DBIx::Roles - Roles for DBI handles


The module provides common API for using roles (AKA mixins/interfaces/plugins) on DBI handles. The problem it solves is that there are a lot of interesting and useful DBIx:: modules on CPAN, that extend the DBI functionality in one or another way, but mostly they insist on wrapping the connection handle themselves, so it is usually not possible to use them together. Also, once in a while, one needs a local nice-to-have hack, which is not really good enough for CPAN, but is still useful - for example, a common DBI->connect() wrapper that reads DSN from the config file. Of course, one might simply write a huge wrapper for all possible add-ons, but this approach is not really scalable. Instead, this module allows to construct your own functionality for the DB connection handle, by picking from various bells and whistles provided by other DBIx::Roles::* modules.

The package is bundled with a set of predefined role modules ( see "Predefined role modules").


There are three ways to use the module for wrapping a DBI connection handle. The best is IMO is this:

   use DBIx::Roles qw(AutoReconnect SQLAbstract);
   my $dbh = DBI-> connect($dsn, $user, $pass);

When the module is imported with a list of roles, it overrides DBI-> connect so that calls within the current package result in creation of DBIx::Roles object, which then behaves identically to the DBI handle. Calls to DBI-> connect outside the package are not affected, moreover, different packages can import DBIx::Roles with different roles.

The more generic syntax can be used to explicitly list the required roles:

   use DBIx::Roles;
   my $dbh = DBIx::Roles->new( qw(AutoReconnect SQLAbstract));
   $dbh-> connect( $dsn, $user, $pass);

or even

   use DBIx::Roles;
   my $dbh = DBIx::Roles-> connect( 
        [qw(AutoReconnect SQLAbstract)], 
        $dsn, $user, $pass

All these are equivalent, and result in construction of an object that plays roles DBIx::Roles::AutoReconnect and DBIx::Roles::SQLAbstract, plus does all DBI functionality.

An example below uses DBIx::Roles to contact a PostgreSQL DB, and then read some backend information:

   use strict;
   use DBIx::Roles qw(SQLAbstract StoredProcedures);
   # connect to a predefined DB template1
   my $d = DBI-> connect( 'dbi:Pg:dbname=template1', 'pgsql', '');
   # StoredProcedures converts pg_backend_pid() into "SELECT * FROM pg_backend_pid()"
   print "Backend PID: ", $d-> pg_backend_pid, "\n";
   # SQLAbstract declares select(), use it to read currently connected clients
   use Data::Dumper;
   my $st = $d-> select( 'pg_stat_activity', '*');
   print Dumper( $st-> fetchall_arrayref );
   # done
   $d-> disconnect;

The roles used in the example are basically syntactic sugar, but there are other roles that do alter the program behavior, if applied. For example, adding AutoReconnect to the list of the imported roles makes select() calls restartable.

Predefined role modules

All modules included in packages have their own manual pages, so only brief descriptions are provided here:

DBIx::Roles::AutoReconnect - Restarts DB call if database connection breaks. Based on idea of DBIx::AutoReconnect

DBIx::Roles::Buffered - Buffers write-only queries. Useful with lots of INSERTs and UPDATEs over slow remote connections.

DBIx::Roles::Default - not a module on its own, but a package that is always imported, and need not to be imported explicitly. Implements actual calls to DBI handle.

DBIx::Roles::Hook - Exports callbacks to override DBI calls.

DBIx::Roles::InlineArray - Flattens arrays passed as parameters to DBI calls into strings.

DBIx::Roles::RaiseError - Change defaults to RaiseError => 1

DBIx::Roles::Shared - Share DB connection handles. To be used instead of DBI-> connect_cached.

DBIx::Roles::SQLAbstract - Exports methods insert,select,update etc in the SQL::Abstract fashion. Inspired by DBIx::Abstract.

DBIx::Roles::StoredProcedures - Treats any method reached AUTOLOAD as a call to a stored procedure.

