Transactions with MariaDB Connector/C++

MariaDB Connector/C++ supports multi-statement transactions with manual commit, rollback, and savepoints by disabling auto-commit on the sql::Connection object.

Developers can use MariaDB Connector/C++ to perform basic DML (Data Manipulation Language) operations inside a transaction with MariaDB database products.

Auto-Commit Behavior

By default, MariaDB Connector/C++ enables auto-commit. When auto-commit is enabled, each SQL statement is executed in its own transaction.

Confirm the auto-commit is enabled by calling sql::Connection::getAutoCommit():

bool isAutoCommit = conn->getAutoCommit();

Multi-Statement Transactions

MariaDB Connector/C++ supports multi-statement transactions when the auto-commit is disabled.

Disable auto-commit by calling sql::Connection::setAutoCommit():

conn->setAutoCommit(false);

When auto-commit is disabled, a new transaction is automatically started when the current transaction is manually committed or rolled back. It means the application does not need to manually start each new transaction with START TRANSACTION or BEGIN.

The transaction can be manually managed by performing the following operations:

• Setting the transaction isolation by calling sql::Connection::setTransactionIsolation() or using SET TRANSACTION ISOLATION LEVEL.

• Set the transaction to read-only by calling sql::Connection::setReadOnly() or using SET TRANSACTION READ ONLY.

• Create a savepoint by calling sql::Connection::setSavepoint() or using SAVEPOINT.

• Release a savepoint by calling sql::Connection::releaseSavepoint() or using RELEASE SAVEPOINT.

• Roll back to a savepoint by calling sql::Connection::rollback() with the savepoint or using ROLLBACK TO SAVEPOINT.

• Commit the transaction by calling sql::Connection::commit() or using COMMIT.

• Roll back the transaction by calling sql::Connection::rollback() or using ROLLBACK.

Code Example: DML in Transaction

UPDATE, INSERT, and DELETE are DML (Data Manipulation Language) operations that modify data in a table.

The following code demonstrates how to execute UPDATE on the example table within a transaction with auto-commit disabled.

To insert or delete data, replace the UPDATE statement in the code example with an INSERT or DELETE statement:

// Includes
#include <iostream>
#include <mariadb/conncpp.hpp>

void updateContact(std::shared_ptr<sql::PreparedStatement> &stmnt,
                   sql::SQLString first_name,
                   sql::SQLString email)
{
   try {
      // Bind variables to prepared statement parameters
      // Note that the index starts at 1--not 0
      stmnt->setString(1, email);
      stmnt->setString(2, first_name);

      // Execute Statement
      stmnt->executeUpdate();
   }

   // Catch Exception
   catch (sql::SQLException &e) {
      std::cerr << "Error updating contact: "
         << e.what() << std::endl;
   }
}

// Main Process
int main(int argc, char **argv)
{
   try {
      // Instantiate Driver
      sql::Driver* driver = sql::mariadb::get_driver_instance();

      // Configure Connection
      // The URL or TCP connection string format is
      // ``jdbc:mariadb://host:port/database``.
      sql::SQLString url("jdbc:mariadb://192.0.2.1:3306/test");

      // Use a properties map for the other connection options
      sql::Properties properties({
            {"user", "db_user"},
            {"password", "db_user_password"},
         });

      // Establish Connection
      // Use a smart pointer for extra safety
      std::unique_ptr<sql::Connection> conn(driver->connect(url, properties));

      // Use Connection to update contacts with a transaction
      try {
         // Disabling ``auto-commit`` mode automatically starts a new user managed transaction.
         conn->setAutoCommit(false);

         // Create a PreparedStatement
         // Use a smart pointer for extra safety
         std::shared_ptr<sql::PreparedStatement> stmnt(conn->prepareStatement(
                  "UPDATE test.contacts SET email=? WHERE first_name = ?"
               )
            );

         std::string contacts[3][2] = {
               { "John", "[e-mail]" },
               { "Jon", "[e-mail]" },
               { "Johnny", "[e-mail]" }
            };

         for (int row { 0 }; row < 3; ++row) {
            updateContact(stmnt, contacts[row][0], contacts[row][1]);
         }

         // Commit the transaction
         conn->commit();
      }
      catch (sql::SQLException &e) {
         std::cerr << "Error updating contact with a transaction: "
            << e.what() << std::endl;

         // Rollback the transaction
         conn->rollback();
      }

      // Close Connection
      conn->close();
   }
   catch (sql::SQLException &e) {
      std::cerr << "SQL exception in the database: "
         << e.what() << std::endl;

      // Exit (Failed)
      return 1;
   }

   // Exit (Success)
   return 0;
}

The query below confirms the UPDATE of the example table:

SELECT * FROM test.contacts;

+----+------------+-----------+-------------------------+
| id | first_name | last_name | email                   |
+----+------------+-----------+-------------------------+
|  1 | John       | Smith     | [e-mail]   |
+----+------------+-----------+-------------------------+
|  2 | Jon        | Smith     | [e-mail]    |
+----+------------+-----------+-------------------------+
|  3 | Johnny     | Smith     | [e-mail] |
+----+------------+-----------+-------------------------+

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