Provided by: libpoe-component-dbiagent-perl_0.26-3.1_all 
NAME
POE::Component::DBIAgent - POE Component for running asynchronous DBI calls.
SYNOPSIS
sub _start {
my ($self, $kernel, $heap) = @_[OBJECT, KERNEL, HEAP];
$heap->{helper} = POE::Component::DBIAgent->new( DSN => [$dsn,
$username,
$password
],
Queries => $self->make_queries,
Count => 3,
Debug => 1,
);
# Queries takes a hashref of the form:
# { query_name => 'select blah from table where x = ?',
# other_query => 'select blah_blah from big_view',
# etc.
# }
$heap->{helper}->query(query_name =>
{ cookie => 'starting_query' },
session => 'get_row_from_dbiagent');
}
sub get_row_from_dbiagent {
my ($kernel, $self, $heap, $row, $cookie) = @_[KERNEL, OBJECT, HEAP, ARG0, ARG1];
if ($row ne 'EOF') {
# {{{ PROCESS A ROW
#row is a listref of columns
# }}} PROCESS A ROW
} else {
# {{{ NO MORE ROWS
#cleanup code here
# }}} NO MORE ROWS
}
}
DESCRIPTION
DBIAgent is your answer to non-blocking DBI in POE.
It fires off a configurable number child processes (defaults to 3) and feeds database queries to it via
two-way pipe (or sockets ... however POE::Component::Wheel::Run is able to manage it). The primary
method is "query".
Usage
After initializing a DBIAgent and storing it in a session's heap, one executes a "query" (or
"query_slow") with the query name, destination session (name or id) and destination state (as well as any
query parameters, optionally) as arguments. As each row of data comes back from the query, the
destination state (in the destination session) is invoked with that row of data in its $_[ARG0] slot.
When there are no more rows to return, the data in $_[ARG0] is the string 'EOF'.
Not EVERY query should run through the DBIAgent. If you need to run a short lookup from within a state,
sometimes it can be a hassle to have to define a whole separate state to receive its value, and resume
processing from there.. The determining factor, of course, is how long your query will take to execute.
If you are trying to retrieve one row from a properly indexed table, use "$dbh->selectrow_array()". If
there's a join involved, or multiple rows, or a view, you probably want to use DBIAgent. If it's a
longish query and startup costs (time) don't matter to you, go ahead and do it inline.. but remember the
whole of your program suspends waiting for the result. If startup costs DO matter, use DBIAgent.
Return Values
The destination state in the destination session (specified in the call to "query()") will receive the
return values from the query in its $_[ARG0] parameter. DBIAgent invokes DBI's "fetch" method
internally, so the value will be a reference to an array. If your query returns multiple rows, then your
state will be invoked multiple times, once per row. ADDITIONALLY, your state will be called one time
with $_[ARG0] containing the string 'EOF'. 'EOF' is returned even if the query doesn't return any other
rows. This is also what to expect for DML (INSERT, UPDATE, DELETE) queries. A way to utilise this might
be as follows:
sub some_state {
#...
if ($enough_values_to_begin_updating) {
$heap->{dbiagent}->query(update_values_query =>
this_session =>
update_next_value =>
shift @{$heap->{values_to_be_updated}}
);
}
}
sub update_next_value {
my ($self, $heap) = @_[OBJECT, HEAP];
# we got 'EOF' in ARG0 here but we don't care... we know that an
# update has been executed.
for (1..3) { # Do three at a time!
my $value;
last unless defined ($value = shift @{$heap->{values_to_be_updated}});
$heap->{dbiagent}->query(update_values =>
this_session =>
update_next_value =>
$value
);
}
}
new()
Creating an instance creates a POE::Session to manage communication with the Helper processes. Queue
management is transparent and automatic. The constructor is named "new()" (surprised, eh? Yeah, me
too). The parameters are as follows:
DSN An arrayref of parameters to pass to DBI->connect (usually a dsn, username, and password).
