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Common Colaborative Coding Protocol

branch: master
README.rst

This project provides a generic server and sub-process agent for implementing cross-editor real-time, character by character collaboration (a la SubEthaEdit, Gobby or Google Docs). Support is currently provided for the following editors:

The editor-specific plugins are extremely tiny and merely delegate all work to an editor-agnostic sub-process. This means that it should be extremely easy to add CCCP support to almost any editor that supports extension.

Currently, the only functionality provided is character-by-character simultaneous co-edit between any number of editors. Future functionality may include things like:

  • Buffer set linking – This would allow collaborators to "follow" each-other's actions not just in terms of edits, but in terms of active buffers and their respective positions. In this case, one collaborator would be the "master" while the others followed along.
  • File discovery – Rather than having to share an opaque file identifier, the server would expose a list of active files to the editors, allowing users to link their buffers by selecting the appropriate file from a list.
  • Commit Coordination – At present, there is no VCS-specific support. This means that when you're ready to commit, you must nominate just one person to do the commit and then sync back up again. This can be a pain. With certain VCS (like Git), it would be possible to run the commit through CCCP and perform the same commit (with the same author information) simultaneously on both ends, resulting in the same commit being shared between collaborators without a separate fetch action.

Usage

If you want to use CCCP, you will first need to build both the server and agent modules. This can be done using the stage task in the SBT build. This task will create a dist/ directory under whatever project you use it against. Inside this directory will be all of the dependencies required to run that particular module, as well as a pair of scripts (for Windows and *nix) which launch that process. If you don't have SBT on your system, you can find instructions on how to get running here: https://github.com/harrah/xsbt/wiki/Getting-Started-Setup. Essentially all you need to do is launch the sbt-launch.jar file in the project directory.

If you don't feel like installing from source, you can use the pre-built binaries available in the Downloads section. Binaries are available for both the server and the agent.

The server process is a very straight forward HTTP server built on BlueEyes, which is in turn built on Netty. When you launch its process, you will need to pass the --configFile <file> option, where <file> is replaced with your server configuration file. The following template should be sufficient:

server {
  port = 8585
  sslPort = 8586
}

(note: you can find these contents in the default.conf file in the server module)

The server will remain up and running until killed with Ctrl+C. Note that Netty will reject the HUP signal if there are still outstanding client connections. You should run the server in some place that is accessible to all clients involved. All edit operations will be proxied through this server, which is handling the server-side OT for CCCP. You can find more details on this process below.

The second module you will need to build is the agent. This is the editor agnostic sub-process that will be used by any editor-specific plugins providing CCCP functionality. You do not run this process directly. Just build it and take note of its output directory.

Once you have all of this setup, you can now configure your editor-specific plugin. This will involve entering the root directory of the agent build (this should be dist/ in the agent module directory) as well as the protocol, host and port to be used to connect to the server.

If everything is working, you should be able to link a buffer in your editor. This process will prompt you for an id for that buffer. This id is used to identify the file you are linking so that collaborators need not use the exact same file names. Linking the file merely creates the registration with the server, it does not upload any data!

With the file linked, you can begin editing. Other collaborators can link with the same id. The critical restriction is that they must start from exactly the same file state as you did when you first linked the file. One good way to ensure this is to make everyone start from the same clean Git revision. Any edits you have performed since linking will be sent down to the new collaborators the moment they link their buffers.

After this point, all edits will be synced character-by-character between all linked buffers. Collaborators can type simultaneously at different points (or even the same point) in the file. Conflicts are resolved by the server in a uniform way, so the protocol never "fails". If your connection is more latent than your keyboard, edits may be chunked together slightly. This is entirely normal. The chunk sizes will automatically adjust themselves immediately in response to the network latency between your agent and the server, making the protocol extremely self-healing. You can even disconnect entirely from the server, perform a large number of edits, and they will all be synced in a large chunk as soon as your connection is re-established.

