Ghee: that thin layer of data change management over Btrfs, using Linux extended attributes (xattrs)

The tastiest way to manage data using Linux extended attributes (xattrs), written in pure Rust and made of delicious, fibrous Open Source code.

Ghee provides tools for manipulating xattrs on individual files as well as for working with the filesystem as a document database where the filesystem paths act as primary keys and extended attributes act as fields.

Data in Ghee tables can be managed using Git-style commits, implemented as Btrfs subvolumes and read-only snapshots.

In this way Ghee leverages the copy-on-write (CoW) nature of Btrfs to efficiently store deltas, even on large binary blobs.

License

This software is licensed under GPL version 3 only.

Conventions

Extended attribute names are parsed in a consistent manner by Ghee. Any xattr not preceded by the trusted, security, system, or user namespace will have the user namespace by default. For example, xattr trusted.uptime remains as is, while uptime would become user.uptime.

Extended attribute values are parsed as f64 numbers if possible; otherwise, they are interpeted as strings.

REPL

Running ghee with no arguments will enter a read-eval-print-loop (REPL), allowing for fluent command input:

  $ ghee
  Ghee 0.6.0

  ghee$ set ./test -s test=1

etc.

Subcommands

Ghee operates through a set of subcommands, each with a primary function. Run ghee --help to list them, and ghee $SUBCMD --help to get usage information for each subcommand.

Examples of each subcommand follow:

Move

Moves xattr values from one path to another.

• ghee mv path1.txt path2.txt: move all xattrs from path1.txt to path2.txt
• ghee mv -f id path1.txt path2.txt: move xattr id from path1.txt to path2.txt
• ghee mv -f id -f url path1.txt path2.txt: move xattrs id and url from path1.txt to path2.txt

Copy

Copies xattr values from one path to another.

• ghee cp path1.txt path2.txt: copy all xattrs from path1.txt to path2.txt
• ghee cp -f id path1.txt path2.txt: copy xattr id from path1.txt to path2.txt
• ghee cp -f id -f url path1.txt path2.txt: copy xattrs id and url from path1.txt to path2.txt

Remove

Removes xattr values, recursively by default.

• ghee rm path.txt: remove all xattrs on path.txt
• ghee rm dir: remove all xattrs from dir and all descendants
• ghee rm --flat dir: remove all xattrs from dir only, not its descendants
• ghee rm -f id path.txt: remove xattr id from path.txt
• ghee rm -f id -f url path1.txt path2.txt path3.txt: remove xattrs id and url from path1.txt, path2.txt, and path3.txt
• ghee rm -f name dir: remove xattr name from dir and all its descendants

Set

Sets xattr values, recursively by default.

• ghee set -s id=123 path1.txt: set xattr id to value 123 on path1.txt
• ghee set -s name=Jama dir: set xattr name to value "Jama" on dir and all descendants
• ghee set -s name=Amira --flat dir: set xattr name to value "Amira" on dir only, not its descendants
• ghee set -s id=123 -s url=http://example.com path1.txt path2.txt path3.txt: set xattr id to value 123 and xattr url to value "http://example.com" on path1.txt, path2.txt, and path3.txt

Get

Recursively get and print xattr values for one or more paths.

By default, the get subcommand outputs a tab-separated table with a column order of path, field, value. The value bytes are written to stdout as-is without decoding.

This excludes the user.ghee prefix unless -a --all is passed.

To opt out of the recursive default, use --flat.

• ghee get dir: print all xattrs for directory dir and all descendant files and directories, as raw (undecoded) TSV
• ghee get -f id path1.txt: print xattr id and its value on path1.txt as raw (undecoded) TSV
• ghee get -f id -f url path1.txt path2.txt path3.txt: print xattrs id and url and their respective values on path1.txt, path2.txt, and path3.txt as raw (undecoded) TSV

The get command can also output JSON - in which case values are decoded as UTF-8, filling in a default codepoint when decoding fails:

• ghee get -j --flat dir: print all xattrs for directory dir itself but not its descendants, as UTF-8 decoded JSON
• ghee get -j -f id path1.txt: print xattr id and its value on path1.txt as UTF-8 decoded JSON
• ghee get -j -f id -f url path1.txt path2.txt path3.txt: print xattrs id and url and their respective values on path1.txt, path2.txt, and path3.txt as JSON

By adding --where (or -w), SQL WHERE-style clauses can be provided to select which files to include in the output. For example, ghee get -w age >= 65 ./patients will select all files under directory ./patients whose user.age attribute is 65 or greater.

