wait_for_it v1.0.0 WaitForIt View Source
WaitForIt provides macros for various ways to wait for things to happen.
Since most Elixir systems are highly concurrent there must be a way to coordinate and synchronize
the concurrent processes in the system. While the language provides features such as
Process.sleep/1
and receive
/after
that can be used implement such synhronization, they are
inconvenient to use for this purpose. WaitForIt builds on top of these language features to
provide convenient and easy-to-use facilities for synchronizing concurrent activities. While
this is likely most useful for test code in which tests must wait for concurrent or asynchronous
activities to complete, it is also useful in any scenario where concurrent processes must
coordinate their activity. Examples include asynchronous event handling, producer-consumer
processes, and time-based activity.
There are three distinct forms of waiting provided:
- The
wait
macro waits until a given expression evaluates to a truthy value. - The
case_wait
macro waits until a given expression evaluates to a value that matches any one of the given case clauses (looks like an Elixircase
expression). - The
cond_wait
macro waits until any one of the given expressions evaluates to a truthy value (looks like an Elixircond
expression).
All three forms accept the same set of options to control their behavior:
:timeout
- the amount of time to wait (in milliseconds) before giving up:frequency
- the polling frequency (in milliseconds) at which to re-evaluate conditions:signal
- disable polling and use a condition variable of the given name instead
The :signal
option warrants further explanation. By default, all three forms of waiting use
polling to periodically re-evaluate conditions to determine if waiting should continue. The
frequency of polling is controlled by the :frequency
option. However, if the :signal
option
is given it disables polling altogether. Instead of periodically re-evaluating conditions at a
particular frequency, a condition variable is used to signal when conditions should be
re-evaluated. It is expected that the signal
macro will be used to unblock the waiting code
in order to re-evaluate conditions. For example, imagine a typical producer-consumer problem in
which a consumer process waits for items to appear in some buffer while a separate producer
process occasionally place items in the buffer. In this scenario, the consumer process might use
the wait
macro with the :signal
option to wait until there are some items in the buffer and
the producer process would use the signal
macro to tell the consumer that it might be time for
it to check the buffer again.
# CONSUMER
# assume the existence of a `buffer_size` function
WaitForIt.wait buffer_size() >= 4, signal: :wait_for_buffer
# PRODUCER
# put some things in buffer, then:
WaitForIt.signal(:wait_for_buffer)
Notice that the same condition variable name :wait_for_buffer
is used in both cases. It is
important to note that when using condition variables for signaling like this, both the wait
invocation and the signal
invocation should be in the same Elixir module. This is because
WaitForIt uses the calling module as a namespace for condition variable names to prevent
accidental name collisions with other registered processes in the application. Also note that
just because a condition variable has been signalled does not necessarily mean that any waiters
on that condition variable can stop waiting. Rather, a signal indicates that waiters should
re-evaluate their waiting conditions to determine if they should continue to wait or not.
Link to this section Summary
Functions
Wait until the given expression
matches one of the case clauses in the given block. Returns
the value of the matching clause or {:timeout, timeout_milliseconds}
Wait until one of the expressions in the given block evaluates to a truthy value. Returns the
value corresponding with the matching expression or {:timeout, timeout_milliseconds}
Send a signal to the given condition variable to indicate that any processes waiting on the condition variable should re-evaluate their wait conditions
Wait until the given expression
evaluates to a truthy value. Returns {:ok, value}
or
{:timeout, timeout_milliseconds}
Link to this section Functions
Wait until the given expression
matches one of the case clauses in the given block. Returns
the value of the matching clause or {:timeout, timeout_milliseconds}
.
The do
block passed to this macro must be a series of case clauses exactly like a built-in
Elixir case
expression. Just like a case
expression, the clauses will attempt to be matched
from top to bottom and the first one that matches will provide the resulting value of the
expression. The difference with case_wait
is that if none of the clauses initially matches it
will wait and periodically re-evaluate the clauses until one of them does match or a timeout
occurs.
Options
See the WaitForIt module documentation for further discussion of these options.
:timeout
- the amount of time to wait (in milliseconds) before giving up:frequency
- the polling frequency (in milliseconds) at which to re-evaluate conditions:signal
- disable polling and use a condition variable of the given name instead
Examples
Wait until queue has at least 5 messages, then return them:
WaitForIt.case_wait Queue.get_messages(queue), timeout: 30_000, frequency: 100 do
messages when length(messages) > 4 -> messages
end
A thermostat that keeps temperature in a small range:
def thermostat(desired_temperature) do
WaitForIt.case_wait get_current_temperature() do
temp when temp > desired_temperature + 2 ->
turn_on_air_conditioning()
temp when temp < desired_temperature - 2 ->
turn_on_heat()
end
thermostat(desired_temperature)
end
Ring the church bells every 15 minutes:
def church_bell_chimes do
count = WaitForIt.case_wait Time.utc_now.minute, frequency: 60_000, timeout: 60_000 * 60 do
15 -> 1
30 -> 2
45 -> 3
0 -> 4
end
IO.puts(String.duplicate(" ding ding ding dong ", count))
church_bell_chimes()
end
Wait until one of the expressions in the given block evaluates to a truthy value. Returns the
value corresponding with the matching expression or {:timeout, timeout_milliseconds}
.
The do
block passed to this macro must be a series of expressions exactly like a built-in
Elixir cond
expression. Just like a cond
expression, the embedded expresions will be
evaluated from top to bottom and the first one that is truthy will provide the resulting value of
the expression. The difference with cond_wait
is that if none of the expressions is initially
truthy it will wait and periodically re-evaluate them until one of them becomes truthy or a
timeout occurs.
Options
See the WaitForIt module documentation for further discussion of these options.
:timeout
- the amount of time to wait (in milliseconds) before giving up:frequency
- the polling frequency (in milliseconds) at which to re-evaluate conditions:signal
- disable polling and use a condition variable of the given name instead
Examples
Trigger an alarm when any sensors go beyond a threshold:
def sound_the_alarm do
WaitForIt.cond_wait timeout: 60_000 * 60 * 24 do
read_sensor(:sensor1) > 9 -> IO.puts("Alarm: :sensor1 too high!")
read_sensor(:sensor2) < 100 -> IO.puts("Alarm: :sensor2 too low!")
read_sensor(:sensor3) < 0 -> IO.puts("Alarm: :sensor3 below zero!")
end
sound_the_alarm()
end
Send a signal to the given condition variable to indicate that any processes waiting on the condition variable should re-evaluate their wait conditions.
The caller of signal
must be in the same Elixir module as any waiters on the same condition
variable since the module is used as a namespace for condition variables. This is to prevent
accidental name collisions as well as to enforce good practice for encapsulation.
Wait until the given expression
evaluates to a truthy value. Returns {:ok, value}
or
{:timeout, timeout_milliseconds}
.
Options
See the WaitForIt module documentation for further discussion of these options.
:timeout
- the amount of time to wait (in milliseconds) before giving up:frequency
- the polling frequency (in milliseconds) at which to re-evaluate conditions:signal
- disable polling and use a condition variable of the given name instead
Examples
Wait until the top of the hour:
WaitForIt.wait Time.utc_now.minute == 0, frequency: 60_000, timeout: 60_000 * 60
Wait up to one minute for a particular record to appear in the database:
case WaitForIt.wait Repo.get(Post, 42), frequency: 1000, timeout: 60_000 do
{:ok, data} -> IO.inspect(data)
{:timeout, timeout} -> IO.puts("Gave up after #{timeout} milliseconds")
end