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MySQL Triggers with Logging

with one comment

Somebody asked why you can’t implement MySQL triggers that write information when you want to stop the DML statement, like autonomous procedures in Oracle. The question was a surprise but I didn’t find anything on it, so here’s how you can do it. This is more or less like an autonomous process by leveraging both the InnoDB and MyISAM engine’s behaviors. This post leverages an earlier explanation of MySQL Triggers.

  1. First you create a MyISAM table, which is a persistent store that auto commits when you’re other InnoDB tables can be transactionally dependent. Here’s a simple MyISAM logger table.
CREATE TABLE logger
( logger_id         INT UNSIGNED AUTO_INCREMENT PRIMARY KEY
, logger_event      VARCHAR(50)
, logger_table      VARCHAR(50)
, logger_instring   VARCHAR(100)
, logger_outstring  VARCHAR(100)
, created_by        INT UNSIGNED
, creation_date     DATE
, last_updated_by   INT UNSIGNED
, last_update_date  DATE) ENGINE=MyISAM;
  1. Next, you create an on-insert trigger that changes an input but doesn’t stop the transaction. It also writes to the logger MyISAM table in the scope of the transaction.
CREATE TRIGGER contact_insert
BEFORE INSERT ON contact
FOR EACH ROW
BEGIN
 
  /* Check if last name contains a white space. */
  IF new.last_name REGEXP '^.* .*$' THEN
 
    /* Insert into an MyISAM table, which auto commits in the scope
       of a transaction. */
    INSERT INTO logger
    VALUES ( null
           ,'insert'
           ,'contact'
           , new.last_name
           , REPLACE(new.last_name,' ','-')
           , new.created_by
           , new.creation_date
           , new.last_updated_by
           , new.last_update_date );
 
    /* Replace the name for the INSERT INTO the CONTACT table. */
    SET new.last_name := REPLACE(new.last_name,' ','-');
  END IF;
END;
$$

  1. Next, you create an on-update trigger that changes an update while aborting the transaction. It also writes to the logger MyISAM table because its outside the InnoDB scope of a transaction and auto committed on insert.
CREATE TRIGGER contact_update
BEFORE UPDATE ON contact
FOR EACH ROW
BEGIN
 
  /* Check if last name contains a white space. */
  IF new.last_name REGEXP '^.* .*$' THEN
 
    /* Insert into an MyISAM table, which auto commits in the scope
       of a transaction. */
    INSERT INTO logger
    VALUES ( null
           ,'update'
           ,'contact'
           , new.last_name
           , null           
           , old.created_by
           , old.creation_date
           , new.last_updated_by
           , new.last_update_date );
 
    /* Throw an exception to force the business user to see they
       can't update a last name with a white space. */
    SIGNAL SQLSTATE '42000';
  END IF;
END;
$$

  1. Next, you create a test case with an INSERT and UPDATE statement that meets the condition of the triggers.
/* Insert a row meeting the trigger condition. */
INSERT INTO contact VALUES
( null, 1001, 1003,'Catherine', null,'Zeta Jones', 1001, UTC_DATE(), 1001, UTC_DATE());
 
/* Update a row meeting the trigger condition. */
UPDATE contact
SET    last_name = 'Zeta Jones'
,      last_updated_by = 1003
,      last_update_date = UTC_DATE()
WHERE  last_name = 'Zeta-Jones';

  1. Last, query the logger table. You have a record inserted for both the allowed behavior and the aborted behavior. This means you have the ability to capture material that should never be inserted or updated into a table and who did it by leveraging the who-audit columns of the table.
SELECT * FROM logger;

It returns:

+-----------+--------------+--------------+-----------------+------------------+------------+---------------+-----------------+------------------+
| logger_id | logger_event | logger_table | logger_instring | logger_outstring | created_by | creation_date | last_updated_by | last_update_date |
+-----------+--------------+--------------+-----------------+------------------+------------+---------------+-----------------+------------------+
|         1 | insert       | contact      | Zeta Jones      | Zeta-Jones       |       1001 | 2013-04-26    |            1001 | 2013-04-26       |
|         2 | update       | contact      | Zeta Jones      | NULL             |       1001 | 2013-04-26    |            1003 | 2013-04-26       |
+-----------+--------------+--------------+-----------------+------------------+------------+---------------+-----------------+------------------+
2 rows in set (0.00 sec)

This effectively delivers in MySQL the equivalent of an autonomous transaction in Oracle. The result from the non-critical trigger records the before and after value, while the results from the critical update trigger only record the before values because the event is aborted by raising an error in the trigger. As always, I hope this helps somebody looking for a solution.

