Software Heritage virtual filesystem (SwhFS) — Design notes

Warning

this document describes design notes for the Software Heritage virtual filesystem (SwhFS), which is still under active development and hence not yet available for general use.

The Software Heritage {ref}data model <data-model> is a Direct Acyclic Graph (DAG) with nodes of different types that correspond to source code artifacts such as directories, commits, etc. Using this FUSE module (SwhFS for short) you can locally mount, and then navigate as a (virtual) file system, parts of the archive identified by {ref}Software Heritage identifiers <persistent-identifiers> (SWHIDs).

To retrieve information about the source code artifacts, SwhFS interacts over the network with the Software Heritage archive via its {ref}Web API <swh-web-api-urls>.

Command-line interface

$ swh fs mount <DIR> [SWHID]...

will mount the Software Heritage archive at the local <DIR>, the SwhFS mount point. From there, the user will be able to lazily load and navigate the archive using SWHID at entry points.

If one or more SWHIDs are also specified, the corresponding objects will be pre- fetched from the archive at mount-time and available at <DIR>/archive/<SWHID>.

For more details see the CLI documentation.

Mount point

The SwhFS mount point contain:

  • archive/: initially empty, this directory is lazily populated with one entry per accessed SWHID, having actual SWHIDs as names.

  • meta/: initially empty, this directory contains one <SWHID>.json file for each <SWHID> entry under archive/. The JSON file contain all available meta information about the given SWHID, as returned by the Software Heritage Web API for that object. Note that, in case of pagination (e.g., snapshot objects with many branches) the JSON file will contain a complete version with all pages merged together.

Todo

Consider sharding <SWHID>/<SWHID>.json files under ab/cd/ dirs to avoid exploding the number of dir entries under archive/ and meta/ (cf. T2694)

File system representation

SWHID are represented differently on the file-system depending on the associated node types in the Software Heritage graph. Details are given below, for each node type.

cnt nodes (blobs)

Content leaves (AKA blobs) are represented on disks as regular files, containing the corresponding bytes, as archived.

Note that permissions are associated to blobs only in the context of directories. Hence, when accessing blobs from the top-level archive/ directory, the permissions of the archive/SWHID file will be arbitrary and not meaningful (e.g., 0x644).

dir nodes (directories)

Directory nodes are represented as directories on the file-system, containing one entry for each entry of the archived directory. Entry names and other metadata, including permissions, will correspond to the archived entry metadata.

Note that SwhFS is mounted read-only, no matter what the permissions say. So it is possible that, in the context of a directory, a file is presented as writable, whereas actually writing to it will fail with EPERM.

rev nodes (commits)

Revision (AKA commit) nodes are represented on the file-system as directories with the following entries:

  • root: source tree at the time of the commit, as a symlink pointing into archive/, to a SWHID of type dir

  • parents/ (note the plural): a virtual directory containing entries named 1, 2, 3, etc., one for each parent commit. Each of these entry is a symlink pointing into archive/, to the SWHID file for the given parent commit

  • parent (note the singular): present if and only if the current commit has a single parent commit (which is the most common case). When present it is a symlink pointing into archive/ to the SWHID for the sole parent commit

  • history: a virtual directory containing all the parents commit until the root commit. Entries are listed as symlinks with the SWHID as directory name, pointing into archive/SWHID, and are returned in a topological ordering similar to git log ordering.

  • meta.json: metadata for the current node, as a symlink pointing to the relevant meta/<SWHID>.json file

rel nodes (releases)

Release nodes are represented on the file-system as directories with the following entries:

  • target: target node, as a symlink to archive/<SWHID>

  • target_type: type of the target SWHID, as a 3-letter code

  • root: present if and only if the release points to something that (transitively) resolves to a directory. When present it is a symlink pointing into archive/ to the SWHID of the given directory

  • meta.json: metadata for the current node, as a symlink pointing to the relevant meta/<SWHID>.json file

snp nodes (snapshots)

Snapshot nodes are represented on the file-system as directories with on entry for each branch in the snapshot.

Branch names are mangled by replacing…

Todo

decide how to do branch name escaping and describe it here

Each entry is a symlink pointing into archive/ to the branch target SWHID.

Caching

SwhFS retrieves both metadata and file contents from the Software Heritage archive via the network. In order to obtain reasonable performances several caches are used to minimize network transfer.

Caches are stored on disk in SQLite DB(s) located under $XDG_CACHE_HOME/swh/fuse/.

Todo

  • potential improvement: store blobs larger than a threshold on disk as files rather than in SQLite, e.g., under $XDG_CACHE_HOME/swh/fuse/objects/

All caches are persistent (i.e., they survive the restart of the SwhFS process) and global (i.e., they are shared by concurrent SwhFS processes).

We assume that no cache invalidation is necessary, due to intrinsic properties of the Software Heritage archive, such as integrity verification and append-only archive changes. To clean the caches one can just remove the corresponding files from disk.

Metadata cache

SWHID → JSON metadata

The metadata cache map each SWHID to the complete metadata of the referenced object. This is analogous to what is available in meta/<SWHID>.json file (and generally used as data source for returning the content of those files).

Cache location on-disk: $XDG_CACHE_HOME/swh/fuse/metadata.sqlite

Blob cache

cnt SWHID → bytes

The blob cache map SWHIDs of type cnt to the bytes of their archived content.

In general, each SWHID that has an entry in the blob cache also has a matching entry in the metadata cache for other blob attributes (e.g., checksums, size, etc.).

The blob cache entry for a given content object is populated, at the latest, the first time the object is open()-d. It might be populated earlier on due to prefetching, e.g., when a directory pointing to the given content is listed for the first time.

Cache location on-disk: $XDG_CACHE_HOME/swh/fuse/blob.sqlite

Dentry cache

dir SWHID → directory entries

The dentry (directory entry) cache map SWHIDs of type dir to the directory entries they contain. Each entry comes with its name as well as file attributes (i.e., all its needed to perform a detailed directory listing).

Additional attributes of each directory entry should be looked up on a entry by entry basis, possibly hitting the metadata cache.

The dentry cache for a given dir is populated, at the latest, when the content of the directory is listed. More aggressive prefetching might happen. For instance, when first opening a dir a recursive listing of it can be retrieved from the remote backend and used to recursively populate the dentry cache for all (transitive) sub-directories.

Parents cache

rev SWHID → parent SWHIDs

The parents cache map SWHIDs of type rev to the list of their parent commits.

The parents cache for a given rev is populated, at the latest, when the content of the revision virtual directory is listed. More aggressive prefetching might happen. For instance, when first opening a rev virtual directory a recursive listing of all its ancestor can be retrieved from the remote backend and used to recursively populate the parents cache for all ancestors.

History cache

rev SWHID → ancestor SWHIDs

The history cache map SWHIDs of type rev to a list of rev SWHIDs corresponding to all its revision ancestors, sorted in reverse topological order. As the parents cache, the history cache is lazily populated and can be prefetched. To efficiently store the ancestor lists, the history cache represents ancestors as graph edges (a pair of two SWHID nodes), meaning the history cache is shared amongst all revisions parents.