Internet-Draft                                                 M. Toomim
Expires: Mar 10, 2024                                  Invisible College
Intended status: Proposed Standard                             G. Little
                                                       Invisible College
                                                               R. Walker
                                                       Invisible College
                                                              B. Bellomy
                                                       Invisible College
                                                               J. Gentle
                                                       Invisible College
                                                            Nov 19, 2023

      \=/====\\   |//===\\=       /=\      =\==\|\=/==  =|====\==
      ||/    |\\  ||\    |\\    /|| \|\        |//      //|    \\\
      |\\    |//  |\\    |//   //|   \\\       \\\      /\/    |||
      \=|====|=   |/====/=\   /=\/====|=\      =\=      \\=    =/=
      //\    /\\  //|    |\\  |/|     |||      \\\      |||    |//
      |||    |||  |\\    |//  |\/     \|/      /|\      |=\    |\\
      =\=\==/=/   ==|    |\=  ||=     /==  ===/=|=\===  |==\===//

                  Braid-HTTP: Synchronization for HTTP


   Braid is a set of extensions that generalize HTTP from a state
   *transfer* protocol into a full state *synchronization* protocol.

   Braid is composed of four independent extensions to HTTP:

     1. VERSIONING of resource history
     2. UPDATES sent as patches
     3. SUBSCRIPTIONS to updates over time
     4. MERGE-TYPES that specify OT or CRDT behavior

   Each extension provides a distinct value for a stand-alone use-case.
   However, they can compose together to support the full power of CRDTs
   and Operational Transforms on web resources.  This allows multiple
   writers to make simultaneous mutations to arbitrary content-types,
   under arbitrary network delays and partitions, while guaranteeing
   consistency across multiple clients and servers.  This improves web
   caching and network performance, and enables natively peer-to-peer,
   collaboratively-editable, local-first web applications.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.  The list of current Internet-Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

Table of Contents

   1.  Introduction ..................................................4
   1.1.  HTTP applications need State Synchronization ................4
   1.2.  Braid-HTTP is four extensions to HTTP .......................4
   2.  Versioning for Resources ......................................6
   2.1.  Comparison with ETag ........................................7
   2.2.  PUT a new version ...........................................7
   2.3.  GET a specific version ......................................8
   2.4.  GET a range of historical versions ..........................9
   2.5.  Rules for Version and Parents headers ......................10
   3.  Updates as Patches or Snapshots ..............................11
   3.1.  PUT an update as a patch ...................................12
   3.2.  GET an update as a patch ...................................13
   3.3.  PUT an update as a set of patches ..........................14
   3.4.  Using Patches: 1 for safe Partial PUTs .....................15
   3.5.  PUT an update with a custom patch-type .....................16
   4.  Subscriptions for GET ........................................17
   4.1.  Creating a Subscription ....................................18
   4.2.  Sending multiple updates per GET ...........................19
   4.3.  Continuing a Subscription ..................................20
   4.4.  Signaling "all caught up" ..................................21
   4.5.  Errors .....................................................21
   5.  Design Goals..................................................22
   6.  Example Use Cases ............................................22
   6.1.  Basic Examples .............................................22
   6.1.1.  Subscribing to the current temperature ...................23
   6.1.2.  Versioning of source code ................................24
   6.1.3.  Patches can append to server logs ........................24
   6.2.  Combination examples .......................................25
   6.2.1.  Resumeable uploads .......................................25  Version-Type: bytestream ...............................25  Protocol for resuming uploads ..........................25
   6.2.2.  Dynamic resources: animating a PNG .......................27
   6.2.3.  Dynamic proxies and caches ...............................28
   6.2.4.  Serverless chat example ..................................28
   7.  Related Work .................................................29
   7.1.  Existing IETF Standards ....................................29
   7.3.  IETF Work in Progress ......................................29
   7.3.  Web Frameworks .............................................30
   8.  IANA Considerations ..........................................31
   8.1.  Header Field Registration ..................................31
   9.  Security Considerations ......................................31
   10.  Conventions .................................................31
   11. Copyright Notice .............................................32
   12. References ...................................................32
   12.1. Normative References .......................................32
   12.2. Informative References .....................................33
   13. Acknowledgements .............................................34
   14. Authors' Addresses ...........................................35

1.  Introduction

1.1.  HTTP applications need state Synchronization, not just Transfer

   HTTP [RFC9110] transfers a static version of state within a single
   request and response.  If the state changes, HTTP does not
   automatically update clients with the new versions.  This design
   satisficed when webpages were mostly static and written by hand;
   however today's websites are dynamic, generated from layers of state
   in databases, and provide realtime updates across multiple clients
   and servers.  Programmers today need to *synchronize*, not just
   *transfer* state, and to do this, they must work around HTTP.

   The web has a long history of such workarounds.  The original web
   required users to click reload when a page changed.  Javascript and
   XMLHTTPRequest [XHR] made it possible to update just part of a page,
   running a GET request behind the scenes.  However, a GET request
   still could not push server-initiated updates.  To work around this,
   web programmers would poll the resource with repeated GETs, which was
   inefficient.  Long-polling was invented to reduce redundant requests,
   but still requires the client to initiate a round-trip for each
   update.  Server-Sent Events [SSE] finally created a standard for the
   server to push events, but SSE provides semantics of an event-stream,
   not an update-stream, and SSE programmers must encode the semantics
   of updating a resource within the event stream.  Today there is still
   no standard to push updates to a resource's state.

