Braid-HTTP: Synchronization for HTTP
draft-toomim-httpbis-braid-http-04
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Michael Toomim , Greg Little , Rafie Walker , Bryn Bellomy , Seph Gentle | ||
| Last updated | 2023-11-18 | ||
| Replaces | draft-toomim-braid | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
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| Send notices to | (None) |
draft-toomim-httpbis-braid-http-04
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
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Braid-HTTP: Synchronization for HTTP
draft-toomim-httpbis-braid-http-04
Abstract
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
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http://datatracker.ietf.org/drafts/current/.
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
https://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
https://www.ietf.org/shadow.html
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
6.2.1.1. Version-Type: bytestream ...............................25
6.2.1.2. 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
messages.
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
open.
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
architecture:
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
situations.
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"):
Request:
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"}}] | |
Response:
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:
Request:
GET /chat
Version: "ej4lhb9z78"
Response:
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.
Request:
GET /chat
Version: "3"
Parents: "1a", "1b"
Response:
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
version.
- 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:
Request:
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"}] | |
Response:
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:
Request:
GET /chat
Version: "g09ur8z74r"
Parents: "ej4lhb9z78"
Response:
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:
Response:
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:
Request:
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} | |
Response:
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:
Request:
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"}] | |
Response:
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.
Request:
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"} | |
] | |
Response:
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.
Request:
GET /chat
Subscribe:
Response:
HTTP/1.1 209 Subscription
Subscribe:
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.
Example:
Initial request:
GET /chat
Subscribe:
Initial response:
HTTP/1.1 209 Subscription
Subscribe:
Version: "ej4lhb9z78" | Update
Content-Type: application/json |
Content-Length: 64 |
|
[{"text": "Hi, everyone!", | | Snapshot
"author": {"link": "/user/tommy"}}] | |
<Client disconnects>
Reconnection request:
GET /chat
Subscribe:
Parents: "ej4lhb9z78"
Reconnection response:
HTTP/1.1 209 Subscription
Subscribe:
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
request.
Request:
GET /chat
Subscribe:
Response:
HTTP/1.1 209 Subscription
Subscribe:
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
individually.
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:
Request:
GET /temperature
Response:
HTTP/1.1 200 OK
Content-Length: 4
70 F
Braid Subscriptions enable a client can stay up-to-date as the
temperature changes:
Request:
GET /temperature
Subscribe: true
Response:
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:
https://code.jquery.com/jquery-3.7.1.js
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:
Request:
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:
Request:
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].
6.2.1.1. 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
`x82ha`".
This creates a direct correspondence to time and space -- at each
timestep, the size of the document is one byte longer.
6.2.1.2. 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:
Request:
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:
Response:
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:
Request:
HEAD /something
Parents: "abwejf-0"
(A) If the server has received everything, it will report a 200 OK at
the final version:
Response:
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:
Response:
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:
Response:
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:
Request:
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.
Request:
GET /animated-braid.png
Subscribe
Response:
HTTP/1.1 209 Subscribe
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
Subscribe:
-------
[ {ip: '13.55.32.158', pubkey: 'x371...8382'},
{ip: '244.38.55.83', 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
_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 - 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
Server
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
synchronization.
8. IANA Considerations
8.1. Header Field Registration
HTTP header fields are registered within the "Hypertext Transfer
Protocol (HTTP) Field Name" registry maintained at
<https://www.iana.org/assignments/http-fields>.
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 (iesg@ietf.org) - Internet
Engineering Task Force".
9. Security Considerations
XXX Todo
10. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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
(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.
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.
<https://xhr.spec.whatwg.org/>
[SSE] Hickson, I. "Server-Sent Events", W3C Recommendation,
February 2015.
<https://www.w3.org/TR/2015/REC-eventsource-20150203/>
[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, <https://www.rfc-editor.org/info/rfc9051>.
[RFC6578] Daboo, C., Quillaud, A., "Collection Synchronization
for Web Distributed Authoring and Versioning (WebDAV)",
RFC 6578, DOI 10.17487/RFC6578, March 2012,
<https://www.rfc-editor.org/info/rfc6578>.
[RFC4791] Daboo, C., Desruisseaux, B., Dusseault, L., "Calendaring
Extensions to WebDAV (CalDAV)", RFC 4791,
DOI 10.17487/RFC4791, March 2007,
<https://www.rfc-editor.org/info/rfc4791>.
[RFC6350] Perreault, S., "vCard Format Specification", RFC 6350,
DOI 10.17487/RFC6350, August 2011,
<https://www.rfc-editor.org/info/rfc6350>.
[RFC8620] Jenkins, N., Newman, C., "The JSON Meta Application
Protocol (JMAP)", RFC 8620, DOI 10.17487/RFC8620,
July 2019, <https://www.rfc-editor.org/info/rfc8620>.
[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
EMail: toomim@gmail.com
Web: https://invisible.college/@toomim
Greg Little
Invisible College, Berkeley
2053 Berkeley Way
Berkeley, CA 94704
EMail: glittle@gmail.com
Web: https://glittle.org/
Rafie Walker
Bard College
EMail: slickytail.mc@gmail.com
Bryn Bellomy
Invisible College, Berkeley
2053 Berkeley Way
Berkeley, CA 94704
EMail: bryn@signals.io
Web: https://invisible.college/@bryn
Joseph Gentle
Invisible College, Berkeley
2053 Berkeley Way
Berkeley, CA 94704
EMail: me@josephg.com
Web: https://josephg.com/