Why Write-Ahead Logging (WAL) is Mandatory
In traditional rollback journal mode, writing to SQLite locks the entire database file, preventing all other connections from reading. This causes immediate bottlenecks in concurrent web apps.
In WAL mode (`PRAGMA journal_mode = WAL;`), modifications are appended to a separate `-wal` file. Readers never block writers, and writers never block readers, enabling concurrency with zero lock contention.
Tuning the RAM Page Cache
By default, SQLite caches only 2MB of database pages in RAM. For modern servers with gigabytes of available memory, this forces continuous unnecessary disk reads.
Setting `PRAGMA cache_size = -64000;` instructs SQLite to allocate 64MB of dedicated RAM for caching B-Tree index pages, resulting in instantaneous query execution.
Step-by-Step Tutorial
Enable WAL Mode Once on Database
Run `PRAGMA journal_mode = WAL;`. This setting is persistent and stored in the database header.
Configure Connection-Level PRAGMAs
Execute `PRAGMA synchronous = NORMAL; PRAGMA cache_size = -64000; PRAGMA temp_store = MEMORY;` whenever a new connection opens.
Always Enable Foreign Keys
Execute `PRAGMA foreign_keys = ON;` because SQLite disables relational cascade constraints by default.
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Frequently Asked Questions
Can SQLite handle concurrent write requests in WAL mode?
In WAL mode, while readers never block writers, writes are still serialized (one writer at a time). However, because writes complete in microseconds with WAL, SQLite can easily handle hundreds of write transactions per second.
What happens to the -wal and -shm files when I close SQLite?
When all database connections cleanly close, SQLite automatically performs a checkpoint, flushing all WAL records into the main `.db` file and truncating the temporary journal files.