# SWH FUSE — Design notes

this document describes design notes for SWH FUSE, which is still under active
development and hence **not yet available** for general use.

The [Software Heritage](https://www.softwareheritage.org/)
{ref}`data model <data-model>` is a [Direct Acyclic
Graph](https://en.wikipedia.org/wiki/Directed_acyclic_graph) (DAG) with nodes of
different types that correspond to source code artifacts such as directories,
commits, etc. Using this
[FUSE](https://en.wikipedia.org/wiki/Filesystem_in_Userspace) module (*SWH FUSE*
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 the FUSE module
interacts over the network with the Software Heritage archive via its
{ref}`Web API <swh-web-api-urls>`.

## Command-line interface

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

will mount the Software Heritage archive at the local `<DIR>`, the *SWH FUSE
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 {ref}`CLI documentation <swh-graph-cli>`.

## Mount point

The SWH FUSE 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.

consider sharding `<SWHID>`/`<SWHID>.json` files under `ab/cd/` dirs to avoid
exploding the number of dir entries under `archive/` and `meta/`

**Zack:** how about making the sharding depth a CLI-option, defaulting to 1?
(i.e., `ab/<SWHID>`, `cd/<SWHID>`, …)

## 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 the FUSE mount is 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
- `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...

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

SWH FUSE 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 a single SQLite DB located at
`$XDG_CACHE_HOME/swh/fuse/cache.sqlite`.

**XXX** potential improvement: use a separate DB for the blob cache, so that
it's easier to delete

**XXX** 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 SWH FUSE
process) and global (i.e., they are shared by concurrent SWH FUSE 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).

### 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.

### 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.

## Examples

show the swh/graph/tests/dataset/example graph image and corresponding mounting
points + examples of different level of information (storage, edge labels, etc.)