The MySQL server can service multiple clients at the same time because it is multithreaded. To deal with contention among clients, the server performs any necessary locking so that two clients cannot modify the same data at once. However, as the server executes statements, it's very possible that successive queries received from a given client will be interleaved with queries from other clients. If a client issues multiple statements that are dependent on each other, the fact that other clients may be updating tables in between those statements can cause difficulties. Statement failures can be problematic, too, if a multiple-statement operation does not run to completion. Suppose you have a flight table containing information about airline flight schedules and you want to update the record for flight 578 by choosing a pilot from among those available. You might do so using three statements as follows:
SELECT @p_val := pilot_id FROM pilot WHERE available = 'yes' LIMIT 1; UPDATE pilot SET available = 'no' WHERE pilot_id = @p_val; UPDATE flight SET pilot_id = @p_val WHERE flight_id = 578;
The first statement chooses one of the available pilots, the second marks the pilot as unavailable, and the third assigns the pilot to the flight. That's straightforward enough in practice, but in principle there are a couple of significant difficulties with the process:
The MySQL server can handle multiple clients at the same time. If two clients want to schedule pilots, it's possible that both of them would run the initial SELECT query and retrieve the same pilot ID number before either of them has a chance to set the pilot's status to unavailable. If that happens, the same pilot would be scheduled for two flights at once.
All three statements must execute successfully as a unit. For example, if the SELECT and the first UPDATE run successfully, but the second UPDATE fails, the pilot's status is set to unavailable without the pilot being assigned a flight. The database will be left in an inconsistent state.
To prevent concurrency and integrity problems in these types of situations, transactions are helpful. A transaction groups a set of statements and guarantees the following properties:
This chapter discusses how to determine whether or not your MySQL server supports transactions and shows the syntax for the SQL statements that begin and end transactions. It also describes how to implement transactional operations from within programs, using error detection to determine whether to commit or roll back. The final section discusses some workarounds you can use if your MySQL server doesn't support transactions.
Scripts related to the examples shown here are located in the transactions directory of the recipes distribution.
Using the mysql Client Program
Writing MySQL-Based Programs
Record Selection Techniques
Working with Strings
Working with Dates and Times
Sorting Query Results
Modifying Tables with ALTER TABLE
Obtaining and Using Metadata
Importing and Exporting Data
Generating and Using Sequences
Using Multiple Tables
Introduction to MySQL on the Web
Incorporating Query Resultsinto Web Pages
Processing Web Input with MySQL
Using MySQL-Based Web Session Management
Appendix A. Obtaining MySQL Software
Appendix B. JSP and Tomcat Primer
Appendix C. References