Using 'Lock' in web applications

Question

A few months ago I was interviewing for a job inside the company I am currently in, I dont have a strong web development background, but one of the questions he posed to me was how could you improve this block of code.

I dont remember the code block perfectly but to sum it up it was a web hit counter, and he used lock on the hitcounter.

lock(HitCounter)
{
   // Bla...
}

However after some discussion he said, lock is good but never use it in web applications!

What is the basis behind his statement? Why shouldnt I use lock in web applications?

Answer 1

First of all, you never want to lock an object that you actually use in any application. You want to create a lock object and lock that:

private readonly object _hitCounterLock = new object();
lock(_hitCounterLock)
{
    //blah
}

As for the web portion of the question, when you lock you block every thread that attempts to access the object (which for the web could be hundreds or thousands of users). They will all be waiting until each thread ahead of them unlocks.

Answer 2

There is no special reason why locks should not be used in web applications. However, they should be used carefully as they are a mechanism to serialize multi-threaded access which can cause blocking if lock blocks are contended. This is not just a concern for web applications though.

What is always worth remembering is that on modern hardware an uncontended lock takes 20 nanoseconds to flip. With this in mind, the usual practice of trying to make code inside of lock blocks as minimal as possible should be followed. If you have minimal code within a block, the overhead is quite small and potential for contention low.

To say that locks should never be used is a bit of a blanket statement really. It really depends on what your requirements are e.g. a thread-safe in-memory cache to be shared between requests will potentially result in less request blocking than on-demand fetching from a database.

Finally, BCL and ASP.Net Framework types certainly use locks internally, so you're indirectly using them anyway.