Sean Egan


POE::Component::LaDBI - POE Component that spawns a perl subprocess to handle non-blocking access to the DBI API.


 use POE::Component::LaDBI;

 POE::Component::LaDBI->create( Alias => "ladbi" );

 $k->call(ladbi => "register",
          OfflineEvent => 'db_offline');

 $k->post(ladbi => "connect",
          SuccessEvent => "connected",
          FailureEvent  => "connect_failed",
          Args => ["dbi:Pg:dbname=$dbname", $user, $passwd],
          UserData => $stuff );

 $k->post(ladbi => "disconnect",
          SuccessEvent => "disconnected",
          FailureEvent  => "disconnect_failed",
          HandleId => $dbh_id,
          UserData => $stuff);

 $k->post("ladbi" => "prepare",
          SuccessEvent => "prepared",
          FailureEvent => "prepare_failed",
          HandleId => $dbh_id,
          Args => [$sql], 
          UserData => $stuff);

 $k->post("ladbi" => "finish",
          SuccessEvent => "finished",
          FailureEvent => "finish_failed",
          HandleId => $sth_id,
          UserData => $stuff);

 $k->post("ladbi" => "execute",
          SuccessEvent => "executed",
          FailureEvent => "execute_failed",
          HandleId => $sth_id,
          Args => [$bind_val0, $bind_val1, ...],
          UserData => $stuff);

 $k->post("ladbi" => "rows",
          SuccessEvent => "rows_found",
          FailureEvent => "rows_failed",
          HandleId => $sth_id,
          UserData => $stuff);

 $k->post("ladbi" => "fetchrow",
          SuccessEvent => "row_fetched",
          FailureEvent => "fetch_failed",
          HandleId => $sth_id,
          UserData => $stuff);

 $k->post("ladbi" => "fetchrow_hash",
          SuccessEvent => "row_fetched",
          FailureEvent => "fetch_failed",
          HandleId => $sth_id,
          UserData => $stuff);

 $k->post("ladbi" => "fetchall",
          SuccessEvent => "all_fetched",
          FailureEvent => "fetchall_failed",
          HandleId => $sth_id,
          Args => [ @optional_indicies ],
          UserData => $stuff);

 $k->post("ladbi" => "fetchall_hash",
          SuccessEvent => "all_fetched",
          FailureEvent => "fetchall_failed",
          HandleId => $sth_id,
          Args => [ @optional_keys ],
          UserData => $stuff);

 $k->post("ladbi" => "ping",
         SuccessEvent => "check_ping_results",
         FailureEvent => "ping_failed",
         HandleId => $dbh_id,
         UserData => $stuff);

 $k->post("ladbi" => "do",
          SuccessEvent => "check_do_results",
          FailureEvent => "do_failed",
          HandleId => $dbh_id,
          Args => [ $sql, $attr_hashref, @bind_values ],
          UserData => $stuff);

 $k->post("ladbi" => "begin_work",
          SuccessEvent => "check_transactions_enabled",
          FailureEvent => "begin_work_failed",
          HandleId => $dbh_id,
          UserData => $stuff);

 $k->post("ladbi" => "commit",
          SuccessEvent => "check_commit",
          FailureEvent => "commit_failed",
          HandleId => $dbh_id,
          UserData => $stuff);

 $k->post("ladbi" => "rollback",
          SuccessEvent => "check_rollback",
          FailureEvent => "rollback_failed",
          HandleId => $dbh_id,
          UserData => $stuff);

 $k->post("ladbi" => "selectall",
          SuccessEvent => "check_results",
          FailureEvent => "selectall_failed",
          HandleId => $dbh_id,
          Args => [ $sql ],
          UserData => $stuff);

 $k->post("ladbi" => "selectall_hash",
          SuccessEvent => "check_results",
          FailureEvent => "selectall_failed",
          HandleId => $dbh_id,
          Args => [ $sql, $key_field ],
          UserData => $stuff);