   In practice, web programmers today often give up on using standards
   for "data that changes", and instead send custom messages over a
   WebSocket -- a hand-rolled synchronization protocol.  Unfortunately,
   this forfeits the benefits of HTTP and ReST, such as caching and a
   uniform interface [REST].  As the web becomes increasingly dynamic,
   web applications are forced to implement additional layers of
   non-standard Javascript frameworks to synchronize changes to state.

1.2.  Braid-HTTP is four extensions to HTTP

   State synchronization implementations come in many forms.  The
   simplest perform one-off transfers of state from one computer to
   another, but as implementations advance, they may develop support for
   pushed updates, delivery guarantees, multiple writers, multiple types
   of edits, expressed as diffs or patches, with automatic conflict
   resolution, offline modes, merge semantics over multiple content
   types, and/or function over different network topologies and
   conditions.  Different systems have different needs, and
   implementations today come in multiple variations, with different
   network protocols.

   Fortunately, it turns out that any implementation's network protocol
   for these features can be decomposed into the same simple set of four
   extensions to HTTP semantics:

     1. Versioning (Section 2)

        Each resource has a history of changes, ordered in time.

     2. Updates as Patches or Snapshots (Section 3)

        Each resource can express updates as either *patches* or
        *snapshots*; in bidirectional client->server and server->client

     3. Subscriptions (Section 4)

        A Subscribe header can be added to GET requests. The server
        responds by pushing updates to the client while the request is

     4. Merge Types [MERGE-TYPES]

        If multiple clients and servers simultaneously mutate the same
        resource, they can guarantee a consistent resulting state by
        implementing the same Merge Type.

   The first three extensions are improvements to existing HTTP
   features, which are valuable in HTTP on their own, aside from from
   state synchronization.  (For examples, see Section 6.)  However, by
   composing these extensions together, HTTP can genrealize into a full
   synchronization protocol, and ReST into a synchronization

         HTTP:         HyperText  *Transfer* Protocol
         becomes:      HyperState *Synchronization* Protocol

         ReST:         Representational State *Transfer*
         becomes:      Representational State *Synchronization*

   Together, they allow an arbitrary set of clients and servers to make
   arbitrary mutations to arbitrary resources, under arbitrary network
   delays and partitions, and merge all mutations consistently,
   receiving updates as soon as they reconnect.  This enables caches to
   support dynamic content, web applications to feature an offline mode,
   and textareas to support collaborative editing.

   The extensions are explained in the following sections, in turn.

2.  Versioning for Resources

   Each Braid resource has a current version, and a version history.
   Versions are specified as a set of one or more strings (called
   "version IDs") in the Structured Headers [RFC8941] format.  Each
   version ID must be unique, to differentiate distinct changes at
   distinct points in time.

   To specify the version of content in a request or response body, a
   Version header MAY be included in a request for a PUT, PATCH or POST,
   or in the response to a GET:

           Version: "dkn7ov2vwg"

   Every version also has a set of parents, denoting the version(s)
   immediately before the version, that it derives from.  Any version
   can be recreated by first merging its parents, and then applying the
   its update onto that merger.  Parents are specified with a Parents
   header in a PUT/PATCH/POST request or GET response:

           Parents: "ajtva12kid", "cmdpvkpll2"

   The full graph of parents forms a Directed Acyclic Graph (DAG),
   representing the partial order of all versions.  A version A is known
   to have occurred before a version B if and only if A is an ancestor
   of B in the partial order.  Braid time is a DAG, rather than a line.

   For any two versions A and B that are specified in a Version or
   Parents header, A cannot be a descendent of B or vice versa.  The
   ordering of versions in the list carries no meaning.

   A Version header is also allowed to contain multiple IDs, to describe
   the version of a merger:

           Version: "dkn7ov2vwg", "v2vwgdkn7o"

   However, any single mutation SHOULD create only a single version ID,
   and mergers themselves need not be announced over the network when
   created.  They only need to be referenced after the fact, in some

   If a client or server does not specify a Version for a resource it
   transfers, the recipient MAY generate and assign it new version IDs.
   If a client or server does not specify a Parents header when
   transferring a new version, the recipient MAY presume that the most
   recent versions it has (the frontier of time) are the parents of the
   new version.  It MAY also ignore or reject the update.

2.1.  Comparison with ETag

   The Version header is similar to an ETag, but has two differences:

   1. ETags are sensitive to Content-Encoding.  If you send the same
      version with a GZip Content-Encoding, it will have a different
      ETag, but the same Version.

   2. A Version marks a unique point in causal graph time -- not unique
      content.  If a resource is changed from version A to B, and then
      to C, such that the contents at A are the same as the contents at
      C, then it is possible for versions A and C to have the same ETag,
      even though they have different Versions.  This can break a CRDT
      or OT merge algorithm.

   Versions can be used in a variety of requests, as we explain next.

