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This article is about how GRDB uses SQLite's WAL mode. The editor thinks it is very practical, so share it with you as a reference and follow the editor to have a look.

GRDB uses SQLite's WAL mode

WAL, whose full name is Write Ahead Logging, is a mechanism for implementing atomic transactions in SQLite. This mode was introduced from SQLite version 3.7.0. Before that, SQLite used the Rollback Journal mechanism to implement atomic transactions. In the Rollback Journal mechanism, when the data needs to be modified, the modified data is backed up first, and then the data of the database is modified. If the transaction succeeds, the backup data is deleted; if the transaction fails, the backup data is restored. The disadvantage of this mechanism is that it reads and writes frequently and cannot be read when writing.

WAL mode works like its name, writing a log file before writing to the database. The log file is called wal. When the transaction succeeds, the modified content is retained in this file. When the transaction fails, it is deleted from this file. When the records of the wal file accumulate to a certain number, it is written to the database at one time. If you read the database, read it from wal before reading the database file. In this way, the write operation does not affect the read operation, thus improving the efficiency.

GRDB directly supports WAL mode. When developers use DatabasePool to establish a database connection, WAL mode is used by default; if DatabaseQueue is used to establish a connection, this mode is not used. If there is too much accumulated data in WAL, writing to the database will degrade the performance of the database, so avoid WAL accumulating too much data.

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