 $k->post("ladbi" => "selectcol",
          SuccessEvent => "check_results",
          FailureEvent => "selectcol_failed",
          HandleId => $dbh_id,
          Args => [ $sql, $attr_hashref ],
          UserData => $stuff);

 $k->post("ladbi" => "selectrow",
          SuccessEvent => "check_results",
          FailureEvent => "selectrow_failed",
          HandleId => $dbh_id,
          Args => [ $sql, $attr_hashref ],
          UserData => $stuff);

 $k->post("ladbi" => "quote",
          SuccessEvent => "use_quote_results",
          FailureEvent => "quote_failed",
          HandleId => $dbh_id,
          Args => [ $value ],
          UserData => $stuff);


LaDBI Session Events


This tells the LaDBI session to shutdown. It takes two optional arguments, $cause and $errstr. Both arguments get posted back to all registered sessions and all outstanding requests.

For registered sessions the OfflineEvent is called with $cause and $errstr as ARG0 and ARG1.

For the outstanding requests FailureEvent is called with $cause as ARG1 and $errstr as ARG2.


register is a callable as well as postable event, which registers your session with the LaDBI session. All the other events you can post to a LaDBI session are request-response events. register allows you to get events posted back to your session when events occur which effect the LaDBI session as a whole.


LaDBI can loose it's sub-process (which is actually doing the DBI calls). In this case an OfflineEvent will be posted to all the client sessions which have registered with this LaDBI session.

The OfflineEvent passes two arguments back to the client session.

 sub db_offline {
   my ($cause, $errstr, $alias) = @_[ARG0,ARG1,ARG2];

The $cause is the either "SHUTDOWN" or the error string from the ErrorEvent of LaDBI's internal POE::Wheel::Run .

The $errstr is the value passed to the "shutdown" event or the ARG0 of the internal POE::Wheel::Run ErrorEvent.

The $alias is the alias of the POE::Component::LaDBI session which has lost it's subprocess and is shutting downn. This allows the registered user of the POE::Component::LaDBI session to track LaDBI sessions from start up to shutdown.

LaDBI Request Events

All request events have the same handler. This is because the handler merely creates a request message and sends it to the perl sub-process which is doing the actuall DBI calls.

The handler takes the same arguments. Not all events use the all the argument fields. The arguments fields/keys are:


UserData is a tool you, the programmer, may use to correlate LaDBI requests with LaDBI responses. Both SuccessEvent and ErrorEvent handlers will be passed the UserData originally submitted in the <$k-post()>>.

UserData must be a scalar. As a scalar it may be a reference to a hash, or array, or object, or anything your twisted mind may come up with. Therefor you may use any data in the scalar to correlate the response to the original request. Further, you may just use this as a clever way to pass data from the subroutine where the request was done to the response handler.


All POE::Component::LaDBI events require SuccessEvent.

The SuccessEvent is fired off if the DBI call returned successfully. "Returning Successfully" means that no exeption was called (as the RaiseError attribute might cause) AND that the return value from the DBI call was a defined() value.

However a SuccessEvent does not mean that the SQL completed in what you might commonly think of as a successful manner. For instance, a SELECT statement might not return anything. In that case, the returned data will be an empty array ref. Further, somecalls, while well formed, will return an error because that feature (like transactions ala $dbh->begin_work) are not implemented in your DBI driver.

The handler for the SuccessEvent is invoked the following arguments.

  sub success_event_handler {
     my ($handle_id, $datatype, $data, $userdata) = @_[ARG0..ARG3];

This is a cookie representing a DBI handle object. There are two kinds of DBI handle objects, database handles and statement handles. You use $handle_id to refer to a DBI handle object you want to call methods on.

Both, connect and prepare generate new handle ids. The SuccessEvent called by the connect handler is passed a new database handle id. The same is true for prepare but the $handle_id represents a DBI statement handler object instead.

All other POE::Component::LaDBI events just return the $handle_id that was used to invoke them. For exmple, the execute event requires a statement handle id. When the SuccessEvent for that execute is called it just returns the same statement handle id.


