Merge pull request #1036 from matrix-org/rav/delete_stateres_draft

Delete state_resolution.rst

drafts/state_resolution.rst

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-State Resolution
-================
-This section describes why we need state resolution and how it works.
-
-
-Motivation
------------
-We want to be able to associate some shared state with rooms, e.g. a room name
-or members list. This is done by having a current state dictionary that maps
-from the pair event type and state key to an event.
-
-However, since the servers involved in the room are distributed we need to be
-able to handle the case when two (or more) servers try and update the state at
-the same time. This is done via the state resolution algorithm.
-
-
-State Tree
-------------
-State events contain a reference to the state it is trying to replace. These
-relations form a tree where the current state is one of the leaf nodes.
-
-Note that state events are events, and so are part of the PDU graph. Thus we
-can be sure that (modulo the internet being particularly broken) we will see
-all state events eventually.
-
-
-Algorithm requirements
-----------------------
-We want the algorithm to have the following properties:
-
-- Since we aren't guaranteed what order we receive state events in, except that
-  we see parents before children, the state resolution algorithm must not depend
-  on the order and must always come to the same result. 
-- If we receive a state event whose parent is the current state, then the
-  algorithm will select it.
-- The algorithm does not depend on internal state, ensuring all servers should
-  come to the same decision.
-
-These three properties mean it is enough to keep track of the current state and
-compare it with any new proposed state, rather than having to keep track of all
-the leafs of the tree and recomputing across the entire state tree.
-
-
-Current Implementation
-----------------------
-The current implementation works as follows: Upon receipt of a newly proposed
-state change we first find the common ancestor. Then we take the maximum
-across each branch of the users' power levels, if one is higher then it is
-selected as the current state. Otherwise, we check if one chain is longer than
-the other, if so we choose that one. If that also fails, then we concatenate
-all the pdu ids and take a SHA1 hash and compare them to select a common
-ancestor.