== Using transactions

=== Transactions with connections

You can execute transaction using SQL `BEGIN`/`COMMIT`/`ROLLBACK`, if you do so you must use
a `link:../../apidocs/io/vertx/sqlclient/SqlConnection.html[SqlConnection]` and manage it yourself.

Or you can use the transaction API of `link:../../apidocs/io/vertx/sqlclient/SqlConnection.html[SqlConnection]`:

[source,java]
----
pool.getConnection()
  // Transaction must use a connection
  .onSuccess(conn -> {
    // Begin the transaction
    conn.begin()
      .compose(tx -> conn
        // Various statements
        .query("INSERT INTO Users (first_name,last_name) VALUES ('Julien','Viet')")
        .execute()
        .compose(res2 -> conn
          .query("INSERT INTO Users (first_name,last_name) VALUES ('Andy','Guibert')")
          .execute())
        // Commit the transaction
        .compose(res3 -> tx.commit()))
      // Return the connection to the pool
      .eventually(() -> conn.close())
      .onSuccess(v -> System.out.println("Transaction succeeded"))
      .onFailure(err -> System.out.println("Transaction failed: " + err.getMessage()));
  });
----

When the database server reports the current transaction is failed (e.g the infamous _current transaction is aborted, commands ignored until
end of transaction block_), the transaction is rollbacked and the `link:../../apidocs/io/vertx/sqlclient/Transaction.html#completion--[completion]` future
is failed with a `link:../../apidocs/io/vertx/sqlclient/TransactionRollbackException.html[TransactionRollbackException]`:

[source,java]
----
tx.completion()
  .onFailure(err -> {
    System.out.println("Transaction failed => rolled back");
  });
----

=== Simplified transaction API

When you use a pool, you can call `link:../../apidocs/io/vertx/sqlclient/Pool.html#withTransaction-java.util.function.Function-[withTransaction]` to pass it a function executed
within a transaction.

It borrows a connection from the pool, begins the transaction and calls the function with a client executing all
operations in the scope of this transaction.

The function must return a future of an arbitrary result:

- when the future succeeds the client will commit the transaction
- when the future fails the client will rollback the transaction

After the transaction completes, the connection is returned to the pool and the overall result is provided.

[source,java]
----
pool.withTransaction(client -> client
  .query("INSERT INTO Users (first_name,last_name) VALUES ('Julien','Viet')")
  .execute()
  .flatMap(res -> client
    .query("INSERT INTO Users (first_name,last_name) VALUES ('Andy','Guibert')")
    .execute()
    // Map to a message result
    .map("Users inserted")))
  .onSuccess(v -> System.out.println("Transaction succeeded"))
  .onFailure(err -> System.out.println("Transaction failed: " + err.getMessage()));
----