There is one important restriction here: you cannot change files outside of the editor. If you do this, the collaboration will get out of sync and the server will reject your changes. If you want to change a file outside the editor, you will need to unlink that buffer, change the file and then relink when you are done.

jEdit

First, you need to install the CCCP plugin. The best way to do this (right now) is to install from source. To do this, clone the project, cd to the root of the project directory and run the following commands:

$ sbt
> project cccp-jedit-client
> stage
> exit
$ cd clients/jedit/
$ ./local-deploy.sh

This assumes that you have set the JEDIT_HOME environment variable. Once you have performed these steps, the plugin will be installed and ready to use in your jEdit installation (note: jEdit may be open during this time). To activate the plugin, open up the Plugin Manager (action plugin-manager) and activate CCCP.jar.

Alternatively, if you prefer not to install from source, you can use the pre-built binary provided in this project's Downloads section. Note that this binary may be slightly out of date, depending on whether or not I have remembered to update it. Copy this JAR file to the jars/ directory in your jEdit settings directory. Note that you must also copy the scala-library.jar file from the Scala 2.9.1 distribution. Make sure it is exactly this version! If you have another plugin that depends on Scala, well, let's hope it depends on the same version.

Once you have the plugin up and running, the first thing you should do is configure it settings. Open up Plugin Options (action plugin-options) and navigate to the CCCP pane (note that the UI here is a work in progress). You need to set four settings:

  • CCCP Agent Home – This should be the directory containing the CCCP agent distribution. You can obtain this from the Downloads area, or built the agent from source using the stage task.
  • Protocol – This should be either http or https. Don't try to get cute with spdy or rsync, they will not work! This is the protocol used to transfer data from the agent to the server.
  • Host – The host name (or IP) hosting the CCCP server instance. Note that IPv6 is supported, but you should bracket the address to avoid any ambiguity.
  • Port – The port number on which the CCCP server is listening.

Once you save the settings, jEdit will reset the CCCP agent! This means that any currently-linked buffers will be unlinked.

Linking

Whenever you want to collaborate on a particular buffer, you must link that buffer with the server. When you link with the server, you will choose a unique identifier for this buffer. Your collaborators will link with your buffer by simply linking with the server using this exact same identifier.

Before you link, you should ensure that your buffer is in a known "start" state. Starting from a clean Git revision that is shared with your collaborators is a good way to go. Any changes made to your buffer after linking will be forwarded to the server. This way, collaborators are free to link with the server using the same buffer id at any point (even after you have been editing for a while). The important point is that their buffer is in the exact same start state as your buffer started in. A simple workflow might go as follows:

  1. User A checks out commit cafebabe
  2. User A opens file Foo.scala and links the buffer using id foo-scala
  3. User A starts editing Foo.scala (note that the save action is not significant and may be performed at will)
  4. User B checks out commit cafebabe
  5. User A is still editing, saving, committing, and generally being productive
  6. User B opens file Foo.scala and links the buffer using id foo-scala
  7. All of the changes performed by User A since linking the buffer initially will be performed on User B's buffer. Note that User B does not need to wait for this to happen; s/he is free to start editing immediately
  8. After a little bit of syncing, User A and User B will be in perfect sync and will see each other's edits in real-time, character-by-character
  9. Eventually, User A decides s/he has had enough of this collaboration nonsense and unlinks the buffer
  10. User B can continue editing, with the changes being forwarded to the server. Since User A has unlinked, its edits will not be forwarded to the server and thus not shared with User B. Likewise, the edits from User B will not be forwarded to User A.

At the end of this string of events, User A may reconsider unlinking and want to relink with the server. In order to do this without corrupting the local state, User A must first discard all local changes and revert back to commit cafebabe. Once they link, all changes (including those performed by User A) will be reapplied to the buffer, bringing it back into sync with the current collaborative state.

