Understanding Coroutines, Event Loops, and Asynchronous I/O

Reading ten files sequentially takes ten minutes, which is simple but slow; the article first presents this serial method with a basic for file in files: result = file.read() process(result) loop and lists its advantages (simplicity, maintainability) and the single drawback of speed.

It then explores parallel reading using ten threads, showing a sketch of thread creation and noting that while this speeds up I/O for a small number of files, creating thousands of threads (e.g., for 10,000 files) incurs resource consumption, scheduling overhead, and limited I/O throughput.

To avoid the thread‑scaling problem, the article introduces event‑driven asynchronous I/O, describing how a single‑threaded event loop can issue many non‑blocking reads and process results as they become ready, exemplified by the following code:

event_loop = EventLoop()

def add_to_event_loop(event_loop, file):
  file.asyn_read()  # file asynchronous read
  event_loop.add(file)

while event_loop:
  file = event_loop.wait_one_IO_ready()
  process(file.result)

This pattern is identified as the Reactor model, where events trigger callbacks. The article explains that callbacks simplify the event‑loop logic but can lead to nested “callback hell” when handling complex workflows.

Finally, coroutines are presented as a solution that retains the non‑blocking benefits of the event loop while allowing code to be written in a linear, synchronous style. Example coroutine code:

def start_IO_type_1(io):
  io.start()  # asynchronous request
  yield      # suspend coroutine
  process_IO_type_1(result)

def add_to_event_loop(io, event_loop):
  coroutine = start_IO_type_1(io)
  next(coroutine)
  event_loop.add(coroutine)

while event_loop:
  coroutine = event_loop.wait_one_IO_ready()
  next(coroutine)

By using coroutines, the I/O handling flow is no longer split into separate callback functions, making the code easier to understand while still achieving high‑performance asynchronous execution. The article concludes with a summary of the evolution from blocking serial I/O to coroutine‑based non‑blocking I/O, highlighting its importance in modern high‑performance servers.