The Braid Protocol: Synchronization for HTTP
The Braid Protocol: Synchronization for HTTP
Internet-Draft                                                 M. Toomim
Expires: Jan 8, 2020                                   Invisible College
Intended status: Proposed Standard                             R. Walker
                                                       Invisible College
                                                            July 8, 2019

              The Braid Protocol: Synchronization for HTTP
                         draft-toomim-braid-00

Abstract

  Braid is a proposal for a new version of HTTP that transforms it from
  a state *transfer* protocol into a state *synchronization* protocol.
  Braid puts the power of Operational Transform and CRDTs onto the web,
  improving network performance and robustness, and enabling
  peer-to-peer web applications.

  At the same time, Braid creates an open standard for the dynamic
  internal state of websites.  Programmers can access state uniformly,
  whether local or on another website.  This creates a separation of UI
  from State, and allows any user to edit or choose their own UI for any
  website's state.

  We have a working prototype of the Braid, and have deployed it with
  production websites.  This memo describes the protocol, how it
  differs from prior versions of HTTP, and a plan to deploy it in a
  backwards-compatible way, where web developers can opt into the new
  synchronization features without breaking the rest of the web.



Status of this Memo

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Synchronization . . . . . . . . . . . . . . . . . . . . . . . . 7
   3.  Deployment and Upgrade Plan . . . . . . . . . . . . . . . . .  10
   4.  Proposed Changes to HTTP. . . . . . . . . . . . . . . . . . .  11
   4.1.  Linked JSON   . . . . . . . . . . . . . . . . . . . . . . .  12
   4.2.  Generalized request/response  . . . . . . . . . . . . . . .  13
   4.3.  Subscriptions   . . . . . . . . . . . . . . . . . . . . . .  14
   4.4.  Versioning  . . . . . . . . . . . . . . . . . . . . . . . .  15
   4.4.1.  Versions  . . . . . . . . . . . . . . . . . . . . . . . .  16
   4.4.2.  Patches   . . . . . . . . . . . . . . . . . . . . . . . .  17
   4.4.3.  Merge Types . . . . . . . . . . . . . . . . . . . . . . .  18
   4.4.4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . .  19
   5.  Network Messages  . . . . . . . . . . . . . . . . . . . . . .  20
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
   8.  Copyright Notice  . . . . . . . . . . . . . . . . . . . . . .  23
   9.  Author's Address  . . . . . . . . . . . . . . . . . . . . . .  23



1. Introduction

  HTTP was initially designed to transfer static pages.  If a page
  changes, it is the client's responsibility to issue another GET
  request.  This made sense when pages were static and written by hand.
  However, today's websites are dynamic, generated from databases, and
  continuously mutate as their state changes.  Now we need state
  *synchronization*, not just state *transfer*.

  Unfortunately, there is no standard way to synchronize.  Instead,
  programmers write non-standard code; wiring together custom protocols
  over WebSockets and long-polling XMLHTTPrequests with stacks of
  Javascript frameworks.  The task of connecting a UI with data is one
  that every dynamic website has to do, but there is no standard way to
  do it.



      ======= HTTP Websites =======      ====== Braid Websites ======

      Today's websites are               Braid generalizes HTTP and
      generated from multiple            REST into a uniform standard
      layers of state across             that synchronizes state
      multiple computers.  Each          within and between dynamic
      layer has a different API.         websites.

        x Non-standard state API          o Standard state API

        _Client__
       /         \
      :  o o o o  :   Webpage DOM          o o o o       State
      :   \|  \|  :                         \|  \|
      :    x   x  :   HTML Templates         o   o       State
      :   /|  /|  :                         /|  /|
      :  x x x x  :   JS Models            o o o o       State
       \ | | | | /                         | | | |
         | | | |                           | | | |
         o o o o    - http:// -            o o o o     - braid:// -
       / | | | | \                         | | | |
      :  x x x x  :   Views                o o o o       State
      :  |  \| |  :                        |  \| |
      :  x   x x  :   Controllers          o   o o       State
      :   \ / \|  :                         \ / \|
      :    x   x  :   Models                 o   o       State
      :     \ /   :                           \ /
       \.... x ../    Database                 o         State
         Server

      Today's programmers have to        On the braid, each piece of
      learn each API, and wire them      state (o) has a URL; whether
      together, making sure that         public, or internal to a
      changes to shared state            client or server.  Any state
      synchronize across all             can be a function of other
      layers and computers.              state, and dynamically
                                         recomputes when its
                                         dependencies change.  Braid
                                         guarantees the network will
                                         synchronize.



