File URL handling

DataLad datasets can record URLs for file content access as metadata. This is a feature provided by git-annex and is available for any annexed file. DataLad improves upon the git-annex functionality in two ways:

  1. Support for a variety of (additional) protocols and authentication methods.

  2. Support for special URLs pointing to individual files located in registered (annexed) archives, such as tarballs and ZIP files.

These additional features are available to all functionality that is processing URLs, such as get, addurls, or download-url.

Extensible protocol and authentication support

DataLad ships with a dedicated implementation of an external git-annex special remote named git-annex-remote-datalad. This is a somewhat atypical special remote, because it cannot receive files and store them, but only supports read operations.

Specifically, it uses the CLAIMURL feature of the external special remote protocol to take over processing of URLs with supported protocols in all datasets that have this special remote configured and enabled.

This special remote is automatically configured and enabled in DataLad dataset as a datalad remote, by commands that utilize its features, such as download-url. Once enabled, DataLad (but also git-annex) is able to act on additional protocols, such as s3://, and the respective URLs can be given directly to commands like git annex addurl, or datalad download-url.

Beyond additional protocol support, the datalad special remote also interfaces with DataLad’s Credential management. It can identify a particular credential required for a given URL (based on something called a “provider” configuration), ask for the credential or retrieve it from a credential store, and supply it to the respective service in an appropriate form. Importantly, this feature neither requires the necessary credential or provider configuration to be encoded in a URL (where it would become part of the git-annex metadata), nor to be committed to a dataset. Hence all information that may depend on which entity is performing a URL request and in what environment is completely separated from the location information on a particular file content. This minimizes the required dataset maintenance effort (when credentials change), and offers a clean separation of identity and availability tracking vs. authentication management.

Indexing and access of archive content

Another git-annex special remote, named git-annex-remote-datalad-archives, is used to enable file content retrieval from annexed archive files, such as tarballs and ZIP files. Its implementation concept is closely related to the git-annex-remote-datalad, described above. Its main difference is that it claims responsibility for a particular type of “URL” (starting with dl+archive:). These URLs encode the identity of an archive file, in terms of its git-annex key name, and a relative path inside this archive pointing to a particular file.

Like git-annex-remote-datalad, only read operations are supported. When a request to a dl+archive: “URL” is made, the special remote identifies the archive file, if necessary obtains it at the precise version needed, and extracts the respected file content from the archive at the correct location.

This special remote is automatically configured and enabled as datalad-archives by the add-archive-content command. This command indexes annexed archives, extracts, and registers their content to a dataset. File content availability information is recorded in terms of the dl+archive: “URLs”, which are put into the git-annex metadata on a file’s content.