Development guidelines

Design principles

  • simple first, complicated only when necessary

  • adopting generic established 3rd party solutions before implementing specific solutions

  • only uni directional dependencies between components/modules, no circles

  • only one language: Python (except, GUI of course)

Source code & Git repository

Code Rules

The are some rules or better strong guidelines for writing code. The following applies to all python code (and were applicable, also to JS and other code):

  • Use an IDE (e.g. vscode or otherwise automatically enforce code (formatting and linting). Use nomad qa before committing. This will run all tests, static type checks, linting, etc.

  • There is a style guide to python. Write pep-8 compliant python code. An exception is the line cap at 79, which can be broken but keep it 90-ish.

  • Test the public API of each sub-module (i.e. python file)

  • Be pythonic and watch this.

  • Document any public API of each sub-module (e.g. python file). Public meaning API that is exposed to other sub-modules (i.e. other python files).

  • Use google docstrings.

  • Add your doc-strings to the sphinx documentation in docs. Use .md, follow the example. Markdown in sphix is supported via recommonmark and AutoStructify

  • The project structure is according to this guid. Keep it!


These guidelines are partially enforced by CI/CD. As part of CI all tests are run on all branches; further we run a linter, pep8 checker, and mypy (static type checker). You can run nomad qa to run all these tests and checks before committing.

The CI/CD will run on all refs that do not start with dev-. The CI/CD will not release or deploy anything automatically, but it can be manually triggered after the build and test stage completed successfully.


The master branch of our repository is protected. You must not (even if you have the rights) commit to it directly. The master branch references the latest official release (i.e. what the current NOMAD runs on). The current development is represented by version branches, named vx.x.x. Usually there are two or more of these branched, representing the development on minor/bugfix versions and the next major version(s). Ideally these version branches are also not manually push to.

Instead you develop on feature branches. These are branches that are dedicated to implement a single feature. They are short lived and only exist to implement a single feature.

The lifecycle of a feature branch should look like this:

  • create the feature branch from the last commit on the respective version branch that passes CI

  • do your work and push until you are satisfied and the CI passes

  • create a merge request on GitLab

  • discuss the merge request on GitLab

  • continue to work (with the open merge request) until all issues from the discussion are resolved

  • the maintainer performs the merge and the feature branch gets deleted

While working on a feature, there are certain practices that will help us to create a clean history with coherent commits, where each commit stands on its own.

git commit --amend

If you committed something to your own feature branch and then realize by CI that you have some tiny error in it that you need to fix, try to amend this fix to the last commit. This will avoid unnecessary tiny commits and foster more coherent single commits. With amend you are basically adding changes to the last commit, i.e. editing the last commit. If you push, you need to force it git push origin feature-branch -f. So be careful, and only use this on your own branches.

git rebase <version-branch>

Lets assume you work on a bigger feature that takes more time. You might want to merge the version branch into your feature branch from time to time to get the recent changes. In these cases, use rebase and not merge. Rebase puts your branch commits in front of the merged commits instead of creating a new commit with two ancestors. It basically moves the point where you initially branched away from the version branch to the current position in the version branch. This will avoid merges, merge commits, and generally leave us with a more consistent history. You can also rebase before create a merge request, basically allowing for no-op merges. Ideally the only real merges that we ever have, are between version branches.

git merge --squash <other-branch>

When you need multiple branches to implement a feature and merge between them, try to use squash. Squashing basically puts all commits of the merged branch into a single commit. It basically allows you to have many commits and then squash them into one. This is useful if these commits where just made for synchronization between workstations or due to unexpected errors in CI/CD, you needed a save point, etc. Again the goal is to have coherent commits, where each commits makes sense on its own.

Often a feature is also represented by an issue on GitLab. Please mention the respective issues in your commits by adding the issue id at the end of the commit message: My message. #123.

We tag releases with vX.X.X according to the regular semantic versioning practices. After releasing and tagging the version branch is removed. Do not confuse tags with version branches. Remember that tags and branches are both Git references and you can accidentally pull/push/checkout a tag.

The main NOMAD GitLab-project (nomad-fair) uses Git-submodules to maintain its parsers and other dependencies. All these submodules are places in the /dependencies directory. There are helper scripts to install (, see setup) and commit changes to all submodules ( After merging or checking out, you have to make sure that the modules are updated to not accidentally commit old submodule commits again. Usually you do the following to check if you really have a clean working directory.

git checkout something-with-changes
git submodule update
git status

Terms and Identifiers

There are is some terminology consistently used in this documentation and the source code. Use this terminology for identifiers.

Do not use abbreviations. There are (few) exceptions: proc (processing); exc, e (exception); calc (calculation), repo (repository), utils (utilities), and aux (auxiliary). Other exceptions are f for file-like streams and i for index running variables. Btw., the latter is almost never necessary in python.


  • upload: A logical unit that comprises one (.zip) file uploaded by a user.

  • calculation: A computation in the sense that is was created by an individual run of a CMS code.

  • raw file: User uploaded files (e.g. part of the uploaded .zip), usually code input or output.

  • upload file/uploaded file: The actual (.zip) file a user uploaded

  • mainfile: The mainfile output file of a CMS code run.

  • aux file: Additional files the user uploaded within an upload.

  • repo entry: Some quantities of a calculation that are used to represent that calculation in the repository.

  • archive data: The normalized data of one calculation in nomad’s meta-info-based format.