2.2.  PUT a new version

   When a PUT request changes the state of a resource, it can specify
   the new version of the resource, the parent version IDs that it was
   directly built upon, and the way multiple simultaneous changes should
   be merged (the "Merge-Type"):


         PUT /chat
         Version: "ej4lhb9z78"                              | Update
         Parents: "oakwn5b8qh", "uc9zwhw7mf"                |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Content-Length: 64                                 |
         [{"text": "Hi, everyone!",                         | | Snapshot
           "author": {"link": "/user/tommy"}}]              | |


         HTTP/1.1 200 OK

   Merge-Types are specified in [MERGE-TYPES].  The Version and Parents
   headers are optional.  If Version is omitted, the recipient may
   assign new version IDs.  If Parents is omitted, the recipient may
   assume that its current version is the version's parents.

   As will be explained in Section 3, we call the set of data that
   updates a resource from one version to another an "update".  An
   update consists of a set of headers and a body.  In this example, the
   update includes a snapshot of the entire new value of the resource.
   However, one can also specify the update as a set of patches.

2.3.  GET a specific version

   A server can allow clients to request historical versions of a
   resource in GET requests by responding to the Version and Parents
   headers.  A client can specify a specific version that it wants with
   the Version header:


         GET /chat
         Version: "ej4lhb9z78"


         HTTP/1.1 200 OK
         Version: "ej4lhb9z78"                              | Update
         Parents: "oakwn5b8qh", "uc9zwhw7mf"                |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Content-Length: 64                                 |
         [{"text": "Hi, everyone!",                         | | Snapshot
           "author": {"link": "/user/tommy"}}]              | |

2.4.  GET a range of historical versions

   A client can request a range of history by including a Parents and a
   Version header together.  The Parents marks the beginning of the
   range (the oldest versions) and the Version marks the end of the
   range (the newest versions) that it requests.


         GET /chat
         Version: "3"
         Parents: "1a", "1b"


         HTTP/1.1 200 OK
         Current-Version: "3"

         Version: "2"                                       | Update
         Parents: "1a", "1b"                                |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Content-Length: 64                                 |
         [{"text": "Hi, everyone!",                         | | Snapshot
           "author": {"link": "/user/tommy"}}]              | |

         Version: "3"                                       | Update
         Parents: "2"                                       |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Content-Length: 117                                |
         [{"text": "Hi, everyone!",                         | | Snapshot
           "author": {"link": "/user/tommy"}}               | |
          {"text": "Yo!",                                   | |
           "author": {"link": "/user/yobot"}]               | |

   To express a range of updates, the response body contains a sequence
   of updates; each with its own content-length.  The format of this
   sequence is defined in the upcoming (Section 4.2) on Subscriptions.

2.5.  Rules for Version and Parents headers

   If a GET request contains a Version header:

     - The Subscribe header (Section 4) MUST be absent.

     - If the Parents header is absent, the server SHOULD return a
       single response, containing the requested version of the resource
       in its body, with the Version response header set to the same

     - If the server does not support historical versions, it MAY ignore
       the Version header and respond as usual, but MUST NOT include the
       Version header in its response.

   If a GET request contains a Parents header:

     - The server SHOULD send the set of versions updating the Parents
       to the specified Version.  If no Version is specified, then it
       should update the client to the server's current version.

       - If the request contains a Subscribe header, then it SHOULD
         additionally leave the request open and subscribe the client to
         future updates.  Otherwise, it should close the connection
         after sending the updates.

     - If the server does not support historical versions, then it MAY
       ignore the Parents header, but MUST NOT include the Parents
       header in its response.

   A server MAY refactor or rebase the version history that it provides
   to a client, so long as it does not affect the resulting state, or
   the result of merges using the history.  (Rules for specifying
   constraints on such rebases are out of scope for this draft.)

   A server does not need to honor historical version requests for all
   documents, for all history. If a server no longer has the historical
   context needed to honor a request, it may respond using an error code
   that will be defined in a subsequent version of this draft.

3.  Updates as Patches or Snapshots

   Whereas today's HTTP sends the current version of a resource as a
   "snapshot" in the body of a GET response or PUT request, and allows
   clients to send a "patch" in a PATCH request, a general state
   synchronization protocol needs updates that travel in both directions
   (both server->client and client->server) and across multiple methods.

   This section describes a general form for bidirectional updates.
   Updates can be sent as snapshots or patches.  When sent as patches, a
   single update can contain a single patch, or multiple patches.
   Unlike the PATCH method, these updates can be sent idempotently by
   including versioning information -- a client or server that receives
   the same update twice, for the same version, can discard the second
   update, and thus maintain idempotence.

   There are two reasons to send an update as a patch:

     - Patches can be smaller and more efficient

     - Patches articulate *how* changes occur, which enables Merge-Types
       to intelligently merge, e.g. in collaborative editing.

   There are two ways to express patches:

     - Custom patch Content-Types: As defined in HTTP PATCH [RFC5789], a
       custom patch format can be specified as a Content-Type.  Any such
       patch can be included in any Braid-HTTP update, by adding its
       Content-Type to its update or patch header.

     - Range Patches: If a Content-Range header is specified on the
       update or patch, then it defines the region of the document that
       is being replaced by the content, as specified in [RANGE-PATCH].

   Every patch MUST include either a Content-Type or a Content-Range.