The value of $datatype is a string that tells you what kind of data structure is contained in $data. $data can be a return code "RC", a return value "RV", an array ref of array refs "TABLE", a hash ref of hash refs "NAMED_TABLE", an array ref representing a row "ROW", a hash ref representing a row "NAMED_ROW", an array ref representing a column "COLUMN", or a string meant to represent a part of a SQL statement "SQL" (like from $dbh-quote()>).

Here is a some rough descriptions of the format of the values of $datatype:


Data is an array ref of array refs to scalars.

  Data = [
          [row0col0, row0col1, ...],
          [row1col0, row1col1, ...],

This one is odd. See the description of selectall_hashref() in DBI. For *_hashref() calls in DBI you have to provide the database table field which will be the hash key into this hash table. The values corresponding to each key is a hash of the rows returned from the select or fetch. I did not invent this and do not quite understand why is is this way.

  Data = {
          row0colX_val => {col0_name => row0_val, col1_name => row0_val, ...},
          row1colX_val => {col0_name => row1_val, col1_name => row1_val, ...},

Data is an array ref of scalars.

  Data = [ elt0, elt1, ... ]

Data is an hash ref containing name-value pairs of each data item in the row; the name is the column name, the value is the column value.

  Data = { col0_name => elt0, col1_name => elt1, ... }
  Data = [ elt0, elt1, ... ]

Return code is a scalar valude returned from the DBI call.

  Data = $rc

Return Value is a scalar value returned from the DBI call.

  Data = $rv

This is the data type for the return value from DBI::quote() call.

  Data = $sql_string

This is scalar value or a reference to a more complex data structure (see above).

Some calls may return successfully with $data a defined value, yet $data may be a "zero-but-true" value. For example look at the DBI description of $dbh->ping.


The scalar passed into the original request which resulted in this response. It is entirely programmer defined what is held in this scalar value. Hint: you can set a hash ref as the user data, aka \%stuff_assoc_with_ladbi_call


All POE::Component::LaDBI events require FailureEvent.

When FailureEvent is invoked, it can be for several different reasons.

One common reason is that your SQL is malformed. This one gets me all the time.

Another common reason is that you did not provide the correct arguements to the POE::Component::LaDBI event. The arguements you provide in the Args field are passed literally to the DBI call. Bad arguements, or the wrong number, will cause the DBI call to throw an execption.

Another reason might be that you are using an invalid $handle_id. The $handle_id might be invalid because it is garbled, or it has be deleted due to a previous use of disconnect or finish.

Finnally, it might just be that something bad happened internally to POE::Component::LaDBI::Engine or DBI itself.

A FailureEvent is provided the following arguments.

  sub failure_event {
    my ($handle_id, $errtype, $errstr, $err, $userdata) =

The argument $handle_id is either undef, a statement handle, or a database handle depending on the type of request which was submitted and now failed.

The argument $errtype can be "SHUTDOWN", "ERROR", "EXCEPTION", "INVALID_REQUEST".

The $errtype eq "EXCEPTION" results from the fact that all the actual DBI command are wrapped in and eval {} and the $@ checked. In this case, $errstr is set to $@ and $err is undefined.

The $errtype eq "ERROR" results from the fact that the results of the DBI command is checked for undef. Then the appropriate DBI $DBI::errstr and $DBI::err are passed back as $errstr and $err respectively.

The $errtype eq "SHUTDOWN" results from abnomal termination of the POE::Component::LaDBI session. $err is set to the cause and $errstr is set to a string explanation of the cause. $errstr can be "signal" or a POE::Wheel::Run operation. And $err is set to the signal type or wheel operation string value of $!.

The $errtype eq "INVALID_REQUEST" means that POE::Component::LaDBI failed to instantiate a POE::Component::LaDBI::Request object. Hence, nothing was sent to the sub-process running POE::Component::LaDBI::Engine for execution. $errstr is set to an empty string and $err is set to undef.

