/*
* Copyright (c) Meta Platforms, Inc. and affiliates.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#pragma once
#include <folly/Random.h>
#include <folly/functional/Invoke.h>
#include <folly/futures/Future.h>
namespace folly {
namespace futures {
/**
* retrying and retryingUnsafe
*
* Given a policy and a future-factory, creates futures according to the
* policy.
*
* The policy must be moveable - retrying will move it a lot - and callable of
* any of the forms:
* - Future<bool>(size_t, exception_wrapper)
* - SemiFuture<bool>(size_t, exception_wrapper)
* - bool(size_t, exception_wrapper)
* Internally, the latter is transformed into the former in the obvious way.
* The first parameter is the attempt number of the next prospective attempt;
* the second parameter is the most recent exception. The policy returns a
* (Semi)Future<bool> which, when completed with true, indicates that a retry
* is desired.
*
* retrying always returns a SemiFuture and enforces transfer onto an executor
* in the implementation. This ensures that recursive policies are safe and
* run in the right place.
* retryingUnsafe returns a Future and restricts both the callable and policy
* to return Future<bool> or bool. SemiFuture forms are not valid.
* As the name suggests, prefer retrying to retryingUnsafe to avoid surprising
* recursion or putting work on the TimeKeeper that should not run there.
* Note that, consistent with other SemiFuture-returning functions retrying
* should be assumed to be lazy: it may do nothing until .wait()/.get() is
* called on the result or an executor is attached with .via.
*
* We provide a few generic policies:
* - Basic
* - CappedJitteredexponentialBackoff
*
* Custom policies may use the most recent try number and exception to decide
* whether to retry and optionally to do something interesting like delay
* before the retry. Users may pass inline lambda expressions as policies, or
* may define their own data types meeting the above requirements. Users are
* responsible for managing the lifetimes of anything pointed to or referred to
* from inside the policy.
*
* For example, one custom policy may try up to k times, but only if the most
* recent exception is one of a few types or has one of a few error codes
* indicating that the failure was transitory.
*
* Cancellation is not supported.
*
* If both FF and Policy inline executes, then it is possible to hit a stack
* overflow due to the recursive nature of the retry implementation
*/
template <class Policy, class FF>
Future<typename isFutureOrSemiFuture<invoke_result_t<FF, size_t>>::Inner>
retryingUnsafe(Policy&& p, FF&& ff);
template <class Policy, class FF>
FOLLY_NODISCARD SemiFuture<
typename isFutureOrSemiFuture<invoke_result_t<FF, size_t>>::Inner>
retrying(Policy&& p, FF&& ff);
namespace detail {
struct retrying_policy_raw_tag {};
struct retrying_policy_fut_tag {};
template <class Policy>
struct retrying_policy_traits {
using result = invoke_result_t<Policy, size_t, const exception_wrapper&>;
using is_raw = std::is_same<result, bool>;
using is_fut = std::is_same<result, Future<bool>>;
using is_semi_fut = std::is_same<result, SemiFuture<bool>>;
using tag = typename std::conditional<
is_raw::value,
retrying_policy_raw_tag,
typename std::conditional<
is_fut::value || is_semi_fut::value,
retrying_policy_fut_tag,
void>::type>::type;
};
template <class Policy, class FF, class Prom>
void retryingImpl(size_t k, Policy&& p, FF&& ff, Prom prom) {
using F = invoke_result_t<FF, size_t>;
using T = typename F::value_type;
auto f = makeFutureWith([&] { return ff(k++); });
std::move(f).thenTry([k,
prom = std::move(prom),