   Finally, it is possible to include multiple patches within a single
   update by including a "Patches: N" header, and setting the body to a
   concatenation of that many patches.

   These scenarios are elaborated below.

3.1.  PUT an update as a patch

   A single Range Patch (see [RANGE-PATCH]) can be constructed as a
   Partial PUT [RFC9110] -- a PUT with a Content-Range header:


         PUT /chat
         Version: "g09ur8z74r"                              | Update
         Parents: "ej4lhb9z78"                              |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Content-Length: 53                                 | | Patch
         Content-Range: json .messages[1:1]                 | |
                                                            | |
         [{"text": "Yo!",                                   | |
           "author": {"link": "/user/yobot"}]               | |


         HTTP/1.1 200 OK

   Note that Partial PUTs can result in data loss if sent to a server
   that does not support them.  Legacy servers ignore headers they do
   not understand, and will interpret the patch as the entire resource,
   replacing the resource's state with just the region being patched.

   To avoid this, only use Partial PUTs on servers that you know support
   them, or do feature detection (to be defined in a subsequent draft)
   to determine what they support.  You can also reformulate a Partial
   PUT with Patches: 1 as in (Section 3.4) below, as a safe alternative.

3.2.  GET an update as a patch

   A GET response can also express a patch, using Content-Range:


         GET /chat
         Version: "g09ur8z74r"
         Parents: "ej4lhb9z78"


         HTTP/1.1 200 OK
         Content-Type: application/json                     | Update
         Merge-Type: sync9                                  |
         Content-Length: 53                                 | | Patch
         Content-Range: json .messages[1:1]                 | |
                                                            | |
         [{"text": "Yo!",                                   | |
           "author": {"link": "/user/yobot"}]               | |

   Or a custom patch type, using Content-Type:


         HTTP/1.1 200 OK
         Merge-Type: sync9                                  | Update
         Content-Length: 53                                 | | Patch
         Content-Type: application/json-patch+json          | |
                                                            | |
         [                                                  | |
          {"op": "test", "path": "/a/b/c", "value": "foo"}, | |
          {"op": "remove", "path": "/a/b/c"},               | |
          {"op": "add", "path": "/a/b/c", "value": []},     | |
          {"op": "replace", "path": "/a/b/c", "value": 42}, | |
          {"op": "move", "from": "/a/b", "path": "/a/d"},   | |
          {"op": "copy", "from": "/a/d", "path": "/a/d/e"}  | |
         ]                                                  | |

3.3.  PUT an update as a set of patches

   To format an update as a set of patches, include a header called
   "Patches" and assign it to the number of patches included, and format
   those patches in the body as a sequence separated by blank lines:


         PUT /chat
         Version: "g09ur8z74r"                              | Update
         Parents: "ej4lhb9z78"                              |
         Content-Length: 189                                |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Patches: 2                                         |
         Content-Length: 53                                 | | Patch
         Content-Range: json .messages[1:1]                 | |
                                                            | |
         [{"text": "Yo!",                                   | |
           "author": {"link": "/user/yobot"}]               | |
         Content-Length: 40                                 | | Patch
         Content-Range: json .latest_change                 | |
                                                            | |
         {"type": "date", "value": 1573952202370}           | |


         HTTP/1.1 200 OK

   To distinguish the boundaries between patches in an update, each
   patch MUST include the following header:

         Content-Length: N

   This length determines where each patch ends, and next begins.

3.4.  Using Patches: 1 for safe Partial PUTs

   The "Patches" header also provides a safe alternative to Partial
   PUTs.  Instead of specifying the patch in the request body of the
   PUT, the request can:

     - set the "Patches: 1" header

     - move the "Content-Length" and "Content-Range" headers into patch
       headers on the request body

   For example, the Partial PUT example of Section 3.1 would translate
   like this:


         PUT /chat
         Version: "g09ur8z74r"                              | Update
         Parents: "ej4lhb9z78"                              |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Patches: 1                                         |
         Content-Length: 53                                 | | Patch
         Content-Range: json .messages[1:1]                 | |
                                                            | |
         [{"text": "Yo!",                                   | |
           "author": {"link": "/user/yobot"}]               | |


         HTTP/1.1 200 OK

   Because this PUT request has no Content-Length, legacy servers will
   not know where the request body ends, and will not recognize a
   complete PUT.  (Any server that accepts a PUT before it is complete
   also risks data corruption from network failures during a PUT.)
   Servers that understand Patches: 1, on the other hand, know to look
   at the patch inside to find the true Content-Length, and will be able
   to complete the request.

3.5.  PUT an update with a custom patch-type

   Since PATCH is not idempotent, a client may want to send a patch
   idempotently using a PUT.  The client SHOULD include a Version and
   Parents header to ensure idempotency.  The server SHOULD discard
   duplicate patches (for the same Version) to satisfy idempotence.


