A tool to compress some state in a Synapse instance's database
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README.md

Compress Synapse State Tables

An experimental tool that reads in the rows from state_groups_state and state_group_edges tables for a particular room and calculates the changes that could be made that (hopefully) will significantly reduce the number of rows.

This tool currently does not write to the database in any way, so should be safe to run. If the -o option is specified then SQL will be written to the given file that would change the tables to match the calculated state. (Note that if -t is given then each change to a particular state group is wrapped in a transaction).

The tool will also ensure that the generated state deltas do give the same state as the existing state deltas.

Algorithm

The algorithm works by attempting to create a tree of deltas, produced by appending state groups to different "levels". Each level has a maximum size, where each state group is appended to the lowest level that is not full.

This produces a graph that looks approximately like the following, in the case of having two levels with the bottom level (L1) having a maximum size of 3:

L2 <-------------------- L2 <---------- ...
^--- L1 <--- L1 <--- L1  ^--- L1 <--- L1 <--- L1

The sizes and number of levels used can be controlled via -l.

Note: Increasing the sum of the sizes of levels will increase the time it takes for to query the full state of a given state group. By default Synapse attempts to keep this below 100.

Example usage

$ synapse-compress-state -p "postgresql://localhost/synapse" -r '!some_room:example.com' -o out.sql -t
Fetching state from DB for room '!some_room:example.com'...
Got initial state from database. Checking for any missing state groups...
Number of state groups: 73904
Number of rows in current table: 2240043
Number of rows after compression: 165754 (7.40%)
Compression Statistics:
  Number of forced resets due to lacking prev: 34
  Number of compressed rows caused by the above: 17092
  Number of state groups changed: 2748
New state map matches old one

# It's finished, so we can now go and rewrite the DB
$ psql synapse < out.data