What is the difference between event driven model and reactor pattern?
From the wikipedia Reactor Pattern article:
The reactor design pattern is an event handling pattern for handling service requests delivered concurrently to a service handler by one or more inputs.
It named a few examples, e.g. nodejs, twisted, eventmachine
But what I understand that above is popular event driven framework, so make them also a reactor pattern framework?
How to differentiate between these two? Or they are the same?
Answer
The reactor pattern is more specific than "event driven programming". It is a specific implementation technique used when doing event driven programming. However, the term is not used with much accuracy in typical conversation, so you should be careful about using it and expecting your audience to understand you, and you should be careful in how you interpret the term when you encounter its use.
One way to look at the reactor pattern is to consider it closely related to the idea of "non-blocking" operations. The reactor sends out notifications when certain operations can be completed without blocking. For example, select(2) can be used to implement the reactor pattern for reading from and writing to sockets using the standard BSD socket APIs (recv(2), send(2), etc). select will tell you when you can receive bytes from a socket instantly - because the bytes are present in the kernel receiver buffer for that socket, for example.
Another pattern you might want to consider while thinking about these ideas is the proactor pattern. In contrast to the reactor pattern, the proactor pattern has operations start regardless of whether they can finish immediately or not, has them performed asynchronously, and then arranges to deliver notification about their completion.
The Windows I/O Completion Ports (IOCP) API is one example where the proactor pattern can be seen. When performing a send on a socket with IOCP, the send operation is started regardless of whether there is any room in the kernel send buffer for that socket. The send operation continues (in another thread, perhaps a thread in the kernel) while the WSASend call completes immediately. When the send actually completes (meaning only that the bytes being sent have been copied into the kernel send buffer for that socket), a callback function supplied to the WSASend call is invoked (in a new thread in the application).
This approach of starting operations and then being notified when they are complete is central to the idea of asynchronous operations. Compare it to non-blocking operations where you wait until an operation can complete immediately before attempting to perform it.
Either approach can be used for event driven programming. Using the reactor pattern, a program waits for the event of (for example) a socket being readable and then reads from it. Using the proactor pattern, the program instead waits for the event of a socket read completing.
Strictly speaking, Twisted misuses the term reactor. The Twisted reactor which is based on select(2) (twisted.internet.selectreactor) is implemented using non-blocking I/O, which is very reactor-like. However, the interface it exposes to application code is asynchronous, making it more proactor-like. Twisted also has a reactor based on IOCP. This reactor exposes the same asynchronous application-facing API and uses the proactor-like IOCP APIs. This hybrid approach, varying from platform to platform in its details, makes neither the term "reactor" nor "proactor" particularly accurate, but since the API exposed by twisted.internet.reactor is basically entirely asynchronous instead of non-blocking, proactor would probably have been a better choice of name.