         PUT /chat
         Version: "up12vyc5ib"                              | Update
         Parents: "2bcbi84nsp"                              |
         Content-Length: 371                                |
         Merge-Type: sync9                                  |
         Patches: 1                                         |
         Content-Length: 288                                | | Patch
         Content-Type: application/json-patch+json          | |
                                                            | |
         [                                                  | |
          {"op": "test", "path": "/a/b/c", "value": "foo"}, | |
          {"op": "remove", "path": "/a/b/c"},               | |
          {"op": "add", "path": "/a/b/c", "value": []},     | |
          {"op": "replace", "path": "/a/b/c", "value": 42}, | |
          {"op": "move", "from": "/a/b", "path": "/a/d"},   | |
          {"op": "copy", "from": "/a/d", "path": "/a/d/e"}  | |
         ]                                                  | |


         HTTP/1.1 200 OK

4.  Subscriptions for GET

   If a GET request includes the Subscribe header, the server can
   "subscribe" the client to the resource, which means it promises to
   keep it up-to-date with the updates necessary to describe new
   versions as it learns about them.

   The server will first send the current version, and then stream the
   updates for future versions.  Each update can express the new content
   either as a snapshot, or a set of patches, as in Section 3.


         GET /chat


         HTTP/1.1 209 Subscription

         Version: "ej4lhb9z78"                              | Update
         Parents: "oakwn5b8qh", "uc9zwhw7mf"                |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Content-Length: 64                                 |
         [{"text": "Hi, everyone!",                         | | Snapshot
           "author": {"link": "/user/tommy"}}]              | |

         Version: "g09ur8z74r"                              | Update
         Parents: "ej4lhb9z78"                              |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Patches: 1                                         |
         Content-Length: 53                                 | | Patch
         Content-Range: json .messages[1:1]                 | |
                                                            | |
         [{"text": "Yo!",                                   | |
           "author": {"link": "/user/yobot"}]               | |

         Version: "2bcbi84nsp"                              | Update
         Parents: "g09ur8z74r"                              |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Patches: 1                                         |
         Content-Length: 58                                 | | Patch
         Content-Range: json .messages[2:2]                 | |
                                                            | |
         [{"text": "Hi, Tommy!",                            | |
           "author": {"link": "/user/sal"}}]                | |

         Version: "up12vyc5ib"                              | Update
         Parents: "2bcbi84nsp"                              |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Patches: 1                                         |
         Content-Length: 288                                | | Patch
         Content-Type: application/json-patch+json          | |
                                                            | |
         [                                                  | |
          {"op": "test", "path": "/a/b/c", "value": "foo"}, | |
          {"op": "remove", "path": "/a/b/c"},               | |
          {"op": "add", "path": "/a/b/c", "value": []},     | |
          {"op": "replace", "path": "/a/b/c", "value": 42}, | |
          {"op": "move", "from": "/a/b", "path": "/a/d"},   | |
          {"op": "copy", "from": "/a/d", "path": "/a/d/e"}  | |
         ]                                                  | |

   Note the blank line after the "Subscribe:" header.  This allows
   subscriptions to be smuggled through existing HTTP clients (such as
   browser fetch() APIs), which will interpret the sequence of updates
   as a long response body of unspecified length.  Application
   programmers can then parse the updates via polyfill libraries on top
   of existing client libraries.  In future versions of this draft, it
   could be advantageous to remove the blank line for upgraded clients,
   which can interpret the initial response headers and body as the
   headers and body for the first update, directly.

   It is RECOMMENDED that updates do not change the Merge-Type for a
   resource, because there is no defined semantics for merging updates
   of different Merge-Types.  If a client observes a change in
   Merge-Type from a server, it is suggested to reload the resource.

4.1.  Creating a Subscription

   A client requests a subscription by issuing a GET request with a
   Subscribe header:

           Subscribe: <Parameters>

   <Parameters> may be blank, set to "true", or contain arbitrary data,
   and is reserved for future use.

   This header modifies the normal GET method's semantics, to request a
   subscription to future updates to the data, rather than only
   returning the current version of the representation data.

   A server implementing Subscribe MUST include a Subscribe header in
   its response.  The server then SHOULD keep the connection open, and
   send updates over it.

   In general, a server that implements subscriptions promises to keep
   its subscribed clients up-to-date by sending changes until the
   connection is closed.  Once closed, a subscription can be resumed by
   the client issuing a subsequent GET request on the same document.

4.2.  Sending multiple updates per GET

   To send multiple updates, a server concatenates multiple updates into
   a single response body.  Each update MUST include headers and a body,
   and MUST specify the end of its body by including at least one of the
   following headers:

      - Content-Length: N
      - Patches: N

   The body may be zero-length.  A server MAY separate each update with
   one or more blank lines.  These lines do not count towards
   Content-Length.  They can by used to visually separate updates, or to
   guide the behavior of certain proxies or clients:

       1. Certain clients or proxies close inactive connections.  A
          server signal that a connection is still active by
          periodically sending additional blank lines between updates.

       2. Some clients (e.g. Firefox) only flush incoming data after a
          receiving a chunk of a certain size.  A server can ensure
          small updates get flushed by padding them with blank lines.

4.3.  Continuing a Subscription

   Once closed, a Braid subscription may be restarted by the client
   issuing a new subscription request.

   When the client reconnects, it may specify its last known version
   using the Parents header.  The server SHOULD then send only the
   updates since that version.


