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!
- •
- Fork Safety
Like Apache::DBI, but unlike "connect_cached()", DBIx::Connector
create a new database connection if a new process has been
"fork"ed. This happens all the time under mod_perl, in POE
applications, and elsewhere. Works best with DBI 1.614 and higher.
- •
- Thread Safety
Unlike Apache::DBI or "connect_cached()", DBIx::Connector will
create a new database connection if a new thread has been spawned. As with
"fork"ing, spawning a new thread can break database
connections.
- •
- Works Anywhere
Unlike Apache::DBI, DBIx::Connector runs anywhere -- inside of mod_perl or
not. Why limit yourself?
- •
- Explicit Interface
DBIx::Connector has an explicit interface. There is none of the magical
action-at-a-distance crap that Apache::DBI is guilty of, and no global
caching. I've personally diagnosed a few issues with Apache::DBI's magic,
and killed it off in two different projects in favor of
"connect_cached()", only to be tripped up by other gotchas. No
more.
- •
- Optimistic Execution
If you use "run()" and "txn()", the database handle will
be passed without first pinging the server. For the 99% or more of the
time when the database is just there, you'll save a ton of overhead
without the ping.
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 $conn = DBIx::Connector->new(@args);
You can store the connection somewhere in your app where you can easily access
it, and for as long as it remains in scope, it will try its hardest to
maintain a database connection. Even across "fork"s (especially with
DBI 1.614 and higher) and new threads, and even calls to
"$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:
- •
- "ping"
- •
- "fixup"
- •
- "no_ping"
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 "fork"ed 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 "ping"s. 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:
- •
- PostgreSQL 8.0
- •
- SQLite 3.6.8
- •
- MySQL 5.0.3 (InnoDB)
- •
- Oracle
- •
- Microsoft SQL Server
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¶
- •
- DBIx::Connector::Driver
- •
- DBI
- •
- DBIx::Class
- •
- Catalyst::Model::DBI
Support¶
This module is managed in an open GitHub repository
<
http://github.com/theory/dbix-connector/>. Feel free to fork and
contribute, or to clone
git://github.com/theory/dbix-connector.git
<
git://github.com/theory/dbix-connector.git> and send patches!
Found a bug? Please post <
http://github.com/theory/dbix-connector/issues>
or email <mailto:bug-dbix-connector@rt.cpan.org> a report!
Authors¶
This module was written and is maintained by:
- •
- David E. Wheeler <david@kineticode.com>
It is based on documentation, ideas, kibbitzing, and code from:
- •
- Tim Bunce <http://tim.bunce.name>
- •
- Brandon L. Black <blblack@gmail.com>
- •
- Matt S. Trout <mst@shadowcat.co.uk>
- •
- Peter Rabbitson <ribasushi@cpan.org>
- •
- Ash Berlin <ash@cpan.org>
- •
- Rob Kinyon <rkinyon@cpan.org>
- •
- Cory G Watson <gphat@cpan.org>
- •
- Anders Nor Berle <berle@cpan.org>
- •
- John Siracusa <siracusa@gmail.com>
- •
- Alex Pavlovic <alex.pavlovic@taskforce-1.com>
- •
- Many other DBIx::Class contributors
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.