OpenNeuro Quickstart Guide: Accessing OpenNeuro datasets via DataLad¶
OpenNeuro is a free and open platform for sharing neuroimaging data. Researchers can upload their own data to share it with others and also download others shared data from OpenNeuro with various methods.
Among other downloading options, OpenNeuro publishes its hosted data as DataLad datasets on GitHub and each dataset can be installed with DataLad as a DataLad dataset. If you want to find out about this option, this section contains basic insights and commands to get you started even if you haven’t used DataLad before.
The entire collection of OpenNeuro datasets can be found at github.com/OpenNeuroDatasets, but each dataset in there is identified via its OpenNeuro dataset ID.
A url to an OpenNeuro dataset on GitHub thus always takes the following form:
After you have installed DataLad, you can obtain the datasets just as any other DataLad dataset with datalad clone or datalad install:
$ datalad clone email@example.com:OpenNeuroDatasets/ds002336.git install(ok): /tmp/ds002336 (dataset) datalad clone firstname.lastname@example.org:OpenNeuroDatasets/ds002336.git
(Note that the above clone command used an “SSH URL” to get the dataset, and this requires that you have a GitHub account and have added an SSH key to your account. If that’s not the case, please switch to an https URL that takes the following form:
Afterwards, you can browse the dataset for files that you need and obtain them with the datalad get command.
datalad get . will download the entire dataset content, while paths or globbing expressions (such as
sub-*/anat/*) can define a precise subset of to-be-retrieved files.
$ datalad get sub-01/anat/ get(ok): /<path-to-dataset>/sub-01/anat/sub-01_T1w.nii.gz (file) [from s3-PUBLIC...] action summary: get (ok: 1)
$ datalad clone ///openneuro/ds002336 # you can also install legacy datasets from openfMRI $ datalad clone ///openfmri/ds000030
What’s DataLad and why should I use it to do this?¶
DataLad is a data management and data publication tool, building up on the tools Git and git-annex. It allows you to version control data alongside to code and even software environments, it can transparently and reproducibly link and share command executions, and it provides transport logistics for dataset consumption, sharing and collaboration. Using DataLad can make your science more open, transparent, and reproducible. You can find a short overview (5 min read) of its main features in the section What you really need to know. Below, we’ve listed a few bullet points on why it may be useful for you to download an OpenNeuro dataset via DataLad.
Potential for small disk usage: Cloning datasets is fast, and typically done in a matter of seconds. The resulting dataset contains the names and content identities of all files in the dataset, but it is only a fraction of the total data size as all those files do not yet contain file content. On demand, you can retrieve file contents of your choice via datalad get which downloads the specified files or directories. If you do not need file contents anymore and want to free up diskspace, a datalad drop drops the file contents, reducing the file size to about 140Kb, but you retain access to the file via datalad get. Thus, if your hard drive overflows, you can drop file contents without losing access to them, or keep a very lightweight reference to your input data next to your analysis results.
Easy updating mechanisms: Should a dataset get updated, you do not need to re-download a new version of the dataset, but run a
datalad update --mergein the dataset instead to automatically obtain the new or updated files.
Open and transparent science: Share analysis together with data by creating a dataset for your code and results and installing your OpenNeuro dataset of choice as a DataLad subdataset inside of it. This links the data you are using in a precise version to your code and results. If you want to, you can even use datalad run or datalad containers-run for provenance-tracked execution of commands or scripts, or the
datalad-containersDataLad extension to attach software environments to your computation. Your analysis dataset can then be shared with others via GitHub or similar services and will not only link your code and analyses, but also the data you have used for it. The section YODA-compliant data analysis projects has an example of how to do this.
These are only a few reasons why DataLad datasets can be beneficial – if you want to find out more about DataLad’s features, this handbook can give you a complete overview of everything the tool can do.
What should I be mindful of when using DataLad datasets?¶
While DataLad datasets – in our opinion – have many advantages, it may be good to know what you shouldn’t do in a dataset if you don’t have much experience with DataLad yet:
Don’t copy or move files out of a dataset without dereferencing. A dataset is a self-contained entity, and its version control features for (large) data work because it handles those files in an internal dataset annex. Opening a file viewer window and moving a file out of its dataset can be very harmful, because in all likelihood this will copy a symlink that points into the dataset annex instead of the actual file. Here’s what you should do if you want to copy or move a file out of a dataset into a non-dataset location: Make sure that the file content is present (datalad get), and copy or move the file with a tool that can dereference (i.e., resolve to canonical paths) symlinks. The command line tool
cpfor copying can do this with the
-L/--dereferenceoption, for example, any command can do it if the file path is wrapped in a
readlink -f <path>command. Alternatively, run datalad unlock prior to moving with any tool of your choice. See also the FAQ on Getting data out of datasets or the section Miscellaneous file system operations.
Don’t force-overwrite files: Many files in datasets are annexed for version control and, by default (on any non-Windows operating system), write-protected to ensure file integrity. If you encounter a file that will not let you change it right away and responds, for example, with a “permission denied” error, it is important to not forcefully modify this data. Instead, whenever you want to modify data, you need to datalad unlock it to remove the write protection. Afterwards, you can save any changes you have made to this file without jeopardizing version control and file integrity. Alternatively, use the datalad run command that can automatically unlock files for you. The chapter Under the hood: git-annex contains the details about this.
Make sure to retrieve data content prior to opening it with any tool: If file content isn’t yet retrieved, many tools emit confusing errors if they try to open these “empty files”. If you encounter a software that can’t find a file, complains that it can’t handle the file type you are giving to it, or behaves strange in another way, the first thing you should check is whether file content is present. If this isn’t a problem, you should try again after running datalad unlock – this command turns the file type from symlink into file and can help tools that don’t operate on symlinks cope.