian and SUSE as well).

For people unfamiliar with Linux Distro packaging you can use the following tools to search for packages:

• Ubuntu - http://packages.ubuntu.com/
• Fedora - https://apps.fedoraproject.org/packages/

For upper-constraints.txt changes

If the change was proposed by the OpenStack CI bot, then if the change has passed CI, only one reviewer is needed and they should +2 +A without thinking about things.

If the change was not proposed by the OpenStack CI bot, and does not include a global-requirements.txt change, then it should be rejected: the CI bot will generate an appropriate change itself. Ask in #openstack-infra if the bot needs to be run more quickly.

Otherwise the change may be the result of recalculating the constraints which changed when a global-requirements.txt change is proposed. In this case, ignore the changes to upper-constraints.txt and review the global-requirements.txt component of the change.