Queries
A hashref of the form Query_Name => "$SQL". For example:
{
sysdate => "select sysdate from dual",
employee_record => "select * from emp where id = ?",
increase_inventory => "update inventory
set count = count + ?
where item_id = ?",
}
As the example indicates, DBI placeholders are supported, as are DML statements.
Count
The number of helper processes to spawn. Defaults to 3. The optimal value for this parameter will
depend on several factors, such as: how many different queries your program will be running, how much
RAM you have, how often you run queries, and most importantly, how many queries you intend to run
simultaneously.
ErrorState
An listref containing a session and event name to receive error messages from the DBI. The message
arrives in ARG0.
query($query_name, [ \%args, ] $session, $state, [ @parameters ])
The "query()" method takes at least three parameters, plus any bind values for the specific query you are
executing.
$query_name
This parameter must be one of the keys to the Queries hashref you passed to the constructor. It is
used to indicate which query you wish to execute.
\%args
This is an OPTIONAL hashref of arguments to pass to the query.
Currently supported arguments:
hash
Return rows hash references instead of array references.
cookie
A cookie to pass to this query. This is passed back unchanged to the destination state in
$_[ARG1]. Can be any scalar (including references, and even POE postbacks, so be careful!). You
can use this as an identifier if you have one destination state handling multiple different
queries or sessions.
delay
Insert a 1ms delay between each row of output.
I know what you're thinking: "WHY would you want to slow down query responses?!?!?" It has to do
with CONCURRENCY. When a response (finally) comes in from the agent after running the query, it
floods the input channel with response data. This has the effect of monopolizing POE's
attention, so that any other handles (network sockets, pipes, file descriptors) keep getting
pushed further back on the queue, and to all other processes EXCEPT the agent, your POE program
looks hung for the amount of time it takes to process all of the incoming query data.
So, we insert 1ms of time via Time::HiRes's "usleep" function. In human terms, this is
essentially negligible. But it is just enough time to allow competing handles (sockets, files)
to trigger "select()", and get handled by the POE::Kernel, in situations where concurrency has
priority over transfer rate.
Naturally, the Time::HiRes module is required for this functionality. If Time::HiRes is not
installed, the delay is ignored.
group
Sends the return event back when "group" rows are retrieved from the database, to avoid event
spam when selecting lots of rows. NB: using group means that $row will be an arrayref of rows,
not just a single row.
$session, $state
These parameters indicate the POE state that is to receive the data returned from the database. The
state indicated will receive the data in its $_[ARG0] parameter. PLEASE make sure this is a valid
state, otherwise you will spend a LOT of time banging your head against the wall wondering where your
query data is.
@parameters
These are any parameters your query requires. WARNING: You must supply exactly as many parameters as
your query has placeholders! This means that if your query has NO placeholders, then you should pass
NO extra parameters to "query".
Suggestions to improve this syntax are welcome.
finish()
The "finish()" method tells DBIAgent that the program is finished sending queries. DBIAgent will shut
its helpers down gracefully after they complete any pending queries. If there are no pending queries,
the DBIAgent will shut down immediately.
NOTES
• Error handling is practically non-existent.
• The calling syntax is still pretty weak... but improving. We may eventually add an optional
attributes hash so that each query can be called with its own individual characteristics.
• I might eventually want to support returning hashrefs, if there is any demand.
• Every query is prepared at Helper startup. This could potentially be pretty expensive. Perhaps a
cached or deferred loading might be better? This is considering that not every helper is going to
run every query, especially if you have a lot of miscellaneous queries.
Suggestions welcome! Diffs more welcome! :-)
AUTHOR
This module has been fine-tuned and packaged by Rob Bloodgood <robb@empire2.com>. However, most of the
queuing code originated with Fletch <fletch@phydeaux.org>, either directly or via his ideas. Thank you
for making this module a reality, Fletch!
However, I own all of the bugs.
This module is free software; you may redistribute it and/or modify it under the same terms as Perl
itself.
perl v5.32.0 2021-01-08 DBIAgent(3pm)