  As the web becomes more dynamic and data-driven, the complexity of the
  non-standard Javascript stack grows, and an increasing amount of data
  is inaccessible to the open web.  The result is a web which is open on
  the surface, but closed internally: websites can link to each other's
  *pages*, but cannot easily share each other's internal *state*.

  We can solve this by generalizing HTTP into a *synchronization*
  protocol, which replaces the complex Javascript stack, while providing
  new features, and making website internal state accessible anywhere
  desired, and realtime synchronized by default.

  We have a working prototype of the Braid protocol, and have deployed
  it with production websites.  The prototype is implemented as a
  polyfill library, which adds Braid features to existing browsers and
  servers.

  This document describes the new protocol, how it differs from prior
  versions of HTTP, and a plan to deploy it in a backwards-compatible
  way, where web developers can opt into the new synchronization
  features without breaking the rest of the web.



2. Synchronization

  Braid incorporates the abilities of Operational Transform and CRDTs.
  These are approaches to solving *synchronization*.

  Synchronization is a problem that occurs whenever two or more
  computers or threads access the same state.  Synchronization code is
  tricky to write, and can result in clobbers, corruptions, or race
  conditions.

  This is a challenging problem, which has seen a number of partial
  attempts in HTTP, such as e-tags, cache control, PATCH, JSON-diff, and
  SSE.

  Luckily, a set of maturing synchronization technologies (such as
  Operational Transform and CRDTs) can now automate and encapsulate
  synchronization within a library.  They can synchronize arbitrary JSON
  data structures across an arbitrary set of computers that make
  arbitrary mutations, and consistently merge their edits into a valid
  result, without a central server, in the face of arbitrary network
  delays and dropouts.  In other words, it is now possible to interact
  with state stored anywhere on a network as if it is a local variable,
  and program as if it is already downloaded and always up-to-date.

  Unfortunately, each synchronizer implements a different protocol, with
  a different set of features and tradeoffs.  Braid proposes a common
  language for synchronizers, so that they can interoperate, and
  implements it as an extension to HTTP.

  This lets multiple synchronizers interoperate, if they agree on a way
  to consistently resolve ambiguities -- a *merge type*.  We have run
  tests that succesfully interoperate a CRDT and OT system over the
  common Braid protocol.



  When applying synchronization to the web, we see the power of
  synchronization manifest in these concrete ways:

    - Caches update automatically and instantly, because servers promise
      to push changes to their subscribers.  This obsoletes the
      `cache-control` and `refresh` headers, and the `max-age`
      heuristic.  Users never need to force-clear their cache.

    - Updates go over the network as diffs, which can be much smaller
      than the resources they modify, significantly reducing network
      usage.

    - Web apps get an offline mode for free.  Edits from multiple
      clients merge automatically once they come online.  Network
      failures recover transparently.

    - *Reload* buttons in browsers become unnecessary, and can be
      removed for braid sites.  Browsers automatically discover and
      display the most recent version on their own.

    - Web apps require roughly 70% less code to build (in our
      experiments), because programmers do not need web frameworks or
      custom logic to wire together a stack of server-state and
      client-state.  This work is automated by the protocol.

    - Every <textarea> can become a collaborative editor (like Google
      Docs) for free.

    - Servers become optional.  Many apps can function without a server,
      because peers can synchronize with one another directly over the
      protocol.

    - In standardizing synchronization, we implicitly create a standard
      for state.  This allows state to be shared between different sites
      without the need for an extra API.

    - Standardizing the representation of *state* allows us to separate
      the representation of UI and state.  Most HTTP websites *inject*
      the state that they receive into templates representing UI
      components.  Braid sites can instead understand UI components as
      lenses through which to view state.  This improves the semantics
      of UI rendering, allows state to be inspected by clients directly,
      and makes it easier to build multiple alternative UIs for a single
      site.


  Standardized state and synchronization allows the topology of content
  on the web to be less centralized.