      Initial request:

         GET /chat

      Initial response:

         HTTP/1.1 209 Subscription

         Version: "ej4lhb9z78"                              | Update
         Content-Type: application/json                     |
         Content-Length: 64                                 |
         [{"text": "Hi, everyone!",                         | | Snapshot
           "author": {"link": "/user/tommy"}}]              | |

      <Client disconnects>

      Reconnection request:

         GET /chat
         Parents: "ej4lhb9z78"

      Reconnection response:

         HTTP/1.1 209 Subscription

         Version: "g09ur8z74r"                              | Update
         Parents: "ej4lhb9z78"                              |
         Content-Type: application/json                     |
         Merge-Type: sync9                                  |
         Patches: 1                                         |
         Content-Length: 53                                 | | Patch
         Content-Range: json .messages[1:1]                 | |
                                                            | |
         [{"text": "Yo!",                                   | |
           "author": {"link": "/user/yobot"}]               | |

4.4.  Signaling "all caught up"

   When starting or resuming a subscription, the server can indicate
   which version is current by specifying a "Current-Version" header
   before starting the stream of versions.  This should contain the
   frontier of time -- the leaves of the currently-known time DAG.  The
   client can use this information to determine when it has caught up
   with the server's version at the time it received the client's


         GET /chat


         HTTP/1.1 209 Subscription
         Current-Version: "ej4lhb9z78"               <-- Current Version

         Version: "b9z78ej4lh"                     | Updates
         Content-Type: application/json            |
         Merge-Type: sync9                         |
         Content-Length: 2                         |
         []                                        |
         Version: "ej4lhb9z78"                     | <-- Current Version
         Parents: "b9z78ej4lh"                     |
         Content-Type: application/json            |
         Merge-Type: sync9                         |
         Content-Length: 64                        |
         [{"text": "Hi, everyone!",                |
           "author": {"link": "/user/tommy"}}]     V
                                                     <-- Now caught up

4.5.  Errors

   If a server has dropped the history that a client requests, the
   server can return a 410 GONE response, to tell the client "sorry, I
   don't have the history necessary to synchronize with you."

5.  Design Goals

   This spec is designed to be:

   1. Backwards-compatible with existing HTTP

   2. Easy to implement simple synchronizers with.  For instance, it
      should be easy to write a read-only synchronizer for an
      append-only log.

   3. Possible to implement arbitrary synchronization algorithms.  For
      instance, these extensions support any Operational Transform or
      CRDT algorithm.

6.  Example Use Cases

   The first three Braid extensions (Versions, Patches, and
   Subscriptions) are useful independently -- without Merge-Types, and
   with or without each other.

   The first section 6.1. gives examples of independent uses.  Then
   6.2. puts them together into more advanced combinations.

6.1.  Basic Examples

   First, we give examples of Subscriptions, Versions, and Patches used

6.1.1.  Subscribing to the current temperature

   Subscriptions are useful independently of versioning, patches, or
   merging.  Suppose that a web server hosts the current temperature:


       GET /temperature


       HTTP/1.1 200 OK
       Content-Length: 4

       70 F

   Braid Subscriptions enable a client can stay up-to-date as the
   temperature changes:


       GET /temperature
       Subscribe: true


       HTTP/1.1 209 Subscription
       Subscribe: true

       Content-Length: 4

       70 F

       Content-Length: 4

       72 F

       Content-Length: 4

       73 F

       Content-Length: 4

       71 F

6.1.2.  Versioning of source code

   Source code is often hosted at URIs with embedded versions, such as:

   However, doing so sacrifices having a universal name for the
   resource, as each version gets its own URI.  there is no standard way
   to know which URIs are the same resource.

   Braid versions can instead be specified with the Version: header:


       GET /jquery.js
       Version: "3.7.1"

   This provides a standard way for clients to access versions of a
   resource programmatically.  One advantage of this approach is that
   the same URI can be used by the programmers to edit the code, via
   full-featured version control (over Braid), as is used to import the
   code as library into other code.

6.1.3.  Patches can append to server logs

   Patches are useful for a variety of stand-alone use-cases.  For
   instance, imagine a server in a datacenter that holds system logs.  A
   client might want to append a notification to the log, and could do
   so with a simple Range-Patch like so:


       PUT /logs
       Content-Range: lines -
       Content-Length: 53
       Content-Type: text/plain

       Notification: process 7726 OOM error on host "buster"

   This range-patch uses the "lines" unit (specified in [RANGE-PATCH])
   at the special position "-" which represents the new line at the end
   of the file.

6.2.  Combination examples

6.2.1.  Resumeable uploads

   Braid semantics are expressive enough to implement resumeable
   uploads.  This provides an alternative approach to the work in
   [draft-ietf-httpbis-resumable-upload].  Version-Type: bytestream

   The uploading resource can be considered an append-only bytestream.
   We can declare this with a header:

           Version-Type: bytestream

   "Bytestream" versions look like:

           Version: "x82ha-344"

   Which refers to "the resource after appending 344 bytes by agent

   This creates a direct correspondence to time and space -- at each
   timestep, the size of the document is one byte longer.  Protocol for resuming uploads

   A client starts an upload by simply specifying this special
   Version-Type, along with the version it will reach when it's
   complete, via the Current-Version header:


           PUT /something
           Current-Version: "abwejf-900"
           Version-Type: bytestream
           Content-Length: 900

           <binary data of length 900>

   Now if the upload succeeds, the server will give a 200 OK as usual:


