Java synchronization options for preventing duplicate orders (file, db locking?)

I have two use cases for placing an order on a website. One is directly submitted from a web front end with a creditcard, and the other is a notification of an external payment from a processor like paypal. In both situations, I need to ensure that the order is only placed one time.

I would like to use the same mechanism for both scenarios if possible, to help with code reuse. In the first use case, the user can submit the order form multiple times and result in different theads trying to place an order. I can use ajax to stop this, but I need a server side solution for certainty. In the second usecase, the notification messages may be sent through in duplicates so I need to protect against that too.

I want the solution to be scalable across a distributed environment, so a memory lock is out of the question. I was looking at saving a unique token to the database to prevent multiple submissions there, but I really don't want to be messing with the existing database transactions. The real solution it seems is to lock on something external like a file in a shared location across jvms.

All orders have a unique long id, so I could use that to synchronize. What would be the best way of doing this? I could potentially create a file per id, or do something fancier with a region of the file. However I don't have much experience with file locking, so if there is a better option I would lo开发者_StackOverflow中文版ve to hear it. Any code samples would help very much.

If you already have a unique long id, nothing better than a simple database table with manually assigned primary keys can't happen to you. Every RDBMS (and also key-value NoSQL databases) will effectively and efficiently discover primary keys clashes. It is basically:

1. Start transaction
2. INSERT INTO orders VALUES (your_unique_id)
3. Commit

Depending on the database, 2. or 3. will throw an exception which you can easily catch.

If you really want to avoid databases (could you elaborate a little bit more why?), you can:

1. Use file locking (nasty and not scalable), don't go that way.

2. In-memory locking with clustering (with Terracotta it's like working with normal boolean that is magically clustered)

3. Queuing requests and having only single consumer.

Using JMS and single-threaded consumer looks promising, however you still have to discover duplicates (but at least you avoid concurrently placed orders) and it might be terribly slow...