Timeouts & Cancellation

Goal: Learn how to prevent the program from freezing on slow responses.

Source code: chapter4/src/uptimemon.nim

Our current program works fine with the well-behaving URIs we've tested so far: all these locations either respond quickly or quickly return an error.

However, not all requests will go smoothly when you face the real web. Poor connections, slow servers, anti-bot checks, and access restrictions result in responses that may take long to complete or even never complete. One "misbehaving" request can negatively affect the entire program.

For example, try adding an IP address that never responds to the list:

const uris = @[
  "https://duckduckgo.com/?q=chronos", "https://mock.codes/403", "http://10.255.255.1",
]

Run the program and you'll see that it'll run for 10+ seconds, stuck on this last IP.

Let's add a timeout to our requests to cancel slow requests before they ruin our app: if a request takes longer than 5 seconds, we cancel it.

import std/sequtils
import chronos/apps/http/httpclient

const uris = @[
  "https://duckduckgo.com/?q=chronos", "https://mock.codes/403", "http://10.255.255.1",
]

proc check(session: HttpSessionRef, uri: string) {.async: (raises: [CancelledError]).} =
  try:
    let response = await session.fetch(parseUri(uri)).wait(5.seconds)

    if response.status == 200:
      echo "[OK] " & uri
    else:
      echo "[NOK] " & uri & ": " & $response.status
  except HttpError, FuturePendingError, AsyncTimeoutError:
    echo "[ERR] " & uri & ": " & getCurrentExceptionMsg()

proc check(uris: seq[string]) {.async: (raises: []).} =
  let
    session = HttpSessionRef.new()
    futures = uris.mapIt(session.check(it))

  try:
    await allFutures(futures)
  except CancelledError:
    await cancelAndWait(futures)
  finally:
    await session.closeWait()

when isMainModule:
  waitFor check(uris)

Here's the part that changed:

proc check(session: HttpSessionRef, uri: string) {.async: (raises: [CancelledError]).} =
  try:
    let response = await session.fetch(parseUri(uri)).wait(5.seconds)

    if response.status == 200:
      echo "[OK] " & uri
    else:
      echo "[NOK] " & uri & ": " & $response.status
  except HttpError, FuturePendingError, AsyncTimeoutError:
    echo "[ERR] " & uri & ": " & getCurrentExceptionMsg()

1. We use the .wait(timeout) modifier on our fetch future.
2. If the request takes longer than the provided duration, .wait() automatically cancels the underlying future and raises an AsyncTimeoutError.
3. We catch this error alongside other expected exceptions in our except block.

Run the program again and you'll see it complete in roughly 5 seconds, i.e. our timeout.