SQLite database is getting locked

I have a piece of code in C++ that creates many threads & each of these threads acc开发者_高级运维ess a common SQLite database & SELECTs & INSERTs data.

All the INSERTs are with in transactions. Many times I get "Database is locked" error.

How can I fix this?

You can use SQLite in a multithreaded way, but for each thread you must open a new connection (sqlite3_open()) and operate on that connection.

There is more info in the official documentation

If you start your transactions only with "BEGIN" on more than one thread and you are then using INSERTs sqlite can deadlock.

That's because sqlite only aquires a read lock on BEGIN. On the INSERT it has to upgrade that read lock to a write lock (which it can only do if there is no read lock). This does not happen with journal_mode=WAL (only the default journal_mode=delete) as wal mode allows read access during writes. (So using WAL mode might be a fix).

Solution: Use BEGIN IMMEDIATE if you plan to use an insert in a transaction. That way sqlite aquires a write lock on the database.

I was mainly trying to point out that as some assume, you should not have multiple users writing to the same SQLite DB as this is not what it is designed for;

(5) Can multiple applications or multiple instances of the same application access a single database file at the same time?

Multiple processes can have the same database open at the same time. Multiple processes can be doing a SELECT at the same time. But only one process can be making changes to the database at any moment in time, however.

SQLite uses reader/writer locks to control access to the database. (Under Win95/98/ME which lacks support for reader/writer locks, a probabilistic simulation is used instead.) But use caution: this locking mechanism might not work correctly if the database file is kept on an NFS filesystem. This is because fcntl() file locking is broken on many NFS implementations. You should avoid putting SQLite database files on NFS if multiple processes might try to access the file at the same time. On Windows, Microsoft's documentation says that locking may not work under FAT filesystems if you are not running the Share.exe daemon. People who have a lot of experience with Windows tell me that file locking of network files is very buggy and is not dependable. If what they say is true, sharing an SQLite database between two or more Windows machines might cause unexpected problems.

We are aware of no other embedded SQL database engine that supports as much concurrency as SQLite. SQLite allows multiple processes to have the database file open at once, and for multiple processes to read the database at once. When any process wants to write, it must lock the entire database file for the duration of its update. But that normally only takes a few milliseconds. Other processes just wait on the writer to finish then continue about their business. Other embedded SQL database engines typically only allow a single process to connect to the database at once.

However, client/server database engines (such as PostgreSQL, MySQL, or Oracle) usually support a higher level of concurrency and allow multiple processes to be writing to the same database at the same time. This is possible in a client/server database because there is always a single well-controlled server process available to coordinate access. If your application has a need for a lot of concurrency, then you should consider using a client/server database. But experience suggests that most applications need much less concurrency than their designers imagine.

When SQLite tries to access a file that is locked by another process, the default behavior is to return SQLITE_BUSY. You can adjust this behavior from C code using the sqlite3_busy_handler() or sqlite3_busy_timeout() API functions.

Source: http://www.sqlite.org/faq.html