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+laptopfs
+========
+
+Benjamin Barenblat, <bbarenblat@gmail.com>  
+October 16, 2021
+
+
+Objective
+---------
+
+Build a highly available network file system.
+
+
+Requirements
+------------
+
+laptopfs is a network file system designed for use on laptop home directories.
+This means it is
+
+  - a file system, allowing the users to manipulate files and directories using
+    established working patterns.
+
+  - highly available and partition-resistant, supporting offline operation just
+    as well as it supports online operation.
+
+  - secure by default, ensuring confidentiality and integrity whenever data
+    leave the laptop, including when they are stored on file servers.
+
+  - designed for multiple concurrent active sessions.
+
+  - disk- and bandwidth-efficient, transferring and storing only the data users
+    actually need.
+
+laptopfs is not
+
+  - designed for collaboration. If two clients modify the file system at the
+    same time, the server will reject one client’s write; while the unfortunate
+    client will likely succeed on a second attempt, this does not scale well and
+    should generally be avoided.
+
+  - a completely POSIX-compatible file system. In particular, laptopfs does not
+    support hard links, device nodes, or IPC mechanisms (named pipes, sockets,
+    etc.).
+
+
+Out of Scope
+------------
+
+Though laptopfs encrypts data whenever they leave the laptop and ensures the
+user’s data are encrypted remotely, it does not encrypt data while they are
+stored on the local disk. If a user requires this level of security, they should
+leverage the functionality built into their operating system.
+
+laptopfs clients use file system modification times to resolve certain types of
+writer–writer races. To ensure that races are resolved in an unsurprising way,
+clients should be well-synchronized to a coherent clock source. NTP (or even
+SNTP) should be sufficient for this.
+
+
+Background
+----------
+
+Distributed computing has captured both public imagination and corporate
+attention for over half a century. The stories of Arthur C. Clarke envision a
+distant future where mainframes are shared between entire neighborhoods. Later,
+Sun Microsystems promised a distributed future, proclaiming that “The network is
+the computer”. So-called distributed operating systems like Plan 9 from Bell
+Labs continue to draw attention decades after their development mostly stopped.
+
+However, the promised land of distributed computing has mostly failed to
+materialize. Comparing visions against reality, it’s easy to see why: Today’s
+world consists not of a vast network of computers communicating at the speed of
+light, but rather a vast archipelago of high-speed, high-reliability network
+fabrics joined by comparatively slow, sketchy, and expensive links. It is
+telling that even security-conscious corporations have had to make firewall
+exceptions for Mosh, a remote terminal protocol that supports low bandwidth and
+high latency, and that the world’s most popular “distributed” version control
+system, Git, is the one that allows users to work productively with no network
+connection at all.
+
+While researchers and developers have made incredible progress building fast and
+secure file systems that operate on individual machines and on high-speed
+network fabrics, there currently exists no file system that truly embraces the
+archipelago model. NFS and AFS can be configured to support low bandwidth and
+high latency, but neither allows disconnected operation. Past efforts to add
+support for true disconnected operation to existing network file systems have
+stalled. Most users thus access their remote files either by creating working
+copies on local disk (cf. Dropbox, Unison) or by offloading computing elsewhere
+(cf. Google Docs, Overleaf). These solutions satisfy most users’ requirements
+but also present considerable drawbacks, ranging from cognitive overhead from
+tracking multiple copies to philosophical frustration from using closed-source
+software.
+
+
+Related Work
+------------
+
+No past effort to create a network file system that supports disconnected
+operation has yet succeeded. Most have definitively failed, crushed under their
+own complexity.
+
+[Coda](http://coda.cs.cmu.edu/), a fork of AFS that aims to support disconnected
+operation, has been under development since 1987. Dormant for many years, it has
+recently received a burst of new activity. Reflecting its origins in the 1980s,
+Coda’s code quality and RPC protocol leave something to be desired, though work
+is occurring to bring it up to modern standards. Despite this work, Coda still
+includes significant known security bugs and is not yet suitable for production
+deployment on the internet.
+
+[Bazil](https://bazil.org/) was an attempt to create a decentralized,
+distributed file system that supported disconnected operation. Though the
+project produced a high-quality [Go FUSE library](https://github.com/bazil/fuse),
+development on Bazil itself seems to have stalled around 2015.
+
+Robert Mustacchi’s bachelor’s thesis, [StashFS: Generalized Disconnected
+Operation](https://cs.brown.edu/research/pubs/theses/ugrad/2010/mustacchi.pdf),
+described a FUSE overlay file system that added disconnected operation to _any_
+network file system. Because it operates as an overlay file system, StashFS is
+somewhat constrained in the assumptions it can make, but it seems fundamentally
+sound. Unfortunately, its code is not publicly available, and the author did not
+respond to email asking for a copy.
+
+[Dropbox](https://dropbox.com/) and [Google Drive](https://drive.google.com/)
+are premier proprietary cloud storage offerings. However, they are
+synchronization services, not file systems, which means they are fairly heavy on
+disk usage. Furthermore, each uses proprietary server software and RPC
+protocols, making them impossible to self-host and difficult to reliably extend.
