Multi-client TCP server with a thread for each client and DB connection pool

Question

I'm creating a Qt TCP (with SSL) server to which multiple clients should connect and use the DB, and this is the main task. So, I think I can use this architecture: each client has its own thread, and clients use DB via DB connection pool.

Client connection part

main.cpp:

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    MyServer server;

    server.listen(QHostAddress::Any, 1234);

    return a.exec();
}

myserver.h:

class MyServer : public QTcpServer
{
    Q_OBJECT

    explicit MyServer(QObject *parent = 0);
    ~MyServer();

protected:
    void incomingConnection(qintptr socketDescriptor) Q_DECL_OVERRIDE;

signals:
    void stopAll();
};

myserver.cpp:

MyServer::MyServer(QObject *parent) : QTcpServer(parent)
{
    //...
}

MyServer::~MyServer(){
    emit stopAll();
}

void MyServer::incomingConnection(qintptr socketDescriptor){
    QThread* clientThread = new QThread;
    MyClient *client = new MyClient(socketDescriptor, this);

    client->moveToThread(clientThread);

    connect(clientThread, SIGNAL(started()), client, SLOT(process()));

    connect(client, SIGNAL(finished()), clientThread, SLOT(quit()));
    connect(this, SIGNAL(stopAll()), client, SLOT(stopFromServer()));
    connect(client, SIGNAL(finished()), client, SLOT(deleteLater()));
    connect(clientThread, SIGNAL(finished()), clientThread, SLOT(deleteLater()));

    clientThread->start();
}

myclient.h:

class MyClient : public QObject
{
    Q_OBJECT
public:
    explicit MyClient(int socketDescriptor, MyServer *server);
    ~MyClient();

private:
    QSslSocket* socket = NULL;

public slots:
    void process();
}

myclient.cpp:

void MyClient::process(){
    //typical connection things
    connect(this->socket, SIGNAL(encrypted()), this, SLOT(ready()));
}

void MyClient::ready(){
    connect(socket,SIGNAL(readyRead()),this, SLOT(newData()));
}

---

DB connection pool part

DAO which will be executed in the client threads:

.cpp:

//DAO method

QList<QMap<QString, QString>> sqlResult = DatabaseService::executeQuery(sqlCommand);

//sqlResult processing

DatabaseService:

.h:

class DatabaseService
{
public:
    //...
    static QMap<QString, QString> executeQuery(QString command);

private:
    static QThreadStorage<QSqlDatabase> mDatabasePool;
    static QSqlDatabase getDatabase();

};

.cpp:

//...
QThreadStorage<QSqlDatabase> DatabaseService::mDatabasePool;

QSqlDatabase DatabaseService::getDatabase()
{
    if(DatabaseService::mDatabasePool.hasLocalData()) {
        return DatabaseService::mDatabasePool.localData();
    } else {
        auto database = QSqlDatabase::addDatabase("QPSQL", QUuid::createUuid().toString());
        database.setHostName("hostName");
        database.setDatabaseName("databaseName");
        database.setUserName("user");
        database.setPassword("password");
        database.open();
        DatabaseService::mDatabasePool.setLocalData(database);
        return database;
    }
}

QMap<QString,QString> DatabaseService::executeQuery(QString command){

    QSqlQuery query (DatabaseService::getDatabase());
    query.exec(command);

    //...

    return result;
}

//...

This is all the code related to the subject.

You also can check my SO question about DB connection pool (please note that the code is not exactly the same).

So, how good is this architecture? What could be fixed and improved and how?

Answer 1

I am not sure how QThreads work:

• Is it a wrapper around an OS thread?
• Are QThreads pooled?

Assuming a QThread is a wrapper and not pooled.

Then this technique is actually very limiting. The trouble is that threads are relatively expensive to create, in reality you can only run a handful of them simultaneously without cost (creating more than 2x the processors is probably a bad idea), In socket programming most of the threads are idle (network is very slow compared to the speed of the processor).

There is a famous paper on the top c10k. Basically given current network speeds and processor speeds you should easily be able to support 10 thousand simultaneous connections. But using this model you will top out supporting a couple of hundred simultaneous connections.

Basically you want to design your server around a single thread model and use pselect() (though this is now outdated by better options poll(), epoll() and finally LibEvent). Don't bother experimenting with the older versions just jump to LibEvent.