The Matrix Part 4 and MySql dilemma - php

Question

I want to do the following:
Basically I have the following design for an events table:
event:
id
code
date
When a new event is created, I want to do the following:
Check if there are any codes already available. A code is available if the date has already passed.
$code1 = select code from event where date_add(date, INTERVAL 7 day) < NOW() AND code NOT IN (select code from event where date_start > NOW()) limit 1
If a code is available, get that code and use that for the new event.
insert into event (code, date) VALUES($code1, NOW())
If a code is not available, then generate a new code.
The problem is I am afraid that when 2 events are created at the same time, they both get the same code. How can I prevent that?
The goal is to assign a code from 1-100 for each event. So because 1-100 is only 100 numbers, I need to recycle codes so that is why I check for old codes to assign to new events. I want to be able to assign codes from 1 to 100 to events by recycling old codes. I don't want to assign the same code to 2 different events.

Answer 1

You ought to lock the table while you work:
LOCK TABLE event WRITE;
SELECT MIN(code) FROM event WHERE date_add(date_end, INTERVAL 7 day) < NOW()
AND code NOT IN (SELECT code FROM event WHERE date_start > NOW());
...
INSERT INTO event ...
UNLOCK TABLES;
You might also keep a table with all active codes (in this case all numbers from 1 to 100). In this case you can do the INSERT with a single statement:
INSERT INTO event ( code, <other fields> )
SELECT MIN(codes.code) AS code, <other values> FROM codes
LEFT JOIN event ON ( codes.code = event code
AND ( event.date_end > DATE_ADD(now(), INTERVAL 7 day)
OR event.date_start >= NOW()) )
WHERE event.code IS NULL;
This selects all codes that are not used in "active" events, and inserts the smallest of them into event (add other fields as needed).
You could also employ a subSELECT ( SELECT DISTINCT code FROM event ) in place of the codes table, but in that case you would only select codes that you already used at least once; any "new" codes would be ignored.
A side effect of the above logic is that larger codes gets reused less often, i.e., if you have twenty active events, chances are that they're using codes from 1 to 20. If you instead want to recycle codes evenly, you can for example SELECT ... ORDER BY RAND() LIMIT 1.

Answer 2

Ok, this is a long shot, and I'm not a database person, so take my suggestion with a grain of salt and do your own research ...
I think what you want is serializable transactions. Basically, you first ask MySQL to make transaction isolation serializable using SET TRANSACTION ISOLATION LEVEL SERIALIZABLE. Then, before your three steps, you begin a transaction by running START TRANSACTION, and after your three steps you run COMMIT.
Some further reading:
http://en.wikipedia.org/wiki/Database_transaction
http://en.wikipedia.org/wiki/Serializability
http://dev.mysql.com/doc/refman/5.0/en/sql-syntax-transactions.html
I am not sure about the overhead the serializability incurs. Hopefully someone more familiar with databases can chip in.
If you can work around your problem, I'd personally rather do that than using transactions.