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Enable WAL for SQLite #13897

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Enable WAL for SQLite #13897

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Enable WAL for SQLite
asymmetric Sep 24, 2022
4e7bbd0
Update changelog.d/13897.feature
asymmetric Sep 27, 2022
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1 change: 1 addition & 0 deletions changelog.d/13897.feature
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@@ -0,0 +1 @@
Enable Write-Ahead Logging for SQLite installs. Contributed by [asymmetric](https://github.com/asymmetric).
4 changes: 4 additions & 0 deletions synapse/storage/engines/sqlite.py
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Expand Up @@ -88,6 +88,10 @@ def on_new_connection(self, db_conn: "LoggingDatabaseConnection") -> None:

db_conn.create_function("rank", 1, _rank)
db_conn.execute("PRAGMA foreign_keys = ON;")

# Enable WAL.
# see https://www.sqlite.org/wal.html
db_conn.execute("PRAGMA journal_mode = WAL;")
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is it correct that we do this on every new connection? According to https://www.sqlite.org/wal.html#persistence_of_wal_mode it seems like it is persisted in the database, so shouldn't it just go in a database schema migration file?

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Indeed, it's sufficient to do this once (although doing it every time won't cause issues). Any pointers on how to implement this as a migration?

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Have a look at https://matrix-org.github.io/synapse/develop/development/database_schema.html#delta-files.

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It doesn't look like the pragma will be dumped when we do a full schema dump :

$ sqlite3 test.db
-- Loading resources from /home/dmr/.sqliterc
SQLite version 3.36.0 2021-06-18 18:36:39
Enter ".help" for usage hints.
sqlite> PRAGMA journal_mode;
journal_mode
delete
sqlite> PRAGMA journal_mode = WAL;
journal_mode
wal
sqlite> .schema
sqlite> .dump --data-only --nosys
sqlite> .dump 
PRAGMA foreign_keys=OFF;
BEGIN TRANSACTION;
COMMIT;
sqlite>

So: if we effect this change via a migration, new installations won't benefit from this the next time we take a schema dump.

db_conn.commit()

def is_deadlock(self, error: Exception) -> bool:
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