多个PHP脚本可以同时运行吗？

Question

I am building a turn-based multiplayer game with Flash and PHP. Sometimes two users may call on the same PHP script at the same time. This script is designed to write some information to the database. But that script should not run if that information has already been written by another user, or else the game will break. If PHP processes these scripts sequentially (similar to how MySQL queues up multiple queries), then only one script should run in total and everything should be fine.

However, I find that around 10% of the time, BOTH user's scripts are executed. My theory is that the server sometimes receives both user requests to run the script at exactly the same time and they both run because neither detected that anything has been written to the database yet. Is it possible that both scripts were executed at the same time? If so, what are the possible solutions to this problem.

Answer 1

That's very plausible that they do. Look into database transactions.

Briefly, database transactions are used to control concurrency between programs that access the database at the same time. You start a transaction, then execute multiple queries and finally commit the transaction. If two scripts overlap each other, one of them will fail.

Note that isolation levels can further give fine grained control of how much the two (or more) competing scripts may share. Typically all are allowed to ready from the database, but only one is allowed to write. So the error will happen at the final commit. This is fine as long as all side effects are happening in the database, but not sufficient if you have external side effects (Such as deleting a file or sending an email). In these cases you may want to lock a table or row for the duration of the transaction or set the isolation level.