pm = static_cast<Policy&&>(p),
ffm = static_cast<FF&&>(ff)](Try<T>&& t) mutable {
if (t.hasValue()) {
prom.setValue(std::move(t).value());
return;
}
auto& x = t.exception();
auto q = makeFutureWith([&] { return pm(k, x); });
std::move(q).thenTry([k,
prom = std::move(prom),
xm = std::move(x),
pm = std::move(pm),
ffm = std::move(ffm)](Try<bool> shouldRetry) mutable {
if (shouldRetry.hasValue() && shouldRetry.value()) {
retryingImpl(k, std::move(pm), std::move(ffm), std::move(prom));
} else if (shouldRetry.hasValue()) {
prom.setException(std::move(xm));
} else {
prom.setException(std::move(shouldRetry.exception()));
}
});
});
}
template <class Policy, class FF>
Future<typename invoke_result_t<FF, size_t>::value_type> retryingFuture(
size_t k, Policy&& p, FF&& ff) {
using F = invoke_result_t<FF, size_t>;
using T = typename F::value_type;
auto prom = Promise<T>();
auto f = prom.getFuture();
retryingImpl(
k, static_cast<Policy&&>(p), static_cast<FF&&>(ff), std::move(prom));
return f;
}
template <class Policy, class FF>
SemiFuture<typename invoke_result_t<FF, size_t>::value_type> retryingSemiFuture(
size_t k, Policy&& p, FF&& ff) {
auto sf = folly::makeSemiFuture().deferExValue(
[k, p = static_cast<Policy&&>(p), ff = static_cast<FF&&>(ff)](
Executor::KeepAlive<> ka, auto&&) mutable {
auto futureP = [p = static_cast<Policy&&>(p), ka](
size_t kk, exception_wrapper e) mutable {
return p(kk, std::move(e)).via(ka);
};
auto futureFF = [ff = static_cast<FF&&>(ff),
ka = std::move(ka)](size_t v) mutable {
return ff(std::move(v)).via(ka);
};
return retryingFuture(k, std::move(futureP), std::move(futureFF));
});
return sf;
}
template <class Policy, class FF>
Future<typename isFutureOrSemiFuture<invoke_result_t<FF, size_t>>::Inner>
retryingFuture(Policy&& p, FF&& ff, retrying_policy_raw_tag) {
auto q = [pm = static_cast<Policy&&>(p)](size_t k, exception_wrapper x) {
return makeFuture<bool>(pm(k, x));
};
return retryingFuture(0, std::move(q), static_cast<FF&&>(ff));
}
template <class Policy, class FF>
SemiFuture<typename isFutureOrSemiFuture<invoke_result_t<FF, size_t>>::Inner>
retryingSemiFuture(Policy&& p, FF&& ff, retrying_policy_raw_tag) {
auto q = [pm = static_cast<Policy&&>(p)](size_t k, exception_wrapper x) {
return makeSemiFuture<bool>(pm(k, x));
};
return retryingSemiFuture(0, std::move(q), static_cast<FF&&>(ff));
}
template <class Policy, class FF>
Future<typename invoke_result_t<FF, size_t>::value_type> retryingFuture(
Policy&& p, FF&& ff, retrying_policy_fut_tag) {
return retryingFuture(0, static_cast<Policy&&>(p), static_cast<FF&&>(ff));
}
template <class Policy, class FF>
SemiFuture<typename invoke_result_t<FF, size_t>::value_type> retryingSemiFuture(
Policy&& p, FF&& ff, retrying_policy_fut_tag) {
return retryingSemiFuture(0, static_cast<Policy&&>(p), static_cast<FF&&>(ff));
}
// jittered exponential backoff, clamped to [backoff_min, backoff_max]
template <class URNG>
Duration retryingJitteredExponentialBackoffDur(
size_t n,
Duration backoff_min,
Duration backoff_max,
double jitter_param,
URNG& rng) {
using dist = std::normal_distribution<double>;
// short-circuit to avoid 0 * inf = nan when computing backoff_rep below
if (FOLLY_UNLIKELY(backoff_min == Duration(0))) {
return Duration(0);
}
auto jitter = jitter_param > 0 ? std::exp(dist{0., jitter_param}(rng)) : 1.;
auto backoff_rep =
jitter * static_cast<double>(backoff_min.count()) * std::pow(2, n - 1);
if (FOLLY_UNLIKELY(backoff_rep >= static_cast<double>(backoff_max.count()))) {
return std::max(backoff_min, backoff_max);
}
auto backoff = Duration(Duration::rep(backoff_rep));
return std::max(backoff_min, std::min(backoff_max, backoff));
