c++ - How to use SQLite in a multi-threaded application?

Question

I'm developing an application with SQLite as the database, and am having a little trouble understanding how to go about using it in multiple threads (none of the other Stack Overflow questions really helped me, unfortunately).

My use case: The database has one table, let's call it "A", which has different groups of rows (based on one of their columns). I have the "main thread" of the application which reads the contents from table A. In addition, I decide, once in a while, to update a certain group of rows. To do this, I want to spawn a new thread, delete all the rows of the group, and re-insert them (that's the only way to do it in the context of my app). This might happen to different groups at the same time, so I might have 2+ threads trying to update the database.

I'm using different transactions from each thread, I.E. at the start of every thread's update cycle, I have a begin. In fact, what each thread actually does is call "BEGIN", delete from the database all the rows it needs to "update", and inserts them again with the new values (this is the way it must be done in the context of my application).

Now, I'm trying to understand how I go about implementing this. I've tried reading around (other answers on Stack Overflow, the SQLite site) but I haven't found all the answers. Here are some things I'm wondering about:

1. Do I need to call "open" and create a new sqlite structure from each thread?
2. Do I need to add any special code for all of this, or is it enough to spawn different threads, update the rows, and that's fine (since I'm using different transactions)?
3. I saw something talking about the different lock types there are, and the fact that I might receive "SQLite busy" from calling certain APIs, but honestly I didn't see any reference that completely explained when I need to take all this into account. Do I need to?

If anyone can answer the questions/point me in the direction of a good resource, I'd be very grateful.

UPDATE 1: From all that I've read so far, it seems like you can't have two threads who are going to write to a database file anyway.

See: http://www.sqlite.org/lockingv3.html. In section 3.0: A RESERVED lock means that the process is planning on writing to the database file at some point in the future but that it is currently just reading from the file. Only a single RESERVED lock may be active at one time, though multiple SHARED locks can coexist with a single RESERVED lock.

Does this mean that I may as well only spawn off a single thread to update a group of rows each time? I.e., have some kind of poller thread which decides that I need to update some of the rows, and then creates a new thread to do it, but never more than one at a time? Since it looks like any other thread I create will just get SQLITE_BUSY until the first thread finishes, anyway.

Have I understood things correctly?

BTW, thanks for the answers so far, they've helped a lot.

Answer 1

Some steps when starting out with SQLlite for multithreaded use:

1. Make sure sqlite is compiled with the multi threaded flag.
2. You must call open on your sqlite file to create a connection on each thread, don't share connections between threads.
3. SQLite has a very conservative threading model, when you do a write operation, which includes opening transactions that are about to do an INSERT/UPDATE/DELETE, other threads will be blocked until this operation completes.
4. If you don't use a transaction, then transactions are implicit, so if you start a INSERT/DELETE/UPDATE, sqlite will try to acquire an exclusive lock, and complete the operation before releasing it.
5. If you do a BEGIN EXCLUSIVE statement, it will acquire an exclusive lock before doing operations in that transaction. A COMMIT or ROLLBACK will release the lock.
6. Your sqlite3_step, sqlite3_prepare and some other calls may return SQLITE_BUSY or SQLITE_LOCKED. SQLITE_BUSY usually means that sqlite needs to acquire the lock. The biggest difference between the two return values:
   • SQLITE_LOCKED: if you get this from a sqlite3_step statement, you MUST call sqlite3_reset on the statement handle. You should only get this on the first call to sqlite3_step, so once reset is called you can actually "retry" your sqlite3_step call. On other operations, it's the same as SQLITE_BUSY
   • SQLITE_BUSY : There is no need to call sqlite3_reset, just retry your operation after waiting a bit for the lock to be released.