Written by maclochlainn

April 25th, 2013 at 11:01 pm

Delay or synchronize it?

with one comment

A couple students in one of my classes ran into a problem when competing Java threads tried to insert new rows in a table. They raised an error when they tried the DELAY keyword to avoid the race (collision) condition in an INSERT statement. It was simple to explain to them that the DELAY keyword doesn’t work with an InnoDB table. Any attempt throws the following error:

ERROR 1616 (HY000): DELAYED OPTION NOT supported FOR TABLE 'message'

Important Update: INSERT DELAYED is gone in MySQL 5.6.6 (announcement) and the whole issue comes down to synchronizing threads (some dislike the solution) or using the ON DUPLICATE KEY clause.

They retested their Java application after redefining the target table using the MyISAM engine. They found it worked but that’s a bad fix in Java (a brief Java/MySQL tutorial post). They really needed to synchronize the Java thread (line #22), leave out the DELAY keyword, and manage the table with the InnoDB engine. Here’s the modified Java code (by the way, they named their project VulcanTech if you’re wondering about the packages in the import statement):

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package vulcantech.vth.server.commands;
 
import java.io.IOException;
import java.net.Socket;
 
import vulcantech.vth.server.combeans.MessageBean;
 
public class MessageHandler implements Handler {
 
  @Override
  public void handleIt(Object... args) {
 
    MessageBean message = (MessageBean) args[0];
    Socket sock = (Socket) args[1];
 
    DatabaseConnection dbconnection = new DatabaseConnection();
 
    String update = new String("INSERT INTO message(message_timestamp, sender, recipient, message, checked) VALUES(\'" 
                  + message.getTimeStamp() + "\', \'" + message.getSender() + "\', \'" 
                  + message.getRecipient() + "\', \'" + message.getMessage() + "\', b\'0\')");
 
    synchronized (this) {
      dbconnection.executeUpdate(update);
    }
 
    try {
      sock.getOutputStream().write(1);
    } catch (IOException e) {
      e.printStackTrace();
    }
    dbconnection.close();
  }
}

Hope this helps those who encounter race conditions against MySQL when you’re writing Enterprise Java Beans (EJBs).

Written by maclochlainn

July 22nd, 2012 at 3:53 pm

Fixing my.cnf on Fedora

with 5 comments

Working with a Fedora 16 VM for my students (next term) and found that the MySQL Server’s my.cnf file worked with a Linux socket as opposed to a listener port, and that several configuration options where missing from the file. Here’s the default /etc/my.cnf file after the package installation from the Red Hat site:

[mysqld]
# Settings user and group are ignored when systemd is used.
# If you need to run mysqld under different user or group,
# customize your systemd unit file for mysqld according to the
# instructions in http://fedoraproject.org/wiki/Systemd
datadir=/var/lib/mysql
socket=/var/lib/mysql/mysql.sock
# Disabling symbolic-links is recommended to prevent assorted security risks
symbolic-links=0
 
[mysqld_safe]
log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid

Without rebuilding the log files, this seemed like the cleanest replacement for the MySQL Server my.cnf for a development instance running on Fedora 16. If you’ve other suggestions, please let me know.

[mysqld]
# Settings user and group are ignored when systemd is used.
# If you need to run mysqld under different user or group,
# customize your systemd unit file for mysqld according to the
# instructions in http://fedoraproject.org/wiki/Systemd
 
# Default directory.
datadir=/var/lib/mysql
 
# The TCP/IP Port the MySQL Server listens on.
# ------------------------------------------------------------
#   Find the machine's IP address with this command run as
#   the root user and use the port number specified in the
#   my.cnf file:
#   [root@localhost ~]# netstat -an | grep 3306
# ------------------------------------------------------------
 
bind-address=nnn.nnn.nnn.nnn
port=3306
 
# The Linux Socket the MySQL Server uses when not using a listener.
# socket=/var/lib/mysql/mysql.sock
 
# Disabling symbolic-links is recommended to prevent assorted security risks
symbolic-links=0
 
# The default storage engine that will be used when creating new tables.
default-storage-engine=INNODB
 
# Set the SQL mode to strict.
sql-mode="STRICT_TRANS_TABLES,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION"
 
# Set the maximum number of connections.
max_connections=100
 
# Set the number of open tables for all threads.
table_cache=256
 
# Set the maximum size for internal (in-memory) temporary tables.
tmp_table_size=26M
 
# Set how many threads should be kept in a cache for reuse.
thread_cache_size=8
 
# MyISAM configuration.
myisam_max_sort_file_size=100G
myisam_sort_buffer_size=52M
key_buffer_size=36M
read_rnd_buffer_size=256K
sort_buffer_size=256K
 
