Software Heritage Filesystem (SwhFS) — Design notes

The Software Heritage 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 filesystem, parts of the archive identified by Software Heritage identifiers (SWHIDs).

To retrieve information about the source code artifacts, SwhFS interacts over the network with the Software Heritage archive via its Web API.

Architecture

SwhFS in context (C4 context diagram):

../_images/arch-context.svg

Main components of SwhFS (C4 container diagram):

../_images/arch-container.svg

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/: virtual directory allowing to mount any artifact on the fly using its SWHID as name. The associated metadata of the artifact from the Software Heritage Web API can also be accessed through the SWHID.json file (in case of pagination, the JSON file will contain a complete version with all pages merged together). Note: the archive directory cannot be listed with ls, but entries in it can be accessed (e.g., using cat or cd).

  • origin/: initially empty, this directory is lazily populated with one entry per accessed origin URL, having encoded URL as names. The URL encoding is done using the percent-encoding mechanism described in RFC 3986.

  • cache/: on-disk representation of locally cached objects and metadata. Via this directory you can browse cached data and selectively remove them from the cache, freeing disk space. (See swh fs clean in the CLI to completely empty the cache). The directory is populated with symlinks to: all artifacts, identified by their SWHIDs and sharded by the first two character of their object id, the metadata identified by a SWHID.json entry, and the origin/ directory.

  • README: file explaining briefly what is SwhFS.

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 at least one parent commit (which is the most common case). When present it is a symlink pointing into parents/1/

  • history: a virtual directory listing all its revision ancestors, sorted in reverse topological order. The history can be listed through by-date/, by-hash/ or by-page/ with each its own sharding policy.

  • meta.json: metadata for the current node, as a symlink pointing to the relevant archive/<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: regular file containing the type of the target SWHID

  • 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 archive/<SWHID>.json file

snp nodes (snapshots)

Snapshot nodes are represented on the file-system as recursive directories following the branch names structure. For example, a branch named refs/tags/v1.0 will be represented as a refs directory containing a tags directory containing a v1.0 symlink pointing to the branch target SWHID.

ori nodes (origins)

Origin nodes are represented on the file-system as directories with one entry for each origin visit.

The visits directories are named after the visit date (YYYY-MM-DD, if multiple visits occur the same day only the first one is kept). Each visit directory contains a meta.json with associated metadata for the origin node, and potentially a snapshot symlink pointing to the visit’s snapshot node.

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, or using a more fine-grained strategy, navigate the cache/ top-level directory and rm <SWHID> to purge specific artifacts.

Metadata cache

Artifact id → JSON metadata

The metadata cache map each artifact to the complete metadata of the referenced object. This is analogous to what is available in archive/<SWHID>.json file (and generally used as data source for returning the content of those files). Artifacts are identified using their SWHIDs, or in the case of origin visits, using their URLs.

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

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.

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

Direntry cache

dir inode → directory entries

The direntry cache map inode representing directories to the 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 direntry 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 direntry cache for all (transitive) sub-directories.

Cache location: in-memory.