+
+
+Detailed Design
+---------------
+
+A laptopfs network consists of one server and zero or more clients. The server
+holds the entire file system and passes hunks of it to interested clients; it
+also notifies connected clients when the file system has changed. The clients
+get hunks of the file system from the server, upload new hunks, and present the
+contents of the file system to users via FUSE.
+
+
+### The Server ###
+
+Fundamentally, the laptopfs server implements an RPC-accessible key-value store.
+It has a few extra bits to facilitate use of that store as a file system, but
+the basic RPC interface to laptopfsd looks an awful like the interface to a hash
+table: You can request a hunk of data by ID, or you can upload a new hunk of
+data. Unlike most hash tables, however, clients see laptopfsd’s key-value store
+as append-only: They cannot delete or modify entries in the store. This prevents
+reader–writer races between clients. (Writer–writer races are addressed later.)
+
+Using the append-only key-value store, it’s fairly easy to implement a
+copy-on-write file system by differentiating between three types of hunks:
+
+  - A data hunk stores raw data.
+
+  - A file hunk is a hunk containing a serialized vector of IDs, offsets, and
+    lengths, each triplet representing a slice of a data hunk. Retrieving those
+    data hunks, slicing them appropriately, and concatenating the slices yields
+    the contents of the file.
+
+  - A directory hunk is a hunk containing a set of file hunk IDs as well as a
+    name, mtime, mode, etc. for each file.
+
+The server tracks the ID of the root directory, and clients can update that ID
+using a special RPC. This is the one destructive operation that a client can
+perform, and it has several safeguards built into it. Notably, an update-root
+RPC does not simply request that the server update the root to a new value; it
+requests that the server update the root _from_ an old value _to_ a new value.
+If the old value does not match the current root ID, the server will reject the
+transaction. This enables a client to detect that its change would have
+clobbered another client’s changes, at which point the client can initiate a
+merge process and try again.
+
+Because mutating the file system necessarily involves at least two changes (one
+to upload the new root directory and one to set it as the root), the server
+allows clients to group mutations into transactions. To avoid round trips during
+the transactions, the server requires clients to generate new IDs; it will
+reject a transaction if the transaction attempts to use an ID already in use.
+
+Since clients see the store as append-only, the server must garbage-collect it
+periodically to avoid unbounded growth. This means the server needs to know the
+links between hunks in the file system. To facilitate this – while leaving the
+server as oblivious as possible to the actual contents of the file system –
+clients use AEAD to reveal the relevant parts of file and directory hunks.
+
+  - Data hunks encrypt everything, leaving the associated data region empty.
+
+  - File hunks encrypt data hunk offsets and lengths, leaving the data hunk IDs
+    in the associated data region.
+
+  - Directory hunks encrypt file names, mtimes, modes, etc., leaving the file
+    hunk IDs in the associated data region.
+
+The server implements a simple mark-and-sweep garbage collection algorithm and
+runs it at thresholds configurable by the administrator. During the collection,
+the server will reject transactions that mutate the file system, but since this
+is strictly more favorable to the client than completely disconnected operation,
+clients can cope.
+
+To allow clients to detect changes, the server allows clients to subscribe for
+updates to the root.
+
+The final RPC interface for the server, in pseudo-gRPC, thus looks like this:
+
+    message Hunk {
+      message AssociatedData {
+        repeated bytes referenced_hunk_ids
+      }
+
+      bytes id
+      AssociatedData associated_data [security = signed]
+      bytes data [security = signed+encrypted]
+    }
+
+    message GetHunkRequest {
+      repeated bytes ids
+    }
+
+    message GetHunkResponse {
+      repeated Hunk hunks  // any requested hunks that exist
+    }
+
+
+    message TransactionOperation {
+      message PutHunk {
+        Hunk hunk
+      }
+
+      message UpdateRoot {
+        bytes old_id
+        bytes new_id
+      }
+
+      oneof op {
+        PutHunk put_hunk
+        UpdateRoot update_root
+      }
+    }
+
+    message ExecuteTransactionRequest {
+      repeated TransactionOperation ops;
+    }
+
+    message ExecuteTransactionResponse {}
+
+
+    message RootIdRequest {
+      bool include_hunk
+    }
+
+    message RootIdResponse {
+      bytes new_root_id
+      optional Hunk new_root
+    }
+
+
+    service Laptopfs {
+      rpc GetHunk(GetHunkRequest) returns (GetHunkResponse) {}
+      rpc ExecuteTransaction(ExecuteTransactionRequest) returns (ExecuteTransactionResponse) {}
+
+      // This returns the current value immediately and then streams updates as
+      // they happen.
+      rpc RootId(RootIdRequest) returns (stream RootIdResponse) {}
+    }
+
+
+### The Client ###
+
+Since the laptopfs server is oblivious to most data in the system, most
+complexity appears in the laptopfs client. The client has three roles: It must
+maintain a local disk cache of the state that exists on the server, it must
+reveal that state to the user through a FUSE interface, and it must push user
+changes back to the server.
