A content-addressable storage backend for Django.
django-castor is a re-usable app for Django which provides a content-addressable storage backend. The main class, djcastor.storage.CAStorage, is a type of FileSystemStorage which saves files under their SHA-1 digest.
- No matter how many times the same file is uploaded, it will only ever be stored once, thus eliminating redundancy.
- Filenames are pseudorandom and made up only of hexadecimal characters, so you don’t have to worry about filename collisions or sanitization.
- django-castor shards files in the uploads directory based on their digests; this prevents filesystem issues when too many files are in a single directory.
For more information on the CAS concept, see the wikipedia page.
pip install django-castor # or easy_install django-castor
Basic usage is as follows:
from django.db import models from djcastor import CAStorage class MyModel(models.Model): ... uploaded_file = models.FileField(storage=CAStorage(), upload_to='uploads')
At the moment, Django requires a non-empty value for the upload_to parameter. Note that CAStorage will not use this value; if you need to customize the destination for uploaded files, use the location parameter (see below).
For extended usage, there are several options you can pass to the CAStorage constructor. The first two are inherited from the built-in FileSystemStorage:
- location: The absolute path to the directory that will hold uploaded files. If omitted, this will be set to the value of the MEDIA_ROOT setting.
- base_url: The URL that serves the files stored at this location. If omitted, this will be set to the value of the MEDIA_URL setting.
CAStorage also adds two custom options:
keep_extension (default True): Preserve the extension on uploaded files. This allows the webserver to guess their Content-Type.
sharding (default (2, 2)): The width and depth to use when sharding digests, expressed as a two-tuple. The following examples show how these parameters affect the sharding:
>>> digest = '1f09d30c707d53f3d16c530dd73d70a6ce7596a9' >>> print shard(digest, width=2, depth=2) 1f/09/1f09d30c707d53f3d16c530dd73d70a6ce7596a9 >>> print shard(digest, width=2, depth=3) 1f/09/d3/1f09d30c707d53f3d16c530dd73d70a6ce7596a9 >>> print shard(digest, width=3, depth=2) 1f0/9d3/1f09d30c707d53f3d16c530dd73d70a6ce7596a9
The first small caveat is that content-addressable storage is not suited to rapidly-changing content. If your website modifies the contents of file fields on a regular basis, it might be a better idea to use a UUID-based storage backend for those fields.
The second, more important caveat with this approach is that if the parent model of a file is deleted, the file will remain on disk. Because individual files may be referred to by more than one model, and django-castor has no awareness of these references, it leaves file deletion up to the developer.
For the most part, you can get away without deleting uploads. In fact, content-addressable storage is often used for long-term archival systems, where files are immutable and must be kept for future auditing (usually for compliance with government regulations).
If disk space is at a premium and you need to delete uploads, there are two approaches you might want to take:
- Garbage collection: write a script that walks through the list of uploaded files and checks references to each one. If no references are found, delete the file.
- Reference counting: denormalize the FileField into a separate model, and keep a count of all the models pointing to it. Once this count reaches zero, delete the file from the filesystem.
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