The core concept to understand is that the server maintains a list of deltas which will take a buffer from a single starting state to the current shared state. If a user attempts to apply those deltas to a starting state other than the starting state assumed by the server, the result will be a buffer that is permanently out of sync. Collaborative edits performed on this buffer will likely be rejected by the server and thus never forwarded onto other users.

Agent Protocol

The agent protocol is based on SWANK, which is the protocol used by SLIME and ENSIME to communicate with Emacs. The essence of the protocol is just sending s-expressions over a raw socket with run-length prefixes. The best description I've found of this process is from the ENSIME manual:

To send an s-expression, determine its ASCII length and then encode that integer as a padded six-digit hexadecimal value. Write this value to the output socket first, then followed by the ASCII form of the s-expression. On the receiving side, the reader loop should read six ASCII characters into a buffer and convert that into an integer, then read that number of ASCII characters from the socket, parsing the result into an s-expression.

http://aemon.com/file_dump/wire_protocol.png

Each SWANK RPC call is of the following form:

(:swank-rpc <form> <call-id>)

For example, if you wanted to invoke the edit-file RPC as call id 42, the s-expression would look like the following:

(:swank-rpc (swank:edit-file "file.txt" (:retain 4 :insert "ing" :retain 1)) 42)

The actual ASCII bytes sent over the socket would be as follows:

000050(:swank-rpc (swank:edit-file "file.txt" (:retain 4 :insert "ing" :retain 1)) 42)

The call id should be unique for each RPC invocation, but beyond that it has no restrictions. Returns for a particular call will use its call id, though this feature is not relevant for CCCP as none of the calls have returns.

Invocations from the agent to the editor are less restricted. Generally, they can be of any agreed-upon form. They still use run-length prefixing and s-expressions, but beyond that any form is allowed. See the Editor API.

Agent API

  • (swank:init-connection (:protocol protocol :host host :port port))

    Initializes the agent's connection to the server. Note that the agent will not actually test this connection, it will merely configure for later HTTP calls. This RPC must be invoked prior to anything else and may only be called once.

  • (swank:link-file id file-name)

    Creates a new buffer linkage for a particular identifier. This identifier will be used whenever the agent sends operations on this buffer to the server. Thus, if you want to link a buffer between two editors, you would simply link them both to the same identifier. The file name is only significant in that it must be the file name included in the swank:edit-file invocations which perform the actual edits. This is done so that the editor plugin does not have to maintain its own internal mapping from file names to identifiers.

    This call must be made prior to editing the file and can only be made once. Note that it is possible to relink buffers after having previously unlinked them. However, this requires that the buffer be in exactly the same state as any buffers that remained linked, or the same state as the last buffer to be unlinked at the point at which it was unlinked. Generally speaking, it is just safer to link on a fresh identifier when relinking a buffer.

  • (swank:unlink-file file-name)

    Removes a linkage for a particular file. Remote updates will not be propagated to the buffer once this call has run. This also frees any resources in the agent that are associated with the linkage. Please note that in cases of high-latency, there may be changes local to the agent that have not yet transmitted to the server. These changes will not be sent if unlink-file happens before such time as that is possible. The editor local buffer will still have the changes, but they will never reach the server.

  • (swank:edit-file file-name (...))

    This is the most important API call. This call should be made on every buffer change. The inner-form is the description of the buffer change and must be an ordered property list of the form (:key1 value1 :key2 value2). The exact schema for this property list should be as follows:

    • :retain – Must correspond to an integer value. Specifies an offset into the file.
    • :insert – Must correspond to a string value. Specifies a text string to insert at the current location.
    • :delete – Must correspond to a string value. Specifies a text string to delete from the current location.

    There are a few things that are important to understand about this format. First, the offsets must span the entire file. Thus, if you add up all of the :retain values, plus the length of the :insert and :delete strings, it must equal the total character length of the buffer. In the case of :insert, this is the total length after application of the operation; in the case of :delete, it is the total length before application of the operation. Note that this metaphor only makes sense if you have either an :insert or a :delete, but not both. This is a weakness in the line of thought, since it is very possible to have an operation which performs both actions (e.g. if text is selected and replaced with some new text in an atomic action). A truer way of looking at operation offsets would be to view the operation as an ordered set of instructions to a cursor walking through the buffer from start to finish. The cursor must traverse the entire document.

