MatrixSynapse/synapse/storage/schema/__init__.py

# Copyright 2021 The Matrix.org Foundation C.I.C.
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SCHEMA_VERSION = 68  # remember to update the list below when updating
"""Represents the expectations made by the codebase about the database schema

This should be incremented whenever the codebase changes its requirements on the
shape of the database schema (even if those requirements are backwards-compatible with
older versions of Synapse).

See https://matrix-org.github.io/synapse/develop/development/database_schema.html
for more information on how this works.

Changes in SCHEMA_VERSION = 61:
    - The `user_stats_historical` and `room_stats_historical` tables are not written and
      are not read (previously, they were written but not read).
    - MSC2716: Add `insertion_events` and `insertion_event_edges` tables to keep track
      of insertion events in order to navigate historical chunks of messages.
    - MSC2716: Add `chunk_events` table to track how the chunk is labeled and
      determines which insertion event it points to.

Changes in SCHEMA_VERSION = 62:
    - MSC2716: Add `insertion_event_extremities` table that keeps track of which
      insertion events need to be backfilled.

Changes in SCHEMA_VERSION = 63:
    - The `public_room_list_stream` table is not written nor read to
      (previously, it was written and read to, but not for any significant purpose).
      https://github.com/matrix-org/synapse/pull/10565

Changes in SCHEMA_VERSION = 64:
    - MSC2716: Rename related tables and columns from "chunks" to "batches".

Changes in SCHEMA_VERSION = 65:
    - MSC2716: Remove unique event_id constraint from insertion_event_edges
      because an insertion event can have multiple edges.
    - Remove unused tables `user_stats_historical` and `room_stats_historical`.

Changes in SCHEMA_VERSION = 66:
    - Queries on state_key columns are now disambiguated (ie, the codebase can handle
      the `events` table having a `state_key` column).

Changes in SCHEMA_VERSION = 67:
    - state_events.prev_state is no longer written to.

Changes in SCHEMA_VERSION = 68:
    - event_reference_hashes is no longer read.
    - `events` has `state_key` and `rejection_reason` columns, which are populated for
      new events.
"""


SCHEMA_COMPAT_VERSION = (
    # we now have `state_key` columns in both `events` and `state_events`, so
    # now incompatible with synapses wth SCHEMA_VERSION < 66.
    66
)
"""Limit on how far the synapse codebase can be rolled back without breaking db compat

This value is stored in the database, and checked on startup. If the value in the
database is greater than SCHEMA_VERSION, then Synapse will refuse to start.
"""