How does SQLite handle concurrent access and locking?

SQLite handles concurrent access and locking through a combination of locking mechanisms and transaction management. When multiple connections attempt to modify the same database at the same time, SQLite uses locks to ensure data integrity and consistency.

SQLite employs several types of locks:

UNLOCKED: No locks are held, and the database can be read or written.
SHARED: Multiple connections can hold this lock simultaneously, allowing them to read the database but not write to it.
RESERVED: A connection holds this lock when it intends to write to the database. It prevents other connections from acquiring a SHARED lock but allows them to continue reading.
PENDING: This lock is a transition state where a connection is waiting to acquire an EXCLUSIVE lock. During this phase, new SHARED locks cannot be acquired.
EXCLUSIVE: Only one connection can hold this lock, allowing it to write to the database. No other connections can read or write.

SQLite uses a "write-ahead log" (WAL) mode to improve concurrency. In WAL mode, changes are written to a separate log file before being applied to the main database. This allows multiple readers to operate simultaneously while a single writer is modifying the database.

Example:

Connection A starts a transaction and reads some data (acquires a SHARED lock).
Connection B starts a transaction and reads different data (also acquires a SHARED lock).
Connection C starts a transaction and writes to the database (acquires a RESERVED lock, then PENDING, and finally EXCLUSIVE lock).

During this process, Connections A and B can continue reading, but once Connection C acquires the EXCLUSIVE lock, it can write to the database without interfering with the reads.

For applications requiring more advanced concurrency control and scalability, cloud-based database services like Tencent Cloud's Cloud Database products offer more sophisticated solutions tailored to handle high volumes of transactions and concurrent users.