    Note that operations sent from the editor to the agent are likely to be single-action operations with a leading and trailing retain. This is extremely unlikely to be the case for operations coming from the agent to the editor. This is because the protocol composes operations together when latency exceeds typist speed (the normal mode of operation). As a result, the editor code which handles operations must be able to handle multiple actions in a single operation. For example:

    (:retain 4 :delete "bar" :insert "foo" :retain 127 :insert "baz" :retain 10)

    The jEdit plugin handles this by converting each :delete and :insert action into its own separate operation with offset and contents. These actions are then applied in order (the ordering bit is very important, otherwise the offsets will not be correct for actions subsequent to the first in the operation).

    Just to give an example of an operation, we would insert the text here at offset 11133 with a total buffer length of 11430 using the following operation:

    (:retain 11133 :insert "here" :retain 297)

    It is very important that operation application and synthesis is implemented correctly in the editor-specific plugins. Bugs in this code will result in incorrectly-synchronized buffers and errors in the agent, the server, or both. For more details on operations, see this article on OT as well as the documentation at http://www.waveprotocol.org. CCCP does not implement the Wave protocol, but it does use Wave's OT algorithms and operation abstractions.

  • (swank:shutdown)

    Causes the agent process to gracefully shutdown. This call should be used instead of just killing the sub-process. While killing the process will work, the swank:shutdown call gives the agent a chance to clean up registrations on the server.

Callbacks

In order for changes to be pushed back from the server to the client, the agent must make a call proactively to the client, not as a response to any particular message. This must also happen for things like errors, malformed messages and similar. SWANK provides a mechanism for reporting errors on specific calls, and thus the only callbacks which are unique (and require documentation) are those synthesized by the agent. Currently, there is only one of these:

  • (:edit-performed file-name (...))

    This call indicates that an operation has been applied to the given file, and that operation is represented by the specified form. The format used to represent an operation is the same as the one used by the swank:edit-file RPC. Note that this call will only take place for operations which need to be applied to the local buffer. Thus, local operations (which are already in the buffer) will not result in this callback, only remote operations. These remote operations will have already passed through the OT process, and thus should be directly insertable into the local buffer.

Gory Details

CCCP fully implements an optimistic concurrency control mechanism called "operational transformation". This is what allows real-time collaborative editing on a single document to proceed without each editor waiting for a server round-trip before inserting or removing characters. Before we dive into how this works, we need to establish a little vocabulary:

  • operation – a command to change the edit buffer consisting of zero or more actions applied in a cursor style, spanning the entire buffer
  • action – an individual component of an operation, indicating that text should be added or removed (depending on the action type)
  • transformation – the process of adjusting or "fixing" operations to that they can be reordered between clients without affecting the net composite
  • composition – the process of taking two operations that apply to the same document and deriving one operation which represents the net change of the two when applied to the original document
  • client – the editor itself
  • agent – the editor sub-process which handles the client-side work
  • server – the server process which handles the server-side work
  • document – a term I will use interchangably with edit buffer

The fundamental problem with real-time collaborative editing is that changes are occuring simultaneously at various positions in the document. Each editor needs to apply its operations locally without delay. This is a critical "feature" as it is what allows input responsiveness in the client. Unfortunately, if editor A inserts two characters at offset 12 while simultaneously editor B inserts five characters at offset 20, there is potential for document corruption.

This is really the classic diamond problem in concurrency control. Editor A applies its operation locally and sends it to B. Meanwhile, editor B applies its operation locally and sends it to editor A. However, when editor A attempts to apply the operation from editor B, it will perform the insertion at offset 20, which is not the location in the document that B intended. The actual intended location has become offset 22 due to the two new characters inserted by A prior to receiving the operation from B. This is the problem that OT solves.

