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Document attachments
====================
.. contents:: Contents:
:local:
Reasoning
---------
The type of a Soledad document's content is `JSON <http://www.json.org/>`_,
which is good for efficient lookup and indexing. On the other hand, this is
particularly bad for storing larger amounts of binary data, because:
* the only way to store data in JSON is as unicode string, and this uses more
space than what is actually needed for binary data storage.
* upon synchronization, the content of a Soledad document needs to be
completelly transferred and decrypted for the document to be available for
use.
Document attachments were introduced as a means to efficiently store large
payloads of binary data while avoiding the need to wait for their transfer to
have access to the documents' contents.
Client-side
-----------
In the client, attachments are stored as (SQLite) BLOBs in a separate SQLCipher
database. Encryption of data before it's sent to the server is the same used
for Soledad documents' content during usual synchronization process (AES-256
GCM mode).
See :ref:`client-side-attachment-api` for reference.
Usage example
^^^^^^^^^^^^^
The attachments API is currently available in the `Document` class, and the
document needs to know about the store to be able to manage attachments. When
you create a new document with soledad, that document will already know about
the store that created it, and can put/get/delete an attachment:
.. code-block:: python
from twisted.internet.defer import inlineCallbacks
@inlineCallbacks
def attachment_example(soledad):
doc = yield soledad.create_doc({})
state = yield doc.get_attachment_state()
dirty = yield doc.is_dirty()
assert state == AttachmentStates.NONE
assert dirty == False
yield doc.put_attachment(open('hackers.txt'))
state = yield doc.get_attachment_state()
dirty = yield doc.is_dirty()
assert state | AttachmentState.LOCAL
assert dirty == True
yield soledad.put_doc(doc)
dirty = yield doc.is_dirty()
assert dirty == False
yield doc.upload_attachment()
state = yield doc.get_attachment_state()
assert state | AttachmentState.REMOTE
assert state == AttachmentState.SYNCED
fd = yield doc.get_attachment()
assert fd.read() == open('hackers.txt').read()
Server-side
-----------
In the server, a simple REST API is served by a `Twisted Resource
<https://twistedmatrix.com/documents/current/api/twisted.web.resource.Resource.html>`_
and attachments are stored in the filesystem as they come in without
modification.
A token is used to allow listing, getting, putting and deleting attachments. It
has to be added as an HTTP auth header, as in::
Authorization: Token <base64-encoded uuid:token>
Check out the :ref:`server-side-attachments-rest-api` for more information on
how to interact with the server using HTTP.
The :ref:`IBlobsBackend <i-blobs-backend>` interface is provided, so in the
future there can be different ways to store attachments in the server side
(think of a third-party storage, for example). Currently, the
:ref:`FilesystemBlobsBackend <filesystem-blobs-backend>` is the only backend
that implements that interface.
Some characteristics of the :ref:`FilesystemBlobsBackend
<filesystem-blobs-backend>` are:
* Configurable storage path.
* Quota support.
* Username, blob_id and user storage directory sanitization.
|