+
+The laptopfs client implements its on-disk cache using both a key-value store
+(like the server’s) and a log. The key-value store is a pure cache of the server
+state; the client never mutates it except in response to an announcement
+received from the server. The log, then, stores a record of all mutations that
+the client has made but not pushed to the server. This allows the client’s file
+system bits to mimic the implementation of a log-structured file system, a
+well-understood design that is both relatively simple and fairly performant.
+
+One set of client worker threads are responsible for maintaining the client’s
+store. At startup, the client starts a `RootId` RPC with the server to get the
+current ID of the root directory. It then executes a breadth-first search of the
+file system, fetching all directory and file hunks. Data hunks are not fetched
+in advance – instead, they’re fetched as the user tries to read them. (The
+client can apply whatever heuristics it wants to try to make reads as
+low-latency as possible, but basic readahead, triggered on open(2), lseek(2),
+and read(2), should cover most basic cases.) The end result is that while file
+access might lag a bit, navigating through the file system itself should always
+feel snappy. As the root directory ID changes, the client restarts its
+breadth-first search, optionally after waiting for the file system to settle.
+
+As the client receives hunks from the server, it decrypts and verifies their
+integrity; before it uploads new hunks to the server, it encrypts them. Data
+hunks are thus stored on disk simply as their contents, while file and directory
+hunks have more structure:
+
+    message Timestamp {
+      uint64 seconds
+      fixed32 nanoseconds
+    }
+
+    message FileHunk {
+      message HunkSlice {
+        uint64 offset
+        uint64 length
+      }
+
+      repeated HunkSlice slices
+
+      // The actual last time the data in the file changed.
+      Timestamp true_modification_time
+    }
+
+    message DirectoryHunk {
+      message FileInfo {
+        message Xattr {
+          bytes name
+          bytes value
+        }
+
+        bytes name
+        uint32 mode
+        Timestamp mtime
+        repeated Xattr xattrs
+      }
+
+      repeated FileInfo files
+
+      // The actual last time the data in the directory hunk changed.
+      Timestamp true_modification_time
+}
+
+As the user interacts with the FUSE frontend, the client computes a view of the
+file system from replaying the log on top of the store and adds entries to the
+log to correspond to user changes. Log entries are all timestamped; each can be
+one of the following operations:
+
+  - WRITE(data, path) (creating the file if it does not yet exist)
+
+  - DELETE(path)
+
+  - RENAME(path, path)
+
+  - SETMTIME(path, timestamp)
+
+  - CHMOD(path, mode)
+
+  - SETXATTR(path, name, value)
+
+The algebra for replaying these changes is based on “last one wins” semantics.
+
+Every few seconds (with the delay configurable by the user), the client updates
+the server on anything that has changed locally. The client first compacts the
+log. It then computes the minimally sized transaction that will apply the
+changes contained in the log to the store. Finally, it attempts to commit this
+transaction to the server. If the transaction succeeds, the client drops the
+uploaded changes from the log. If it fails, the client does nothing and tries
+again later.
+
+Like the server, the client uses a mark-and-sweep garbage collector to prevent
+its store from growing arbitrarily large. The client also can drop data to keep
+its disk usage small; it implements a simple least recently used scheme to
+determine when to drop data the user is likely no longer interested in.
+
+laptopfs supports one important operation beyond the standard Unix file system
+API: the ability to pin files to local disk. The client maintains a list of
+files the user would like to have locally at all times and fetches them whenever
+it discovers they have changed. These files are exempt from cache drops.
+
+
+### Future Work ###
+
+This design is highly extensible. Provided clients and servers all implement the
+RPC interface with the same semantics, multiple client and server
+implementations can coexist. The RPC interface is even transport-agnostic, and
+one could envision a client that supported multiple transports via plugins.
+
+The client has virtually unlimited latitude to heuristically prefetch and drop
+data, and heuristics will be an area of active exploration after the initial
+implementation.
+
+
+Security and Privacy Considerations
+-----------------------------------
+
+laptopfs encrypts most data stored in the file system using 256-bit AES-GCM with
+a key accessible only to the user. This produces fairly good privacy properties:
+
+  - Integrity: Neither the server nor an active adversary can modify data that
+    the user has uploaded. A malicious server could serve old data, but clients
+    can detect if it throws away a transaction (as it will not signal a root
+    invalidation after a transaction successfully completes).
+
+  - Confidentiality: The file system data are fully confidential, but metadata
+    are not. In particular, the server knows the general tree structure of the
+    file system and can estimate the number of files in each directory. The
+    server does not, however, have access to file names and can only make
+    educated guesses at file sizes.
+
+  - Availability: The server or an active adversary can deny service at any
+    time. On the other hand, laptopfs is designed to work with a disconnected
+    server.
+
+
+Work Estimates
+--------------
+
+You can always stop at any time.
+
+
+TODO: We need a slightly better log interface – this one doesn’t do very well
+when you’re trying to change a small part of a very large file (e.g., updating
+metadata in a ripped Blu-Ray).
diff --git a/LICENSE b/LICENSE
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