MySQL Table Locking

http://www.mysqltutorial.org/mysql-table-locking/

Summary: in this tutorial, you will learn how to use MySQL locking for cooperating table accesses between sessions.

A lock is a flag associated with a table. MySQL allows a client session to explicitly acquire a table lock for preventing other sessions from accessing the same table during a specific period. A client session can acquire or release table locks only for itself. It cannot acquire or release table locks for other sessions.

Before going into detail, we create a table named tbl for practicing the table locking statements.

CREATE TABLE tbl (
  id INT NOT NULL AUTO_INCREMENT,
  col INT NOT NULL,
  PRIMARY KEY (id)
) Engine = InnoDB;

LOCK and UNLOCK TABLES syntax

The following statement explicitly acquires a table lock:

LOCK TABLES table_name [READ | WRITE]

To lock a table, you specify its name after the LOCK TABLES keywords. In addition, you specify the type of lock, either  READ or WRITE.

To release a lock for a table, you use the following statement:

UNLOCK TABLES;

Read Locks

A READ lock has the following features:

• A READ lock for a table can be acquired by multiple sessions at the same time. In addition, other sessions can read data from the table without acquiring the lock.
• The session that holds the READ lock can only read data from the table, but cannot write. In addition, other sessions cannot write data to the table until the READ lock is released. The write operations from another session will be put into the waiting states until the READ lock is released.
• If the session is terminated, either normally or abnormally, MySQL will release all the locks implicitly. This feature is also relevant for the WRITE lock.

Let’s take a look at how the READ lock works in the following scenario.

In the first session, first, connect to the sampledb database and use the CONNECTION_ID()function to get the current connection id as follows:

SELECT CONNECTION_ID();

Then, insert a new row into the tbl table.

INSERT INTO tbl(col) VALUES(10);

Next, query the data the tbl table.

SELECT * FROM tbl;

After that, to acquire a lock, you use the LOCK TABLE statement.

LOCK TABLE tbl READ;

Finally, in the same session, if you try to insert a new row into the tbl table, you will get an error message.

INSERT INTO tbl(col) VALUES(11);

Error Code: 1099. Table 'tbl' was locked with a READ lock and can't be updated.

So the once READ lock is acquired, you cannot write data into the table within the same session.

Let’s check the READ lock from a different session.

First, connect to the sampledb and check the connection id:

SELECT CONNECTION_ID();

Then, retrieve data from the tbl  table:

SELECT * FROM tbl;

Next, insert a new row into the tbl table:

INSERT INTO tbl(col) VALUES(20);

The insert operation from the second session is in the waiting state because a READ lock is already acquired on the tbl table by the first session and it has not released yet.

You can see the detailed information from the SHOW PROCESSLIST statement.

SHOW PROCESSLIST;

After that, go back to the first session and release the lock by using the UNLOCK TABLESstatement. After you release the READ lock from the first session, the INSERT operation in the second session executed.

Finally, check it the data of the tbl table to see if the INSERT operation from the second session really executed.

SELECT * FROM tbl;

Write Locks

A WRITE lock has the following features:

• The only session that holds the lock of a table can read and write data from the table.
• Other sessions cannot read data from and write data to the table until the WRITElock is released.

Let’s go into detail to see how the WRITE lock works.

First, acquire a WRITE lock from the first session.

LOCK TABLE tbl WRITE;

Then, insert a new row into the tbl table.

INSERT INTO tbl(col) VALUES(11);

It works.

Next, read data from the tbl table.

SELECT * FROM tbl;

It also works.

After that, from the second session, try to write and read data:

INSERT INTO tbl(col) VALUES(21);
SELECT * FROM tbl;

MySQL puts these operations into a waiting state. You can check it using the SHOW PROCESSLIST statement.

SHOW PROCESSLIST

Finally, release the lock from the first session.

UNLOCK TABLES;

You will see all pending operations from the second session executed and the following picture illustrates the result:

Read vs. Write locks

• Read locks are “shared” locks which prevent a write lock is being acquired but not other read locks.
• Write locks are “exclusive ” locks that prevent any other lock of any kind.

In this tutorial, you have learned how to lock and unlock tables for reading and writing to cooperate the table accesses between sessions.