ESMValTool policy on backward compatibility#


Development of recipes or conducting project-related work may require a rather long period of time during which new versions of the ESMValTool might become available. For a good user experience and a seamless workflow, users and developers need to know before upgrading to a new version if and how their work might be affected (backward compatibility). This includes, for instance, information about changes to technical features such as syntax of recipes and configuration files, and interfaces of shared functions, but also changes that affect the results of an ESMValTool run, e.g. modification of algorithms or changes to the order of operators. It is therefore essential that users and developers have the best advice on how and when to upgrade to new versions.

While trying to minimise the impact of new code developments on users and developers by maintaining backward compatibility where possible, this cannot always be guaranteed. A very restrictive policy might delay the ESMValTool development and make it more complex for developers to contribute.

This document outlines the key principles of an updated ESMValTool policy on backward compatibility.


Release: A numbered version of ESMValCore / ESMValTool that has been released to the community, e.g. 2.4.0. This policy relates only to backward compatibility of releases, not to interim revisions of the main branch. Release numbers are of the format x.y.z, where:

  • x indicates a major release

  • y indicates a minor release

  • z indicates a patch release

Backward-incompatible change: A change in ESMValCore or ESMValTool that causes a recipe to no longer run successfully (a breaking change), or which results in scientifically significant changes in results (a science change).

Breaking change: A change which causes a previously working recipe to no longer run successfully.

Science change: A change that alters scientific results. We do not formally distinguish between trivial science changes (e.g. from changes in the order of calculations) and more significant changes that would affect interpretation, although the detail that we communicate will share any understanding that we have regarding expected impact.

Benign third-party dependency changes: A change over which we have no control, but which we believe will only have trivial technical impacts (such as a change in font). Such changes are outside of the scope of this policy, though we will communicate about those we are aware of.

Developer of backward-incompatible change: For the purpose of this policy, developer is the individual that is responsible for the pull request (PR) that is not backward compatible.

Recipe developer: Someone who is developing a recipe that is not (yet) integrated into the repository.

Recipe user: For the purpose of this policy, a recipe user is anyone who runs a recipe using a release of ESMValTool. In this context, someone can be both a recipe developer and a recipe user, but they perform different activities in each capacity.

Recipe maintainer: First contact point for integrated recipes in case of problems with that recipe (see also Maintaining a recipe).

Integrated recipes: Recipes that are contained within the main branch of the ESMValTool repository, and can therefore be updated by any developer in line with the above guidance. Note that the recipe can be updated by someone other than the original author.

User recipes: Recipes developed by any developer outside of the main branch of the repository (i.e. on a dev/feature branch or outside the repository completely), and therefore cannot be updated by anyone else.


The ESMValTool and ESMValCore policy on backward compatibility aims at balancing two competing needs: the occasional need of improvements or maintenance to break backward compatibility and the need for stability for existing users and developers. The following aspects are covered by this policy:

  • Key principles and approaches to backward compatibility

  • Guidelines and requirements for developers of backward-incompatible changes

  • Communication with users and developers about backward-incompatible changes

Not within the scope of this policy are:

  • Versioning scheme of ESMValTool

  • Breakage of recipes due to changes in input data or dependencies. This is covered by the broken recipe policy.

Expectations of developers, users & funders#

Stakeholders and their expectations and aims:

