The observations made above can all be considered when designing a more broadly applicable project template that may be used in a variety of contexts. To this end, it is helpful to conceptualize some core guiding principles that should be followed by all data analysis projects, particularly under the Open Science paradigm.

Because data analysis projects often involve writing new software, a data analysis project structure requires support for both data analysis proper and software development. Software development methods fall outside the scope of this work, but some concepts are useful in the context of data analysis, particularly for ad hoc data analysis. For instance, many programming languages require specific folder layouts to create self-contained distributed software. For instance, to create a package with the Python programming language ( https://python.org/), a specific project layout must be followed. ⁹ This is also visible in Figure 1, with the presence of the project folder and many files specific to Python packages, crucially, in the locations required by Python build backends. Something similar occurs for many programming languages, such as R ¹⁰ and Rust, ¹¹ among others.

However, a researcher may not want to create self-contained, distributed software. Languages such as Python and R ( https://www.r-project.org/) can interpret and execute single-file scripts to achieve certain goals (i.e., “scripting”). As scripting is fast, convenient, and easy to perform, it is the most common method of data analysis. Scripting provides flexibility during the development process; however, this typically exacerbates the fragmentation of project structures. In particular, the environment of execution now becomes much more relevant: which packages are installed and at which versions, the order in which the scripts were read and executed, and, potentially, even the order of which lines are (manually) run becomes important to the success of the overall analysis.

This increased flexibility is obviously useful for the research process, which requires the ability to change quickly to adapt to new findings, especially during hypothesis-generating “exploratory” research. The principles presented here aim to retain this essential requirement of adaptability but, at the same time, push for increased standardization of methods, avoiding the most common and dangerous pitfalls that can be encountered during data analysis.

1. Use a version control system

At its core, software is a collection of text files, and this includes data analysis software. While producing code, it is important to record the differences between the different versions of these files. This is very useful, especially during the research process, to “retrace our steps” or to attempt new methodologies without the fear of losing any previous work. Such records are also useful as provenance information and potentially as proof of authorship, similar to what a laboratory notebook does for a “wet-lab” experimental researcher.

There is consolidated software that can be used as a version control system. An overwhelming majority of projects use git ( https://git-scm.com/) for this purpose, although others exist. Platforms that integrate git, such as GitHub ( github.com) and GitLab ( gitlab.com), are increasingly used for data analysis, both as a collaboration tool during the project and as a sharing platform afterwards.

The first principle should therefore be this: use a version control system, such as git.

A few practical observations stem from this principle: •

version control encourages good development practices, such as atomic commits, meaningful commit messages, and more, reducing the number of mistakes made while programming, and increasing efficiency by making debugging easier;

The use of a version control system also has implications for FAIR-ness. Leveraging remote platforms is fundamental to both Findability and Accessibility. Integrations of platforms such as GitHub with archives such as Zenodo ( https://zenodo.org/) allow developers to easily archive for long-term preservation their data analysis code, promoting Accessibility, Findability, and Reusability.

2. Documentation is essential

When working on a data analysis project, documentation is important for both the experimenter themselves and external users. Through ideal documentation, the rationale, process, and potentially the result of the analysis are presented to the user, together with practical steps on how to actually reproduce the work. As with all other aspects of data analysis, documentation takes many different forms, but is the most difficult thing to standardize for one simple reason: documentation is written by humans for human consumption. Documentation is therefore allowed high flexibility in structure, content, form, and delivery method.

Even though rigid standardization is impossible, some guidelines on how to write effective documentation can still be drawn, often from best practices in the much wider world of open-source software. We have already highlighted the fundamental role of the README file and its widespread adoption. This file contains high-level information about the project and is usually the first, and perhaps only, documentation that all users encounter and read. It is therefore essential that core aspects of the project are delivered through the README file, such as the following: •

the aim of the project, in clear, accessible language;

Other aspects of the project, such as a list of contributors, may also be included in the README file. The README file may also be called DESCRIPTION, although README is a much more widely accepted standard.

Additional documentation can be added to the project in several ways (see Figure 1). A common documentation file is the CONTRIBUTING file, which contains information on how to contribute to the project, how authorship of eventual publications will be assigned, and other community-level information. The CODE_OF_CONDUCT file contains guidelines and policies on how the project is managed, the expected conduct of project members, and potentially how arising issues between project members are resolved. Such a file can be important to either projects open to collaboration from the public or large consortium-level projects. Another important documentation file in the Open Source community is the CHANGELOG file. It contains information on how the project changed over time and its salient milestones. For data analysis, it could be used to inform collaborators of important changes in the codebase, methodology, or any other news that might be important to announce and record. Additionally, together with the commit history, CHANGELOG files can be useful for tracking the provenance of the analysis, as we have already mentioned.

