2 * Copyright 2015 Facebook, Inc.
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
8 * http://www.apache.org/licenses/LICENSE-2.0
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
18 #include <folly/futures/Future.h>
22 /// This namespace is for utility functions that would usually be static
23 /// members of Future, except they don't make sense there because they don't
24 /// depend on the template type (rather, on the type of their arguments in
25 /// some cases). This is the least-bad naming scheme we could think of. Some
26 /// of the functions herein have really-likely-to-collide names, like "map"
29 /// Returns a Future that will complete after the specified duration. The
30 /// Duration typedef of a `std::chrono` duration type indicates the
31 /// resolution you can expect to be meaningful (milliseconds at the time of
32 /// writing). Normally you wouldn't need to specify a Timekeeper, we will
33 /// use the global futures timekeeper (we run a thread whose job it is to
34 /// keep time for futures timeouts) but we provide the option for power
37 /// The Timekeeper thread will be lazily created the first time it is
38 /// needed. If your program never uses any timeouts or other time-based
39 /// Futures you will pay no Timekeeper thread overhead.
40 Future<Unit> sleep(Duration, Timekeeper* = nullptr);
43 * Set func as the callback for each input Future and return a vector of
44 * Futures containing the results in the input order.
46 template <class It, class F,
47 class ItT = typename std::iterator_traits<It>::value_type,
49 = typename decltype(std::declval<ItT>().then(std::declval<F>()))::value_type>
50 std::vector<Future<Result>> map(It first, It last, F func);
52 // Sugar for the most common case
53 template <class Collection, class F>
54 auto map(Collection&& c, F&& func)
55 -> decltype(map(c.begin(), c.end(), func)) {
56 return map(c.begin(), c.end(), std::forward<F>(func));
59 } // namespace futures
62 Make a completed Future by moving in a value. e.g.
65 auto f = makeFuture(std::move(foo));
69 auto f = makeFuture<string>("foo");
72 Future<typename std::decay<T>::type> makeFuture(T&& t);
74 /** Make a completed void Future. */
75 Future<Unit> makeFuture();
77 /** Make a completed Future by executing a function. If the function throws
78 we capture the exception, otherwise we capture the result. */
80 auto makeFutureWith(F&& func)
81 -> Future<typename Unit::Lift<decltype(func())>::type>;
83 /// Make a failed Future from an exception_ptr.
84 /// Because the Future's type cannot be inferred you have to specify it, e.g.
86 /// auto f = makeFuture<string>(std::current_exception());
88 DEPRECATED Future<T> makeFuture(std::exception_ptr const& e);
90 /// Make a failed Future from an exception_wrapper.
92 Future<T> makeFuture(exception_wrapper ew);
94 /** Make a Future from an exception type E that can be passed to
95 std::make_exception_ptr(). */
96 template <class T, class E>
97 typename std::enable_if<std::is_base_of<std::exception, E>::value,
99 makeFuture(E const& e);
101 /** Make a Future out of a Try */
103 Future<T> makeFuture(Try<T>&& t);
106 * Return a new Future that will call back on the given Executor.
107 * This is just syntactic sugar for makeFuture().via(executor)
109 * @param executor the Executor to call back on
110 * @param priority optionally, the priority to add with. Defaults to 0 which
111 * represents medium priority.
113 * @returns a void Future that will call back on the given executor
115 inline Future<Unit> via(
117 int8_t priority = Executor::MID_PRI);
119 /// Execute a function via the given executor and return a future.
120 /// This is semantically equivalent to via(executor).then(func), but
121 /// easier to read and slightly more efficient.
122 template <class Func>
123 auto via(Executor*, Func func)
124 -> Future<typename isFuture<decltype(func())>::Inner>;
126 /** When all the input Futures complete, the returned Future will complete.
127 Errors do not cause early termination; this Future will always succeed
128 after all its Futures have finished (whether successfully or with an
131 The Futures are moved in, so your copies are invalid. If you need to
132 chain further from these Futures, use the variant with an output iterator.
134 This function is thread-safe for Futures running on different threads. But
135 if you are doing anything non-trivial after, you will probably want to
136 follow with `via(executor)` because it will complete in whichever thread the
137 last Future completes in.
139 The return type for Future<T> input is a Future<std::vector<Try<T>>>
141 template <class InputIterator>
142 Future<std::vector<Try<
143 typename std::iterator_traits<InputIterator>::value_type::value_type>>>
144 collectAll(InputIterator first, InputIterator last);
146 /// Sugar for the most common case
147 template <class Collection>
148 auto collectAll(Collection&& c) -> decltype(collectAll(c.begin(), c.end())) {
149 return collectAll(c.begin(), c.end());
152 /// This version takes a varying number of Futures instead of an iterator.
153 /// The return type for (Future<T1>, Future<T2>, ...) input
154 /// is a Future<std::tuple<Try<T1>, Try<T2>, ...>>.
155 /// The Futures are moved in, so your copies are invalid.
156 template <typename... Fs>
157 typename detail::CollectAllVariadicContext<
158 typename std::decay<Fs>::type::value_type...>::type
159 collectAll(Fs&&... fs);
161 /// Like collectAll, but will short circuit on the first exception. Thus, the
162 /// type of the returned Future is std::vector<T> instead of
163 /// std::vector<Try<T>>
164 template <class InputIterator>
165 Future<typename detail::CollectContext<
166 typename std::iterator_traits<InputIterator>::value_type::value_type
168 collect(InputIterator first, InputIterator last);
170 /// Sugar for the most common case
171 template <class Collection>
172 auto collect(Collection&& c) -> decltype(collect(c.begin(), c.end())) {
173 return collect(c.begin(), c.end());
176 /// Like collectAll, but will short circuit on the first exception. Thus, the
177 /// type of the returned Future is std::tuple<T1, T2, ...> instead of
178 /// std::tuple<Try<T1>, Try<T2>, ...>
179 template <typename... Fs>
180 typename detail::CollectVariadicContext<
181 typename std::decay<Fs>::type::value_type...>::type
184 /** The result is a pair of the index of the first Future to complete and
185 the Try. If multiple Futures complete at the same time (or are already
186 complete when passed in), the "winner" is chosen non-deterministically.