Projects / Funders

  • Aim to facilitate scientific discovery

  • Expect deliverables, e.g. new features/recipes

  • Expect reproducible results

Recipe users

  • Expect the recipe to work

  • Expect the recipe to be easy to run

  • Expect reproducible results

  • Expect easy installation of ESMValTool

Recipe developers

  • Develop recipes

  • Expect their recipe to keep working with every new release of ESMValCore

  • Expect ESMValCore bugfixes and new features to become available quickly

  • Expect reproducible results

  • Expect easy installation of ESMValTool

Core developers and recipe maintainers

  • Fix bugs

  • Add ESMValCore features requested by recipe developers

  • Try to accommodate ESMValCore features contributed to by recipe developers

  • Maintain existing recipes

  • Add new recipes

  • Try to help (other) recipe developers with contributing their recipe

  • Try to make installation as easy as possible

There is a tension between making new features available and keeping everything as is. New features facilitate scientific discovery because they enable recipe developers to do new research (e.g. analyse more data, new data, or perform a different analysis). Ensuring that every recipe ever made works with every new feature is technically a lot of work, more than we have funding for. Therefore we need to make sure that new features are added regularly, but we respect the timescale on which recipe developers work when removing outdated features. Writing a paper and getting it published may take up to a year, so this seems a good timescale for larger changes. For changes that only affect a few users, shorter timescales could be acceptable. It is also good to note that we are part of a large software ecosystem (ESMValTool currently depends on over 500 different software packages), so we may not always be able to control at what pace changes are made to the software that we depend upon.

Two-way communication about new and removed features is needed to make this work. This requires active involvement from both the people developing the new features and the recipe developers. ESMValTool core developers and ESMValCore core developers need to make sure they clearly communicate changes. In the first place, this is done by writing good descriptions in issues and pull requests on GitHub, but some of this material also makes it to the changelog (where the GitHub pull requests are linked). It is highly recommended to communicate a relevant selection (e.g. important new, scheduled for removal, and removed features) also by other means, to ensure we reach as many people potentially affected as possible (see Guidance on handling *backward-incompatible changes* section below). We organize monthly community meetings where recipe developers can learn about the latest developments and everyone is welcome to join, ask questions, and provide feedback.

To meet the needs of users and funders, we should take reproducibility of older results seriously, but this should not hold us back from advancing our tools. We can support this by uploading a well tested container image to an archive that provides a DOI and by providing clear instructions on how to use such containers.

Helping developers to upgrade#

Recipe users of ESMValTool should be able to successfully run integrated recipes using a release, since all backward-incompatible changes introduced between releases will have been fixed before the release is created. Please note the broken recipe policy.

However, recipe developers working on user recipes must be provided with information to enable them to adapt their code to resolve issues related to backward-incompatible changes when backward-incompatible changes are introduced to the main branch / when a release of ESMValTool is created.

Guidance on handling backward-incompatible changes#

As well as helping users to handle backward-incompatible changes, the policy and surrounding tools must help developers avoid making backward-incompatible changes. Not many ideas are developed on this yet, but components should include:

  • Testing; backward-incompatible changes should be discovered as early in the development process as possible. This motivates continued investment in automated testing. To discover backward-incompatible changes early on in the development cycle, every night a selection of recipes is run on CircleCI. A recipe can be added to the test suite by adding it to the directory esmvaltool/recipes/testing. Only add recipes that require a small amount of data, i.e. considerably less than a gigabyte.

  • Guidance on how to minimise the likelihood of introducing backward-incompatible changes and how to use deprecation warnings when needed (see developer guidance).

  • Instructions on how to provide text for the release notes to assist recipe developers to adapt their recipe in light of the backward-incompatible change

  • General instructions for recipe developers working on user recipes to enable them to adapt their code related to backward-incompatible changes (see ESMValTool_Tutorial: issue #263).

  • The developer or reviewer must tag the core development team to notify them of the backward-incompatible change, and give at least 2 weeks for objections to be raised before merging to the main branch. If a strong objection is raised the backward-incompatible change should not be merged until the objection is resolved.

Guidance on releasing backward-incompatible changes#

During the release process, the following information must be provided:

  • Release notes: The release notes are already documented in the ESMValTool Changelog and ESMValCore Changelog, and “backward-incompatible changes” is the first section after “Highlights”.

    • backward-incompatible changes: This section must include clear instructions detailing how a recipe developer should adapt their code for each item in this section, whether the adapted code would introduce a science change, and the list of affected or fixed integrated recipes that had to be updated due to the backward-incompatible changes, if applicable (to provide further examples to recipe developers working on user recipes of how to adapt code).

    • Developer guidance: Developers of backward-incompatible changes must:

      • write and include the information required for the “backward-incompatible changes” section in the PR that introduces the backward-incompatible change

      • share details of the backward-incompatible change at the next monthly ESMValTool community meeting

    • Communication: The release notes must be shared with the community (for example, via the User mailing list and the Community repository) at the point the first release candidate is made, highlighting the “backward-incompatible changes” section. The User Engagement Team should organise the communication of new releases together with the Release manager.