Concurrency

Concurrency is provided in Nim, using async/await syntax. This feature is powered by the asyncdispatch module.

Note that, concurrency is not parallelism. All the examples here are async functions that run on a single thread. This is mainly useful for IO intensive tasks.

A general example looks like this:

import asyncdispatch

proc ioManager(id: string) {.async.} =
  for i in 1..10:
    # wait for some some async process
    await sleepAsync(10)
    echo id & " - run: " & $i

let
  ma = ioManager("a")
  mb = ioManager("b")

waitFor ma and mb

You should see an output like:

a - run: 1
b - run: 1
a - run: 2
b - run: 2
a - run: 3
b - run: 3

showing interleaved function completions.

Async functions are tagged with the {.async.} pragma. These functions can now use the await keyword to wait for async procedures.

In the example above, we have used waitFor on 2 async functions, so the execution blocks until both functions are run to completion.

An alternative option is:

runForever()

which blocks, waiting indefinitely for all asynchronous functions.

You may also find Peter’s article on async programming helpful.

Higher Async modules

The underlying asyncdispatch modules is used by several higher level modules:

1. asyncfile for async file operations.
2. asyncnet for async networking operations.
3. asynchttpserver for an async http server.