# InnoDB configuration.
innodb_data_home_dir=/var/lib/mysql
innodb_additional_mem_pool_size=2M
innodb_flush_log_at_trx_commit=1
innodb_log_buffer_size=1M
innodb_buffer_pool_size=25M
innodb_log_file_size=5M
innodb_thread_concurrency=8
 
[mysqld_safe]
log-error=/var/log/mysqld.log
pid-file=/var/run/mysqld/mysqld.pid

As always, I hope this helps somebody.

Written by maclochlainn

December 7th, 2011 at 1:15 am

MySQL Sunday

without comments

The merry-go-round is running and Oracle Open World 2010 has begun with MySQL Sunday. The merry-go-round is a fixture at the northwest corner of the Moscone South building, at the intersection shared between the Moscone North building.

The keynote was interesting because Oracle confirmed that they have and will continue to invest in MySQL. The MySQL 5.5 Candidate Release is now available for download. Key features that you’ll note are 200% to 300% improved performance, the InnoDB is now the default engine, backup and recovery are dramatically improved, and the enterprise model is integrated to provide more information about internals and performance.

A little research for the comprehensive new feature list for MySQL 5.5 found a well documented page in the MySQL 5.5 Reference. Examples of things left out of the presentation, not an inclusive list of all features, are: improved operation on the Solaris platform (surprise ;-)), support for semisynchronous replication, support for SQL standard SIGNAL and RESIGNAL statements, support for Unicode character sets, a LOAD XML statement, expanded partitioning options – including the ability to truncate only a partition of a table, and may new command options.

Ronald Bradford gave a good presentation on MySQL idiosyncrasies. I thought spelling out behaviors common to Windows and Mac platforms that differ from Linux and Unix platforms was great. He’s got a new book that he co-authored on MySQL and PHP. I’ve look into the book yet.

I also enjoyed Giuseppe Maxia’s presentation on partitioning. His regular web site is here, but beware if you’ve a slow web connection because the photos delay page loading. ;-)

Update on MySQL Sunday

Here are some links provided by Oracle to presentations:

  • Edward Screven’s presentation
  • Live Webinar: ‚ÄúDelivering Scalability and High Availability with MySQL 5.5 Replication Enhancements

Written by maclochlainn

September 19th, 2010 at 2:30 pm

Posted in InnoDB,LAMP,MAMP,MySQL

Decoding 1005 on MySQL

with 17 comments

If you’re using referential integrity on MySQL, you can easily run into this error with the InnoDB engine. It frequently seems to occur with an ALTER TABLE statement. It can mean many things but typically it means the data types don’t match between the foreign key and primary key column. Likewise, it can mean one of the column data types disagrees in a multiple-column foreign to multiple-column primary key constraint.

The error by itself isn’t very handy. This is a sample:

ERROR 1005 (HY000): Can't create table 'sampledb.#sql-4a0_2' (errno: 150)

You can try SHOW WARNINGS but you’ll discover more about the error by running the following command as the root superuser:

mysql> SHOW engine innodb STATUS;

It returns a dump of the InnoDB’s activity. You can see it by unfolding the complete log, if you’re interested in the details. The significant part of the log to solve this type of problem is:

------------------------
LATEST FOREIGN KEY ERROR
------------------------
100130 17:16:57 Error IN FOREIGN KEY CONSTRAINT OF TABLE sampledb/#sql-4a0_2:
FOREIGN KEY(member_type)
REFERENCES common_lookup(common_lookup_id):
Cannot find an INDEX IN the referenced TABLE WHERE the
referenced COLUMNS appear AS the FIRST COLUMNS, OR COLUMN types
IN the TABLE AND the referenced TABLE do NOT MATCH FOR CONSTRAINT.

While this is more information, it isn’t necessarily enough to solve the problem when you’re new to MySQL. The first place you should look is whether the data types agree between the foreign key and primary key columns.

The most common variation that I’ve run into is where the primary key column uses a int unsigned data type and the foreign key column uses an int data type. It’s quite nice that the InnoDB Engine stops this cold. Naturally, you fix it by changing the foreign key data type to match the int unsigned data type.

Written by maclochlainn

January 30th, 2010 at 6:58 pm

Posted in InnoDB,MySQL,sql