Throughout nomad, we use different ids. If something is called id, it is usually a random uuid and has no semantic connection to the entity it identifies. If something is called a hash than it is a hash build based on the entity it identifies. This means either the whole thing or just some properties of said entities.

  • The most common hashes is the calc_hash based on mainfile and auxfile contents.

  • The upload_id is a UUID assigned at upload time and never changed afterwards.

  • The mainfile is a path within an upload that points to a main code output file. Since, the upload directory structure does not change, this uniquely ids a calc within the upload.

  • The calc_id (internal calculation id) is a hash over the mainfile and respective upload_id. Therefore, each calc_id ids a calc on its own.

  • We often use pairs of upload_id/calc_id, which in many context allow to resolve a calc related file on the filesystem without having to ask a database about it.

  • The pid or (coe_calc_id) is an sequential interger id.

  • Calculation handle or handle_id are created based on those pid. To create hashes we use nomad.utils.hash().

NOMAD-coe Dependencies

We currently use git submodules to maintain references to NOMAD-coe dependencies. All dependencies are python packages and installed via pip to your python environement.

This allows us to target (e.g. install) individual commits. More importantly, we can address c ommit hashes to identify exact parser/normalizer versions. On the downside, common functions for all dependencies (e.g. the python-common package, or nomad_meta_info) cannot be part of the nomad-FAIRDI project. In general, it is hard to simultaneously develop nomad-FAIRDI and NOMAD-coe dependencies.

Another approach is to integrate the NOMAD-coe sources with nomad-FAIRDI. The lacking availability of individual commit hashes, could be replaces with hashes of source-code files.

We use the branch nomad-fair on all dependencies for nomad-FAIRDI specific changes.


There are several steps to take, to wrap a NOMAD-coe parser into a nomad@FAIRDI parser:

  • Implement nomadcore.baseclasses.ParserInterface or a class with a similar constructutor and parse method interface.

  • Make sure that the meta-info is only loaded for each parse instance, not for each parser run.

  • Have a root package that bears the parser name, e.g. vaspparser

  • The important classes (e.g. the parser interface implementation) in the root module (e.g. vaspparser/

  • Only use sub-modules were necessary. Try to avoid sub-directories

  • Have a test module. Don’t go overboard with the test data.

  • Make it a pypi-style package, i.e. create script.

  • The package name should be the parser name, e.g. vaspparser.

  • Let the parser logging as it is. We will catch it with a handler installed on the root logger. This handler will redirect all legacy log events and put it though the nomad@FAIRDI treatment described below.

  • Remove all scala code.


We are rewriting all NOMAD-coe normalizers, see nomad.normalizing.


There are three important prerequisites to understand about nomad-FAIRDI’s logging:

  • All log entries are recorded in a central elastic search database. To make this database useful, log entries must be sensible in size, frequence, meaning, level, and logger name. Therefore, we need to follow some rules when it comes to logging.

  • We use an structured logging approach. Instead of encoding all kinds of information in log messages, we use key-value pairs that provide context to a log event. In the end all entries are stored as JSON dictionaries with @timestamp, level, logger_name, event plus custom context data. Keep events very short, most information goes into the context.

  • We use logging to inform about the state of nomad-FAIRDI, not about user behavior, input, data. Do not confuse this when determining the log-level for an event. For example, a user providing an invalid upload file, for example, should never be an error.

Please follow the following rules when logging:

  • If a logger is not already provided, only use nomad.utils.get_logger() to acquire a new logger. Never use the build-in logging directly. These logger work like the system loggers, but allow you to pass keyword arguments with additional context data. See also the structlog docs.

  • In many context, a logger is already provided (e.g. api, processing, parser, normalizer). This provided logger has already context information bounded. So it is important to use those instead of acquiring your own loggers. Have a look for methods called get_logger or attributes called logger.

  • Keep events (what usually is called message) very short. Examples are: file uploaded, extraction failed, etc.

  • Structure the keys for context information. When you analyse logs in ELK, you will see that the set of all keys over all log entries can be quit large. Structure your keys to make navigation easier. Use keys like nomad.proc.parser_version instead of parser_version. Use module names as prefixes.

  • Don’t log everything. Try to anticipate, how you would use the logs in case of bugs, error scenarios, etc.

  • Don’t log sensitive data.

  • Think before logging data (especially dicts, list, numpy arrays, etc.).

  • Logs should not be abused as a printf-style debugging tool.

Used log keys

The following keys are used in the final logs that are piped to Logstash. Notice that the key name is automatically formed by a separate formatter and may differ from the one used in the actual log call.

Keys that are autogenerated for all logs:

  • @timestamp: Timestamp for the log

  • @version: Version of the logger

  • host: The host name from which the log originated

  • path: Path of the module from which the log was created

  • tags: Tags for this log

  • type: The message_type as set in the LogstashFormatter

  • level: The log level: DEBUG, INFO, WARNING, ERROR

  • logger_name: Name of the logger

  • nomad.service: The service name as configured in

  • nomad.release: The release name as configured in

Keys that are present for events related to processing an entry:

  • nomad.upload_id: The id of the currently processed upload

  • nomad.calc_id: The id of the currently processed entry

  • nomad.mainfile: The mainfile of the currently processed entry

Keys that are present for events related to exceptions:

  • exc_info: Stores the full python exception that was encountered. All uncaught exceptions will be stored automatically here.

  • digest: If an exception was raised, the last 256 characters of the message are stored automatically into this key. If you wish to search for exceptions in Kibana, you will want to use this value as it will be indexed unlike the full exception object.