1Promises::Cookbook::ChaUisneirngCAonndtPriipbeuPltrieondmiinPsgee(rs3l:):DCoocoukmbeonotka:t:iCohnainingAndPipelining(3)
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NAME

6       Promises::Cookbook::ChainingAndPipelining - Examples of
7       chaining/pipelining of asynchronous operations
8

VERSION

10       version 1.04
11

SYNOPSIS

13         my $cv = AnyEvent->condvar;
14
15         fetch_it(
16             'http://rest.api.example.com/-/user/search?access_level=admin'
17         )->then(
18             sub {
19                 my $admins = shift;
20                 collect(
21                     map {
22                         fetch_it( 'http://rest.api.example.com/-/user/' . url_encode( $_->{user_id} ) )
23                     } @$admins
24                 );
25             }
26         )->then(
27             sub { $cv->send( @_ ) },
28             sub { $cv->croak( 'ERROR' ) }
29         );
30
31         my @all_admins = $cv->recv;
32

DESCRIPTION

34       So one of the real benefits of the Promise pattern is how it allows you
35       to write code that flows and reads more like synchronous code by using
36       the chaining nature of Promises. In example above we are first fetching
37       a list of users whose access level is 'admin', in our fictional web-
38       service we get back a list of JSON objects with only minimal
39       information, just a user_id and full_name for instance.  From here we
40       can then loop through the results and fetch the full user object for
41       each one of these users, passing all of the promises returned by
42       "fetch_it" into "collect", which itself returns a promise.
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44       So despite being completely asynchronous, this code reads much like a
45       blocking synchronous version would read, from top to bottom.
46
47         my @all_admins;
48         try {
49             my $admins = fetch_it( 'http://rest.api.example.com/-/user/search?access_level=admin' );
50             @all_admins = map {
51                 fetch_it( 'http://rest.api.example.com/-/user/' . url_encode( $_->{user_id} ) )
52             } @$admins;
53         } catch {
54             die $_;
55         };
56         # do something with @all_admins ...
57
58       The only difference really are the "then" wrappers and the way in which
59       we handle errors, but even that is very similar since we are not
60       including an error callback in the first "then" and allowing the errors
61       to bubble till the final "then", which maps very closely to the
62       "try/catch" block. And of course the Promise version runs
63       asynchronously and reaps all the benefits that brings.
64

AUTHOR

66       Stevan Little <stevan.little@iinteractive.com>
67

69       This software is copyright (c) 2020, 2019, 2017, 2014, 2012 by Infinity
70       Interactive, Inc.
71
72       This is free software; you can redistribute it and/or modify it under
73       the same terms as the Perl 5 programming language system itself.
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77perl v5.36.0                      2P0r2o3m-i0s1e-s2:0:Cookbook::ChainingAndPipelining(3)
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