Nested indices are always ignored in get output, though they will be used as appropriate to shortcut traversal when WHERE-style predicates are specified.

Init

Initializes a directory as a table with a specified primary key, optionally inserting records from JSON where each line is parsed independently---see people.json in the repository for an example.

Examples:

• ghee init -k name ./people: marks the ./people directory as a table with primary key of name
• ghee init -k state -k id ./people-by-state-and-id: marks the ./people-by-state-and-id directory as a table with a compound primary key of [state, id].
• ghee init -k sauce ./pizza < ./pizzas.json: marks the ./pizza directory as a table with primary key sauce, importing records from ./pizzas.json

Create

Exactly like init, but creates the directory first, or errors if it already exists.

Insert

Inserts JSON-formatted records into a table.

Records are read one per line from stdin.

• ghee ins ./people < ./people.json: inserts the records from ./people.json into the table at ./people, indexed by its primary key
• ghee ins ./people ./people.json: same as the above, but not depending on the shell for redirection

Delete

Deletes records from a table.

They are unlinked from all table indices.

The records to be deleted are specified by providing either the components of the primary key or SQL-style WHERE clauses.

• ghee del ./people Von: because the table's primary key is name, deletes the record where name=Von from ./people and all indices.
• ghee del ./people -w name=Von: deletes ./people/Von as above, unlinking from all indices.

Index

Indexes a table.

When Ghee acts on a directory as if it were a database table, each file acts as a relational "record" with the primary key inferred from its subpath under the table directory.

Each file's extended attributes act as the relational attributes.

Table directories created by Ghee also contain a special xattr user.ghee.tableinfo which stores the primary key and related indices (including itself) of a table.

If no index location is provided, it will be placed in a default path under the table being indexed.

Examples:

• ghee idx -k name ./people ./people-by-name: recursively reindex the contents of ./people into a new directory ./people-by-name with primary key coming from xattr name and files hardlinked to the corresponding files in ./people.

  That means the ./people-by-name directory's files will have filenames taken from the names of the people as defined in xattr name.

  The user.ghee.tableinfo xattr for ./people records ./people-by-name as a related index, and the reciprocal is true as well: the user.ghee.tableinfo xattr for ./people-by-name records ./people as a related index.

  Queries such as get and del will be opportunistically accelerated using available indices.

• ghee idx -k region -k name -s ./people-by-name ./people-by-region-and-name: recursively reindex the contents of ./people-by-name into a new directory ./people-by-region-and-name with primary key being the compound of xattr region and xattr name (in that order) and files hardlinked to the corresponding files in ./people, resolved via the hardlinks in ./people-by-name.

  The user.ghee.tableinfo xattrs of both directories will be updated to reflect their relationship.

List

Like the ls command, lists directory contents, but annotated from Ghee's point of view.

Each path is marked as either a table or a record. For tables, the primary key is given.

• ghee ls: lists the current directory's contents
• ghee ls example: lists the contents of ./example

Commit

Stores the current state of the table in a Btrfs snapshot, identified by a UUID.

Optionally, a message describing the changes made since the last snapshot (if any) can be provided.

• ghee commit -m "Update README.md"

The UUID of the commit is outputted.

Log

Displays past commits.

• ghee log: Lists all past commits in the current table

Touch

Similar to the Unix touch command, creates an empty file at the specified path; if the path is part of a Ghee table, xattrs are inferred from the path and written to the new file.

This is a convenient way to add new records to new tables.

With -p / --parents, parent directories will be created.

• ghee touch /pizza/pepperoni: creates an empty file at /pizza/pepperoni, setting xattr topping to pepperoni because the key of the /pizza table is topping.

Restore

Restores paths to their state in the HEAD commit.

• ghee restore README.md

Reset

Resets all files in the table to their state in a specified commit.

• ghee reset add133b4-f58b-a64e-992a-46f983a0e7ed