defmodule Retry do
@moduledoc """
Provides a convenient interface to retrying behavior. All durations are
specified in milliseconds.
Examples
use Retry
import Stream
retry with: exponential_backoff |> randomize |> cap(1_000) |> expiry(10_000) do
# interact with external service
end
retry with: linear_backoff(10, 2) |> cap(1_000) |> take(10) do
# interact with external service
end
retry with: cycle([500]) |> take(10) do
# interact with external service
end
The first retry will exponentially increase the delay, fudging each delay up
to 10%, until the delay reaches 1 second and then give up after 10 seconds.
The second retry will linearly increase the retry by a factor of 2 from 10ms giving up after 10 attempts.
The third example shows how we can produce a delay stream using standard
`Stream` functionality. Any stream of integers may be used as the value of
`with:`.
"""
@default_retry_options [atoms: [:error], rescue_only: [RuntimeError]]
@doc false
defmacro __using__(_opts) do
quote do
import Retry
import Retry.DelayStreams
end
end
@doc """
Retry a block of code delaying between each attempt the duration specified by
the next item in the `with` delay stream.
If the block returns any of the atoms specified in `atoms`, a retry will be attempted.
Other atoms or atom-result tuples will not be retried. If `atoms` is not specified,
it defaults to `[:error]`.
Similary, if the block raises any of the exceptions specified in `rescue_only`, a retry
will be attempted. Other exceptions will not be retried. If `rescue_only` is
not specified, it defaults to `[RuntimeError]`.
The `after` block evaluates only when the `do` block returns a valid value before timeout.
On the other hand, the `else` block evaluates only when the `do` block remains erroneous after timeout.
Example
use Retry
retry with: exponential_backoff() |> cap(1_000) |> expiry(1_000), rescue_only: [CustomError] do
# interact with external service
after
result -> result
else
error -> error
end
"""
defmacro retry(
[{:with, stream_builder} | opts],
do: do_clause,
after: after_clause,
else: else_clause
) do
opts = Keyword.merge(@default_retry_options, opts)
atoms = Keyword.get(opts, :atoms)
quote generated: true do
fun = unquote(block_runner(do_clause, opts))
unquote(delays_from(stream_builder))
|> Enum.reduce_while(nil, fn delay, _last_result ->
:timer.sleep(delay)
fun.()
end)
|> case do
{:exception, e} ->
case e do
unquote(else_clause)
end
e = {atom, _} when atom in unquote(atoms) ->
case e do
unquote(else_clause)
end
e when is_atom(e) and e in unquote(atoms) ->
case e do
unquote(else_clause)
end
result ->
case result do
unquote(after_clause)
end
end
end
end
defmacro retry(_stream_builder, _clauses) do
raise(ArgumentError, ~s(invalid syntax, only "retry", "after" and "else" are permitted))
end
@doc """
Retry a block of code until `halt` is emitted delaying between each attempt
the duration specified by the next item in the `with` delay stream.
The return value for `block` is expected to be `{:cont, result}`, return
`{:halt, result}` to end the retry early.
An accumulator can also be specified which might be handy if subsequent
retries are dependent on the previous ones.
The initial value of the accumulator is given as a keyword argument `acc:`.
When the `:acc` key is given, its value is used as the initial accumulator
and the `do` block must be changed to use `->` clauses, where the left side
of `->` receives the accumulated value of the previous iteration and
the expression on the right side must return the `:cont`/`:halt` tuple
with new accumulator value as the second element.
Once `:halt` is returned from the block, or there are no more elements,
the accumulated value is returned.
Example
retry_while with: linear_backoff(500, 1) |> take(5) do
call_service
|> case do
result = %{"errors" => true} -> {:cont, result}
result -> {:halt, result}
end
end
Example with `acc:`
retry_while acc: 0, with: linear_backoff(500, 1) |> take(5) do
acc ->
call_service
|> case do
%{"errors" => true} -> {:cont, acc + 1}
result -> {:halt, result}
end
end
"""
defmacro retry_while([with: stream_builder], do: block) do
quote generated: true do
unquote(delays_from(stream_builder))
|> Enum.reduce_while(nil, fn delay, _last_result ->
:timer.sleep(delay)
unquote(block)
end)
end
end
defmacro retry_while(args = [with: _stream_builder, acc: _acc_initial], do: block),
do: do_retry_value(Enum.reverse(args), do: block)
defmacro retry_while(args = [acc: _acc_initial, with: _stream_builder], do: block),
do: do_retry_value(args, do: block)
defp do_retry_value([acc: acc_initial, with: stream_builder], do: block) do
quote generated: true do
unquote(delays_from(stream_builder))
|> Enum.reduce_while(unquote(acc_initial), fn delay, acc ->
:timer.sleep(delay)
case acc do
unquote(block)
end
end)
end
end
@doc """
Wait for a block of code to be truthy delaying between each attempt
the duration specified by the next item in the delay stream.
The `after` block evaluates only when the `do` block returns a truthy value.
On the other hand, the `else` block evaluates only when the `do` block remains falsy after timeout.
Example
wait linear_backoff(500, 1) |> take(5) do
we_there_yet?
after
_ ->
{:ok, "We have arrived!"}
else
_ ->
{:error, "We're still on our way :("}
end
"""
defmacro wait(stream_builder, do: do_clause, after: after_clause, else: else_clause) do
quote generated: true do
unquote(delays_from(stream_builder))
|> Enum.reduce_while(nil, fn delay, _last_result ->
:timer.sleep(delay)
case unquote(do_clause) do
result when result in [false, nil] -> {:cont, result}
result -> {:halt, result}
end
end)
|> case do
x when x in [false, nil] ->
case x do
unquote(else_clause)
end
x ->
case x do
unquote(after_clause)
end
end
end
end
defmacro wait(_stream_builder, _clauses) do
raise(ArgumentError, ~s(invalid syntax, only "wait", "after" and "else" are permitted))
end
defp block_runner(block, opts) do
atoms = Keyword.get(opts, :atoms)
exceptions = Keyword.get(opts, :rescue_only)
quote generated: true do
fn ->
try do
case unquote(block) do
{atom, _} = result ->
if atom in unquote(atoms) do
{:cont, result}
else
{:halt, result}
end
result ->
if is_atom(result) and result in unquote(atoms) do
{:cont, result}
else
{:halt, result}
end
end
rescue
e ->
if e.__struct__ in unquote(exceptions) do
{:cont, {:exception, e}}
else
reraise e, __STACKTRACE__
end
end
end
end
end
defp delays_from(stream_builder) do
quote generated: true do
delays = unquote(stream_builder)
[0] |> Stream.concat(delays)
end
end
end
