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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.

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.

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.