David E. Wheeler > DBIx-Connector-0.53 > DBIx::Connector

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Name ^

DBIx::Connector - Fast, safe DBI connection and transaction management

Synopsis ^

  use DBIx::Connector;

  # Create a connection.
  my $conn = DBIx::Connector->new($dsn, $username, $password, {
      RaiseError => 1,
      AutoCommit => 1,
  });

  # Get the database handle and do something with it.
  my $dbh  = $conn->dbh;
  $dbh->do('INSERT INTO foo (name) VALUES (?)', undef, 'Fred' );

  # Do something with the handle more efficiently.
  $conn->run(fixup => sub {
      $_->do('INSERT INTO foo (name) VALUES (?)', undef, 'Fred' );
  });

Description ^

DBIx::Connector provides a simple interface for fast and safe DBI connection and transaction management. Connecting to a database can be expensive; you don't want your application to re-connect every time you need to run a query. The efficient thing to do is to hang on to a database handle to maintain a connection to the database in order to minimize that overhead. DBIx::Connector lets you do that without having to worry about dropped or corrupted connections.

You might be familiar with Apache::DBI and with the DBI's connect_cached() constructor. DBIx::Connector serves a similar need, but does a much better job. How is it different? I'm glad you asked!

DBIx::Connector's other feature is transaction management. Borrowing an interface from DBIx::Class, DBIx::Connector offers an API that efficiently handles the scoping of database transactions so that you needn't worry about managing the transaction yourself. Even better, it offers an API for savepoints if your database supports them. Within a transaction, you can scope savepoints to behave like subtransactions, so that you can save some of your work in a transaction even if part of it fails. See txn() and svp() for the goods.

Usage ^

Unlike Apache::DBI and connect_cached(), DBIx::Connector doesn't cache database handles. Rather, for a given connection, it makes sure that the connection is just there whenever you want it, to the extent possible. The upshot is that it's safe to create a connection and then keep it around for as long as you need it, like so:

  my $conn = DBIx::Connector->new(@args);

You can store the connection somewhere in your app where you can easily access it, and for as long as it remains in scope, it will try its hardest to maintain a database connection. Even across forks (especially with DBI 1.614 and higher) and new threads, and even calls to $conn->dbh->disconnect. When you don't need it anymore, let it go out of scope and the database connection will be closed.

The upshot is that your code is responsible for hanging onto a connection for as long as it needs it. There is no magical connection caching like in Apache::DBI and connect_cached().

Execution Methods

The real utility of DBIx::Connector comes from the use of the execution methods, run(), txn(), or svp(). Instead of this:

  $conn->dbh->do($query);

Try this:

  $conn->run(sub { $_->do($query) }); # returns retval from the sub {...}

The difference is that the run() optimistically assumes that an existing database handle is connected and executes the code reference without pinging the database. The vast majority of the time, the connection will of course still be open. You therefore save the overhead of a ping query every time you use run() (or txn()).

Of course, if a block passed to run() dies because the DBI isn't actually connected to the database you'd need to catch that failure and try again. DBIx::Connector provides a way to overcome this issue: connection modes.

Connection Modes

When calling run(), txn(), or svp(), each executes within the context of a "connection mode." The supported modes are:

Use them via an optional first argument, like so:

  $conn->run(ping => sub { $_->do($query) });

Or set up a default mode via the mode() accessor:

  $conn->mode('fixup');
  $conn->run(sub { $_->do($query) });

The return value of the block will be returned from the method call in scalar or array context as appropriate, and the block can use wantarray to determine the context. Returning the value makes them handy for things like constructing a statement handle:

  my $sth = $conn->run(fixup => sub {
      my $sth = $_->prepare('SELECT isbn, title, rating FROM books');
      $sth->execute;
      $sth;
  });

In ping mode, run() will ping the database before running the block. This is similar to what Apache::DBI and the DBI's connect_cached() method do to check the database connection, and is the safest way to do so. If the ping fails, DBIx::Connector will attempt to reconnect to the database before executing the block. However, ping mode does impose the overhead of the ping ever time you use it.

In fixup mode, DBIx::Connector executes the block without pinging the database. But in the event the block throws an exception, if DBIx::Connector finds that the database handle is no longer connected, it will reconnect to the database and re-execute the block. Therefore, the code reference should have no side-effects outside of the database, as double-execution in the event of a stale database connection could break something:

  my $count;
  $conn->run(fixup => sub { $count++ });
  say $count; # may be 1 or 2

fixup is the most efficient connection mode. If you're confident that the block will have no deleterious side-effects if run twice, this is the best option to choose. If you decide that your block is likely to have too many side-effects to execute more than once, you can simply switch to ping mode.

