CODEWITHSUNDEEP
c++linuxmultithreadingsqlite
baam
baam

Reputation: 1162

SQLite3 Multithreading in C++ (linux)

My applikations runs in several threads. Up to four threads need to access my sqlite3 database. To implement this, I decided to run the database in a seperate thread. In this thread I have a queue (simply a vector of strings). This queue is filled through a pipe from the other four worker threads. After a timelimit or a max. number of elements in my queue I execute it.

Is this an acceptable solution? Or does anyone have a better one?

Upvotes: 0

Views: 2536

Answers (2)

Dariusz
Dariusz

Reputation: 22241

SQLite fully supports multithreading. By implementing your own management system you may improve performance and responsiveness of your application, but it is also probable that... you will not.

All you need to know about sqlite multithreading is in sqlite threadsafe documentation. I suggest you consider information in that document and make a prudent decision.

And if you decide to make your own solution, you can then use the information in the document above to disable unnecesary sqlite internal synchronization.

Side note: there are some useful information in this question/answers: How to use SQLite in a multi-threaded application?

Upvotes: 4

Some programmer dude
Some programmer dude

Reputation: 409166

A common way to handle message queues is to actually have two queues, one where the sender adds its message, and one for the reply. Then instead of using simple strings in the queue, you have a structure which contains the data (the string in your case) and the queue where to add the replies and maybe some kind of identifier to know what message the reply is for.

It's some extra memory overhead as the number of queues increases, but considering this scheme was used on computers in the mid 1980's with only 512 kb RAM (or even less) one can conclude that the overhead is not very big.

As for the implementation, you could create a simple class wrapping most of the functionality, even making it a templated class if you want different queues to be able to receive different data.

Maybe something similar to this:

template<typename Td>
class message_queue
{
public:
    static const message_queue<Td> none{};

    // Post a message to the queue, with an optional reply queue
    void post(const Td& data, message_queue<Td>& reply_queue = none);

    // Get the first message from the queue
    // The second member of the pair is an optional message queue
    // where the reply is posted, it's `none` if no reply needed
    std::pair<Td, message_queue<Td>> get();

    // Wait for a message to arrive in the queue
    void wait() const;

private:
    // The actual message queue
    std::queue<std::pair<Td, message_queue<Td>> queue;
}

The class above can be used something like this:

message_queue<std::string> sql_queue;

// ...

message_queue<std::string> reply_queue;
sql_queue.post("SELECT * FROM some_table", reply_queue);
reply_queue.wait();
std::string data = reply_queue.get().first;

Upvotes: 1

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