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