From: Daira Hopwood Date: Mon, 18 Jan 2016 15:39:53 +0000 (+0000) Subject: Add multi-party-conflict-detection.rst. X-Git-Url: https://git.rkrishnan.org/?p=tahoe-lafs%2Ftahoe-lafs.git;a=commitdiff_plain;h=7cfff0b5b090a416fe818d7c15bf982eb0c5afd4 Add multi-party-conflict-detection.rst. Signed-off-by: Daira Hopwood --- diff --git a/docs/proposed/magic-folder/multi-party-conflict-detection.rst b/docs/proposed/magic-folder/multi-party-conflict-detection.rst new file mode 100644 index 00000000..d99b62c1 --- /dev/null +++ b/docs/proposed/magic-folder/multi-party-conflict-detection.rst @@ -0,0 +1,375 @@ +Multi-party Conflict Detection +============================== + +The current Magic-Folder remote conflict detection design does not properly detect remote conflicts +for groups of three or more parties. This design is specified in the "Fire Dragon" section of this document: +https://github.com/tahoe-lafs/tahoe-lafs/blob/2551.wip.2/docs/proposed/magic-folder/remote-to-local-sync.rst#fire-dragons-distinguishing-conflicts-from-overwrites + +This Tahoe-LAFS trac ticket comment outlines a scenario with +three parties in which a remote conflict is falsely detected: + +.. _`ticket comment`: https://tahoe-lafs.org/trac/tahoe-lafs/ticket/2551#comment:22 + + +Summary and definitions +======================= + +Abstract file: a file being shared by a Magic Folder. + +Local file: a file in a client's local filesystem corresponding to an abstract file. + +Relative path: the path of an abstract or local file relative to the Magic Folder root. + +Version: a snapshot of an abstract file, with associated metadata, that is uploaded by a Magic Folder client. + +A version is associated with the file's relative path, its contents, and +mtime and ctime timestamps. Versions also have a unique identity. + +Follows relation: +* If and only if a change to a client's local file at relative path F that results in an upload of version V', +was made when the client already had version V of that file, then we say that V' directly follows V. +* The follows relation is the irreflexive transitive closure of the "directly follows" relation. + +The follows relation is transitive and acyclic, and therefore defines a DAG called the +Version DAG. Different abstract files correspond to disconnected sets of nodes in the Version DAG +(in other words there are no "follows" relations between different files). + +The DAG is only ever extended, not mutated. + +The desired behaviour for initially classifying overwrites and conflicts is as follows: + +* if a client Bob currently has version V of a file at relative path F, and it sees a new version V' + of that file in another client Alice's DMD, such that V' follows V, then the write of the new version + is initially an overwrite and should be to the same filename. +* if, in the same situation, V' does not follow V, then the write of the new version should be + classified as a conflict. + +The existing `Magic Folder design for remote-to-local sync`_ defines when an initial overwrite +should be reclassified as a conflict. + +The above definitions completely specify the desired solution of the false +conflict behaviour described in the `ticket comment`_. However, they do not give +a concrete algorithm to compute the follows relation, or a representation in the +Tahoe-LAFS file store of the metadata needed to compute it. + +We will consider two alternative designs, proposed by Leif Ryge and +Zooko Wilcox-O'Hearn, that aim to fill this gap. + +.. _`Magic Folder design for remote-to-local sync`: remote-to-local-sync.rst + + + +Leif's Proposal: Magic-Folder "single-file" snapshot design +=========================================================== + +Abstract +-------- + +We propose a relatively simple modification to the initial Magic Folder design which +adds merkle DAGs of immutable historical snapshots for each file. The full history +does not necessarily need to be retained, and the choice of how much history to retain +can potentially be made on a per-file basis. + +Motivation: +----------- + +no SPOFs, no admins +``````````````````` + +Additionally, the initial design had two cases of excess authority: + +1. The magic folder administrator (inviter) has everyone's write-caps and is thus essentially "root" +2. Each client shares ambient authority and can delete anything or everything and + (assuming there is not a conflict) the data will be deleted from all clients. So, each client + is effectively "root" too. + +Thus, while it is useful for file synchronization, the initial design is a much less safe place +to store data than in a single mutable tahoe directory (because more client computers have the +possibility to delete it). + + +Glossary +-------- + +- merkle DAG: like a merkle tree but with multiple