DBIx::Roles::Transaction - Allow nested transactions like DBIx::Transaction does.

Programming interfaces

The interface that faces the caller is not fixed. Depending on the functionality provided by roles, the methods can be added, deleted, or completely changed. For example, the mentioned before hack that would want to connect to a database using a DSN being read from a config file, wouldn't need the first three parameters to connect to be present, and rather would modify the connect call so that instead of

   connect( $dsn, $user, $pass, [$attr])

it might look like

   connect( [$attr])

Using this fictional module, I'll try to illustrate to how a DBI interface can be changed.

Writing a new role

To be accessible, a new role must reside in a unique module ( and usually a unique package). The DBIx::Roles prefix is not required, but is a convenience hack, and is added by default if the imported role name does not contain colons. So, if the role is to be imported as

    use DBIx::Roles qw(Config);

then it must be declared as

    package DBIx::Roles::Config;

Modifying parameters passed to DBI methods

To modify the parameters passed the role must define rewrite method to transform the parameters:

    sub rewrite
        my ( $self, $storage, $method, $parameters) = @_;
        if ( $method eq 'connect') {
             my ( $dsn, $user, $pass) = read_from_config;
             unshift @$parameters, $dsn, $user, $pass;
        return $self-> super( $method, $parameters);

The method is called before any call to DBI methods, so parameters are translated to the DBI syntax.

Overloading DBI methods

If a particular method call is needed to be overloaded, for example, ping, the package must define a method with the same name:

    sub ping 
       my ( $self, $storage, @parameters) = @_;

Since all roles are called recursively, one inside another, a role that wishes to propagate the call further down the line, must call

    return $self-> super( @parameters)

as it is finished. If, on the contrary, the role decides to intercept the call, super need not to be called. Also, in case one needs to intercept not just one but many DBI calls, it is possible to declare a method that is called when any DBI call is issued:

    sub dbi_method
       my ( $self, $storage, $method, @parameters) = @_;
       print "DBI method $method called\n";
       return $self-> super( $method, @parameters);

Note: super is important, and forgetting to call it leads to strange errors

Overloading DBI attributes

Changes to DBI attributes such as PrintError and RaiseError can be caught by STORE method:

    sub STORE
        my ( $self, $storage, $key, $val) = @_;
        print "$key is about to be set to $val, but I won't allow that\n";
        if ( rand 2) {
            $val_ref = 42; # alter
        } else {
            return;  # deny change
        return $self-> super( $key, $val);

Declaring own attributes, methods, and private storage

If a module needs its own attributes, method, or private storage, it needs to declare initialize method:

   sub initialize
       my ( $self ) = @_;
       return {
           # external attributes
           ConfigName => '/usr/local/etc/mydbi.conf',
       }, {
           # private storage
           inifile => Config::IniFile->new,
           loaded  => 0, 
       # external methods
       qw(print_config load_config);

The method is expected to return at least 2 references, first is a hash reference to the external attributes and the second is the private storage. Additional names are exported so these can be called directly.

In the example, the code that uses the role can change attributes as

    $dbh-> {ConfigName} = 'my.conf';

Changes to the attributes can be detected in STORE, as described above. Also, the exported methods can be accessed by the caller directly:

    $dbh-> print_conf;

Note that if roles with clashing attributes or method namespaces are applied to the same DBIx::Roles object, an exception is generated on the loading stage.

Finally, private storage is available as the second argument in all method calls to the role ( it is referred here as $storage ).

Overloading AUTOLOAD

If module declares any method, all calls that are caught in AUTOLOAD are dispatched to it:

   sub any
       my ( $self, $storage, $method, @parameters) = @_;
       if ( 42 == length $method) {
           return md5( @parameters);
       return $self-> super( $method, @parameters);

DBIx::Role::StoredProcedures uses this technique to call stored procedures.

