2017-09-14

dg filesystem layer

unsupported. tagged fuse filesystem

a fuse filesystem for working with file and tag content of a sph-dg database

design for a tag filesystem

status: has not been updated to the new sph-dg version, but otherwise the code should be in a relatively good state

features

instead of storing files in directory hierarchies, files can be in multiple directories, and queries can be made based on which files are included in which directories

works for many programs that expect traditional filesystem semantics, particulary for cases of reading, browsing and writing

supports nested set relation queries in path names, the ones specified in (sph storage dg relation path-find)

copying files from other filesystems into the filesystem and copying from this filesystem into others

all regular-file properties of the underlying filesystem, for example permissions and other stat information

installation

it is part of sph-lib-dg

install sph-lib-dg and the "dg" command-line program. the "dg" program has a "mount" option

dg mount --help

semantics

browsing

entering a directory appends to the path, which selects all files that have every path-segment as tag

root

list all existent tags, not files

sub-level

tags and files

tags are directories and shown are all tags of filtered files

paths

can identify one file if ending with a file name or be a filter query

directories are not identified by paths with multiple elements

directory names are unique

file properties

files do not have persistent customisable names, they only have associated tags

the display format for regular files is comma followed by a hexadecimal number

custom named files can be created, in which case the full given path to the file becomes a temporary alias for the id name. the custom named files are not listed because they would conflict with tags. the alias is removed with the next mount

a stat system call returns appropriate information, except the inode is currently the database identifier and not the underlying filesystem inode

system calls

rmdir

/a

delete tag

/a/b

delete relation (> b (> a *))

deletes tag "b" from all files that have "a" and "b"

/a.b.c

delete (> a b c)

/a/b.c

delete (> (> b c) (> a *))

mkdir

/a

create tag

/a/b

create relation (> b (> a *))

adds tag "b" to all files that have "a"

rename

/a /b

modify tag name

/a/b/c /a/b/d

relink (> c (> (a b) *)) (> (> (a b) *))

/a/b/c /d/e/f

replaces tags for files matched by the first path

(> c (> (a b) *)) (f (> (d e) *))

/a /b/c

replaces tags for files matched by the first path

(> a *) (> (b c) *)

/a/b /b

"mv" will give error about existence

removes tags from files matched by the first path

changes (> (a b) *) (> b *)

create

create file with given directory/tag relations

accepts arbitrary filenames - there is still a new unique file element created, but the custom named files stay accessible via the given file-path as long as the filesystem is mounted

unlink

delete the target file completely

read

read from file

write

write to file

readdir

list right elements

parsed-path syntax

parsed-path: parsed-path-element ["/" parsed-path-element]

parsed-path-element: condition/tag/file

file: ,hexadecimal-number

tag: string

condition: (condition-name tag ...)

caveats

be careful when deleting, only use rmdir to delete directories. file managers delete directories and the contained files, yet files could be part of other directories. if you want to delete all files then use find with maxdepth 2

tag permissions would have to be managed in the database by other means

tagged: filesystem-layer uses dg-tagged because sph-dg supports integer and binary values in interns, which may coexist with strings for tags

excluded

hierarchical tag relations

there has been an idea for using a dot as a part separator in addition to /, to designate a hierachical relation. this has not been implemented for the following reasons

currently too much work because of more complicated parsing and evaluation

while sph-dg supports hierarchical tags, benefits are not clear considering that tags are unique

parenthesised display format for files

"(identifier tag ...)"

round brackets would have to be quoted on typical shell command-lines

program support

programs that usually work well with this system

file managers (except recursive directory operations)

media players

editors and viewers

programs that can create problems

more complicated document editors like libreoffice that do not just save, but try to create backups, locks and whatnot in the same directory

versioning tools - symlinks or hardlinks are not supported, as well as duplicate file names if they may occur

possible enhancements

exclude tags that every matched file has from the listing

tag permissions

setxattr support for setting relation weight

alternatives

using "dg link" to create direct tag-named links to the database files, though this does only allow editing and the list of files needs to be refreshed

tagsistant


tags: application overview start specification computer filesystem sph-dg description filesystem-layer