1Future::Mutex(3pm) User Contributed Perl Documentation Future::Mutex(3pm)
2
6 "Future::Mutex" - mutual exclusion lock around code that returns
7 Futures
8
10 use Future::Mutex;
11 my $mutex = Future::Mutex->new;
13 sub do_atomically
15 {
16 return $mutex->enter( sub {
17 ...
18 return $f;
19 });
20 }
21
23 Most Future-using code expects to run with some level of concurrency,
24 using future instances to represent still-pending operations that will
25 complete at some later time. There are occasions however, when this
26 concurrency needs to be restricted - some operations that, once
27 started, must not be interrupted until they are complete. Subsequent
28 requests to perform the same operation while one is still outstanding
29 must therefore be queued to wait until the first is finished. These
30 situations call for a mutual-exclusion lock, or "mutex".
31 A "Future::Mutex" instance provides one basic operation, which will
33 execute a given block of code which returns a future, and itself
34 returns a future to represent that. The mutex can be in one of two
35 states; either unlocked or locked. While it is unlocked, requests to
36 execute code are handled immediately. Once a block of code is invoked,
37 the mutex is now considered to be locked, causing any subsequent
38 requests to invoke code to be queued behind the first one, until it
39 completes. Once the initial code indicates completion (by its returned
40 future providing a result or failing), the next queued code is invoked.
41 An instance may also be a counting mutex if initialised with a count
43 greater than one. In this case, it can keep multiple blocks outstanding
44 up to that limit, with subsequent requests queued as before. This
45 allows it to act as a concurrency-bounding limit around some operation
46 that can run concurrently, but an application wishes to apply overall
47 limits to stop it growing too much, such as communications with
48 external services or executing other programs.
49
51 new
52 $mutex = Future::Mutex->new( count => $n )
53 Returns a new "Future::Mutex" instance. It is initially unlocked.
55 Takes the following named arguments:
57 count => INT
59 Optional number to limit outstanding concurrency. Will default
60 to 1 if not supplied.
61
63 enter
64 $f = $mutex->enter( \&code )
65 Returns a new "Future" that represents the eventual result of calling
67 the code. If the mutex is currently unlocked, the code will be invoked
68 immediately. If it is currently locked, the code will be queued waiting
69 for the next time it becomes unlocked.
70 The code is invoked with no arguments, and is expected to return a
72 "Future". The eventual result of that future determines the result of
73 the future that "enter" returned.
74 available
76 $avail = $mutex->available
77 Returns true if the mutex is currently unlocked, or false if it is
79 locked.
80
82 Paul Evans <leonerd@leonerd.org.uk>
83perl v5.38.0 2023-07-20 Future::Mutex(3pm)