Various ways of waiting for things to happen.

WaitForIt lets you wait on the results of asynchronous or remote operations using intuitive, familiar syntax built on Elixir's own control-flow constructs (if, case, cond, and with). It is equally at home coordinating concurrent processes in production code and taming flaky timing in tests.

# Wait until a record shows up, and bind it directly:
{:ok, user} = WaitForIt.match_wait({:ok, %User{}}, Repo.fetch(User, id), timeout: 2_000)

Elixir provides several language and standard library features — such as Process.sleep/1, receive/1/after, and Task.async/1/Task.await/2 — that can be used to implement waiting, but they are inconvenient for the purpose. WaitForIt builds on top of them to provide convenient, expressive facilities for waiting on specific conditions. This is most obviously useful in tests that must wait for concurrent or asynchronous activity to complete, but it is just as useful anywhere concurrent processes coordinate their activity — asynchronous event handling, producer-consumer processes, and time-based activity.

To use WaitForIt, require WaitForIt or import WaitForIt.

If you are just getting started, the task-focused guides walk through the most common scenarios: Waiting in tests, Polling vs signaling, Composing waits, Recipes, and Telemetry.

The five forms of waiting

Each form has a ! variant (wait!/2, match_wait!/3, …) that raises WaitForIt.TimeoutError on timeout instead of returning a falsy value or raising the matching built-in error.

wait

wait/2 waits until a given expression evaluates to a truthy value.

# Wait up to one minute for a file to exist, then print its contents.
if WaitForIt.wait(File.exists?("data.csv"), timeout: :timer.minutes(1)) do
  IO.puts(File.read!("data.csv"))
else
  IO.warn("Stopped waiting for the file to exist")
end

match_wait

match_wait/3 waits until a given expression matches a given pattern, and binds out of it. It is the most convenient form when waiting for a tagged result such as {:ok, value}.

{:ok, user} = WaitForIt.match_wait({:ok, %User{}}, Repo.fetch(User, id), timeout: 2_000)

case_wait

case_wait/3 waits until a given expression matches one of the given case clauses. It looks and acts like a case/2 expression, except that it can take an optional else clause.

WaitForIt.case_wait(File.stat("data"), timeout: :timer.seconds(30)) do
  {:ok, %File.Stat{type: :directory}} ->
    File.write!("data/greeting.txt", "Hello, world!")
else
  {:ok, %File.Stat{type: type}} ->
    IO.warn("Expected 'data' to be a directory but its type is #{inspect(type)}")

  {:error, reason} ->
    IO.warn("Could not stat 'data': #{inspect(reason)}")
end

cond_wait

cond_wait/2 waits until any one of the given expressions evaluates to a truthy value. It looks and acts like a cond/1 expression, except that it can take an optional else clause.

WaitForIt.cond_wait(timeout: :timer.seconds(10), interval: 500) do
  File.exists?("data/process.json") -> IO.puts("Processing...")
  NaiveDateTime.utc_now().second == 0 -> IO.puts("Top of the minute!")
else
  IO.warn("Stopped waiting since neither condition ever became truthy")
end

with_wait

with_wait/3 composes several waits in a pipeline. It looks and acts like a with/1 expression, except that its <~ clauses wait until their expression matches.

WaitForIt.with_wait on(
  {:ok, account} <~ {load_account(token), timeout: 2_000},
  {:ok, balance} <~ fetch_balance(account)
) do
  {:ok, balance}
else
  not_ready -> {:error, {:timed_out, not_ready}}
end

Options

All forms of waiting accept the same options:

See the Polling-based waiting and Signal-based waiting sections below for the :interval and :signal options. The :interval option may also be a WaitForIt.Backoff function for exponential or custom backoff.

Timeout behavior

The forms of waiting differ in what happens when a wait times out. This table summarizes the behavior; the non-bang forms mirror the corresponding built-in Elixir construct, while every bang form raises a WaitForIt.TimeoutError.

Waitable expressions and waiting conditions

A waitable expression is any Elixir expression that can be evaluated one or more times to produce a value. A waiting condition decides, from that value, whether to keep waiting or to halt with a result. For wait/2 the waiting condition is implicit (the truthiness of the expression); for case_wait/3 and with_wait/3 it is the case clauses or <~ patterns; for cond_wait/2 it is the truthiness of each branch.

Because a waitable expression is re-evaluated until its waiting condition is met, idempotent expressions are of little use — they would either halt immediately or never halt. It is expected that the value may change on each re-evaluation, and that evaluation may have side effects. Any such side effects must be safe to repeat, since the expression may be evaluated an indeterminate number of times while waiting.

Polling-based waiting

By default, WaitForIt polls: it re-evaluates the waitable expression at a fixed interval until the waiting condition is met or the timeout elapses. The interval is controlled by the :interval option (default 100 ms; the legacy alias :frequency is also accepted).

The :interval option may also be a 1-arity function of the attempt number, which enables backoff strategies — for example, polling less aggressively as time goes on so as not to hammer a struggling dependency. See WaitForIt.Backoff for ready-made strategies such as exponential backoff with jitter.

Signal-based waiting

Signal-based waiting removes the polling loop: instead of re-checking on a timer, a waiter blocks until it receives a signal telling it to re-evaluate. Opt in with the :signal option, naming a signal (any term, typically an atom), and have the code that changes the condition call signal/1.

Imagine a producer-consumer problem in which a consumer waits for items to appear in a buffer while a separate producer occasionally places items in the buffer:

# CONSUMER process
WaitForIt.wait(Buffer.count() >= 4, signal: :buffer_filled)

# PRODUCER process — after putting some things in the buffer, signal waiters
Buffer.put(item)
WaitForIt.signal(:buffer_filled)

Both sides share the same signal name, which binds the producer to the consumer. A signal does not mean the condition is now satisfied — only that waiters should re-evaluate. The wait halts when its condition is met, or continues until the next signal or the timeout.

See the Polling vs signaling guide for guidance on choosing between the two modes.

Telemetry

Every wait emits :telemetry events under the [:wait_for_it, :wait] prefix — :start, :stop, and :exception — so you can observe how long waits take, how many evaluations they require, and how often they time out. The :stop event reports the wait duration, the number of evaluations, and whether the wait :matched or hit a :timeout.

See the Telemetry guide for the full measurement and metadata reference, plus examples of attaching handlers and wiring up Telemetry.Metrics.

Using WaitForIt in tests

Tests — especially integration and end-to-end tests — are one of the most common places to wait on asynchronous work. The WaitForIt.Test module provides ExUnit assertions (assert_eventually/2, refute_eventually/2, and assert_always/2) that wait and re-evaluate and, on timeout, fail with a regular ExUnit.AssertionError that includes the source expression and the last value seen:

defmodule MyApp.SomeTest do
  use ExUnit.Case
  use WaitForIt.Test

  test "the user is eventually confirmed" do
    assert_eventually {:ok, %User{confirmed: true}} = Repo.fetch(User, user_id)
  end
end

The waiting macros can also be used directly in tests when you want their exact return values or timeout semantics — wait/2, for example, returns its value and so drops straight into an assert. See the Waiting in tests guide for a full walkthrough.

A note on "catch-all" clauses

It is common to include "catch-all" clauses in normal case/2 and cond/1 expressions — a final _ clause, or a final always-truthy true condition. When using case_wait/3 and cond_wait/2, avoid such catch-all clauses: because they always match, they would halt the wait on the very first evaluation. Use an else clause instead, which is only evaluated on timeout and lets you customize the behavior and return value when a wait gives up.