Blobs, Trees, and Refs
Want stores all data using a format called the Git-Like Filesystem (GLFS). GLFS is not identical or even a superset of the formats used by Git the version control software. However, it is very similar in terms of the shape of the datastructure.
GLFS is an open source project, distinct from Want, which you can explore here
The first step in a build is to import the entire root module from the local filesystem into Want where it is represented as a GLFS filesystem.
Blobs
A Blob is an arbitrarily large sequence of bytes. It supports random access, so you can read from anywhere in the sequence efficiently. Blobs are immutable, changing the bytes of a Blob would produce a different Blob.
Terminal nodes in a POSIX Filesystem (regular files and symbolic links) are represented as Blobs in GLFS. A Blob does not contain any references to any other filesystem objects.
Refs
Ref stands for Reference. It is information which can be used to retrieve a filesystem object, but is typically much smaller than the object itself. A Ref is like an IOU for an immutable filesystem snapshot.
Refs contain a cryptographic hash of the content that they point to.
They also contain the Type of the data that they point to. The Type is either tree or blob.
Refs to Trees and Refs to blobs contain an equivalent amount of information. Phrased differently, a Ref to a Tree will not take up more or less space than a Ref to a Blob.
Throughout Want, Refs are used extensively. All filesystem data is passed around as Refs. Expressions in the build language evaluate to Refs in the output.
Trees
A Tree is the GLFS analog to a POSIX filesystem directory. A Tree is a set of entries, where each entry contains:
- Name
- Mode
- Ref
Names cannot contain the character /, and must be unique across all the other entires in a tree.
Mode contains permission bits; they are mostly needed for compatibility when converting back to a POSIX filesystem.
The Ref refers to another object in the filesystem, as was discussed above.
Trees form a recursive datastructure because they can contain references to other Trees. Trees cannot contain a reference to themselves or transitively contain trees with a reference to themselves. This is enforced by the cryptographic hash used in each Ref.