The $errtype eq "INVALIE_HANDLE_ID" means that a POE::Component::LaDBI::Request object was create and the message sent to the sub-process, but the POE::Component::LaDBI::Engine object in the sub-process did not have a record of that handle id. $errstr is set to empty string and $err is set to undef.


This is either a database handle id, a statement handle id to use, or undef if a DBI handle type is not required for the LaDBI command.


This is always an array ref. The array is exact the arguemnts to pass to the appropriate DBI method. You are required to pass the correct ones, else you will recieve a FailureEvent, probably of EXCEPTION type.


  use strict;
  use warnings;

  use POE;
  use POE::Component::LaDBI;

  my $LADBI_ALIAS = "ladbi";

  my $DSN = "dbi:Pg:dbname=test";
  my $USER = "dbuser";
  my $PASSWD = "secret";

  my $SQL = "SELECT * FROM contacts";

  POE::Component::LaDBI->create(Alias => $LADBI_ALIAS)
    or die "Failed to create a POE::Component::LaDBI session\n";

    (args => [$DSN, $USER, $PASSWD, $SQL],
     inline_states =>
       _start          => sub {
         my ($dsn, $user, $passwd, $sql) = @_[ARG0..ARG3];
         print STDERR "_start: args=($dsn,$user,$passwd)\n";
         $_[HEAP]->{sql} = $sql;
         $_[KERNEL]->post($LADBI_ALIAS => "connect",
                          SuccessEvent => "selectall",
                          FailureEvent => "dberror",
                          Args => [ $dsn, $user, $passwd ]);

       _stop           => sub {
         print STDERR "_stop: client session ended.\n";

       shutdown        => sub {
         print STDERR "shutdown: sending shutdown to $LADBI_ALIAS\n";
         $_[KERNEL]->post($LADBI_ALIAS => "shutdown");

       selectall       => sub {
         my ($dbh_id, $datatype, $data) = @_[ARG0..ARG2];
         $_[HEAP]->{dbh_id} = $dbh_id;
         print STDERR "selectall: dbh_id=$dbh_id\n";
         $_[KERNEL]->post($LADBI_ALIAS => "selectall",
                          SuccessEvent => "display_results",
                          FailureEvent => "dberror",
                          HandleId     => $dbh_id,
                          Args         => [ $_[HEAP]->{sql} ] );

       display_results => sub {
         my ($dbh_id, $datatype, $data) = @_[ARG0..ARG2];
         print STDERR "display_results: dbh_id=$dbh_id\n";
         for my $row ( @$data ) {
           print join(",", @$row), "\n";
         $_[KERNEL]->post($LADBI_ALIAS => "disconnect",
                          SuccessEvent => "shutdown",
                          FailureEvent => "dberror",
                          HandleId     => $dbh_id);

       dberror         => sub {
         my ($dbh_id, $errtype, $errstr, $err) = @_[ARG0..ARG3];
         print STDERR "dberror: dbh_id  = $dbh_id\n";
         print STDERR "dberror: errtype = $errtype\n";
         print STDERR "dberror: errstr  = $errstr\n";
         print STDERR "dberror: err     = $err\n" if $errtype eq "ERROR";
      } #end: inline_states
    ) #end: POE::Session->create()
  or die "Failed to instantiate POE::Session\n";


  exit 0;


If the environment variable LADBI_DEBUG is set to a true value (perl-wise), or the ":DEBUG" symbol is in the use statement import list (eg use POE::Component::LaDBI qw(:DEBUG)), then debugging will be turned on.

When debuggind is turned on, POE::Component::LaDBI->run() will open and log messages to a file whos name is indicated in $POE::Component::LaDBI::DEBUG_FILE.

The debug log is set to "ladbi_run.log" by default.


None by default.


Sean M. Egan, <seanegan:bigfoot_com>


perl, POE, DBI, POE::Component::LaDBI::Engine, POE::Component::LaDBI::Request, POE::Component::LaDBI::Response