A common place to store documentation is the top level of the project repository, but some templates use the docs folder, also from guidelines used in the Python community (to use tools such as Sphinx ¹⁴ ).

We can conclude by reiterating that the second principle states that documentation is essential.

3. Be logical, obvious, and predictable

When a project layout is logical, obvious, and predictable, human users can easily and quickly understand and interact with it.

To be logical, a layout should categorize files based on their content and logically arrange them according to such categories. To be obvious, this categorization should make sense at a glance, even for non-experts. For instance, a folder named “scripts” should contain scripts (to be obvious) and only scripts (to be logical). To be predictable, a layout should adhere to community standards, so that it “looks” similar to other projects. This creates minimal friction when a user first encounters the project and desires to interact with it.

This principle is also present in aspects of project structure other than layout. For instance, the structure of documentation can also benefit from the same principles but in a different context: logically arranged, obvious in structure, and similar to other projects.

This might be the most difficult principle to follow because it largely depends on the community as a whole. For this reason, we hope that the analysis shown above, especially in Figure 1, and our proposed minimal structure (presented in the next sections) will be useful as guides to effectively implement this principle.

An additional benefit of standardizing project structure is that it allows for tools to leverage it, helping data analysts in repetitive tasks. For example, if all log files created during a workflow run were to be saved in the same folder, a tool could be configured to quickly delete them if the need arises. Were such files dispersed among input, output and even source file, this task may be more difficult and/or error-prone.

We can summarize this third principle like this: be logical, obvious, and predictable.

4. Promote (easy) reproducibility

Scientific Reproducibility has been and still is a central issue, particularly in the field of biomedical research. ^{15, 16} Scientific software developers hold crucial responsibility toward the scientific community of creating reproducible data analysis software.

“Reproducibility” can be understood as the ability of a third-party user to understand the research issue investigated by the project, how it was addressed, and practically execute the analysis proper again to obtain a hopefully similar and ideally identical result to the original author(s). This has two benefits: a reproducible analysis evokes more confidence in those who read and review it, and it makes it much easier to repurpose the analysis to similar data in the future.

In the modern era, scientists are equipped with powerful tools to enable reproducibility, such as containerization and virtualization. While a discussion on how reproducibility can be achieved eludes the scope of this article, the project layout can promote it, especially when all other principles presented here are respected. This increased adoption can be promoted by including obvious and easily implementable reproducibility methods in the project layout directly.

Workflow managers, such as Nextflow, ⁶ Snakemake, ⁷ and the Common Workflow Language (CWL), ⁸ are key tools to enable reproducibility. They allow a researcher to describe in detail the workflow used, from input files to the final output, offloading the burden of execution to the workflow manager. This allows greater transparency in the methodology used and even makes reproducibility a possibility in more complex data analysis scenarios. Additionally, some workflow managers are structured to promote the reusability of the analysis code, even in different architectures or high-performance computing environments. ⁸

Usage of these tools requires specific training. Tools like CWL are invaluable when performing complex, distributed data analysis tasks that require leveraging, for instance, distributed, high-performance computing infrastructures. In these cases, bioinformaticians and other data analysts would immediately see the inherent value of such technologies. However, untrained or low-skill analysts might not be aware of, or able to, use them. As we already mentioned in the Introduction, simple tools with low entry barriers would render reproducibility more accessible.

We conclude this section by stating the fourth and last principle: be (easily) reproducible.

We designed a very simple but powerful and flexible project layout together with a project management tool aimed at upholding the principles outlined in the previous section. We named this tool “Kerblam!”.

In particular, Kerblam! encourages the use of version control with Git (principle 1), allows documentation to be written near the code that it describes (principle 2), invites the analyst to use a simple, logical, obvious and predictable folder structure (principle 3) and makes using containerization tools such as Docker easier and less burdensome by the end user (principle 4). Additionally, it encourages the use of remote file storage (pushing for FAIR-er data), and allows researchers to create and publish readily executable container images to the public to re-run pipelines for reproducibility purposes (see Figure 2C).

