c - SQLite3 and multiple processes

Question

How should one ensure correctness when multiple processes access one single SQLite database file?

Answer 1

First, avoid concurrent access to sqlite database files. Concurrency is one of sqlite's weak points and if you have a highly concurrent application, consider using another database engine.

If you cannot avoid concurrency or drop sqlite, wrap your write transactions in BEGIN IMMEDIATE; ... END;. The default transaction mode in sqlite is DEFERRED which means that a lock is acquired only on first actual write attempt. With IMMEDIATE transactions, the lock is acquired immediately, or you get SQLITE_BUSY immediately. When someone holds a lock to the database, other locking attempts will result in SQLITE_BUSY.

Dealing with SQLITE_BUSY is something you have to decide for yourself. For many applications, waiting for a second or two and then retrying works quite all right, giving up after n failed attempts. There are sqlite3 API helpers that make this easy, e.g. sqlite3_busy_handler() and sqlite3_busy_timeout() but it can be done manually as well.

You could also use OS level synchronization to acquire a mutex lock to the database, or use OS level inter-thread/inter-process messaging to signal when one thread is done accessing the database.