188 This function is thread-safe for Futures running on different threads.
190 template <class InputIterator>
193 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>
194 collectAny(InputIterator first, InputIterator last);
196 /// Sugar for the most common case
197 template <class Collection>
198 auto collectAny(Collection&& c) -> decltype(collectAny(c.begin(), c.end())) {
199 return collectAny(c.begin(), c.end());
202 /** when n Futures have completed, the Future completes with a vector of
203 the index and Try of those n Futures (the indices refer to the original
204 order, but the result vector will be in an arbitrary order)
208 template <class InputIterator>
209 Future<std::vector<std::pair<
211 Try<typename std::iterator_traits<InputIterator>::value_type::value_type>>>>
212 collectN(InputIterator first, InputIterator last, size_t n);
214 /// Sugar for the most common case
215 template <class Collection>
216 auto collectN(Collection&& c, size_t n)
217 -> decltype(collectN(c.begin(), c.end(), n)) {
218 return collectN(c.begin(), c.end(), n);
221 /** window creates up to n Futures using the values
222 in the collection, and then another Future for each Future
225 this is basically a sliding window of Futures of size n
227 func must return a Future for each value in input
229 template <class Collection, class F,
230 class ItT = typename std::iterator_traits<
231 typename Collection::iterator>::value_type,
232 class Result = typename detail::resultOf<F, ItT&&>::value_type>
233 std::vector<Future<Result>>
234 window(Collection input, F func, size_t n);
236 template <typename F, typename T, typename ItT>
237 using MaybeTryArg = typename std::conditional<
238 detail::callableWith<F, T&&, Try<ItT>&&>::value, Try<ItT>, ItT>::type;
240 template<typename F, typename T, typename Arg>
241 using isFutureResult = isFuture<typename std::result_of<F(T&&, Arg&&)>::type>;
243 /** repeatedly calls func on every result, e.g.
244 reduce(reduce(reduce(T initial, result of first), result of second), ...)
246 The type of the final result is a Future of the type of the initial value.
248 Func can either return a T, or a Future<T>
250 func is called in order of the input, see unorderedReduce if that is not
253 template <class It, class T, class F>
254 Future<T> reduce(It first, It last, T&& initial, F&& func);
256 /// Sugar for the most common case
257 template <class Collection, class T, class F>
258 auto reduce(Collection&& c, T&& initial, F&& func)
259 -> decltype(reduce(c.begin(), c.end(), std::forward<T>(initial),
260 std::forward<F>(func))) {
264 std::forward<T>(initial),
265 std::forward<F>(func));
268 /** like reduce, but calls func on finished futures as they complete
269 does NOT keep the order of the input
271 template <class It, class T, class F,
272 class ItT = typename std::iterator_traits<It>::value_type::value_type,
273 class Arg = MaybeTryArg<F, T, ItT>>
274 Future<T> unorderedReduce(It first, It last, T initial, F func);
276 /// Sugar for the most common case
277 template <class Collection, class T, class F>
278 auto unorderedReduce(Collection&& c, T&& initial, F&& func)
279 -> decltype(unorderedReduce(c.begin(), c.end(), std::forward<T>(initial),
280 std::forward<F>(func))) {
281 return unorderedReduce(
284 std::forward<T>(initial),
285 std::forward<F>(func));
293 * Given a policy and a future-factory, creates futures according to the
296 * The policy must be moveable - retrying will move it a lot - and callable of
297 * either of the two forms:
298 * - Future<bool>(size_t, exception_wrapper)
299 * - bool(size_t, exception_wrapper)
300 * Internally, the latter is transformed into the former in the obvious way.
301 * The first parameter is the attempt number of the next prospective attempt;
302 * the second parameter is the most recent exception. The policy returns a
303 * Future<bool> which, when completed with true, indicates that a retry is
306 * We provide a few generic policies:
308 * - CappedJitteredexponentialBackoff
310 * Custom policies may use the most recent try number and exception to decide
311 * whether to retry and optionally to do something interesting like delay
312 * before the retry. Users may pass inline lambda expressions as policies, or
313 * may define their own data types meeting the above requirements. Users are
314 * responsible for managing the lifetimes of anything pointed to or referred to
315 * from inside the policy.
317 * For example, one custom policy may try up to k times, but only if the most
318 * recent exception is one of a few types or has one of a few error codes
319 * indicating that the failure was transitory.
321 * Cancellation is not supported.
323 template <class Policy, class FF>
324 typename std::result_of<FF(size_t)>::type
325 retrying(Policy&& p, FF&& ff);
328 * generic retrying policies
332 std::function<bool(size_t, const exception_wrapper&)>
336 template <class Policy, class URNG>
337 std::function<Future<bool>(size_t, const exception_wrapper&)>
338 retryingPolicyCappedJitteredExponentialBackoff(
340 Duration backoff_min,
341 Duration backoff_max,
347 std::function<Future<bool>(size_t, const exception_wrapper&)>
348 retryingPolicyCappedJitteredExponentialBackoff(
350 Duration backoff_min,
351 Duration backoff_max,
352 double jitter_param);