The default is no_ping, but you likely won't ever use it directly, and isn't recommended in any event.

Simple, huh? Better still, go for the transaction management in txn() and the savepoint management in svp(). You won't be sorry, I promise.

Rollback Exceptions

In the event of a rollback in txn() or svp(), if the rollback itself fails, a DBIx::Connector::TxnRollbackError or DBIx::Connector::SvpRollbackError exception will be thrown, as appropriate. These classes, which inherit from DBIx::Connector::RollbackError, stringify to display both the rollback error and the transaction or savepoint error that led to the rollback, something like this:

    Transaction aborted: No such table "foo" at foo.pl line 206.
    Transaction rollback failed: Invalid transaction ID at foo.pl line 203.

For finer-grained exception handling, you can access the individual errors via accessors:

error

The transaction or savepoint error.

rollback_error

The rollback error.

For example:

  use Try::Tiny;
  $conn->txn(sub {
      try {
          # ...
      } catch {
          if (eval { $_->isa('DBIx::Connector::RollbackError') }) {
              say STDERR 'Transaction aborted: ', $_->error;
              say STDERR 'Rollback failed too: ', $_->rollback_error;
          } else {
              warn "Caught exception: $_";
          }
      };
  });

If a svp() rollback fails and its surrounding txn() rollback also fails, the thrown DBIx::Connetor::TxnRollbackError exception object will have the the savepoint rollback exception, which will be an DBIx::Connetor::SvpRollbackError exception object in its error attribute:

  use Try::Tiny;
  $conn->txn(sub {
      try {
          $conn->svp(sub { # ... });
      } catch {
          if (eval { $_->isa('DBIx::Connector::RollbackError') }) {
              if (eval { $_->error->isa('DBIx::Connector::SvpRollbackError') }) {
                  say STDERR 'Savepoint aborted: ', $_->error->error;
                  say STDERR 'Its rollback failed too: ', $_->error->rollback_error;
              } else {
                  say STDERR 'Transaction aborted: ', $_->error;
              }
              say STDERR 'Transaction rollback failed too: ', $_->rollback_error;
          } else {
              warn "Caught exception: $_";
          }
      };
  });

But most of the time, you should be fine with the stringified form of the exception, which will look something like this:

    Transaction aborted: Savepoint aborted: No such table "bar" at foo.pl line 190.
    Savepoint rollback failed: Invalid savepoint name at foo.pl line 161.
    Transaction rollback failed: Invalid transaction identifier at fool.pl line 184.

This allows you to see you original SQL error, as well as the errors for the savepoint rollback and transaction rollback failures.

Interface ^

And now for the nitty-gritty.

Constructor

new

  my $conn = DBIx::Connector->new($dsn, $username, $password, {
      RaiseError => 1,
      AutoCommit => 1,
  });

Constructs and returns a DBIx::Connector object. The supported arguments are exactly the same as those supported by the DBI. Default values for those parameters vary from the DBI as follows:

RaiseError

Defaults to true if unspecified, and if HandleError is unspecified. Use of the RaiseError attribute, or a HandleError attribute that always throws exceptions (such as that provided by Exception::Class::DBI), is required for the exception-handling functionality of run(), txn(), and svp() to work properly. Their explicit use is therefor recommended if for proper error handling with these execution methods.

AutoInactiveDestroy

Added in DBI 1.613. Defaults to true if unspecified. This is important for safe disconnects across forking processes.

In addition, explicitly setting AutoCommit to true is strongly recommended if you plan to use txn() or svp(), as otherwise you won't get the transactional scoping behavior of those two methods.

If you would like to execute custom logic each time a new connection to the database is made you can pass a sub as the connected key to the Callbacks parameter. See "Callbacks" in DBI for usage and other available callbacks.

Other attributes may be modified by individual drivers. See the documentation for the drivers for details:

DBIx::Connector::Driver::MSSQL
DBIx::Connector::Driver::Oracle
DBIx::Connector::Driver::Pg
DBIx::Connector::Driver::SQLite
DBIx::Connector::Driver::mysql

Class Method

connect

  my $dbh = DBIx::Connector->connect($dsn, $username, $password, \%attr);

Syntactic sugar for:

  my $dbh = DBIx::Connector->new(@args)->dbh;

Though there's probably not much point in that, as you'll generally want to hold on to the DBIx::Connector object. Otherwise you'd just use the DBI, no?