           Closed Networks

        -  -              -  -       Before the web, people used
     /        \        /        \    closed networks like America
    |   x   x  |      |   x  x   |   Online.  Content was
    |          |      |          |   encapsulated behind
     \    x   /        \    x   /    proprietary protocols.
        -  -              -  -
        Aol            Compuserve



             HTTP Websites

        -  o--------------o  -       The web lets any site define
     /        o--------o        \    *pages* at URLs.  A site can
    |   x   x  o------o   x  x   |   *link* to another site's pages,
    |          o------o          o   adding value to both sites.
     \    x   o        o    x   /
        -  o--------------o  -
       Facebook         NY Times



             Braid Websites

        o--o    _---------o  o       The braid lets any site define
        |/    o'-------o  |  |       *states* at URLs.  A state can
    o---o---o---------o---o--o---o   be a *function* of other states.
       / \ / \ o------o    \  \      When a state changes, the others
      o   o---o \__.   o    o   o    automatically synchronize with it,
        o--o        `-----o--o       like a spreadsheet.
      My Stuff         Your Stuff




3. Deployment and Upgrade Plan

  Braid makes fundamental changes to HTTP and REST, which creates an
  opportunity to unify a number of disparate features (SSE, E-tags,
  Cache-control, PATCH, JSON-Diff) within a simple integrated design.

  Rather than shoe-horn these changes into the existing HTTP semantics,
  we propose a new simpler layer, with a backwards-compatible mapping to
  HTTP's existing semantics.  This allows existing applications to
  interoperate, but new applications can opt-in to a simpler web API
  that provides more synchronization features.

  We can deploy these semantics in two phases:

    1. The first phase requires no changes to existing web browsers,
       making it easy for users to experiment with the protocol's
       semantics in existing websites.

       In this phase, browsers fetch the initial HTML page over the
       existing HTTP protocol, which includes Javascript code that
       initiates a WebSocket connection that runs the Braid protocol.
       This WebSocket version of the Braid protocol provides the full
       synchronization functionality, but is less performant --
       requiring an extra round-trip to initiate.

    2. If and when the WebSocket protocol stabilizes and achieves
       real-world adoption, we can add the Braid semantics into HTTP
       itself, in a new layer via an HTTP Upgrade header [RFC 2616,
       section 14.42].

  Both versions of the protocol can maintain backwards-compatibility
  with existing HTTP clients and servers.  Any client accessing a Braid
  server via HTTP 1, 2, or 3 will be able to GET, PUT, and POST Braid
  state, but without full synchronization capabilities.  Likewise, Braid
  clients can access state on HTTP servers, but will have to poll the
  server for updates.




4. Proposed Changes to HTTP

  Braid is composed of a set of opt-in changes that any browser or
  server can implement.

  First, whereas HTTP is explicitly client/server, Braid is capable of
  running peer-to-peer.  To do this, it generalizes the explicit
  request/response pattern of HTTP into a set of common messages, opened
  over a persistent two-way connection.

  Additionally, Braid specifies a new content-type for a resource:
  Linked JSON.  This provides a standard format for dynamic state, akin
  to how HTML specifies a standard for the presented content of a
  webpage.

  This section first describes Linked JSON, and then the changes to HTTP
  networking methods, and finally the versioning features that ensure a
  peer-to-peer network of edits converge to the same version.



4.1. Linked JSON

  Whereas HTML defines a common format for the presented content of web
  pages, braid defines a common format for their internal data, or
  state: Linked JSON.  This extends standard JSON with two additional
  datatypes:

    - A *link*, which lets one piece of JSON, at one URL, to link to
      another piece of JSON at another URL

    - A *binary blob*, which lets one encode a binary file (such as an
      image) as a value.

  We encode links within JSON like:

    {
      "foo": 3,
      "bar": 5,
      "something else": {"link": "braid://foo.com/something"}
    }

  Any object with a field named "link" is special, and interpreted as a
  link.  To encode an actual field named link, you prefix it with an
  underscore, like:

    {"_link": "this is not a link"}

  To encode an underscore, you use two underscores, like "__", and so
  on.

  Links allow programmers to combine data across multiple services, even
  on multiple websites, and to represent non-tree data as JSON, such as
  circular graphs and relational tables.  Foreign keys can be specified
  as links to other queries.  Cycles can be specified as a link back to
  the root of an object.

  Binary data is encoded similarly, as:

    {"binary": "<base64-encoded string>", "content-type": "<type>"}

  The content-type is optional.



4.2. Generalized request/response

  In HTTP, a client sends a *request* to the server, and that request is
  met with a *response*.  By contrast, a Braid connection is two-way, so
  messages can be initiated by either party.  Rather than giving a
  response to a message, a Braid server sends a separate message that
  acts as the response.  It turns out that a GET response message has
  the same effect on a peer as a SET request message-- both set the
  state on the recipient.