roots, and with each node potentially having multiple parents +- magic folder: a logical directory that can be synchronized between many clients + (devices, users, ...) using a Tahoe-LAFS storage grid +- client: a Magic-Folder-enabled Tahoe-LAFS client instance that has access to a magic folder +- DMD: "distributed mutable directory", a physical Tahoe-LAFS mutable directory. + Each client has the write cap to their own DMD, and read caps to all other client's DMDs + (as in the original Magic Folder design). +- snapshot: a reference to a version of a file; represented as an immutable directory containing + an entry called "content" (pointing to the immutable file containing the file's contents), + and an entry called "parent0" (pointing to a parent snapshot), and optionally parent1 through + parentN pointing at other parents. The Magic Folder snapshot object is conceptually very similar + to a git commit object, except for that it is created automatically and it records the history of an + individual file rather than an entire repository. Also, commits do not need to have authors + (although an author field could be easily added later). +- deletion snapshot: immutable directory containing no content entry (only one or more parents) +- capability: a Tahoe-LAFS diminishable cryptographic capability +- cap: short for capability +- conflict: the situation when another client's current snapshot for a file is different than our current snapshot, and is not a descendant of ours. +- overwrite: the situation when another client's current snapshot for a file is a (not necessarily direct) descendant of our current snapshot. + + +Overview +-------- + +This new design will track the history of each file using "snapshots" which are +created at each upload. Each snapshot will specify one or more parent snapshots, +forming a directed acyclic graph. A Magic-Folder user's DMD uses a flattened directory +hierarchy naming scheme, as in the original design. But, instead of pointing directly +at file contents, each file name will link to that user's latest snapshot for that file. + +Inside the dmd there will also be an immutable directory containing the client's subscriptions +(read-caps to other clients' dmds). + +Clients periodically poll each other's DMDs. When they see the current snapshot for a file is +different than their own current snapshot for that file, they immediately begin downloading its +contents and then walk backwards through the DAG from the new snapshot until they find their own +snapshot or a common ancestor. + +For the common ancestor search to be efficient, the client will need to keep a local store (in the magic folder db) of all of the snapshots +(but not their contents) between the oldest current snapshot of any of their subscriptions and their own current snapshot. +See "local cache purging policy" below for more details. + +If the new snapshot is a descendant of the client's existing snapshot, then this update +is an "overwrite" - like a git fast-forward. So, when the download of the new file completes it can overwrite +the existing local file with the new contents and update its dmd to point at the new snapshot. + +If the new snapshot is not a descendant of the client's current snapshot, then the update is a +conflict. The new file is downloaded and named $filename.conflict-$user1,$user2 (including a list +of other subscriptions who have that version as their current version). + +Changes to the local .conflict- file are not tracked. When that file disappears +(either by deletion, or being renamed) a new snapshot for the conflicting file is +created which has two parents - the client's snapshot prior to the conflict, and the +new conflicting snapshot. If multiple .conflict files are deleted or renamed in a short +period of time, a single conflict-resolving snapshot with more than two parents can be created. + +! I think this behavior will confuse users. + +Tahoe-LAFS snapshot objects +--------------------------- + +These Tahoe-LAFS snapshot objects only track the history of a single file, not a directory hierarchy. +Snapshot objects contain only two field types: +- ``Content``: an immutable capability of the file contents (omitted if deletion snapshot) +- ``Parent0..N``: immutable capabilities representing parent snapshots + +Therefore in this system an interesting side effect of this Tahoe snapshot object is that there is no +snapshot author. The only notion of an identity in the Magic-Folder system is the write capability of the user's DMD. + +The snapshot object is an immutable directory which looks like this: +content -> immutable cap to file content +parent0 -> immutable cap to a parent snapshot object +parent1..N -> more parent snapshots + + +Snapshot Author Identity +------------------------ + +Snapshot identity might become an important feature so that bad actors +can be recognized and other clients can stop "subscribing" to (polling for) updates from them. + +Perhaps snapshots could be signed by the user's Magic-Folder write key for this purpose? Probably a bad idea to reuse the write-cap key for this. Better to introduce ed25519 identity keys which can (optionally) sign snapshot contents and store the signature as another member of the immutable directory. + + +Conflict Resolution +------------------- + +detection of conflicts +`````````````````````` + +A Magic-Folder client updates a given file's current snapshot link to a snapshot which is a descendent +of the previous snapshot. For a given file, let's say "file1", Alice can detect that Bob's DMD has a "file1" +that links to a snapshot which conflicts. Two snapshots conflict if one is not an ancestor of the other. + + +a possible UI for resolving conflicts +````````````````````````````````````` + +If Alice links a conflicting snapshot object for a file named "file1", +Bob and Carole will see a file in their Magic-Folder called "file1.conflicted.Alice". +Alice conversely will see an additional file called "file1.conflicted.previous". +If Alice wishes to resolve the conflict with her new version of the file then +she simply deletes the file called "file1.conflicted.previous". If she wants to +choose the other version then she moves it into place: + + mv file1.conflicted.previous file1 + + +This scheme works for N number of conflicts. Bob for instance could choose +the same resolution for the conflict, like this: + + mv file1.Alice file1 + + +Deletion propagation and eventual Garbage Collection +---------------------------------------------------- + +When a user deletes a file, this is represented by a link from their DMD file +object to a deletion snapshot. Eventually all users will link this deletion +snapshot into their DMD. When all users have the link then they locally cache +the deletion snapshot and remove the link to that file in their DMD. +Deletions can of course be undeleted; this means creating a new snapshot +object that specifies itself a descent of the deletion snapshot. + +Clients periodically renew leases to all capabilities recursively linked +to in their DMD. Files which are unlinked by ALL the users of a +given Magic-Folder will eventually be garbage collected. + +Lease expirey duration must be tuned properly by storage servers such that +Garbage Collection does not occur too frequently. + + + +Performance Considerations +-------------------------- + +local changes +````````````` + +Our old scheme requires two remote Tahoe-LAFS operations per local file modification: +1. upload new file contents (as an immutable file) +2. modify mutable directory (DMD) to link to the immutable file cap + +Our new scheme requires three remote operations: +1. upload new file contents (as in immutable file) +2. upload immutable directory representing Tahoe-LAFS snapshot object +3. modify mutable directory (DMD) to link to the immutable snapshot object + +remote changes +`````````````` + +Our old scheme requires one remote Tahoe-LAFS operation per remote file modification (not counting the polling of the dmd): +1. Download new file content + +Our new scheme requires a minimum of two remote operations (not counting the polling of the dmd) for conflicting downloads, or three remote operations for overwrite downloads: +1. Download new snapshot object +2. Download the content it points to +3. If the download is an overwrite, modify the DMD to indicate that the downloaded version is their current version. + +If the new snapshot is not a direct descendant of our current snapshot or the other party's previous snapshot we saw, we will also need to download more snapshots to determine if it is a conflict or an overwrite. However, those can be done in +parallel with the content download since we will need to download the content in either case. + +While the old scheme is obviously more efficient, we think that the properties provided by the new scheme make it worth the additional cost. + +Physical updates to the DMD overiouslly need to be serialized, so multiple logical updates should be combined when an update is already in progress. + +conflict detection and local caching +```````````````````````````````````` + +Local caching of snapshots is important for performance. +We refer to the client's local snapshot cache as the ``magic-folder db``. + +Conflict detection can be expensive because it may require the client +to download many snapshots from the other user's DMD in order to try +and find it's own current snapshot or a descendent. The cost of scanning +the remote DMDs should not be very high unless the client conducting the +scan has