MySQL replication isn’t perfect and sometimes our data gets out of sync, either by a failure in replication or human intervention. We are all familiar with Percona Toolkit’s pt-table-checksum and pt-table-sync to help us check and fix data inconsistencies – but imagine the following scenario where we mix regular replication with the Tungsten Replicator:

We have regular replication going from master (db1) to 4 slaves (db2, db3, db4 and db5), but also we find that db3 is also master of db4 and db5 using Tungsten replication for 1 database called test. This setup is currently working this way because it was deployed some time ago when multi-source replication was not possible using regular MySQL replication. This is now a working feature in …

I’ve started my long journey from Florianópolis, Brazil, to Santa Clara, California and I type this words while waiting for a connection flight. Next Wednesday, Daniel Guzmán Burgos and I will be presenting in the Percona Live MySQL Conference and Expo (PLMCE).

I’m so excited with the new MySQL 101 program that has been added to this year’s event! Along the years I’ve been working as a Support Engineer at Percona I’ve heard two very distinct types of comments amongst others from some people, customers and community in general, about PLMCE:

1) That they went and it was awesome but they found it hard to follow as most of the …

Talking with Percona Live attendees last year I heard a couple of common themes. First, people told me that there is a lot of great advanced content at Percona Live but there is not much for people just starting to learn the ropes with MySQL. Second, they would like us to find a way to make such basic content less expensive.

I’m pleased to say we’re able to accommodate both of these wishes this year at Percona Live! We have created a two-day intensive track called “MySQL 101” that runs April 15-16. MySQL 101 is designed for developers, system administrators and DBAs familiar with other databases but not with MySQL. And of course it’s ideal for anyone else who would like to expand their professional experience to include MySQL. The sessions are designed to lay a solid foundation on many aspects of MySQL development, design and …

Here on the Percona Support team we often ask customers to retrieve disk stats to monitor disk IO and to measure block devices iops and latency. There are a number of tools available to monitor IO on Linux. iostat is one of the popular tools and Percona Toolkit, which is free, contains the pt-diskstats tool for this purpose. The pt-diskstats tool is similar to iostat but it’s more interactive and contains extended information. pt-diskstats reports current disk activity and shows the statistics for the last second (which by …

So far we've looked at many aspects of MySQL, not in any great depth, but hopefully with enough information to get you started and whet your appetite for more.  Now we start to look into areas that aren't in the basic tutorials.

Replication is the technology that allows data to be stored on multiple servers. Typically this is used in "scale out" applications.  "Scale out" is used in contrast to "Scale up" where to scale a solution you buy a bigger box to run it on, where "scale out" means you buy more boxes.  Each has its benefits and drawbacks, with the usual benefit of scale out being that you get more bang for your buck.

The way replication works in MySQL is pretty simple.  One server is identified as the master, and writes every transaction to a file, the binary log.  Other servers (and there may be many) act as slaves and request information from the master. The slave keeps track of where it got up to …

In our last episode we looked at transactions and how to create them.  In this episode I'll look at some of the implications of transactions, especially in a web application.

Transactions and Replication

We will discuss replication in depth later in the series, however it is sufficient for the moment to say that replication allows you to copy data in near real-time between MySQL servers and keep them synchronised.  What gets transferred are the changes that are made to your tables and data. So what about rolled-back (aborted) transactions?  Since the state after the rollback is essentially the same as the state before the transaction started, there seems little point in replicating those statements.  Indeed they are not replicated. Only completed transactions are.

I briefly mentioned that …

We've now come a long way since our first steps at creating our online bookshop database. Now we need to start to think about how to sell the books and store details about the sales.  This is the time we need to start understanding database transactions.

Database transactions are very similar to real world transactions. They define a set of steps required to happen together in order for a transaction to be complete.  A real-world example might be that you buy a trinket from a store.  You find the trinket, then take it to the counter, find out the price, hand over the cash and receive your trinket.  That is a completed transaction.  Should you not have the available cash, the transaction would not be able to be completed and you would need to return the item, effectively rolling back that …

In our last episode we learned how to modify data and table definitions.  This will come in handy as we look at building in referential integrity constraints into our database.  To begin we will need the database definition resulting from last episode's changes. You can download it here »

A word on Storage Engines

Before we can begin we need to understand a little about MySQL Storage Engines.  MySQL actually does only part of the job of parsing SQL, creating query plans, executing them and returning data sets.  Where the data is stored and retrieved there is a Storage Engine at work.  The original storage engine was MyISAM, based on the industry stalwart of ISAM …

In our last episode we covered sorting, searching and grouping. We found out that using the COUNT(*) can be problematic when we have unexpected NULL data. Now we look at how to resolve data issues by updating the data, and perhaps even the table schema. We'll use the same database we did for the last episode. You can download it here ».

Updating Data

Let's recap.  If we pull the list of books, and authors, we find that "The Broken Shore" from Peter Temple has no price.  Not that it has a zero price, but it has a NULL value.

mysql> SELECT CONCAT_WS(' ',`author`.`first_name`,`author`.`last_name`) AS `author`,
 `book`.`title`, `book`.`price`
 FROM `author` INNER JOIN `book` ON `author`.`id` = `book`.`author`
 ORDER BY `book`.`price`
 LIMIT 1; …

In our last episode we were able to select some information from our bookshop database, this time we look at putting this into some semblence of order.  To fully investigate this topic we need a few more entries in our database, so rather than detail them here, I've put together this SQL file you can download and build your database to follow along.

To install the database, unpack the SQL file from its ZIP archive, and use the SOURCE command to pull the data into your database:

SOURCE mysql101_bookshop_20110912.sql;

You can also pass the file to the mysql command line interpreter from the shell:

mysql -uroot -p bookshop < …