Methods for storing metadata associated with individual files?

Given a collection of files which will have associated metadata, what are the recommended methods for storing this metadata?

Some files formats support storing metadata internally (EXIF,ID3,etc), but not all file formats support this, so what are more general options?

Some of the metadata would almost certainly be unique (titles/descriptions/etc), whilst some would be repetitive to varying degrees (categories/tags/etc).
It may also be useful to group the metadata, if different types of attribute are required.

Ideally, solutions should cover concepts, rather than specific language implementations.

From stackoverflow

• One option might be a relational database, structured like this:

  FILE
  f_id
  f_location
  f_title
  f_description
  
  ATTRIBUTE
  a_id
  a_label
  
  VALUE
  v_id
  v_label
  
  METADATA
  md_file
  md_attribute
  md_value
  

  This implementation has some unique information (title/description), but is primarily targetted at repetitive groups of data.

  For some requirements, other less generic tables may be more useful.

  ---

  This has advantages of this being that relational databases are very common, and obviously very good at handling relationships and storing lots of data.

  However, for some uses a database server brings an overhead which might not be desirable. Also, the database server is distinct from the files - they do not sit together, and require different methods of interaction.

  Databases do not (easily) sit under version control - which may be a good or bad thing, depending on your point of view and specific needs.

• Plain text has some obvious advantages over anything else. Something like

  FileName = 'ferrari.gif'
  Title = 'My brand new car'
  Tags = 'cars', 'cool'
  Related = 'michaelknight.mp3'
  

  Picasa's Picasa.ini files are a good example for this kind of metadata. Also, instead of inventing your own format, XML might be worth considering. There are plenty of readily available DOM processors to deal with this format.

  Then again, if the amount of files and relations between them is huge, databases may be better.

• To store metadata in database has some advantages but main problem with database is that metadata are not directly connected to your data. It is more robust if metada stay with data - like special file in the directory or something like that.

  Some filesystems offer special functionality that can be used for metadata - like NTFS Alternate streams. Unfortunately, this can be used for metadata storage in special cases only, because those streams can be easily lost when copying data to storage system that does not support it. I believe that linux filesystems have also similar storage mechanism.

  Anyway, most common solutions are :

  • separate hidden file(s) (per directory) that hold metadata
  • some application use special hidden directory with metadata (like subversion, cvs etc).
  • or database (of various kinds) for all application specific metada - this database can be used also for caching purposes in most cases

  IMO there is no general purpose solution. I would choose storage of metadata in hidden file (robustness) with use of the database for fast access and caching.

• I think the "solution" depends greatly upon what you're going to be doing with the metadata.

  For example, almost all of the metadata we store (Multiple datasets of scientific data) are all chopped up and stored in a database. This allows us to create datasets to preserve the common metadata between the files (as you say, categories and tags) while we have file specific structures (title, start/stop time, min/max values etc.) While we could keep these in hidden files, we do a lot of searching and open our interface to outside consumers via web services.

  If you're storing metadata that isn't going to be searched on, hidden files or a dedicated .xml file per "real" file isn't a bad route to take. It's readable by basically anything, can be converted to different formats easily, and won't be lost if you decide to change your storage mechanism.

  Metadata should help you, not hinder you. I've seen (and been a part of) systems where metadata storage has become more burdensome than storing the actual data, and became a liability. Just keep in mind what you are trying to do with it, and don't over extend yourself with "what ifs."