How does node.js/libuv support async io using the reactor pattern

Question

The reactor pattern which is utilized by libuv for handling IO is synchronous by design but libuv supports async io. How is this possible? Does libuv extend the reactor's design somehow to support async io? Does using multiple threads/event loops aid in achieving this?

Answer 1

The I/O model of Node and libuv is very similar to what nginx does internally.

The libuv uses a single-threaded event loop and non-blocking asynchronous I/O. All functions are synchronous in a way that they run to completion but some clever hackery with promises and generators can be used to appear that they don't (when in fact both the invocation of the generator function is non-blocking and returns the generator object immediately and the generator methods like .next() run to completion), plus the new async/await syntax makes it very convenient.

For operations that cannot be accomplished in a non-blocking way Node uses a thread pool to run the blocking operations in separate threads but this is done transparently and it is never exposed to the application code written in JavaScript (you need to step down to C++ to work with that directly).

See: http://docs.libuv.org/en/v1.x/design.html

Unlike network I/O, there are no platform-specific file I/O primitives libuv could rely on, so the current approach is to run blocking file I/O operations in a thread pool. [...]

libuv currently uses a global thread pool on which all loops can queue work on. 3 types of operations are currently run on this pool:

• File system operations
• DNS functions (getaddrinfo and getnameinfo)
• User specified code via uv_queue_work()

See also those answers for more details:

• what is mean by event loop in node.js ? javascript event loop or libuv event loop? *NodeJS event loop internal working
• Prevent NodeJS from exiting event-loop
• How node.js server serve next request, if current request have huge computation?
• Which would be better for concurrent tasks on node.js? Fibers? Web-workers? or Threads?
• Speed up setInterval
• Async.js - Is parallel really parallel?
• Node.js: Asynchronous Callback Execution. Is this Zalgo?

See the links and illustration in those answers. There are a lot of resources to read bout that topic.