           200 OK

   However, if the upload fails mid-stream, the client will want to
   re-initiate the upload.  It first asks the header how much has been
   received by issueing a HEAD request, and seeing what version of the
   bytestream the the server is at:


           HEAD /something
           Parents: "abwejf-0"

   (A) If the server has received everything, it will report a 200 OK at
       the final version:


           200 OK
           Parents: "abwejf-0"
           Version: "abwejf-900"

   (B) If server has only received a part, it will respond with a
       version partway through the bytestream:


           206 Partial Content
           Parents: "abwejf-0"
           Version: "abwejf-400"

   (C) If the server has received nothing, it will not have any versions
       of the bytestream starting with the version "abwejf-0" initiated
       by the client:


           416 Range Not Satisfiable

   The client now responds accordingly:

     - In case (A), the client is done.
     - In case (B), the client resumes the upload as follows:


           PUT /something
           Current-Version: "abwejf-900"
           Parents: "abwejf-400"
           Content-Range: bytes 400-900/900
           Content-Length: 500

           <binary data from 400-900>

     - In case (C), the client restarts the upload from scratch.

   This simple protocol allows the functionality of resumeable uploads
   to be implemented on top of existing synchronization semantics and
   libraries.  A server that implements Braid-HTTP along with the
   "bytestream" version-type can support resumeable uploads for free.

6.2.2.  Dynamic Resources: Animating a PNG

   Braid allows resources to become inherently dynamic -- able to change
   over time.  You can use this to make a resource animate.

   In this example, a server streams changes to a PNG file in a sequence
   of patches.  When the client renders the new state of the PNG after
   each patch, a new frame of animation is displayed.


       GET /animated-braid.png


       HTTP/1.1 209 Subscribe
       Content-Type: image/png         | Update
       Content-Length: 170763          |
       <binary data>                   | | Snapshot

       Content-Type: image/png         | Update
       Patches: 2                      |
       Content-Length: 1239            | | Patch
       Content-Range: bytes 100-200    | |
                                       | |
       <binary data>                   | |
       Content-Length: 62638           | | Patch
       Content-Range: bytes 348-887    | |
                                       | |
       <binary data>                   | |

6.2.3.  Dynamic proxies and caches

   Since updates aren't pushed, today's web often uses timeouts to
   trigger a cache becoming stale.  Unfortunately, sometimes the timeout
   is wrong, and caches become out-of-date, and we have to wait for an
   unknown cache to timeout before we can see the new version of
   something.  As a result, programmers have learned to force-reload
   pages habitually, and caches become less efficient than necessary.

   A cache supporting the Braid extensions, however, will automatically
   update whenever a change occurs.  If a client starts a GET
   Subscription with a proxy, the proxy will then start and maintain a
   GET Subscription with the origin server for that resource.  The
   origin server will promise to send the proxy updates over its GET
   Subscription, and the proxy will then relay these changes to all
   connected clients.  If a set of clients and servers all support
   Braid, they will never need to force-reload caches for any data
   amongst them.

6.2.4.  Serverless chat example

   A Braid web application can operate offline.  A user can use the app
   from an airplane, and their edits can synchronize when they regain
   internet connections.  Additionally, the Braid protocol can be
   expressed over peer-to-peer transports (e.g. WebRTC) to support a a a
   a peer-to-peer synchronization without a server.  For example, a chat
   application might be served and synchronized on Braid-HTTP, while
   also establishing redundant peer-to-peer connections on WebRTC, and
   translating all Braid-HTTP messages over the WebRTC connections, and
   vice versa.  The server could then be shut down, and users of the
   chat app could continue to send messages to one another.

   Imagine the server serves the current set of trusted clients' IP
   addresses at the /peers state.  Each client then subscribes to the
   /peers state with:

      GET /peers
      [ {ip: '', pubkey: 'x371...8382'},
        {ip: '', pubkey: 'o2u8...2s73'},

   Each peer can then choose a set of those peers with whom to establish
   a WebRTC connection.  It will then exchange Braid messages with those
   peers over that connection.

7.  Related Work

7.1.  Existing IETF Standards

   A number of IETF specifications already standardize aspects of
   synchronization for specific domains.  IMAP [RFC9051] provides
   synchronization of email.  WebDAV provides the synchronization of
   "collections" [RFC6578], and has been extended specifically for
   calendar data in CalDAV [RFC4791], and vCards in [RFC6350].  More
   recently, JMAP [RFC8620] provides an updated method of
   synchronization, supporting mail, calendars, and contacts.

7.2.  IETF Work in Progress

   We wish to integrate this work with the excellent related efforts
   already underway:

   - Per Resource Events [draft-gupta-httpbis-per-resource-events] also
     provides subscriptions for HTTP resources, and also provides a
     mechanism to define types of updates, including patches.

   - Resumable Uploads [draft-ietf-httpbis-resumable-upload] can be
     expressed in Braid-HTTP as a sequence of patches, from client to
     server.  Each patch can specify a version in an "uploading" branch
     of time.  Once the upload is complete, the branch can be "merged"
     back into the main version history.  A subsequent version of this
     draft will provide examples.

   - Byte-Range-Patch [draft-wright-http-patch-byterange] also enables
     general patches using Content-Range, and has useful mechanisms to
     avoid Partial PUTs on legacy servers.

