-Magic Folders local filesystem integration design
-=================================================
+Magic Folder local filesystem integration design
+================================================
*Scope*
This document describes how to integrate the local filesystem with Magic
-Folders in an efficient and reliable manner. For now we ignore Remote to
+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
"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.
+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
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 acquires a lock at arbitrary times.)
+because Magic Folder would acquire a lock at arbitrary times.)
*Eventual consistency property*
-It is not possible to prevent local writes to a file while it is being
-read in order to upload it. Such writes will result in temporary
-inconsistency. 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.
+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
+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, as well as events
-indicating that a file has been written.
+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 the inotify API in
-terms of ``ReadDirectoryChangesW``, allowing most of the code to be
-shared across platforms.
+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.
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
*User interface*
-The Magic Folders local filesystem integration will initially have a
+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 indepently of the rest of
+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::
+replace the current ``[drop_upload]`` section::
- [magic_folders]
+ [magic_folder]
enabled = true
local.directory = "/home/human"
-When a filesystem directory is first "added" to Magic Folders, the user
-needs to create the remote Tahoe-LAFS directory using ``tahoe mkdir``,
+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
projects / tahoe-lafs / tahoe-lafs.git / commitdiff