Introducing FileStores

Catmandu is always our tool of choice when working with structured data. Using the Elasticsearch or MongoDB Catmandu::Store-s it is quite trivial to store and retrieve metadata records. Storing and retrieving a YAML, JSON (and by extension XML, MARC, CSV,…) files can be as easy as the commands below:

$ catmandu import YAML to database < input.yml
$ catmandu import JSON to database < input.json
$ catmandu import MARC to database < marc.data
$ catmandu export database to YAML > output.yml

catmandu.yml  configuration file is required with the connection parameters to the database:

$ cat catmandu.yml
---
store:
  database:
    package: ElasticSearch
    options:
       client: '1_0::Direct' 
       index_name: catmandu
...

Given these tools to import and export and even transform structured data, can this be extended to unstructured data? In institutional repositories like LibreCat we would like to manage metadata records and binary content (for example PDF files related to the metadata).  Catmandu 1.06 introduces the Catmandu::FileStore as an extension to the already existing Catmandu::Store to manage binary content.

A Catmandu::FileStore is a Catmandu::Store where each Catmandu::Bag acts as a “container” or a “folder” that can contain zero or more records describing File content. The files records themselves contain pointers to a backend storage implementation capable of serialising and streaming binary files. Out of the box, one Catmandu::FileStore implementation is available Catmandu::Store::File::Simple, or short File::Simple, which stores files in a directory.

Some examples. To add a file to a FileStore, the stream command needs to be executed:


$ catmandu stream /tmp/myfile.pdf to File::Simple --root /data --bag 1234 --id myfile.pdf

In the command above: /tmp/myfile.pdf is the file up be uploaded to the File::Store. File::Simple is the name of the File::Store implementation which requires one mandatory parameter, --root /data which is the root directory where all files are stored.  The--bag 1234 is the “container” or “folder” which contains the uploaded files (with a numeric identifier 1234). And the --id myfile.pdf is the identifier for the new created file record.

To download the file from the File::Store, the stream command needs to be executed in opposite order:

$ catmandu stream File::Simple --root /data --bag 1234 --id myfile.pdf to /tmp/file.pdf

or

$ catmandu stream File::Simple --root /data --bag 1234 --id myfile.pdf > /tmp/file.pdf

On the file system the files are stored in some deep nested structure to be able to spread out the File::Store over many disks:


/data
  `--/000
      `--/001
          `--/234
              `--/myfile.pdf

A listing of all “containers” can be retreived by requesting an export of the default (index) bag of the File::Store:


$ catmandu export File::Simple --root /data to YAML
_id: 1234
...

A listing of all files in the container “1234” can be done by adding the bag name to the export command:

$ catmandu export File::Simple --root /data --bag 1234 to YAML
_id: myfile.pdf
_stream: !!perl/code '{ "DUMMY" }'
content_type: application/pdf
created: 1498125394
md5: ''
modified: 1498125394
size: 883202
...

Each File::Store implementation supports at least the fields presented above:

  • _id: the name of the file
  • _stream: a callback function to retrieve the content of the file (requires an IO::Handle as input)
  • content_type: the MIME-Type of the file
  • created: a timestamp when the file was created
  • modified: a timestamp when the file was last modified
  • size: the byte length of the file
  • md5: optional a MD5 checksum

We envision in Catmandu that many implementations of FileStores can be created to be able to store files in GitHub, BagIts, Fedora Commons and more backends.

Using the Catmandu::Plugin::SideCar  Catmandu::FileStore-s and Catmandu::Store-s can be combined as one endpoint. Using Catmandu::Store::Multi and Catmandu::Store::File::Multi many different implementations of Stores and FileStores can be combined.

This is a short introduction, but I hope you will experiment a bit with the new functionality and provide feedback to our project.

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