Concurrency

• The main loop is single-threaded and all async calls are managed by libuv library.

libuv

• libuv is written in C language.
• libuv sets up a thread pool to handle concurrency.
• libuv handles System Events
• It has a queue and event loop implemented
• This is the reference for event loop implementation
• Resource to learn about libuv: http://nikhilm.github.io/uvbook/
• Reactor pattern resource: https://subscription.packtpub.com/book/programming/9781783287314/1/ch01lvl1sec09/the-reactor-pattern

Synchronous functions

• Node.js provide some functions like fs.readFileSync() which is blocking
• It is not possible to implement synchronous code like the above since it is implemented internally
• Since Node.js philosophy is Non-blocking I/O hence functions like xxxSync() goes against it.
• We should never call xxxSync() functions in an active server code since this could block server.

Why Node.js is Non-Blocking I/O

• I/O is slow:
  • Accessing RAM is in order of nanoseconds (10^-9)
  • Accessing data on disk or network is in order of milliseconds (10^-3)
• Not good for server

Busy Waiting

• Busy waiting simply refers to polling the non-blocking asynchronous resources until some actual data is returned;
• In this mechanism, a call to resource is immediately returned and we poll the resource again and again
• This is very inefficient since for most of the time the resource is no ready, hence Polling algorithms usually result in a huge amount of wasted CPU time.

Event de-multiplexer

• aka Synchronouse Event demultiplexer or Event notification Interface
• Each operating system has its own interface for the Event Demultiplexer:
  • epoll: Linux
  • kqueue: Mac
  • I/O Completion Port API (IOCP): Windows
• Each I/O operation can behave quite differently depending on the type of the resource, even within the same OS.

Similar Concepts

• Event Driven Architecture (EDA)
• Reactor Pattern (has Event demultiplexer??)
• Reactive Programming
• Observer Pattern
• Pub/Sub Pattern