The most basic skeleton of the project layout implemented by Kerblam! is shown in Figure 2A. The kerblam.toml file contains configuration information for Kerblam! and marks the folder as a Kerblam-managed project. Kerblam! provides a number of utility features out of the box on projects that adapt to the layout presented in Figure 2A or any other project structure after proper configuration.

Data management

Kerblam! can be used to manage a project’s data. It automatically distinguishes between input, output, and intermediate data, based on which folder the data files are saved in: the data folder contains intermediate data produced during the execution of the workflows, the data/in contains input data, and similarly, data/out contains output data. Furthermore, the user can define in the kerblam.toml configuration which input data files can be fetched remotely and from which endpoint. This allows Kerblam! to both fetch these files upon request ( kerblam fetch) and distinguish between remotely available input files and local-only files. Local-only files are deemed “precious” because they cannot be recreated easily. All other data files are “fragile,” as they can be deleted without repercussion to save disk space ( Figure 2B).

These distinctions between data types enable further functions of Kerblam!. kerblam data shows the number and size of files of all types to quickly check how much disk space is being used by the project. Fragile data can be deleted to save disk space with kerblam data clean and precious input data can be exported easily with kerblam data pack. kerblam data pack can also be used to export output data quickly to be shared with colleagues.

Allowing Kerblam! to manage the project’s data using these tools can offload several chores, usually performed manually by the experimenter.

Workflow management

Kerblam! can manage multiple workflows written for any workflow manager. At its core, it can spawn shell subprocesses that then execute a particular workflow manager, potentially one configured by the user. This allows Kerblam! to manage other workflow managers, making them transparent to the user and with a single access point.

Kerblam! can also act before and after the workflow manager proper to aid in several tasks. First, it can manage workflows in the src/workflows folder as if they were written in the root of the project. This is achieved by moving the workflow files from the said folder to the root of the repository just before execution. This allows for slimmer workflows that do not crowd the root of the repository or conflict with each other, thus being more consistent.

Second, it allows the concept of input data profiles. Data profiles are best explained using an example. Imagine an input file, input.csv, containing some data to be analyzed. The experimenter may wish to test the workflows that they have written with a similar, but, say, smaller test_input.csv. Kerblam! allows hot-swapping of these files just before the execution of the workflow manager through profiles. By configuring them in the kerblam.toml file, the experimenter can execute a workflow manager (with kerblam run) specifying a profile: Kerblam! will then swap these two files just before and just after the execution of the workflow to seamlessly use exactly the same workflow but with different input data, in this case for testing purposes.

Kerblam! supports out of the box GNU make as its workflow manager of choice ( https://www.gnu.org/software/make/). Indeed, makefiles can be run directly through it, with no further configuration by the user. Any other workflow manager can be used by writing tiny shell wrappers with the proper invocation command. The range of workflow managers supported out of the box by Kerblam! could increase in the future.

To run workflows, Kerblam! simply creates subshells, executes the proper commands to launch the workflow (or the user-configured shell script), and eventually pipes the outputs of the subshell to the parent shell. This leaves complete control of the workflow to the workflow manager, making the Kerblam! integration seamless.

Containerization support

Containers can be managed directly using Kerblam!. By writing container recipes in src/dockerfiles, Kerblam! can automatically execute workflow managers inside the containers, seamlessly mounting data paths and performing other housekeeping tasks before running the container. As previously stated, Kerblam! works “above” workflow managers. Therefore, the reader might question the usefulness of a containerization wrapper at the level of Kerblam! if the workflow manager of choice already supports it. This containerization feature is meant to be used when a workflow manager would be inappropriate. For instance, very small analyses might not warrant the increased development overhead to use tools such as CWL. Kerblam! allows even shell scripts to be containerized anyway, making even the smallest analyses reproducible.

With these capabilities, Kerblam! promotes reproducibility and allows experienced and inexperienced users alike to perform even the simplest analyses in a reproducible manner.

Pipeline export

Workflows managed by Kerblam! with an available container can be automatically exported in a reproducible package through kerblam package. This creates a preconfigured container image ready to be uploaded to a container registry of choice together with a compressed tarball containing information on how to (automatically) replay the input analysis: the “replay package”.

The process automatically strips all unneeded project files, leading to small container images.

The replay package can be inspected manually by a potential examiner and either re-run manually or through the convenience function kerblam replay, which recreates the same original project layout, fetches the input container, and runs the packaged workflow.