  --------------------------------------------------------------
  | HTTP         | Braid       | Meaning                       |
  | ------------ | ----------- | ----------------------------- |
  | Get Request  | Get message | "I want this"                 |
  | Get Response | Set message | "This is the current version" |
  | Put Request  | Set message | "This is the current version" |
  | Put Response | Ack message | "I accept this version"       |
  --------------------------------------------------------------



Subscriptions

  In the Braid model, whenever a client requests some state, it also
  subscribes to new versions of that state.  This requires only minor
  changes to the semantics of HTTP.

  In the Braid protocol, a GET message not only returns the current
  value of state, but also *subscribes* to future updates.  The
  subscription continues until the client sends a FORGET.  Finally,
  Braid unifies the PUT, POST, and PATCH methods in to a single
  SET method, which is able to both create state and change state.

  ---------------------------------------------------------------------
  | HTTP method    | Braid method | What's new                        |
  | -------------- | ------------ | --------------------------------- |
  | Get            | Get          | Also subscribes to future updates |
  | - n/a -        | Forget       | Ends a "Get" subscription         |
  | Put/Post/Patch | Set          | Also updates all subscribers      |
  | Delete         | Delete       | Also updates all subscribers      |
  ---------------------------------------------------------------------

  The traditional distinction between PUT and POST is that PUT requests
  are idempotent, allowing them to be cached and retried, whereas POST
  is often not.  However, the Braid protocol allows idempotence to be
  distinguished by re-using a version on Set -- if two messages set a
  state to the same version, they are idempotent, and equivalent to a
  PUT.

  In their simplest forms, these messages are otherwise semantically
  identical to their corresponding HTTP methods.  When robust
  synchronization is required, these messages will include optional
  *versioning* features.



4.4. Versioning

  Even though there are many synchronizers, it is possible for them to
  communicate in a common language.  Different synchronizers use
  different data structures internally, and have different network
  messages-- however, the *information* they send can all be represented
  with a common set of objects:

   - VERSIONS  define points in time,  irrespective of space
   - LOCATIONS define points of space, irrespective of time
   - PATCHES replace regions of space, across spans of time

  These three objects are enough to represent any type of change to a
  JSON data structure.  We have verified this experimentally, by
  implementing a translation algorithm that converts the network
  messages of ShareDB (an Operational Transform synchronizer) and
  Automerge (a CRDT synchronizer) into these objects, and back again,
  and verifying that the synchronizers still work in fuzz testing.

  However, synchronizers also differ in how they resolve conflicting
  changes to the same region of state.  We can generalize the behavior
  of these resolvers by defining *merge types*:

   - MERGE TYPES define how edits to the same location resolve

  If a synchronizer expresses state changes using versions, locations,
  and patches, and specifies its merge types, then it can synchronize
  with any other braid synchronizer implementing the same merge types,
  no matter their internal implementation.

  Finally, synchronizers also broadcast *acknowledgements* of the
  versions they have received, in order to tell their peers that they
  have moved forward in time, and will no longer refer to old history
  when sending patches.  This allows their peers to prune their history
  logs, and free up unused memory:

   - ACKNOWLEDGEMENTS of versions allow peers to prune historical memory

  The rest of this section explains how these concepts work together.



4.4.1. VERSIONS

  Time on a network is ambiguous as a result of latency.  If multiple
  peers edit the same state at the same time, we cannot say that one
  happened before the other, and time forks.  When they communicate
  their changes over the network, they merge their edits, and time
  merges.

         (O)    All peers are aware of version (O).
         / \
        /   \
      (A)   (B) Two peers make simultaneous edits creating versions (A)
        \   /   and (B) respectively, and time forks.
         \ /
         (M)    Once a peer sees both of the edits, time merges again.
                Until a peer makes an edit with (A) and (B) as parents,
                the merge is only implicit-- there is no need for to
                assign an ID to it.

  Thus the shape of time is not a line but a Directed Acyclic Graph-- a
  DAG.  Every change to state creates a new version.  This version has
  *parents*: all recent (leaf) versions that the client had seen when it
  made its edit.  The edit becomes the *child* of its parents.  We can
  say that one version came before another only if the first version is
  an *ancestor* of the second.

  Every version is identified by a unique ID.  There is no requirement
  on the format of the ID, only that it be a unique string.

  When a peer makes an edit, it broadcasts the edit's version ID, its
  parents version IDs, and a patch from its parents state to its state.



4.4.2. Patches

  When a peer changes some state, it encodes that change as a patch.  A
  SET message includes 'patches' as an array of patches.  All patches
  are *replace* operations, that replace one region of space with a new
  value.  The region being replaced is specified as a start and end
  index of the previous state, which is computed by merging all the
  parents of the version using its merge type.

