并发：处理Web应用程序中的多个提交

Question

This is a recent interview question to my friend:

How would you handle a situation where users enter some data in the screen and let's say 5 of them clicked on the Submit button * the SAME time ? *

(By same time,the interviewer insisted that they are same to the level of nanoseconds)

My answer was just to make the method that handles the request synchronized and only one request can acquire the lock on the method at a given time.

But it looks like the interviewer kept insisting there was a "better way" to handle it .

One other approach to handle locking at the database level, but I don't think it is "better".

Are there any other approaches. This seems to be a fairly common problem.

Answer 1

If you have only one network card, you can only have one request coming down it at once. ;)

The answer he is probably looking for is something like

• Make the servlet stateless so they can be executed concurrently.
• Use components which allow thread safe concurrent access like Atomic* or Concurrent*
• Use locks only where you obsolutely have to.

What I prefer to do is to make the service so fast it can respond before the next resquest can come in. ;) Though I don't have the overhead of Java EE or databases to worry about.

Answer 2

Does it matter that they click at the same time eg are they both updating the same record on a database?

A synchronized method will not cut it, especially if it's a webapp distributed amongst multiple JVMs. Also the synchronized method may block, but then the other threads would just fire after the first completes and you'd have lost writes.

So locking at database level seems to be the option here ie if the record has been updated, report an error back to the users whose updates were serviced after the first.

Answer 3

You do not have to worry about this as web server launches each request in isolated thread and manages it.

But if you have some shared resource like some file for logging then you need to achieve concurrency and put thread lock on it in request and inter requests