*Why is an Event Library needed at all?
Let us figure it out through a series of Q&As.
Q: What do you expect a network server to be doing all the time?
A: Watch for inbound connections on the port its listening and accept them.
Q: Calling accept yields a descriptor. What do I do with it?
A: Save the descriptor and do a non-blocking read/write operation on it.
Q: Why does the read/write have to be non-blocking?
A: If the file operation ( even a socket in Unix is a file ) is blocking how could the server for example accept other connection requests when its blocked in a file I/O operation.
Q: I guess I have to do many such non-blocking operations on the socket to see when it's ready. Am I right?
A: Yes. That is what an event library does for you. Now you get it.
Q: How do Event Libraries do what they do?
A: They use the operating system's polling facility along with timers.
Q: So are there any open source event libraries that do what you just described?
libev are two such event libraries that I can recall off the top of my head.
Q: Does Redis use such open source event libraries for handling socket I/O?
A: No. For various reasons Redis uses its own event library.