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Blocking the event loop

Category: Async · Status: stub · Lessons: chapter 06, Async (planned)

One line: A task that computes for a long time, or makes a blocking call, on the runtime's thread stops every other task on that thread until it is done.

Also called: blocking in async code.

How it connects

flowchart LR
  n_blocking_the_event_loop["Blocking the event loop"]
  n_cooperative_scheduling["Cooperative scheduling"]
  n_starvation["Starvation"]
  n_blocking_the_event_loop -->|can cause| n_starvation
  n_cooperative_scheduling -->|can cause| n_blocking_the_event_loop
  classDef center stroke-width:3px
  class n_blocking_the_event_loop center
  classDef outside stroke-dasharray: 4 3
  class n_cooperative_scheduling,n_starvation outside

In each language
Rust tokio's spawn_blocking ↗ runs blocking work on a thread dedicated to it, because a future that does not yield keeps the executor from driving other futures
Go there is no event loop to block, and since Go 1.14 ↗ goroutines are asynchronously preemptible, so even a loop without function calls cannot hold up the scheduler
Java a virtual thread that blocks releases its platform thread; JEP 491 ↗ (JDK 24) made that happen inside synchronized too
Python blocking (CPU-bound) code should not be called directly; loop.run_in_executor or asyncio.to_thread ↗ moves it off the loop (asyncio guide ↗)
JavaScript Node runs callbacks on the Event Loop and expensive tasks on a Worker Pool; a thread blocked on behalf of one client cannot serve any other (guide ↗)

Where to read more