Instance Methods

dbh

  my $dbh = $conn->dbh;

Returns the connection's database handle. It will use a an existing handle if there is one, if the process has not been forked or a new thread spawned, and if the database is pingable. Otherwise, it will instantiate, cache, and return a new handle.

When called from blocks passed to run(), txn(), and svp(), dbh() assumes that the pingability of the database is handled by those methods and skips the ping(). Otherwise, it performs all the same validity checks. The upshot is that it's safe to call dbh() inside those blocks without the overhead of multiple pings. Indeed, it's preferable to do so if you're doing lots of non-database processing in those blocks.

run

  $conn->run(ping => sub { $_->do($query) });

Simply executes the block, setting $_ to and passing in the database handle. Returns the value returned by the block in scalar or array context as appropriate (and the block can use wantarray to decide what to do).

An optional first argument sets the connection mode, overriding that set in the mode() accessor, and may be one of ping, fixup, or no_ping (the default). See "Connection Modes" for further explication.

For convenience, you can nest calls to run() (or txn() or svp()), although the connection mode will be invoked to check the connection (or not) only in the outer-most block method call.

  $conn->txn(fixup => sub {
      my $dbh = shift;
      $dbh->do($_) for @queries;
      $conn->run(sub {
          $_->do($expensive_query);
          $conn->txn(sub {
              $_->do($another_expensive_query);
          });
      });
  });

All code executed inside the top-level call to txn() will be executed in a single transaction. If you'd like subtransactions, nest svp() calls.

It's preferable to use dbh() to fetch the database handle from within the block if your code is doing lots of non-database stuff (shame on you!):

  $conn->run(ping => sub {
      parse_gigabytes_of_xml(); # Get this out of the transaction!
      $conn->dbh->do($query);
  });

This is because dbh() will better ensure that the database handle is active and fork- and thread-safe, although it will never ping() the database when called from inside a run(), txn() or svp() block.

txn

  my $sth = $conn->txn(fixup => sub { $_->do($query) });

Starts a transaction, executes the block, setting $_ to and passing in the database handle, and commits the transaction. If the block throws an exception, the transaction will be rolled back and the exception re-thrown. Returns the value returned by the block in scalar or array context as appropriate (and the block can use wantarray to decide what to do).

An optional first argument sets the connection mode, overriding that set in the mode() accessor, and may be one of ping, fixup, or no_ping (the default). In the case of fixup mode, this means that the transaction block will be re-executed for a new connection if the database handle is no longer connected. In such a case, a second exception from the code block will cause the transaction to be rolled back and the exception re-thrown. See "Connection Modes" for further explication.

As with run(), calls to txn() can be nested, although the connection mode will be invoked to check the connection (or not) only in the outer-most block method call. It's preferable to use dbh() to fetch the database handle from within the block if your code is doing lots of non-database processing.

svp

Executes a code block within the scope of a database savepoint if your database supports them. Returns the value returned by the block in scalar or array context as appropriate (and the block can use wantarray to decide what to do).

You can think of savepoints as a kind of subtransaction. What this means is that you can nest your savepoints and recover from failures deeper in the nest without throwing out all changes higher up in the nest. For example:

  $conn->txn(fixup => sub {
      my $dbh = shift;
      $dbh->do('INSERT INTO table1 VALUES (1)');
      eval {
          $conn->svp(sub {
              shift->do('INSERT INTO table1 VALUES (2)');
              die 'OMGWTF?';
          });
      };
      warn "Savepoint failed\n" if $@;
      $dbh->do('INSERT INTO table1 VALUES (3)');
  });

This transaction will insert the values 1 and 3, but not 2.

  $conn->svp(fixup => sub {
      my $dbh = shift;
      $dbh->do('INSERT INTO table1 VALUES (4)');
      $conn->svp(sub {
          shift->do('INSERT INTO table1 VALUES (5)');
      });
  });

This transaction will insert both 4 and 5.

