Notes on the development of raspberry pie (17): QT multi-user connection operation on raspberry pie 4b+ MySQL database synchronization (pessimistic lock of single data)

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Preface

   Installed mysq database , Finally, in order to realize the synchronization of multi-user and multi process operations on a raspberry pie , Avoid some errors in data concurrency , This chapter installs remote services and describes how to use Qt Pessimistic lock for update operation , Example of synchronous query from the command line .

Other operating

   Here is just a little mention , For details, please refer to the blogger's raspberry pie series blog , Very detailed .
   Remote login interface

sudo apt-get install tightvncserver
sudo apt-get install xrdp
sudo service xrdp restart
sudo ufw allow 3389
sudo service ufw restart

   And then you can use window Remote desktop login ：
   Default user name ：pi
   Default password ：raspberry
  
  

install qt5

sudo apt-get install qt5-default
sudo apt-get install qtcreator

   After installed , There are more remote desktop programs qtcreator 了 ：
  
  
   Create an interface project , And then run ：
  （ Compile faster than a few years ago 3B+ faster , Test it in the subsequent development process , Can I ignore 3B+ Cross compilation of ）
  
   Check the database driver ：
  
   No, mysql Database driver for .

sudo apt-get install libqt5sql5-mysql

  

Qt operation mariadb database

QSqlDatabase db;
db = QSqlDatabase::addDatabase("QMYSQL");
db.setHostName("127.0.0.1");
db.setPort(3306);
db.setDatabaseName("data");
db.setUserName("root");
db.setPassword("a1234567");

if(db.open())
{
    
    LOG << "Succeed to open db";
}else{
    
    LOG << "Failed to open db:" << db.lastError().text();
    return;
}

QString cmd = "select * from student;";
QSqlQuery query = db.exec(cmd);
while(query.next())
{
    
    LOG << query.value(0).toString()
        << query.value(1).toString()
        << query.value(2).toString()
        << query.value(3).toString();
}

  

Multi user operation

   It is intended for multi-user operation , Then you need to add a read lock when reading , Write lock is required when writing .
   Two users read a record in the database at the same time , At this time, the user A The value of one of the fields was modified and submitted , Later users B This field has also been modified , user B The submission of will overwrite the user A Submitted value .
  

Lock type

Pessimistic locking

   Every time I go to get the data , Very pessimistic , I think it will be modified by others , So it's locked when you get the data . in short , Shared resources are used by only one thread at a time , Other threads are blocking , After the first thread runs out, transfer the resources to other threads .synchronized and ReentranLock And so on are the embodiment of the pessimistic lock thought .

Optimism lock

   Every time I go to get the data , They are optimistic , I don't think it will be modified . So I don't lock it every time , But in When it's updated , We will see if others have updated the data during this period , If there is any update, get it again , Then judge , Cycle all the time , Until you get the data that has not been modified .（mysql You need to realize the optimistic lock yourself ）.

for update Use scenarios （ Pessimistic locking ）

  for update You can add an exclusive lock to a row of data in the database . When the operation of a transaction is not completed , Other transactions can read but cannot write or update .
   If the project has requirements for the accuracy of certain data , And the project has concurrency （ Not necessarily high concurrency ）, You need to use for update.
   such as ： user A Use the balance to buy goods , At this time, the user B To the user A Initiate a transfer , If it happens at the same time , It may cause users to A Final balance error . You need to use for update Lock the data to prevent errors .
   In this case , Even if concurrency is small , But there is also a certain probability that you will encounter , The error of the balance is intolerable even if it is a penny short , So this particular scenario , Even if it is not highly concurrent , You should also use for update Avoid problems .

for update usage

begin;
select * from XXX where XXX for update;
...
commit;

  for update Must be in a transaction to take effect .

Qt test

Use 127.0.0.1 Of ip Connect （ Local connection ）

QSqlDatabase db;
db = QSqlDatabase::addDatabase("QMYSQL");
db.setHostName("127.0.0.1");
db.setPort(3306);
db.setDatabaseName("data");
db.setUserName("root");
db.setPassword("a1234567");

if(db.open())
{
    
    LOG << "Succeed to open db";
}else{
    
    LOG << "Failed to open db:" << db.lastError().text();
    return;
}

if(db.transaction())
{
    
    QString cmd = "select * from student for update;";
    QSqlQuery query = db.exec(cmd);
    while(query.next())
    {
    
        LOG << query.value(0).toString()
            << query.value(1).toString()
            << query.value(2).toString()
            << query.value(3).toString();
    }
    for(int index = 0; index < 10; index++)
    {
    
        QThread::sleep(1);
        LOG << "sleep:" << index;
    }
    if(!db.commit())
    {
    
        LOG << "Failed to commit";
    }
}

  
   thus , Our lock was successfully added , It is convenient for everyone to start to develop the synchronous development of multi-user process operation database .

Using LAN ip Connect （ Remote connection ）

QSqlDatabase db;
db = QSqlDatabase::addDatabase("QMYSQL");
db.setHostName("192.168.0.103");
db.setPort(3306);
db.setDatabaseName("data");
db.setUserName("root");
db.setPassword("a1234567");

if(db.open())
{
    
    LOG << "Succeed to open db";
}else{
    
    LOG << "Failed to open db:" << db.lastError().text();
    return;
}

if(db.transaction())
{
    
    QString cmd = "select * from student for update;";
    QSqlQuery query = db.exec(cmd);
    while(query.next())
    {
    
        LOG << query.value(0).toString()
            << query.value(1).toString()
            << query.value(2).toString()
            << query.value(3).toString();
    }
    for(int index = 0; index < 10; index++)
    {
    
        QThread::sleep(1);
        LOG << "sleep:" << index;
    }
    if(!db.commit())
    {
    
        LOG << "Failed to commit";
    }
}

   Can't connect ：
  
   At this time, you need to modify the configuration ：

sudo vi /etc/mysql/mariadb.conf.d/50-server.cnf

  

   Restart the service ：

systemctl restart mysql.service

  
   Continue testing , Here is how to use the local area network with raspberry pie ip（ You can't connect before 127.0.0.1）：
  
   Successful connection , And then on the LAN pc On the connection , But the transaction cannot be opened ：
  
   Revise it ：
  

   This is actually related to mysql Drive related , Because I got it a few years ago libmysql.dll and libmysqld.dll, I won't delve into this , It can be operated remotely .

Last one ：《 Notes on raspberry pie development （ sixteen ）： Raspberry pie 4B+ install mariadb database (mysql Open source branch ) And test the basic operation 》
Next ： Coming soon …

This article blog address ：https://hpzwl.blog.csdn.net/article/details/125188715