Flywheel Delivers Reproducibility

Flywheel is committed to supporting reproducible research computations.  We make many software design decisions guided by this commitment. This document explains some key reproducibility challenges and our decisions. 

Reproducibility challenges

Flywheel’s scientific advisory board member, Victoria Stodden, writes that reproducible research must enable people to check each other’s work. In simpler times, research articles could provide enough information so that scientists skilled in the art could check published results by repeating the experiments and computations. But the increased complexity of modern research and software makes the methods section of a published article insufficient to support such checking. The recognition of this problem has motivated the development of many tools.

Reproducibility and data

A first requirement of reproducibility is a clear and well-defined system for sharing data and critical metadata. Data management tools are a strength of the Flywheel software. The tools go far beyond file formats and directory trees, advancing data management for reproducible research and the FAIR principles.

Through experience working with many labs, Flywheel recognized the limitations of modern tools and what new technologies might help. Many customers wanted to begin managing data the moment they were acquired rather than waiting until they were ready to upload fully analyzed results. Flywheel built tools that acquire data directly from imaging instruments – from the scanner to the database. In some MRI sites, Flywheel even acquires the raw scanner data and implements site-specific image reconstruction. The system can also store and search through an enormous range of metadata including DICOM tags as well as project-specific custom annotations and tags.

Reproducibility and containers

A second requirement of reproducibility is sharing open-source software in a repository, such as GitHub or BitBucket. Researchers, or reviewers, can read the source code and in some cases they can download, install and run it. 

Based on customer feedback, Flywheel learned that (a) downloading and installing software – even from freely available open-source code on GitHub! – can be daunting, (b) customers often had difficulty versioning and maintaining software, as students and postdocs come and go, and (c) they would run the software many times, often changing key parameters, and have difficulty keeping track of the work they had done and the work that remained to be done. 

To respond to these challenges, Flywheel implemented computational tools based on container technology (Docker and Singularity). Implementing mature algorithms in a container nearly eliminates the burden of downloading, compiling, and installing critical pieces of software.  Containers include the compiled code along with all the dependencies, such as libraries in small virtual machines that can be run on many operating systems (PC, Mac, Linux, each with different variants). These small virtual machines (containers) can be run on a local machine or on a cloud system. This eliminates the burden of having to find the code, update all the dependencies, and compile.

Reproducibility and analyses: Introducing Gears

Once an algorithm is implemented in a container, Flywheel users run it. A lot. They wanted ways to record the precise input data as well as the algorithm version parameters that were used as they explored the data. The outputs also needed to be recorded. Such a complete record is difficult for individuals to implement; having such a record is necessary for reproducibility.

Flywheel solves these problems by creating a computational system for managed application containers, which we call Gears. The Gear is structured to record every parameter needed to perform an analysis. When the user runs a Gear, the input data, specific version of the container, all the parameters needed to run the container, and the output data are all recorded in the database. This is called an ‘Analysis’ and users perform and store hundreds of Analyses on a data set.

Because all the information about an Analysis is stored in the database associated with the study, people can re-run precisely the same Gear. It is also straightforward to run the same Gear using different data, or to explore the consequences of re-running the Gear after selecting slightly different parameters. Making Analyses searchable also helps people keep track of which Gears were run and which still need to be run. 

Reproducibility and documentation

Clear writing is vitally important to making scientific work reproducible. Tools that support clear and organized notes during the experiments are also very valuable. During the initial development, Flywheel partnered with Fernando Perez and the Jupyter (then iPython) team to implement tools that built on shared software. Flywheel continues to find ways to support these tools. Flywheel tools permit users to link their data to published papers, write documentation about projects and sessions, and add notes. This documentation is part of the searchable database, and Flywheel will continue to support users to incorporate clean and thorough documentation.