mirror of
https://mau.dev/maunium/synapse.git
synced 2024-11-17 15:31:19 +01:00
51 lines
2.1 KiB
ReStructuredText
51 lines
2.1 KiB
ReStructuredText
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.
|