1DBIAgent(3)           User Contributed Perl Documentation          DBIAgent(3)
2
3
4

NAME

6       POE::Component::DBIAgent - POE Component for running asynchronous DBI
7       calls.
8

SYNOPSIS

10        sub _start {
11           my ($self, $kernel, $heap) = @_[OBJECT, KERNEL, HEAP];
12
13           $heap->{helper} = POE::Component::DBIAgent->new( DSN => [$dsn,
14                                                      $username,
15                                                      $password
16                                                     ],
17                                              Queries => $self->make_queries,
18                                              Count => 3,
19                                              Debug => 1,
20                                            );
21
22               # Queries takes a hashref of the form:
23               # { query_name => 'select blah from table where x = ?',
24               #   other_query => 'select blah_blah from big_view',
25               #   etc.
26               # }
27
28           $heap->{helper}->query(query_name =>
29                                  { cookie => 'starting_query' },
30                                  session => 'get_row_from_dbiagent');
31
32        }
33
34        sub get_row_from_dbiagent {
35           my ($kernel, $self, $heap, $row, $cookie) = @_[KERNEL, OBJECT, HEAP, ARG0, ARG1];
36           if ($row ne 'EOF') {
37
38        # {{{ PROCESS A ROW
39
40               #row is a listref of columns
41
42        # }}} PROCESS A ROW
43
44           } else {
45
46        # {{{ NO MORE ROWS
47
48               #cleanup code here
49
50        # }}} NO MORE ROWS
51
52           }
53
54        }
55

