Dependency Management for OpenStack Projects¶
Why do we have a global requirements list?¶
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
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 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.
The mechanics of the solution are relatively simple. We maintain a
central list of all the requirements (
that are allowed in OpenStack projects. This is enforced for
doc/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.
Each project team may also optionally maintain a list of “lower
bounds” constraints for the dependencies used to test the project in a
lower-constraints.txt file. If the file exists, the requirements
check job will ensure that the values it contains match the minimum
values specified in the local requirements files, so when the minimums
lower-constraints.txt will need to be updated at the
same time. Per-project test jobs can be configured to use the file for
unit or functional tests.
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. Environment markers
and comments are permitted. Version specifiers are only allowed for excluding
versions, not setting minimum required versions (minimum
required versions may optionally be specified in
per-project). 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 uses the pip
-c option to pin all the libraries to known good
edit-constraints can be used to unpin a single constraint, and
this is done to install libraries from git.
Enforcement in Projects¶
All projects that have accepted the requirements contract (as listed
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
check-requirements job should
be merged in infra before proposing the change to
Updating dependency settings can be a two-step process. If you create
both patches at the same time, #2 can use
Depends-On to link it to
Adding a new dependency¶
Add the dependency to
openstack/requirements, including any instructions for excluding versions or choosing different versions for python 2 or 3. As part of the same review, run the following command
tox -e generate. Be sure to only update or add constraints related to your addition.
Add the dependency to the appropriate requirements file(s) within the project tree, providing a minimum version specifier. If the
lower-constraints.txtfile exists in the project tree, then update it at the same time.
Removing a dependency¶
Remove the dependency from the requirements and constraints files within the project tree. Be sure to check the docs folder as well.
Check for other projects using the dependency. If none do, update
openstack/requirementsto remove the item from
Updating the minimum version of a dependency¶
openstack/requirements. If the version there is lower than the desired version, prepare a patch to update the setting.
Update the minimum version in the relevant requirements file(s) in the project tree. If the
lower-constraints.txtfile exists, then update it in the same patch.
Excluding a version of a dependency¶
We need to maintain a consistent set of exclusions across all projects
to ensure that the
upper-constraints.txt list of versions stays
openstack/requirements. If it does not exclude the version, prepare a patch to update the specifiers for the dependency. If the excluded version is currently being used in
upper-constraints.txt, update that file in the same patch.
Lowering the value in upper-constraints.txt may result in excluding a version that another project depends on. Check for this situation before proceeding.
Update the relevant requirements files in the project tree to add the exclusion. It is not necessary to copy the exclusion to every project that uses the dependency.
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.
Library specifications should not contain a minimum version
Individual projects may want to start with different “lower bound” versions of dependencies, so we do not track those explicitly in the
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 excluded - 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 excluding.
Reviews that only update
projects.txtshould be workflow approved alongside or before other reviews in order to have the OpenStack Proposal Bot propagation be useful as soon as possible for the other projects. For project removal or addition, the +1 from the current PTL (or core if the PTL proposed the change) should be enough.
Reviews proposed by the OpenStack Proposal Bot to
requirements.txtare allowed to approved and workflowed by a single core reviewer.
Per project requirements allows the review process to stay the same during the
freeze. This is due to the proposal bot not proposing changes to projects
requirements.txt. Projects are responsible for their own
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.txtwe 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
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.
Is the library required for OpenStack project or related dev or infrastructure setup? (Answer to this should be Yes, of course) Which?
Please provide details such as gerrit change request or launchpad bug/blueprint specifying the need for adding this library.
If the library release is managed by the Openstack release process does it use the cycle-with-intermediary release type?
This is needed to ensure that updated releases that consume requirements updates are available for integration/coninstallability tests with other projects.
Do I need to update anything else?
When new library is added, initial version of release needs to be added to
upper-constraints.txt. After that, OpenStack Proposal Bot will propose updates.
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, Gentoo, and SUSE as well).
For people unfamiliar with Linux Distro packaging you can use the following tools to search for packages:
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 only
upper-constraints.txt entry for a new library release,
then the change should be approved if it passes the tests. See the
README.rst in openstack/releases for more details of the release
If the change was not proposed by the OpenStack CI bot, and is not
related to releasing one of our libraries, 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.
Most of the work is done by stable-maint in the releases project. The releases project ensures valid stable releases (little to no API level changes, bugfix only, etc). Once released, the new version is requested to be updated in requirements. The following restrictions are in place to help ensure stable branches do not break.
In stable branches, we usually only update constraints for projects managed within the OpenStack community. Exceptions are made for other projects when there are gate issues. Those updates must be proposed by hand.
The requirements team also verifies the new version’s requirements changes line up with the requirements in the stable branch (GR and UC).
These should be few and far between on stable branches, mainly masking known bad versions or in extreme adding a maximum version allowable for a package. We work to remove these caps as well. Raising effective minimums is only acceptable during Phase I, and only due to security issues.
In nearly all cases this is not allowed. An example where this is allowed would be: A dependency of a dependency has an issue that impacts OpenStack. It wasn’t listed in global-requirements.txt but it is required. In order to block the affected releases and still be able to keep requirements in sync, we list the library in global-requirements.txt and update all projects that require it.
All tools require
openstack_requirements to be installed (e.g. in a Python
virtualenv). All tools have the
--help option, which is the authoritative
documentation for that command.
Compile a constraints file showing the versions resulting from installing all
generate-constraints -p /usr/bin/python2.7 -p /usr/bin/python3.6 \ -r global-requirements.txt -b blacklist.txt --version-map 3.6:3.4 \ --version-map 3.6:3.5 > new-constraints.txt
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"
Combine multiple lower-constraints.txt files to produce a list of the highest version of each package mentioned in the files. This can be used to produce the “highest minimum” for a global lower constraints list (a.k.a., the “TJ Maxx”).:
build-lower-constraints input1.txt input2.txt
Where the input files are lower-constraints.txt or requirements.txt files from one or more projects.
If the inputs are requirements files, a lower constraints list for the requirements is produced. If the inputs are lower-constraints.txt, the output includes the highest version of each package referenced in the files.
Run the validation checks from the
requirements-check job locally
requirements-check tox environment (test is run via ansible
with non-installed playbooks).:
tox -e requirements-check -- /path/to/repo/to/test
Tox & Stable Branches¶
The community relies on
tox for test automation, but managing its
installation has changed depending the versions of other tools being used.
Most projects adopted a script to provide a facade for developers to invoke in
tox.ini file. The script, named
to be install and managed the installation of dependencies needed for tests.
The script had issues with newer versions of pip, which ended up being smarter about how to install dependencies while adhering to constraint files.
tox_install.sh in my project, what should I do with it?¶
If you’re project has a copy of
tox_install.sh, you should remove it. All
references to the script should be converted to use appropriate upper
constraint files, which is typically found in the project’s
An example can be found here.
Why are stable branches failing due to issues with
Depending on the state of a project’s stable branches, you might notice the following error:
ERROR: You must give at least one requirement to install (see "pip help install")
This error is caused by a newer version of pip being used on a stable branch
that isn’t compatible with the
You can fix the issue one of two ways.
The first way is by removing
tox_install.sh all together from the stable
branch and convert the branch to use constraints like you did with master.
The second way, which might be required depending on the extent of the changes
being made to the stable branch, is to patch
tox_install.sh to make it
compatible with newer versions of pip. An example of how to do that can be
found in this patch.