1RateLimiter(3) User Contributed Perl Documentation RateLimiter(3)
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6 Schedule::RateLimiter - prevent events from happening too quickly.
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9 use Schedule::RateLimiter;
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11 # Don't let this event happen more than 5 times in a 60 second period.
12 my $throttle = Schedule::RateLimiter->new ( iterations => 5,
13 seconds => 60 );
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15 # Cycle forever, but not too fast.
16 while ( 1 ) {
17 $throttle->event();
18 &do_something;
19 }
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22 This module provides a way to voluntarily restrict how many times a
23 given action may take place within a specified time frame. Such a tool
24 may be useful if you have written something which periodically polls
25 some public resource and want to ensure that you do not overburden that
26 resource with too many requests.
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28 Initially, one might think that solving this problem would be as simple
29 as sleeping for the number of seconds divided by the number of
30 iterations in between each event. However, that would only be correct
31 if the event took no time at all.
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33 If you know exactly how much time each event is going to take then you
34 could build an even more complicated one-liner such as this:
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36 sleep( (seconds / iterations) - single_event_time )
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38 This module is intended to address the other cases when the exact run-
39 time of each event is unknown and variable. This module will try very
40 hard to allow an event to happen as many times as possible without
41 exceeding the specified bounds.
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43 For example, suppose you want to write something that checks an
44 'incoming' directory once a minute for files and then does something
45 with those files if it finds any. If it takes you two seconds to
46 process those files, then you want to wait 58 seconds before polling
47 the directory again. If it takes 30 seconds to process those files,
48 then you only want to wait 30 seconds. And if it takes 3 minutes, then
49 you want to poll the directory again immediately as soon as you are
50 done.
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52 my $throttle = Schedule::RateLimiter->new ( seconds => 60 );
53 &poll_and_process while ( $throttle->event );
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56 " new() "
57 Creates and returns a new Schedule::RateLimiter object.
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59 The constructor takes up to three parameters:
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61 • block (default: true)
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63 This parameter accepts a true or false value to set the default
64 "block" behavior on future calls to event(). It makes it more
65 convenient to turn blocking off for an entire object at a time.
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67 • iterations (default: 1)
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69 This specifies the number of times an event may take place within
70 the given time period. This must be a positive, non-zero integer.
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72 • seconds (required)
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74 This specifies the minimum number of seconds that must transpire
75 before we will allow (iterations + 1) events to happen. A value of
76 0 disables throttling. You may specify fractional time periods.
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78 example:
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80 my $throttle = Schedule::RateLimiter->new ( iterations => 2,
81 seconds => 10 );
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83 # Event 1
84 $throttle->event();
85 # Event 2
86 $throttle->event();
87 # Event 3
88 $throttle->event();
89 # 10 seconds will have transpired since event 1 at this point.
90 # Event 4
91 $throttle->event();
92 # 10 seconds will have transpired since event 2 at this point.
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94 " event() "
95 Called to signal the beginning of an event. This method will return
96 true or false to indicate if it is ok to proceed with the event. This
97 method uses Time::HiRes to do its calculations and sleeping, so the
98 precision of this method will be the same as the precision of
99 Time::HiRes on your platform.
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101 Takes one (optional) parameter:
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103 • block (default: true)
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105 If set to a false value, this method will do a non-blocking check
106 to see if it is ok for the event to occur. If it is not ok, this
107 method will return a false value and assume that the event did not
108 take place. Otherwise, this method will return a true value and
109 assume that the event did take place.
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111 example:
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113 # Stop when the code moves too fast.
114 while ( 1 ) {
115 if ($throttle->event( block => 0 )) {
116 &do_something;
117 } else {
118 die 'I went too fast!';
119 }
120 }
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123 This module needs to keep a record of when every iteration took place,
124 so if you are allowing a large number of iterations to happen in the
125 given time period, this could potentially use a lot of memory.
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128 If you have multiple iterations that typically happen very quickly, and
129 you want to limit them in a long period of time, they will "clump"
130 together. That is, they all happen at just about the same time, and
131 then the system waits for a long period before doing the same "clump"
132 again. That's just the nature of the best-fit algorithm. Anything
133 that is done to try to separate single events with longer waits than
134 necessary will potentially create a sub-optimal situation if an event
135 in the future takes longer than expected. If you really want all of
136 your events to start at even time periods apart from each other, then
137 set the number of iterations to 1 and adjust the number of seconds
138 accordingly.
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141 Daniel J. Wright, <wright@pair.com>
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144 The POE module provides a more heavyweight solution to this problem as
145 well.
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147 perl.
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151perl v5.36.0 2022-07-22 RateLimiter(3)