Magic Folder local filesystem integration design
================================================

*Scope*

This document describes how to integrate the local filesystem with Magic
Folder in an efficient and reliable manner. For now we ignore Remote to
Local synchronization; the design and implementation of this is scheduled
for a later time. We also ignore multiple writers for the same Magic
Folder, which may or may not be supported in future. The design here will
be updated to account for those features in later Objectives. Objective 3
may require modifying the database schema or operation, and Objective 5
may modify the User interface.

Tickets on the Tahoe-LAFS trac with the `otf-magic-folder-objective2`_
keyword are within the scope of the local filesystem integration for
Objective 2.

.. _otf-magic-folder-objective2: https://tahoe-lafs.org/trac/tahoe-lafs/query?status=!closed&keywords=~otf-magic-folder-objective2

*Local scanning and database*

When a Magic-Folder-enabled node starts up, it scans all directories
under the local directory and adds every file to a first-in first-out
"scan queue". When processing the scan queue, redundant uploads are
avoided by using the same mechanism the Tahoe backup command uses: we
keep track of previous uploads by recording each file's metadata such as
size, ``ctime`` and ``mtime``. This information is stored in a database,
referred to from now on as the magic folder db. Using this recorded
state, we ensure that when Magic Folder is subsequently started, the
local directory tree can be scanned quickly by comparing current
filesystem metadata with the previously recorded metadata. Each file
referenced in the scan queue is uploaded only if its metadata differs at
the time it is processed. If a change event is detected for a file that
is already queued (and therefore will be processed later), the redundant
event is ignored.

To implement the magic folder db, we will use an SQLite schema that
initially is the existing Tahoe-LAFS backup schema. This schema may
change in later objectives; this will cause no backward compatibility
problems, because this new feature will be developed on a branch that
makes no compatibility guarantees. However we will have a separate SQLite
database file and separate mutex lock just for Magic Folder. This avoids
usability problems related to mutual exclusion. (If a single file and
lock were used, a backup would block Magic Folder updates for a long
time, and a user would not be able to tell when backups are possible
because Magic Folder would acquire a lock at arbitrary times.)


*Eventual consistency property*

During the process of reading a file in order to upload it, it is not
possible to prevent further local writes. Such writes will result in
temporary inconsistency (that is, the uploaded file will not reflect
what the contents of the local file were at any specific time). Eventual
consistency is reached when the queue of pending uploads is empty. That
is, a consistent snapshot will be achieved eventually when local writes
to the target folder cease for a sufficiently long period of time.


*Detecting filesystem changes*

For the Linux implementation, we will use the inotify Linux kernel
subsystem to gather events on the local Magic Folder directory tree. This
implementation was already present in Tahoe-LAFS 1.9.0, but needs to be
changed to gather directory creation and move events, in addition to the
events indicating that a file has been written that are gathered by the
current code.

For the Windows implementation, we will use the ``ReadDirectoryChangesW``
Win32 API. The prototype implementation simulates a Python interface to
the inotify API in terms of ``ReadDirectoryChangesW``, allowing most of
the code to be shared across platforms.

The alternative of using `NTFS Change Journals`_ for Windows was
considered, but appears to be more complicated and does not provide any
additional functionality over the scanning approach described above.
The Change Journal mechanism is also only available for NTFS filesystems,
but FAT32 filesystems are still common in user installations of Windows.

.. _`NTFS Change Journals`: https://msdn.microsoft.com/en-us/library/aa363803%28VS.85%29.aspx

When we detect the creation of a new directory below the local Magic
Folder directory, we create it in the Tahoe-LAFS filesystem, and also
scan the new local directory for new files. This scan is necessary to
avoid missing events for creation of files in a new directory before it
can be watched, and to correctly handle cases where an existing directory
is moved to be under the local Magic Folder directory.


*User interface*

The Magic Folder local filesystem integration will initially have a
provisional configuration file-based interface that may not be ideal from
a usability perspective. Creating our local filesystem integration in
this manner will allow us to use and test it independently of the rest of
the Magic Folder software components. We will focus greater attention on
user interface design as a later milestone in our development roadmap.

The configuration file, ``tahoe.cfg``, must define a target local
directory to be synchronized. Provisionally, this configuration will
replace the current ``[drop_upload]`` section::

 [magic_folder]
 enabled = true
 local.directory = "/home/human"

When a filesystem directory is first configured for Magic Folder, the user
needs to create the remote Tahoe-LAFS directory using ``tahoe mkdir``,
and configure the Magic-Folder-enabled node with its URI (e.g. by putting
it in a file ``private/magic_folder_dircap``). If there are existing
files in the local directory, they will be uploaded as a result of the
initial scan described earlier.