lots of history to download because of being offline for a long +time while many new snapshots were distributed. + + +local cache purging policy +`````````````````````````` + +The client's current snapshot for each file should be cached at all times. +When all clients' views of a file are synchronized (they all have the same +snapshot for that file), no ancestry for that file needs to be cached. +When clients' views of a file are *not* synchronized, the most recent +common ancestor of all clients' snapshots must be kept cached, as must +all intermediate snapshots. + + +Local Merge Property +-------------------- + +Bob can in fact, set a pre-existing directory (with files) as his new Magic-Folder directory, resulting +in a merge of the Magic-Folder with Bob's local directory. Filename collisions will result in conflicts +because Bob's new snapshots are not descendent's of the existing Magic-Folder file snapshots. + + +Example: simultaneous update with four parties: + +1. A, B, C, D are in sync for file "foo" at snapshot X +2. A and B simultaneously change the file, creating snapshots XA and XB (both descendants of X). +3. C hears about XA first, and D hears about XB first. Both accept an overwrite. +4. All four parties hear about the other update they hadn't heard about yet. +5. Result: + - everyone's local file "foo" has the content pointed to by the snapshot in their DMD's "foo" entry + - A and C's DMDs each have the "foo" entry pointing at snapshot XA + - B and D's DMDs each have the "foo" entry pointing at snapshot XB + - A and C have a local file called foo.conflict-B,D with XB's content + - B and D have a local file called foo.conflict-A,C with XA's content + +Later: + + - Everyone ignores the conflict, and continue updating their local "foo". but slowly enough that there are no further conflicts, so that A and C remain in sync with eachother, and B and D remain in sync with eachother. + + - A and C's foo.conflict-B,D file continues to be updated with the latest version of the file B and D are working on, and vice-versa. + + - A and C edit the file at the same time again, causing a new conflict. + + - Local files are now: + + A: "foo", "foo.conflict-B,D", "foo.conflict-C" + + C: "foo", "foo.conflict-B,D", "foo.conflict-A" + + B and D: "foo", "foo.conflict-A", "foo.conflict-C" + + - Finally, D decides to look at "foo.conflict-A" and "foo.conflict-C", and they manually integrate (or decide to ignore) the differences into their own local file "foo". + + - D deletes their conflict files. + + - D's DMD now points to a snapshot that is a descendant of everyone else's current snapshot, resolving all conflicts. + + - The conflict files on A, B, and C disappear, and everyone's local file "foo" contains D's manually-merged content. + + +Daira: I think it is too complicated to include multiple nicknames in the .conflict files +(e.g. "foo.conflict-B,D"). It should be sufficient to have one file for each other client, +reflecting that client's latest version, regardless of who else it conflicts with. + + +Zooko's Design (as interpreted by Daira) +======================================== + +A version map is a mapping from client nickname to version number. + +Definition: a version map M' strictly-follows a mapping M iff for every entry c->v +in M, there is an entry c->v' in M' such that v' > v. + + +Each client maintains a 'local version map' and a 'conflict version map' for each file +in its magic folder db. +If it has never written the file, then the entry for its own nickname in the local version +map is zero. The conflict version map only contains entries for nicknames B where +"$FILENAME.conflict-$B" exists. + +When a client A uploads a file, it increments the version for its own nickname in its +local version map for the file, and includes that map as metadata with its upload. + +A download by client A from client B is an overwrite iff the downloaded version map +strictly-follows A's local version map for that file; in this case A replaces its local +version map with the downloaded version map. Otherwise it is a conflict, and the +download is put into "$FILENAME.conflict-$B"; in this case A's +local version map remains unchanged, and the entry B->v taken from the downloaded +version map is added to its conflict version map. + +If client A deletes or renames a conflict file "$FILENAME.conflict-$B", then A copies +the entry for B from its conflict version map to its local version map, deletes +the entry for B in its conflict version map, and performs another upload (with +incremented version number) of $FILENAME. + + +Example: + A, B, C = (10, 20, 30) everyone agrees. + A updates: (11, 20, 30) + B updates: (10, 21, 30) + +C will see either A or B first. Both would be an overwrite, if considered alone. + + +