1. A brief overview of DataLad¶
There can be numerous reasons why you ended up with this handbook in front of you – We do not know who you are, or why you are here. You could have any background, any amount of previous experience with DataLad, any individual application to use it for, any level of maturity in your own mental concept of what DataLad is, and any motivational strength to dig into this software.
All this brief section tries to do is to provide a minimal, abstract explanation of what DataLad is, to give you, whoever you may be, some idea of what kind of tool you will learn to master in this handbook, and to combat some prejudices or presumptions about DataLad one could have.
To make it short, DataLad (www.datalad.org) is a software tool developed to aid with everything related to the evolution of digital objects.
It is not only keeping track of code, it is not only keeping track of data, it is not only making sharing, retrieving and linking data (and metadata) easy, but it assists with the combination of all things necessary in the digital workflow of data and science.
1.1. On data¶
Everyone uses data. But once it exists, it does not suffice for most data to simply reside unchanged in a single location for eternity.
Most data need to be shared – may it be a digital collection of family photos, a genomics database between researchers around the world, or inventory lists of one company division to another. Some data are public and should be accessible to everyone. Other data should circulate only among a select few. There are various ways to distribute data, from emailing files to sending physical storage media, from pointers to data locations on shared file systems to using cloud computing or file hosting services. But what if there was an easy, generic way of sharing and obtaining data?
Most data changes and evolves. A scientist extends a data collection or performs computations on it. When applying for a new job, you update your personal CV. The documents required for an audit need to comply to a new version of a common naming standard and the data files are thus renamed. It may be easy to change data, but it can be difficult to revert a change, get information on previous states of this data, or even simply find out how a piece of data came into existence. This latter aspect, the provenance of data – information on its lineage and how it came to be in its current state – is often key to understanding or establishing trust in data. In collaborative fields that work with small-sized data such as Wikipedia pages or software development, version control tools are established and indispensable. These tools allow users to keep track of changes, view previous states, or restore older versions. How about a version control system for data?
If data are shared as copies of one state of their history, keeping all shared copies up-to-date once the original data change or evolve is at best tedious, but likely impossible. What about ways to easily update data and its shared copies?
The world is full of data. The public and private sector make use of it to understand, improve, and innovate the complex world we live in. Currently, this process is far from optimal. In order for society to get the most out of public data collections, public data need to be FAIR: Findable, Accessible, Interoperable, and Reusable. Apart from easy ways to share or update shared copies of data, extensive metadata is required to identify data, link data collections together, and make them findable and searchable in a standardized way. Can we also easily attach metadata to our data and its evolution?
DataLad is a general purpose tool for managing everything involved in the digital workflow of using data – regardless of the data’s type, content, size, location, generation, or development. It provides functionality to share, search, obtain, and version control data in a distributed fashion, and it aids managing the evolution of digital objects in a way that fulfills the FAIR principles.
1.2. The DataLad philosophy¶
From a software point of view, DataLad is a command line tool, with an additional Python API to use its features within your software and scripts. While being a general, multi-purpose tool, there are also plenty of extensions that provide helpful, domain specific features that may very well fit your precise use case.
But beyond software facts, DataLad is built up on a handful of principles. It is this underlying philosophy that captures the spirit of what DataLad is, and here is a brief overview on it.
DataLad only cares (knows) about two things: Datasets and files. A DataLad dataset is a collection of files in folders. And a file is the smallest unit any dataset can contain. Thus, a DataLad dataset has the same structure as any directory on your computer, and DataLad itself can be conceptualized as a content-management system that operates on the units of files. As most people in any field work with files on their computer, at its core, DataLad is a completely domain-agnostic, general-purpose tool to manage data. You can use it whether you have a PhD in Neuroscience and want to share one of the largest whole brain MRI images in the world, organize your private music library, keep track of all cat memes on the internet, or anything else.
A dataset is a Git repository. All features of the version control system Git also apply to everything managed by DataLad – plus many more. If you do not know or use Git yet, there is no need to panic – there is no necessity to learn all of Git to follow along in learning and using DataLad. You will experience much of Git working its magic underneath the hood when you use DataLad, and will soon start to appreciate its features. Later, you may want to know more on how DataLad uses Git as a fundamental layer and learn some of Git.
A DataLad dataset can take care of managing and version controlling arbitrarily large data. To do this, it has an optional annex for (large) file content. Thanks to this annex, DataLad can easily track files that are many TB or PB in size (something that Git could not do, and allows you to transform, work with, and restore previous versions of data, while capturing all provenance, or share it with whomever you want). At the same time, DataLad does all of the magic necessary to get this awesome feature to work quietly in the background. The annex is set-up automatically, and the tool git-annex (https://git-annex.branchable.com) manages it all underneath the hood. Worry-free large-content data management? Check!
Deep in the core of DataLad lies the social principle to minimize custom procedures and data structures. DataLad will not transform your files into something that only DataLad or a specialized tool can read. A PDF file (or any other type of file) stays a PDF file (or whatever other type of file it was) whether it is managed by DataLad or not. This guarantees that users will not lose data or access if DataLad would vanish from their system (or from the face of the Earth). Using DataLad thus does not require or generate data structures that can only be used or read with DataLad – DataLad does not tie you down, it liberates you.
Furthermore, DataLad is developed for complete decentralization. There is no required central server or service necessary to use DataLad. In this way, no central infrastructure needs to be maintained (or paid for). Your own laptop is the perfect place for your DataLad project to live, as is your institution’s web server, or any other common computational infrastructure you might be using.
Simultaneously, though, DataLad aims to maximize the (re-)use of existing 3rd-party data resources and infrastructure. Users can use existing central infrastructures should they want to. DataLad works with any infrastructure from GitHub to Dropbox, Figshare or institutional repositories, enabling users to harvest all of the advantages of their preferred infrastructure without tying anyone down to central services.
These principles hopefully gave you some idea of what to expect from DataLad, cleared some worries that you might have had, and highlighted what DataLad is and what it is not. The section What you really need to know will give you a one-page summary of the functionality and commands you will learn with this handbook. But before we get there, let’s get ready to use DataLad. For this, the next section will show you how to use the handbook.