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lib/absinthe/middleware/async.ex
defmodule Absinthe.Middleware.Async do
@moduledoc """
This plugin enables asynchronous execution of a field.
See also `Absinthe.Resolution.Helpers.async/1`
# Example Usage:
Using the `Absinthe.Resolution.Helpers.async/1` helper function:
```elixir
field :time_consuming, :thing do
resolve fn _, _, _ ->
async(fn ->
{:ok, long_time_consuming_function()}
end)
end
end
```
Using the bare plugin API
```elixir
field :time_consuming, :thing do
resolve fn _, _, _ ->
task = Task.async(fn ->
{:ok, long_time_consuming_function()}
end
{:middleware, #{__MODULE__}, task}
end
end
```
This module also serves as an example for how to build middleware that uses the
resolution callbacks.
See the source code and associated comments for further details.
"""
@behaviour Absinthe.Middleware
@behaviour Absinthe.Plugin
# A function has handed resolution off to this middleware. The first argument
# is the current resolution struct. The second argument is the function to
# execute asynchronously, and opts we'll want to use when it is time to await
# the task.
#
# This function suspends resolution, and sets the async flag true in the resolution
# accumulator. This will be used later to determine whether we need to run resolution
# again.
#
# This function inserts additional middleware into the remaining middleware
# stack for this field. On the next resolution pass, we need to `Task.await` the
# task so we have actual data. Thus, we prepend this module to the middleware stack.
def call(%{state: :unresolved} = res, {fun, opts}) do
task_data = {Task.async(fun), opts}
%{res |
state: :suspended,
acc: Map.put(res.acc, __MODULE__, true),
middleware: [{__MODULE__, task_data} | res.middleware]
}
end
# This is the clause that gets called on the second pass. There's very little
# to do here. We just need to await the task started in the previous pass.
#
# Finally, we apply the result to the resolution using a helper function that ensures
# we handle the different tuple results.
#
# The `put_result` function handles setting the appropriate state.
# If the result is an `{:ok, value} | {:error, reasoon}` tuple it will set
# the state to `:resolved`, and if it is another middleware tuple it will
# set the state to unresolved.
def call(%{state: :suspended} = res, {task, opts}) do
result = Task.await(task, opts[:timeout] || 30_000)
res
|> Absinthe.Resolution.put_result(result)
end
# We must set the flag to false because if a previous resolution iteration
# set it to true it needs to go back to false now. It will be set
# back to true if any field uses this plugin again.
def before_resolution(acc) do
Map.put(acc, __MODULE__, false)
end
# Nothing to do after resolution for this plugin, so we no-op
def after_resolution(acc), do: acc
# If the flag is set we need to do another resolution phase.
# otherwise, we do not
def pipeline(pipeline, acc) do
case acc do
%{__MODULE__ => true} ->
[Absinthe.Phase.Document.Execution.Resolution | pipeline]
_ ->
pipeline
end
end
end