  Insertions are implemented as replacing a zero-length region with a
  non-zero-length region, and deletes replace a non-zero length region
  with a zero-length one.

  Here are some example patches targeting object 'obj'
  (ie, {set: 'obj', patches: [...]} )

    .foo[0].bar = null       # Replace obj.foo[0].bar with null

    [3:3] = "asdf"           # Insert the string 'asdf' at index 3 of
                               string obj.  Illegal if array.
    [3] = "a"                # Set char 3 of string obj to 'a'
                               or element 3 of array obj to 'a'
    [3] = "asdf"             # Illegal if string.
                               If array, set obj[3] to 'asdf'
    [3:4] = [1, 3, 5]        # Splice [1,3,5] into array obj, replacing
                               element 3.  Illegal if string.
    [4:4] = [{msg: "hi"}]    # Insert an object at the end of array obj

    [3:10] = ""              # Delete characters 3-10 in string obj

    = false                  # Set the entire object obj to false

    .foo[0].bar = undefined  # Delete obj.foo[0].bar



4.4.3. Merge Types

  Different applications want to resolve conflicts in different ways.
  For instance, strings in a collaborative text editor will want to
  merge clobbering edits by inserting everything typed, and deleting
  everything deleted, and breaking ordering ties arbitrarily; but if two
  debits to a bank account balance occur in parallel, we will want to
  merge the debits by adding the differences together.

  A "merge type" specifies how any two edits made in parallel are merged
  together.  If two synchronizers implement the same merge type for some
  state, they will converge to a consistent result after arbitrary
  merges.  Merge types are specified by unique strings, such as
  "sync9-string" or "sharedb-rich-text".

     LWW(vid)       # Last-write-wins, sorted by version ID
     text(vid)      # Merges text edits like Google Docs
     counter        # Merges additions and subtractions by summing

  All peers synchronizing with a piece of state will need to implement
  the same merge types.  We envision a handful of merge types will
  likely cover most situations on the web.  Each field in a JSON object
  can merge using a different merge type.  One way to specify merge
  types of JSON objects is with a schema:

      {
          id: <string>            : LWW(vid)
          body: <string>          : text(vid)
          authors: <array>        : text(vid) [
              author_id: <string> : LWW(vid)
          ]
          likes: <int>            : counter
      }

  However, we have not yet implemented configurable merge types in our
  prototype, or settled on a way to communicate them in the protocol.
  Thus, we do not specify merge types in the network examples given
  later.



4.4.4. Acknowledgements

  In order merge two versions, a synchronizer generally needs enough
  history to trace a path of time back from both versions through a
  common "fork point."  Thus, in order to synchronize perfectly, peers
  need to store historical versions in time back to any fork point from
  which they expect another peer to send an edit.

  To allow other peers to prune history, any peer thus needs to inform
  them that it no longer intends to base edits onto versions from the
  past.  A general way to do this is for peers to agree to always make
  edits to the most recent versions they have.  Then, a peer will be
  able to prune old history as long as it knows which versions of
  history all other peers have seen.  Once all peers have seen a more
  recent version, a peer can know that they will not base an edit on one
  of its ancestors.

  In the Braid protocol, peers can communicate which versions they have
  seen using ACK messages.  Each ACK specifies the URL and version of
  the state that the peer has seen.  When a peer sends an ACK, it means
  "everyone who I have sent this version to has also acknowledged its
  receipt."  Then, once the original sender has received all
  acknowledgements from all peers, it sends out a final "ack-complete"
  message, which communicates "everyone in the entire network has
  acknowledged receipt of this version."  This is enough information for
  any peer to then know that no peer will be sending an edit based on a
  prior ancestor.

  However, this spec does not yet handle the case where a peer goes on
  and offline in a peer-to-peer network.  We are currently working on
  implementing a solution to this and will update this draft when the
  spec has been finished and tested.




5. Network Messages

  We present some examples of the Braid protocol, from the perspective
  of a client communicating with a server.

