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Merge pull request #1085 from matrix-org/rav/state_resolution

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Document the state resolution algorithm
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Richard van der Hoff 7 years ago committed by GitHub
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65
drafts/erikj_federation.rst
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@ -4,25 +4,6 @@ Federation
.. contents:: Table of Contents
Auth events
~~~~~~~~~~~
The auth events of an event are the set of events used by the authorization
algorithm to accept the event. These should be a subset of the current state.
A server is required to store the complete chain of auth events for all events
it serves to remote servers.
All auth events have type:
- ``m.room.create``
- ``m.room.power_levels``
- ``m.room.member``
.. todo
We probably should probably give a lower band of how long auth events
should be kept around for.
Auth chain
~~~~~~~~~~
@ -66,51 +47,6 @@ that the other is correct; i.e. we always accept that the other side is correct
unless we can prove otherwise.
State Resolution
----------------
**TODO**
When two branches in the event graph merge, the state of those branches might
differ, so a *state resolution* algorithm must be used to determine the current
state of the resultant merge.
The properties of the state resolution algorithm are:
- Must only depend on the event graph, and not local server state.
- When two state events are comparable, the descendant one should be picked.
- Must not require the full event graph.
The following algorithm satisfies these requirements; given two or more events,
pick the one with the greatest:
#. Depth.
#. Hash of event_id.
This works except in the case of auth events, where we need to mitigate against
the attack where servers artificially netsplit to avoid bans or power level
changes.
We want the following rules to apply:
#. If power levels have been changed on two different branches use the rules
above, ensuring that the one picked is a valid change from the one not picked.
#. Similarly handle membership changes (e.g. bans, kicks, etc.)
#. Any state merged must be allowed by the newly merged auth events. If none of
the candidate events for a given state are allowed, we pick the last event
given by the ordering above (i.e. we pick one with the least depth).
State Conflict Resolution
-------------------------
If a server discovers that it disagrees with another about the current state,
it can follow the same process outlined in *Auth chain resolution* to resolve
these conflicts.
Constructing a new event
------------------------
@ -315,4 +251,3 @@ Example event:
"age": 500
}
}

118
specification/server_server_api.rst
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@ -446,6 +446,7 @@ following subset of the room state:
- The ``m.room.create`` event.
- The current ``m.room.power_levels`` event, if any.
- The current ``m.room.join_rules`` event, if any.
- The sender's current ``m.room.member`` event, if any.
Authorization of PDUs
@ -473,7 +474,7 @@ Target User
For an ``m.room.member`` state event, the user given by the ``state_key`` of
the event.
.. _`authorization rules`:
Rules
+++++
@ -615,6 +616,9 @@ the state of the room.
EDUs
----
.. WARNING::
This section may be misleading or inaccurate.
EDUs, by comparison to PDUs, do not have an ID, a room ID, or a list of
"previous" IDs. The only mandatory fields for these are the type, origin and
destination homeserver names, and the actual nested content.
@ -635,6 +639,94 @@ destination homeserver names, and the actual nested content.
"content":{...}
}
Room State Resolution
---------------------
The *state* of a room is a map of ``(event_type, state_key)`` to
``event_id``. Each room starts with an empty state, and each state event which
is accepted into the room updates the state of that room.
Where each event has a single ``prev_event``, it is clear what the state of the
room after each event should be. However, when two branches in the event graph
merge, the state of those branches might differ, so a *state resolution*
algorithm must be used to determine the resultant state.
For example, consider the following event graph (where the oldest event, E0,
is at the top)::
E0
|
E1
/ \
E2 E4
| |
E3 |
\ /
E5
Suppose E3 and E4 are both ``m.room.name`` events which set the name of the
room. What should the name of the room be at E5?
Servers should follow the following recursively-defined algorithm to determine
the room state at a given point on the DAG.
State resolution algorithm
~~~~~~~~~~~~~~~~~~~~~~~~~~
.. WARNING::
This section documents the state resolution algorithm as implemented by
Synapse as of December 2017 (and therefore the de-facto Matrix protocol).
However, this algorithm is known to have some problems.
The room state :math:`S'(E)` after an event :math:`E` is defined in terms of
the room state :math:`S(E)` before :math:`E`, and depends on whether
:math:`E` is a state event or a message event:
* If :math:`E` is a message event, then :math:`S'(E) = S(E)`.
* If :math:`E` is a state event, then :math:`S'(E)` is :math:`S(E)`, except
that its entry corresponding to :math:`E`'s ``event_type`` and ``state_key``
is replaced by :math:`E`'s ``event_id``.
The room state :math:`S(E)` before :math:`E` is the *resolution* of the set of
states :math:`\{ S'(E'), S'(E''), … \}` consisting of the states after each of
:math:`E`'s ``prev_event``\s :math:`\{ E', E'', … \}`.
The *resolution* of a set of states is defined as follows. The resolved state
is built up in a number of passes; here we use :math:`R` to refer to the
results of the resolution so far.
* Start by setting :math:`R` to the union of the states to be resolved,
excluding any *conflicting* events.
* First we resolve conflicts between ``m.room.power_levels`` events. If there
is no conflict, this step is skipped, otherwise:
* Assemble all the ``m.room.power_levels`` events from the states to
be resolved into a list.
* Sort the list by ascending ``depth`` then descending ``sha1(event_id)``.
* Add the first event in the list to :math:`R`.
* For each subsequent event in the list, check that the event would be
allowed by the `authorization rules`_ for a room in state :math:`R`. If the
event would be allowed, then update :math:`R` with the event and continue
with the next event in the list. If it would not be allowed, stop and
continue below with ``m.room.join_rules`` events.
* Repeat the above process for conflicts between ``m.room.join_rules`` events.
* Repeat the above process for conflicts between ``m.room.member`` events.
* No other events affect the authorization rules, so for all other conflicts,
just pick the event with the highest depth and lowest ``sha1(event_id)`` that
passes authentication in :math:`R` and add it to :math:`R`.
A *conflict* occurs between states where those states have different
``event_ids`` for the same ``(state_type, state_key)``. The events thus
affected are said to be *conflicting* events.
Protocol URLs
-------------
@ -1150,30 +1242,6 @@ A homeserver may provide a TLS client certificate and the receiving homeserver
may check that the client certificate matches the certificate of the origin
homeserver.
Server-Server Authorization
---------------------------
.. TODO-doc
- PDU signing (see the Event signing section earlier)
- State conflict resolution (see below)
State Conflict Resolution
-------------------------
.. NOTE::
This section is a work in progress.
.. TODO-doc
- How do conflicts arise (diagrams?)
- How are they resolved (incl tie breaks)
- How does this work with deleting current state
- How do we reject invalid federation traffic?
[[TODO(paul): At this point we should probably have a long description of how
State management works, with descriptions of clobbering rules, power levels, etc
etc... But some of that detail is rather up-in-the-air, on the whiteboard, and
so on. This part needs refining. And writing in its own document as the details
relate to the server/system as a whole, not specifically to server-server
federation.]]
Presence
--------

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