}
template <class Policy, class URNG>
std::function<Future<bool>(size_t, const exception_wrapper&)>
retryingPolicyCappedJitteredExponentialBackoff(
size_t max_tries,
Duration backoff_min,
Duration backoff_max,
double jitter_param,
URNG&& rng,
Policy&& p) {
return [pm = static_cast<Policy&&>(p),
max_tries,
backoff_min,
backoff_max,
jitter_param,
rngp = static_cast<URNG&&>(rng)](
size_t n, const exception_wrapper& ex) mutable {
if (n == max_tries) {
return makeFuture(false);
}
return pm(n, ex).thenValue(
[n, backoff_min, backoff_max, jitter_param, rngp = std::move(rngp)](
bool v) mutable {
if (!v) {
return makeFuture(false);
}
auto backoff = detail::retryingJitteredExponentialBackoffDur(
n, backoff_min, backoff_max, jitter_param, rngp);
return futures::sleep(backoff).toUnsafeFuture().thenValue(
[](auto&&) { return true; });
});
};
}
template <class Policy, class URNG>
std::function<Future<bool>(size_t, const exception_wrapper&)>
retryingPolicyCappedJitteredExponentialBackoff(
size_t max_tries,
Duration backoff_min,
Duration backoff_max,
double jitter_param,
URNG&& rng,
Policy&& p,
retrying_policy_raw_tag) {
auto q = [pm = static_cast<Policy&&>(p)](
size_t n, const exception_wrapper& e) {
return makeFuture(pm(n, e));
};
return retryingPolicyCappedJitteredExponentialBackoff(
max_tries,
backoff_min,
backoff_max,
jitter_param,
static_cast<URNG&&>(rng),
std::move(q));
}
template <class Policy, class URNG>
std::function<Future<bool>(size_t, const exception_wrapper&)>
retryingPolicyCappedJitteredExponentialBackoff(
size_t max_tries,
Duration backoff_min,
Duration backoff_max,
double jitter_param,
URNG&& rng,
Policy&& p,
retrying_policy_fut_tag) {
return retryingPolicyCappedJitteredExponentialBackoff(
max_tries,
backoff_min,
backoff_max,
jitter_param,
static_cast<URNG&&>(rng),
static_cast<Policy&&>(p));
}
} // namespace detail
template <class Policy, class FF>
Future<typename isFutureOrSemiFuture<invoke_result_t<FF, size_t>>::Inner>
retryingUnsafe(Policy&& p, FF&& ff) {
using tag = typename detail::retrying_policy_traits<Policy>::tag;
return detail::retryingFuture(
static_cast<Policy&&>(p), static_cast<FF&&>(ff), tag());
}
template <class Policy, class FF>
SemiFuture<typename isFutureOrSemiFuture<invoke_result_t<FF, size_t>>::Inner>
retrying(Policy&& p, FF&& ff) {
using tag = typename detail::retrying_policy_traits<Policy>::tag;
return detail::retryingSemiFuture(
static_cast<Policy&&>(p), static_cast<FF&&>(ff), tag());
}
inline std::function<bool(size_t, const exception_wrapper&)>
retryingPolicyBasic(size_t max_tries) {
return [=](size_t n, const exception_wrapper&) { return n < max_tries; };
}
template <class Policy, class URNG>
std::function<Future<bool>(size_t, const exception_wrapper&)>
retryingPolicyCappedJitteredExponentialBackoff(
size_t max_tries,
Duration backoff_min,
Duration backoff_max,
double jitter_param,
URNG&& rng,
Policy&& p) {
using tag = typename detail::retrying_policy_traits<Policy>::tag;
return detail::retryingPolicyCappedJitteredExponentialBackoff(
max_tries,
backoff_min,
backoff_max,
jitter_param,
static_cast<URNG&&>(rng),
static_cast<Policy&&>(p),
tag());
}
inline std::function<Future<bool>(size_t, const exception_wrapper&)>
retryingPolicyCappedJitteredExponentialBackoff(
size_t max_tries,
Duration backoff_min,
Duration backoff_max,
double jitter_param) {
auto p = [](size_t, const exception_wrapper&) { return true; };
return retryingPolicyCappedJitteredExponentialBackoff(
max_tries,
backoff_min,
backoff_max,
jitter_param,
ThreadLocalPRNG(),
std::move(p));
}
} // namespace futures
} // namespace folly
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