The first step in solving this problem is to handle the simple diamond problem illustrated above. Two editors apply operations a and b simultaneously. We need to derive two transformed operations a' and b' such that a + b' = b + a'. This process is mostly just adjusting offsets and shuffling text in one direction or another, and it is fully implemented by the Wave OT algorithm. The exact details of this process are beyond the scope of this README.

There is one slight niggling detail here: what happens if we have three editors, A, B and C? A key insight of the Jupiter collaboration system (the primary theoretical foundation for Wave) is that it is possible to collapse this problem into the two-editor case by introducing a client-server architecture. Effectively, there are only ever two editors at a time: the client and the server. When operations are applied on the server, they are mirrored back to every other client. This also provides a uniform way of resolving conflicts: just find in favor of the server every time. Naturally, this is a race condition, and it may result in unexpected document states surrounding simultaneous edits at the same offset, but the point is that the document states will be uniform across all clients, and so users are able to simply cursor back and "fix" the change as they see fit.

Unfortunately, solving the one-step diamond is insufficient to enable real-time collaborative editing. The reason for this is best illustrated with an example. Editor A applies an operation a1 and then immediately follows it up with a2. Perhaps A is typing at more than one or two characters per second. Meanwhile, the server has applied an operation from editor B, b1. A sends a1 to the server while the server simultaneously sends b1 to A. This will result in an application of OT to derive a1' (on the server) and b1' (on the client), and that's all well and good. However, A also needs to send operation a2 to the server, and this is where we hit a snag.

The problem is that a2 is an operation that applies to the document state following a1, not respecting b1! Thus, a2 requires a document state that the server does not have. A will send a2 to the server and the server will be unable to apply, transform or otherwise make use of the operation, resulting in editor state corruption.

There are two ways to solve this problem. The first, and the one used by Jupiter and almost every other OT-based collaborative system is for the server to track every individual client's state in vector space. Basically, the server must not only apply a1' to its internal state, it must also apply a1 to an earlier state, creating an in-memory fork of the server state that will be preserved until A comes back into sync with the server. In the case where multiple editors are typing simultaneously, this could potentially take a very long time. The normal state for editors using OT is to be walking entirely different state spaces from each other, only coming back into full sync once everything "calms down". This produces a very nice user experience, but it also means that the server would need to track the full (and potentially lengthy) histories for every single client, producing a large amount of overhead.

This doesn't scale well. Google's key innovation with Wave was to restrict client behavior so that A can never send a2 directly to the server. Instead, A must wait for the confirmation that the server has applied a1, at which point A will use the operations it has received from the server in the interim to infer the current state of the server's document and edit history. Using this information, A will transform a2 into a2' and send that operation to the server. Now, the server may still need to transform a2' against subsequent operations that hadn't been received by A at the time of transmission, but that's not a problem. As long as a2' is rooted in server state space, the server will be able to perform this transformation and will only need to track its own history.

In terms of version control systems, you can think of this like the clients constantly rebasing their history against a central repository, rather than pushing an entire branch and attempting to merge at the end. It's a great deal more work for the clients, but it means that the server only needs to maintain a linear history, regardless of the number of clients.

Unfortunately, Wave doesn't provide this for us. Its code for this purpose is Wave-specific, and so cannot be repurposed for other things. For this reason, CCCP has to provide its own implementation of this logic (state.scala).

At the end of the day, the result is a collaborative editing system that allows character-by-character changes to be shared in real time across any number of clients with varying latencies. The protocol heals itself and degrades gracefully, chunking together updates when the server is taking a long time to report back with the confirmation of the previous operation. This self-healing is so flexible that you can actually take your editor completely offline for any length of time! The edits will simply buffer up, awaiting confirmation. Once the network connection is reestablished, the confirmation will finally arrive, the buffer will flush to the server in one chunk and everything will sync-up once again.

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