does starting a new transaction forces the current transaction to commit?

Question

I have server that uses transactions to write data to database and if all the queries are successful it will commit otherwise it will rollback. Now I want to have two instances of the server work at the same time on the same database and tables. When I was reading mysql's transaction documentation I noticed this sentence: "Beginning a transaction causes any pending transaction to be committed". Does this mean that if server A start transaction A and while transaction A is not completed yet, the server B start transaction B, transaction A is forced to commit? This doesn't make sense to me. If this is the case how I can insure that transaction B is not executed until transaction A is completed normally? Would SET autocommit = 0 be an alternative to this problem?

Answer 1

Assuming when you say you want two instances of the server to work at the same time you mean two separate sessions running on the same server.

The sentence "Beginning a transaction causes any pending transaction to be committed" refers to any pending transaction within the same session only. From http://dev.mysql.com/doc/refman/5.7/en/implicit-commit.html

The statements listed in this section (and any synonyms for them) implicitly end any transaction active in the current session, as if you had done a COMMIT before executing the statement.

So, if Session B starts a transaction before Session A is committed it will not force Session A to commit.

Answer 2

mysql> CREATE TEMPORARY TABLE super(id int);
Query OK, 0 rows affected (0.04 sec)

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO super VALUE(1);
Query OK, 1 row affected (0.00 sec)

mysql> START TRANSACTION;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT * FROM super;
+------+
| id   |
+------+
|    1 |
+------+
1 row in set (0.00 sec)