7.3.  Web Frameworks

   Web applications typically synchronize the state of a client and
   server with layers of models, views, and controllers in web
   frameworks.  By automating synchronization within HTTP, programmers
   have to write fewer layers of code on top of it.

      ====== Legacy Websites ======      ====== Braid Websites ======

      Today's webpages are               Braid generalizes HTTP
      generated from multiple            into a standard for
      layers of state.  Each layer       synchronizing state within
      has a different API.               and between websites.

        x Non-standard state API          o Standard state API

       /         \
      :  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     - http:// -
       / | | | | \                         | | | |
      :  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

      Today's programmers have to        Each piece of Braid state (o)
      learn each API, and wire them      has a URI; whether public or
      together, making sure that         internal.  State can be a
      changes to shared state            function of other state, and
      synchronize across all             and automatically recompute
      layers and computers.              when its dependencies change.
                                         Braid guarantees network

8. IANA Considerations

8.1.  Header Field Registration

   HTTP header fields are registered within the "Hypertext Transfer
   Protocol (HTTP) Field Name" registry maintained at

   This document defines the following HTTP header fields, so their
   associated registry entries have been updated according to the
   permanent registrations below (see [BCP90]):

   | Header Field Name   | Status       | Reference   |
   | Version             | experimental | Section 2   |
   | Parents             | experimental | Section 2   |
   | Merge-Type          | experimental | Section 2.2 |
   | Patches             | experimental | Section 3.3 |
   | Subscribe           | experimental | Section 4   |
   | Current-Version     | experimental | Section 4.4 |

   The change controller is: "IETF ( - Internet
   Engineering Task Force".

9. Security Considerations

   XXX Todo

10.  Conventions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

11.  Copyright Notice

   Copyright (c) 2023 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
   ( 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.

12.  References

12.1.  Normative References

   [RFC5789]  "PATCH Method for HTTP", RFC 5789.

   [RFC9110]  "HTTP Semantics", RFC 9110.

   [RFC9111]  "HTTP Caching", RFC 9111.

   [RFC9112]  "HTTP/1.1", RFC 9112.

   [RFC8941]  "Structured Field Values for HTTP"

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [MERGE-TYPES] draft-toomim-httpbis-merge-types-00

   [RANGE-PATCH] draft-toomim-httpbis-range-patch-00

12.2.  Informative References

   [XHR]      Van Kestern, A., Aubourg, J., Song, J., and R. M.
              Steen, H.  "XMLHttpRequest", September 2019.

   [SSE]      Hickson, I.  "Server-Sent Events", W3C Recommendation,
              February 2015.

   [REST]     Fielding, R.  "Architectural Styles and the Design of
              Network-based Software Architectures"  Doctoral
              dissertation, University of California, Irvine, 2000.

   [RFC9051]  Melnikov, Ed., Leiba, Ed., "Internet Message Access
              Protocol - Version 4rev2", RFC 9051, DOI 10.17487/RFC9051,
              August 2021, <>.

   [RFC6578]  Daboo, C., Quillaud, A., "Collection Synchronization
              for Web Distributed Authoring and Versioning (WebDAV)",
              RFC 6578, DOI 10.17487/RFC6578, March 2012,

   [RFC4791]  Daboo, C., Desruisseaux, B., Dusseault, L., "Calendaring
              Extensions to WebDAV (CalDAV)", RFC 4791,
              DOI 10.17487/RFC4791, March 2007,

   [RFC6350]  Perreault, S., "vCard Format Specification", RFC 6350,
              DOI 10.17487/RFC6350, August 2011,

   [RFC8620]  Jenkins, N., Newman, C., "The JSON Meta Application
              Protocol (JMAP)", RFC 8620, DOI 10.17487/RFC8620,
              July 2019, <>.

   [RFC6902]  Bryan, P., Nottingham, M., "Javascript Object Notation
              (JSON) Patch", RFC 6902.

   [RFC9110]  Fielding, R., Nottingham, M., Reschke, J., "HTTP
              Semantics", RFC 9110

   [BCP90]    Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

13.  Acknowledgements

   In addition to the authors, this spec contains intellectual
   contributions from the following people:

      - Mitar Milutinovic
      - Sarah Allen
      - Duane Johnson
      - Travis Kriplean
      - Marius Nita
      - Paul Pham
      - Morgan Dixon
      - Karthik Palaniappan

   We thank the following people for key feedback on previous drafts:

      - Austin Wright
      - Martin Thomson
      - Eric Kinnear
      - Olli Vanhoja
      - Julian Reschke
      - Chris Lemmons
      - Rahul Gupta

   We also thank Mark Nottingham, Fred Baker, Adam Roach, and Barry
   Leiba for facilitating a productive environment within the IETF.

14.  Authors' Addresses

   For more information, the authors of this document are best contacted
   via Internet mail:

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


   Greg Little
   Invisible College, Berkeley
   2053 Berkeley Way
   Berkeley, CA 94704


   Rafie Walker
   Bard College


   Bryn Bellomy
   Invisible College, Berkeley
   2053 Berkeley Way
   Berkeley, CA 94704


   Joseph Gentle
   Invisible College, Berkeley
   2053 Berkeley Way
   Berkeley, CA 94704