  Basic session with no versioning:

    Send: {get: "text"}                                    [1]
    Recv: {set: "text", val: "Hello"}                      [2]
    Send: {set: "text", val: "Hello, World!"}              [3]
    Send: {forget: "text"}                                 [4]

    [1]: The client requests the most recent version of "text"
    [2]: The server sends the client the value of "text"
    [3]: The client changes the value to "Hello, World!"
    [4]: The client unsubscribes from edits to "text"

   Basic session with versioning:

    Send: {get: "text"}                                    [1]
    Recv: {set: "text", val: "Hello", version: "v1"}       [2]
    Send: {set: "text", version: "v2",                     [3]
           patches: ['[5:5] = ", World!"'],
           parents: ["v1"]}
    Send: {forget: "text"}                                 [4]

    [1]: The client requests the most recent version of "text"
    [2]: The server sends the client the value of "text"
    [3]: The client adds ", World!" on to the end of "text"
    [4]: The client unsubscribes from edits to "text"



  JSON locations and more complex versioning:

    Send: {get: "user/fred", parents: ['62347']}           [1]
    Recv: {set: "user/fred",                               [2]
           patches: ['.name[0] = "F"'],
           version: '2h38a',
           parents: ['62347']}
    Send: {ack: "user/fred", version: "2h38a"}             [3]
    Send: {set: "user/fred",                               [4]
           patches: [".name[4:4] = \" Wilson\""],
           version: "36x02",
           parents: ["2h38a"]}
    Recv: {ack: "user/fred", version: "36x02"}             [5]
    Send: {forget: "user/fred"}                            [6]

    [1]: The client requests the current version of "/current_user",
         as a patch based on version 62347, which it has in cache.
    [2]: The server responds with a SET containing a patch.
    [3]: The client acknowledges receipt of the new version.
         This enables the server to prune its history of old versions.
    [4]: The client updates the current user's name to "Fred Wilson".
    [5]: The server acknowledges receipt of the new version.
         This enables the client to prune its history.
    [6]: The client is done with current_user, and unsubscribes.



  GET semantics:

    Send: {get: "val"}                                     [1a]
    Recv: {set: "val", val: ..., version: "vX"}            [1b]
    ...
    Send: {get: "val", version: "vX"}                      [2a]
    Recv: {set: "val", version: "vX", val: ...}            [2b]
    ...
    Send: {get: "val", parents: ["vPA", "vPB"]}            [3a]
    Send: {get: "val", parents: ["vPA", "vPB"],            [3b]
           version: "vX", patches: ...}
    ...
    Send: {get: "val", version: "vX",                      [4a]
           parents: ["vPA", "vPB"]}
    Send: {get: "val", version: "vX",                      [4b]
           parents: ["vPA", "vPB"], patches: ...}

    [1a]: The client requests the document "val".
    [1b]: The server sends the client the most recent version, giving
          the explicit value of the document as well as the ID of the
          most recent version.  The server also subscribes the client
          to new updates.

    [2a]: The client requests version "vX" of document "val".
    [1b]: The server sends the client version "vX", giving the explicit
          value of the document at version "vX" as well as the ID "vX".
          The server does NOT subscribe the client to new updates.

    [3a]: The client requests the document "val" as a patch based on
          parent versions "vPA" and "vPB".
    [3b]: The server sends the client the most recent version, giving
          its value as a patch against the implicit merge of the given
          parents "vPA" and "vPB", even if these are not the original
          parents of version "vX".  The server also subscribes the
          client to new updates.

    [4a]: The client requests version "vX" of document "val" as a patch
          based on parent versions "vPA" and "vPB".
    [3b]: The server sends the client version "vX", giving its value as
          a patch against the implicit merge of the given parents "vPA"
          and "vPB", even if these are not the original parents of
          version "vX".  The server does NOT subscribe the client to new
          updates.



6. Security Considerations

  Although this protocol enables and encourages web programmers to make
  more internal available and shared, it has the same fundamental
  security model as HTTP.  State at any URL can have access control
  controlling who can access it.

  However, additional work will be needed to analyze the security
  concerns of specific uses of the protocol.

7. IANA Considerations

  This document has no actions for IANA.

8. Copyright Notice

  Copyright (c) 2019 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  This document is subject to BCP 78 and the IETF Trust's Legal
  Provisions Relating to IETF Documents
  (http://trustee.ietf.org/license-info) in effect on the date of
  publication of this document.  Please review these documents
  carefully, as they describe your rights and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Simplified BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  in the Simplified BSD License.


9. Author's Address

  Michael Toomim
  Invisible College, Berkeley
  2053 Berkeley Way
  Berkeley, CA 94704

  Email: toomim@gmail.com
  Web:   https://invisible.college/@toomim

  Rafie Walker
  Invisible College, Berkeley
  2053 Berkeley Way
  Berkeley, CA 94704

  Email: slickytail.mc@gmail.com