Issuing DBI calls

The underlying DBI handle can be reached ( and changed ) by dbh method:

    my $dbh = $self-> dbh;
    $self-> dbh( DBI-> connect( ... ));

but calling methods on it is not always the right thing to do. Instead of a direct call, it is often preferable to call a the method so that it is re-injected through dispatch, and travels through all roles. For example

    sub my_fancy_select { shift-> selectall_arrayref( "SELECT ....") }

is better than

    sub my_fancy_select { shift-> dbh-> selectall_arrayref( "SELECT ....") }

because if gives chance to the other roles to override the call.

Also, it is also possible to reach to the external layer of the object:

    $self-> object-> selectall_arrayref(...)

but there's no guarantee that other roles won't change syntax of the call, so calls on object are not advisable.

Issuing DBI::connect

Calls to DBI->connect are allowed be made directly, but there's another level of flexibility:

    $self-> DBI_connect()

does the same thing by default, but can be overridden, and thus is preferred to the hardcoded DBI-> connect.

Dispatching calls to role methods

There are two methods that cycle through list of applied roles, and call a method, if available:

dispatch $self, $method, @parameters

Calls $method in each role namespace, returns values returned by the first role in the role chain.

dispatch_dbi_method $self, $wantarray, $method, @parameters

Same principle as dispatch, but first calls for $method, and then, for dbi_method, so that when the last role's $method calls super, the call is dispatched to the first role's dbi_method.

Restarting DBI calls

If the next role method is needed to be called indirectly, one can get a reference to the next method by calling

    ( $ref, $private_storage) = $self-> get_super;

which returns the code reference and an extra parameter for the method. If the method is to be called repeatedly, it should be noted that inside that call super can also be called repeatedly. To save and restore the call context, use read-write method context:

   my $ctx = $self-> context;
   AGAIN: eval { $ref->( $self, $private_storage, @param); }
   if ( $@) {
       $self-> context( $ctx);
       goto AGAIN;

Note: DBIx::Roles::AutoReconnect restarts DBI calls when failed, check out its source code.

Hiding the list of roles

It is possible to create a package that exports a particular set of roles, without requiring the caller to list them. Consider code for module MyDBI:

   package MyDBI;

   sub import
        local $DBIx::Roles::ExportDepth = 1;
        import DBIx::Roles qw(InlineArray Buffered StoredProcedures);

This module, if use'd, overloads the package of the caller so that calls to DBI->connect return a DBIx::Roles object with the list of roles predefined by MyDBI.

It is also possible to define local roles, without exporting these to a separate module. Hacking $DBIx::Roles::loaded_packages prevents DBIx::Role from loading modules listed there:

   package MyDBI;
   $DBIx::Roles::loaded_packages{'DBIx::Roles::My_DBI_Role'} = 1;

   sub import
        local $DBIx::Roles::ExportDepth = 1;
        import DBIx::Roles qw(My_DBI_Role InlineArray Buffered StoredProcedures);

   package DBIx::Roles::My_DBI_Role;

   sub connect { .. read from config, for example ... }

Dynamically disable and enable roles

A pair of methods, disable_roles and enable_roles accepts a list of roles and disables/enables these in an incremental fashion, so that

   $self-> disable_roles(qw(MyRole));
   $self-> disable_roles(qw(MyRole));
   $self-> enable_roles(qw(MyRole));

leaves the role disabled. The methods don't fail if there's no corresponding role(s).

Accessing the internals

DBIx::Roles defines method instance that returns the underlying object with API described above. All management of list of roles, call propagation, etc etc is possible via this reference. In particular, the underlying DB connection handle can be reached by reading $db-> instance-> dbh .


DBI-> connect_cached is not supported. Use DBIx::Roles::Shared> instead.


Dependencies - DBI, SQL::Abstract

Similar or related modules - DBIx::Abstract, DBIx::AutoReconnect, DBIx::Simple, DBIx::SQLEngine


Copyright (c) 2005 catpipe Systems ApS. All rights reserved.

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


Dmitry Karasik <>