[[!toc]] # metadata Attach an arbitrary set of metadata to a key. Store in git-annex branch, next to location log files. Metadata can be tags, but it can also be fields with values (ie, date=xxx, conference=yyy). Fields can have multiple values, for example multiple authors. Storage needs to support union merging, including removing tags, and changing values. ## automatically added metadata git annex add should automatically attach the current mtime of a file when adding it. Could also automatically attach permissions. A git hook could be run by git annex add to gather more metadata. For example, by examining MP3 metadata. Also auto adds metadata when adding files to filter branches. See below. ## derived metadata From the ctime, some additional metadata is derived, at least year=yyyy and probably also month, etc. This is probably not stored anywhere. It's computed on demand by a pure function from the other metadata. From the set of tags a file has, a "tag" field is derived, which has the value of each tag. See example below. Should be a general mechanism for this. (It probably generalizes to sql queries if we want to go that far.) # filtered branches `git annex filter year=2014 talk` should create a new branch filtered/year=2014/talk containing only files tagged with that, and have git check it out. In this example, all files appear in top level directory of repo; no subdirs. `git annex fadd haskell` switches to branch filtered/year=2014/talk/haskell with only the haskell talks. `git annex fadd year=2013 year=2012` switches to branch filtered/year=2012,2013,2014/talk/haskell. This has subdirectories 2012, 2013 and 2014 with the matching talks. Patterns can be used in both the values of fields, and in matching tags. So, `year=20*` could be used to match years, and `foo/*` matches any tag in the foo namespace. Or even `*` to match *all* tags. `git annex frm haskell` switches to filtered/year=2012,2013,2014/talk, which has all available talks in it. `git annex fadd conference=fosdem conference=icfp` switches to branch filtered/year=2012,2013,2014/talk/conference=fosdem,icfp. Now there are nested subdirectories. They follow the format of the branch, so 2013/icfp, 2014/fosdem, etc. `git annex filter tag=haskell,debian` uses the "tag" field that is automatically derived from the set of tags. So this yields a branch with haskell and debian subdirectories, containing the files tagged with either. To see all tags, `git annex filter tag=*` ! Files not matching the filter can be included, by using `git annex filter --unmatched=other`. That puts all such files into the subdirectory other. Note that old filter branches can be deleted when switching to a new one. There is no need to retain them. Unless the user has committed non-annexed files to them, In which case, urk. The only reason to use specially named filtered branches is because it makes self-documenting how the repository is currently filtered. ## operations while on filtered branch * If files are removed and git commit called, git-annex should remove the relevant metadata from the files. **possibly** It's not clear that removing a file should nuke all the metadata used to filter it into the branch (especially if it's derived metadata like the year). Also, this is not usable in direct mode because deleting the file.. actually deletes it. * If a file is moved into a new subdirectory while in a filter branch, a tag is added with the subdir name. This allows on the fly tagging. * `git annex sync` should avoid pushing out the filter branch, but it should check if there are changes to the metadata pulled in, and update the branch to reflect them. * If `git annex add` adds a file, it gets all the metadata of the filter branch it's added to. If it's in a relevent directory (like fosdem-2014), it gets that metadata automatically recorded as well. # other uses for metadata Uses are not limited to filter branches. `git annex checkoutmeta year=2014 talk` in a subdir of master could create the same tree of files filter would. The user can then commit that if desired. Or, they could run additional commands like `git annex fadd` to refine the tree of files in the subdir. Metadata can be used for configuring numcopies. One way would be a numcopies=n value attached to a file. But perhaps better would be to make the numcopies.log allow configuring numcopies based on which files have other metadata. Other programs could query git-annex for the metadata of files in the work tree, and do whatever it wants with it. # filenames The hard part of this is actually getting a useful filename to put in the filter branch, since git-annex only has a key which the user will not want to see. * Could use filename metadata for the key, recorded by git-annex add (which may not correspond to filenames being used in regular git branches like master for the key). * Could use the .map files to get a filename, but this is somewhat arbitrary (.map can contain multiple filenames), and is only currently supported in direct mode. Note that any of these filenames can in theory conflict. May need to use `.variant-*` like sync does on conflict to allow 2 files with same name in same filtered branch. # efficient metadata lookup Looking up metadata for filtering so far requires traversing all keys in the git-annex branch. This is slow. A fast cache is needed. # direct mode issues Checking out a filter branch can result in any number of copies of a file appearing in different directories. No problem in indirect mode, but in direct mode these are real, expensive copies. But, it's worth supporting direct mode! So, possible approaches: * Before checking out a filter branch, calculate how much space will be used by duplicates and refuse if not enough is free. * Only check out one file, and omit the copies. Keep track of which files were omitted, and make sure that when committing on the branch, that metadata is not removed. Has the downside that files can seem to randomly move around in the tree as their metadata changes. * Disallow filter branch checkouts that have duplicate files. This would cripple it some, but perhaps not too badly? # gotchas * Checking out a filter branch can remove the current subdir. May be worth detecting when this happens and leaving behind an empty directory so the user can navigate back up. * Git has a complex set of rules for what is legal in a ref name. Filter branch names will need to filter out any illegal stuff. * Filesystems that are not case sensative (including case preserving OSX) will cause problems if filter branches try to use different cases for 2 directories representing the value of some metadata. But, users probably want at least case-preserving metadata values. Solution might be to compare metadata case-insensitively, and pick one representation consistently, so if, for example an author field uses mixed case, it will be used in the filter branch. Alternatively, it could escape `A` to `_A` when such a filesystem is detected and avoid collisions that way (double `_` to escape it). This latter option is ugly, but so are non-posix filesystems.. and it also solves any similar issues with case-colliding filenames.