DESCRIPTION

57       DBIAgent is your answer to non-blocking DBI in POE.
58
59       It fires off a configurable number child processes (defaults to 3) and
60       feeds database queries to it via two-way pipe (or sockets ... however
61       POE::Component::Wheel::Run is able to manage it).  The primary method
62       is "query".
63
64   Usage
65       After initializing a DBIAgent and storing it in a session's heap, one
66       executes a "query" (or "query_slow") with the query name, destination
67       session (name or id) and destination state (as well as any query
68       parameters, optionally) as arguments.  As each row of data comes back
69       from the query, the destination state (in the destination session) is
70       invoked with that row of data in its $_[ARG0] slot.  When there are no
71       more rows to return, the data in $_[ARG0] is the string 'EOF'.
72
73       Not EVERY query should run through the DBIAgent.  If you need to run a
74       short lookup from within a state, sometimes it can be a hassle to have
75       to define a whole seperate state to receive its value, and resume
76       processing from there..  The determining factor, of course, is how long
77       your query will take to execute.  If you are trying to retrieve one row
78       from a properly indexed table, use "$dbh->selectrow_array()".  If
79       there's a join involved, or multiple rows, or a view, you probably want
80       to use DBIAgent.  If it's a longish query and startup costs (time)
81       don't matter to you, go ahead and do it inline.. but remember the whole
82       of your program suspends waiting for the result.  If startup costs DO
83       matter, use DBIAgent.
84
85   Return Values
86       The destination state in the destination session (specified in the call
87       to "query()") will receive the return values from the query in its
88       $_[ARG0] parameter.  DBIAgent invokes DBI's "fetch" method internally,
89       so the value will be a reference to an array.  If your query returns
90       multiple rows, then your state will be invoked multiple times, once per
91       row.  ADDITIONALLY, your state will be called one time with $_[ARG0]
92       containing the string 'EOF'. 'EOF' is returned even if the query
93       doesn't return any other rows.  This is also what to expect for DML
94       (INSERT, UPDATE, DELETE) queries.  A way to utilise this might be as
95       follows:
96
97        sub some_state {
98            #...
99            if ($enough_values_to_begin_updating) {
100
101                $heap->{dbiagent}->query(update_values_query =>
102                                         this_session =>
103                                         update_next_value =>
104                                         shift @{$heap->{values_to_be_updated}}
105                                        );
106            }
107        }
108
109        sub update_next_value {
110            my ($self, $heap) = @_[OBJECT, HEAP];
111            # we got 'EOF' in ARG0 here but we don't care... we know that an
112            # update has been executed.
113
114            for (1..3) {               # Do three at a time!
115                my $value;
116                last unless defined ($value = shift @{$heap->{values_to_be_updated}});
117                $heap->{dbiagent}->query(update_values =>
118                                         this_session =>
119                                         update_next_value =>
120                                         $value
121                                        );
122            }
123
124        }
125
126   new()
127       Creating an instance creates a POE::Session to manage communication
128       with the Helper processes.  Queue management is transparent and
129       automatic.  The constructor is named "new()" (surprised, eh?  Yeah, me
130       too).  The parameters are as follows:
131
132       DSN An arrayref of parameters to pass to DBI->connect (usually a dsn,
133           username, and password).
134
135       Queries
136           A hashref of the form Query_Name => "$SQL".  For example:
137
138            {
139              sysdate => "select sysdate from dual",
140              employee_record => "select * from emp where id = ?",
141              increase_inventory => "update inventory
142                                     set count = count + ?
143                                     where item_id = ?",
144            }
145
146           As the example indicates, DBI placeholders are supported, as are
147           DML statements.
148
149       Count
150           The number of helper processes to spawn.  Defaults to 3.  The
151           optimal value for this parameter will depend on several factors,
152           such as: how many different queries your program will be running,
153           how much RAM you have, how often you run queries, and most
154           importantly, how many queries you intend to run simultaneously.
155
156       ErrorState
157           An listref containing a session and event name to receive error
158           messages from the DBI.  The message arrives in ARG0.
159
160   query($query_name, [ \%args, ] $session, $state, [ @parameters ])
161       The "query()" method takes at least three parameters, plus any bind
162       values for the specific query you are executing.
163
164       $query_name
165           This parameter must be one of the keys to the Queries hashref you
166           passed to the constructor.  It is used to indicate which query you
167           wish to execute.
168
169       \%args
170           This is an OPTIONAL hashref of arguments to pass to the query.
171
172           Currently supported arguments:
173
174           hash
175               Return rows hash references instead of array references.
176
177           cookie
178               A cookie to pass to this query.  This is passed back unchanged
179               to the destination state in $_[ARG1].  Can be any scalar
180               (including references, and even POE postbacks, so be careful!).
181               You can use this as an identifier if you have one destination
182               state handling multiple different queries or sessions.
183
184           delay
185               Insert a 1ms delay between each row of output.
186
187               I know what you're thinking: "WHY would you want to slow down
188               query responses?!?!?"  It has to do with CONCURRENCY.  When a
189               response (finally) comes in from the agent after running the
190               query, it floods the input channel with response data.  This
191               has the effect of monopolizing POE's attention, so that any
192               other handles (network sockets, pipes, file descriptors) keep
193               getting pushed further back on the queue, and to all other
194               processes EXCEPT the agent, your POE program looks hung for the
195               amount of time it takes to process all of the incoming query
196               data.
197
198               So, we insert 1ms of time via Time::HiRes's "usleep" function.
199               In human terms, this is essentially negligible.  But it is just
200               enough time to allow competing handles (sockets, files) to
201               trigger "select()", and get handled by the POE::Kernel, in
202               situations where concurrency has priority over transfer rate.
203
204               Naturally, the Time::HiRes module is required for this
205               functionality.  If Time::HiRes is not installed, the delay is
206               ignored.
207
208           group
209               Sends the return event back when "group" rows are retrieved
210               from the database, to avoid event spam when selecting lots of
211               rows. NB: using group means that $row will be an arrayref of
212               rows, not just a single row.
213
214       $session, $state
215           These parameters indicate the POE state that is to receive the data
216           returned from the database.  The state indicated will receive the
217           data in its $_[ARG0] parameter.  PLEASE make sure this is a valid
218           state, otherwise you will spend a LOT of time banging your head
219           against the wall wondering where your query data is.
220
221       @parameters
222           These are any parameters your query requires.  WARNING: You must
223           supply exactly as many parameters as your query has placeholders!
224           This means that if your query has NO placeholders, then you should
225           pass NO extra parameters to "query".
226
227           Suggestions to improve this syntax are welcome.
228
229   finish()
230       The "finish()" method tells DBIAgent that the program is finished
231       sending queries.  DBIAgent will shut its helpers down gracefully after
232       they complete any pending queries.  If there are no pending queries,
233       the DBIAgent will shut down immediately.
234

NOTES

236       •   Error handling is practically non-existent.
237
238       •   The calling syntax is still pretty weak... but improving.  We may
239           eventually add an optional attributes hash so that each query can
240           be called with its own individual characteristics.
241
242       •   I might eventually want to support returning hashrefs, if there is
243           any demand.
244
245       •   Every query is prepared at Helper startup.  This could potentially
246           be pretty expensive.  Perhaps a cached or deferred loading might be
247           better?  This is considering that not every helper is going to run
248           every query, especially if you have a lot of miscellaneous queries.
249
250       Suggestions welcome!  Diffs more welcome! :-)
251

AUTHOR

253       This module has been fine-tuned and packaged by Rob Bloodgood
254       <robb@empire2.com>.  However, most of the queuing code originated with
255       Fletch <fletch@phydeaux.org>, either directly or via his ideas.  Thank
256       you for making this module a reality, Fletch!
257
258       However, I own all of the bugs.
259
260       This module is free software; you may redistribute it and/or modify it
261       under the same terms as Perl itself.
262
263
264
265perl v5.32.1                      2021-01-27                       DBIAgent(3)
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