Race condition

SOURCE: https://www.nodejsdesignpatterns.com/blog/node-js-race-conditions/

A race condition is a type of programming error that can occur when multiple processes or threads are accessing the same shared resource, for instance, a file on a file system or a record in a database, and at least one of them is trying to modify the resource.

Let's try to present an example. Imagine that while a thread is trying to rename a file, another thread is trying to delete the same file. In this case, the second thread will receive an error because, when it's trying to delete the file, the file has already been renamed. Or, the other way around, while one thread is trying to rename the file, the file was already deleted by the other thread and it's not available on the filesystem anymore.

In other cases, race conditions can be more subtle, because they wouldn't result in the program crashing, but they might just be the source of an incorrect or inconsistent behaviour. In these cases, since there is no explicit error and no stack trace, the issue is generally much harder to troubleshoot and fix.

A classic example is when 2 threads are trying to update the same data source and the new information is the result of a function applied to the current value.

Let's pretend we are building a Roman Empire simulation game in which we can manage some cash flow and we have a global balance in aureus (a currency used in the Roman Empire around 100 B.C.E.). Now, let's say that our initial balance is 0 aurei and that there are two independent game components (possibly running on separate threads) that are trying to increase the balance by 50 aurei each, we should expect that in the end, the balance is 100 aurei, right?

If we implement this in a naive way, we might have the two components performing three distinct operations each:

    Read the current value for balance
    Add 50 aurei to it
    Save the resulting value into balance

Since the two components are running in parallel, without any synchronisation mechanism, the following case could happen:
In the picture above you can see that Component 2 ends up having a stale view of the balance: the balance gets changed by Component 1 after Component 2 has read the balance. For this reason, when Component 2 performs its own update, it is effectively overriding any change previously made by Component 1. This is why we have a race condition: the two components are effectively racing to complete their own tasks and they might end up stepping onto each other's toes! 

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Asynchronous JavaScript
Asynchronous JavaScript: Asynchronous code allows the program to be executed immediately where the synchronous code will block further execution of the remaining code until it finishes the current one.