The Kerblam! analysis flow

Kerblam! favors a very specific methodology when analyzing data, starting with an empty git repository. First, upload the input data to a remote archive (in theory, promoting FAIR-er data). Then, configure Kerblam! to download the input data and write code and workflows for its analysis, potentially in isolated containers or with specific workflow management tools. During development, periodically clean out intermediate and output files to check whether the correct execution of the analysis has become dependent on the local-only state. Finally, package the results and pipelines into the respective environments and share them with the wider public (e.g., as a GitHub release or in an archive such as Zenodo).

We believe that this methodology is simple yet flexible and robust, allowing for high-quality analyses in a wide variety of scenarios. This workflow can also be thought of as “stateless”, as it attempts to steer the user away from storing and relying upon previous runs of the same workflows. This philosophy makes replication easier and less error-prone as, essentially, every workflow run is independent from any other.

This “analysis flow” contrasts with other tools available online that attempt to solve or ameliorate the same issues that Kerblam! addresses. We have selected two of them to highlight these differences: “data science operations”, or DSO ( https://github.com/Boehringer-Ingelheim/dso) and “Data Analysis Project management”, or DAP ( https://github.com/molinerisLab/dap).

DSO, similarly to Kerblam!, leverages other tools to provide its functions, such as Data Version Control (DVC, https://github.com/iterative/dvc), Quarto ( https://quarto.org/), and Git ( https://git-scm.com), among others. It revolves around the concept of “stage”, a single step of the analysis workflow, with predefined inputs and outputs. To isolate each step, it creates individual “stage folders” with input, output, source and report folders as well as configuration files. By leveraging extensive configuration and DVC, the authors of DSO seem to prefer an analysis flow that preserves and version-controls both code and data, syncing them both with remote endpoints for collaboration. DVC, in particular, is aimed at the management of Machine Learning models. In such models, recording exactly which input data was used to train a specific version of a model is essential, so such an approach may be warranted. Indeed, DVC lets users “commit” and remotely store data similarly to code. However, we argue that for most data analysis tasks such versioning is not necessary, as the input data stays largely the same and the computational demands of the analysis are not so high to require careful storage of all output artifacts.

DAP focuses on the Python programming language, leveraging Anaconda ( https://anaconda.org/) for environment management and Snakemake ⁷ for workflow management. DAP also uses direnv ( https://direnv.net/) to update PATH variable with useful shortcuts. DAP creates two main folders to structure the project: a “workflow” folder, with code and configuration files, and a “workspaces” folder, with different project “versions”. DAP “versions” fulfill similar goals as DSO/DVC commits do. Each version of the project is a different workspace folder, which contains symbolic links to a specific (sub) set of files in the “workflow” folder for this specific “version” of the project. A DAP project also uses Git ( https://git-scm.com/) to provide overall version control. Similar to DSO, by laying out different versions of the same project next to each other, DAP also promotes a “state-based” workflow of sorts. When changes to workflows are required, the whole project is copied over, edited, and preserved alongside older copies.

While this is indubitably valuable if one often needs to refer to previous versions of the project, we believe it adds an additional layer of complexity in file and folder structure. This is especially due to the extensive usage of symbolic links, references and other opaque techniques that, while increasing development speed, might hinder the accessibility of the project to new collaborators or reproducers.

Kerblam! is a command-line tool. This means that experience with command line tools is required to use it. This is indeed quite an obstacle when approaching researchers not trained in computer science, as the real and perceived complexity of the command line triggers repudiation of the whole practice. The development of a graphical user interface, as well as interactive and accessible training materials may help surmount this barrier.

Similarly, the usage of Git from the command line may incur the same issues. Fortunately, many graphical interfaces to Git exist. The Git website hosts a list of them, divided by operating system: https://git-scm.com/downloads/guis.

While external dependencies were kept at a minimum, the installation of Kerblam! still requires the user to autonomously install other tools, such as Git and Docker. This might provide additional friction when beginning to use the tool. Methods to completely overcome these issues are unclear.

Availability

Kerblam! is a free and open-source software available on GitHub at https://github.com/MrHedmad/kerblam. It is written in Rust and may be compiled to support GNU/Linux-flavored operating systems, MacOS, and Windows. Alternatively, GitHub releases provide precompiled artifacts for both operating systems. The full documentation of Kerblam! is available at https://kerblam.dev. Active support for Kerblam! and its development are guaranteed for the foreseeable future.