Superficially, svp() resembles run() and txn(), including its support for the optional connection mode argument, but in fact savepoints can only be used within the scope of a transaction. Thus svp() will start a transaction for you if it's called without a transaction in-progress. It simply redispatches to txn() with the appropriate connection mode. Thus, this call from outside of a transaction:

  $conn->svp(ping => sub {
      $conn->svp( sub { ... } );
  });

Is equivalent to:

  $conn->txn(ping => sub {
      $conn->svp( sub { ... } );
  })

Savepoints are supported by the following RDBMSs:

For all other RDBMSs, svp() works just like txn(): savepoints will be ignored and the outer-most transaction will be the only transaction. This tends to degrade well for non-savepoint-supporting databases, doing the right thing in most cases.

mode

  my $mode = $conn->mode;
  $conn->mode('fixup');
  $conn->txn(sub { ... }); # uses fixup mode.
  $conn->mode($mode);

Gets and sets the connection mode attribute, which is used by run(), txn(), and svp() if no mode is passed to them. Defaults to "no_ping". Note that inside a block passed to run(), txn(), or svp(), the mode attribute will be set to the optional first parameter:

  $conn->mode('ping');
  $conn->txn(fixup => sub {
      say $conn->mode; # Outputs "fixup"
  });
  say $conn->mode; # Outputs "ping"

In this way, you can reliably tell in what mode the code block is executing.

connected

  if ( $conn->connected ) {
      $conn->dbh->do($query);
  }

Returns true if currently connected to the database and false if it's not. You probably won't need to bother with this method; DBIx::Connector uses it internally to determine whether or not to create a new connection to the database before returning a handle from dbh().

in_txn

  if ( $conn->in_txn ) {
     say 'Transacting!';
  }

Returns true if the connection is in a transaction. For example, inside a txn() block it would return true. It will also work if you use the DBI API to manage transactions (i.e., begin_work() or AutoCommit.

Essentially, this is just sugar for:

  $con->run( no_ping => sub { !$_->{AutoCommit} } );

But without the overhead of the code reference or connection checking.

disconnect_on_destroy

  $conn->disconnect_on_destroy(0);

By default, DBIx::Connector calls $dbh->disconnect when it goes out of scope and is garbage-collected by the system (that is, in its DESTROY() method). Usually this is what you want, but in some cases it might not be. For example, you might have a module that uses DBIx::Connector internally, but then makes the database handle available to callers, even after the DBIx::Connector object goes out of scope. In such a case, you don't want the database handle to be disconnected when the DBIx::Connector goes out of scope. So pass a false value to disconnect_on_destroy to prevent the disconnect. An example:

  sub database_handle {
       my $conn = DBIx::Connector->new(@_);
       $conn->run(sub {
           # Do stuff here.
       });
       $conn->disconnect_on_destroy(0);
       return $conn->dbh;
  }

Of course, if you don't need to do any work with the database handle before returning it to your caller, you can just use connect():

  sub database_handle {
      DBIx::Connector->connect(@_);
  }

disconnect

  $conn->disconnect;

Disconnects from the database. Unless disconnect_on_destroy() has been passed a false value, DBIx::Connector uses this method internally in its DESTROY method to make sure that things are kept tidy.

driver

  $conn->driver->begin_work( $conn->dbh );

In order to support all database features in a database-neutral way, DBIx::Connector provides a number of different database drivers, subclasses of DBIx::Connector::Driver, that offer methods to handle database communications. Although the DBI provides a standard interface, for better or for worse, not all of the drivers implement them, and some have bugs. To avoid those issues, all database communications are handled by these driver objects.

This can be useful if you want more fine-grained control of your transactionality. For example, to create your own savepoint within a transaction, you might do something like this:

  use Try::Tiny;
  my $driver = $conn->driver;
  $conn->txn(sub {
      my $dbh = shift;
      try {
          $driver->savepoint($dbh, 'mysavepoint');
          # do stuff ...
          $driver->release('mysavepoint');
      } catch {
          $driver->rollback_to($dbh, 'mysavepoint');
      };
  });

Most often you should be able to get what you need out of txn() and svp(), but sometimes you just need the finer control. In those cases, take advantage of the driver object to keep your use of the API universal across database back-ends.

See Also ^

Support ^

This module is managed in an open GitHub repository. Feel free to fork and contribute, or to clone git://github.com/theory/dbix-connector.git and send patches!

Found a bug? Please post or email a report!

Authors ^

This module was written and is maintained by:

It is based on documentation, ideas, kibbitzing, and code from:

Copyright and License ^

Copyright (c) 2009-2010 David E. Wheeler. Some Rights Reserved.

This module is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

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