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DBD::Gofer(3pm)       User Contributed Perl Documentation      DBD::Gofer(3pm)



NAME
       DBD::Gofer - A stateless-proxy driver for communicating with a remote
       DBI

SYNOPSIS
         use DBI;

         $original_dsn = "dbi:..."; # your original DBI Data Source Name

         $dbh = DBI->connect("dbi:Gofer:transport=$transport;...;dsn=$original_dsn",
                             $user, $passwd, \%attributes);

         ... use $dbh as if it was connected to $original_dsn ...

       The "transport=$transport" part specifies the name of the module to use
       to transport the requests to the remote DBI. If $transport doesn't
       contain any double colons then it's prefixed with
       "DBD::Gofer::Transport::".

       The "dsn=$original_dsn" part must be the last element of the DSN
       because everything after "dsn=" is assumed to be the DSN that the
       remote DBI should use.

       The "..." represents attributes that influence the operation of the
       Gofer driver or transport. These are described below or in the
       documentation of the transport module being used.

DESCRIPTION
       DBD::Gofer is a DBI database driver that forwards requests to another
       DBI driver, usually in a seperate process, often on a separate machine.
       It tries to be as transparent as possible so it appears that you are
       using the remote driver directly.

       DBD::Gofer is very similar to DBD::Proxy. The major difference is that
       with DBD::Gofer no state is maintained on the remote end. That means
       every request contains all the information needed to create the
       required state. (So, for example, every request includes the DSN to
       connect to.) Each request can be sent to any available server. The
       server executes the request and returns a single response that includes
       all the data.

       This is very similar to the way http works as a stateless protocol for
       the web.  Each request from your web browser can be handled by a
       different web server process.

       Use Cases

       This may seem like pointless overhead but there are situations where
       this is a very good thing. Let's consider a specific case.

       Imagine using DBD::Gofer with an http transport. Your application calls
       connect(), prepare("select * from table where foo=?"), bind_param(),
       and execute().  At this point DBD::Gofer builds a request containing
       all the information about the method calls. It then uses the httpd
       transport to send that request to an apache web server.

       This 'dbi execute' web server executes the request (using
       DBI::Gofer::Execute and related modules) and builds a response that
       contains all the rows of data, if the statement returned any, along
       with all the attributes that describe the results, such as
       $sth->{NAME}. This response is sent back to DBD::Gofer which unpacks it
       and presents it to the application as if it had executed the statement
       itself.

       Advantages

       Okay, but you still don't see the point? Well let's consider what we've
       gained:

       Connection Pooling and Throttling

       The 'dbi execute' web server leverages all the functionality of web
       infrastructure in terms of load balancing, high-availability,
       firewalls, access management, proxying, caching.

       At its most basic level you get a configurable pool of persistent
       database connections.

       Simple Scaling

       Got thousands of processes all trying to connect to the database? You
       can use DBD::Gofer to connect them to your smaller pool of 'dbi
       execute' web servers instead.

       Caching

       Client-side caching is as simple as adding ""cache=1"" to the DSN.
       This feature alone can be worth using DBD::Gofer for.

       Fewer Network Round-trips

       DBD::Gofer sends as few requests as possible (dependent on the policy
       being used).

       Thin Clients / Unsupported Platforms

       You no longer need drivers for your database on every system.
       DBD::Gofer is pure perl.

CONSTRAINTS
       There are some natural constraints imposed by the DBD::Gofer
       'stateless' approach.  But not many:

       You can't change database handle attributes after connect()

       You can't change database handle attributes after you've connected.
       Use the connect() call to specify all the attribute settings you want.

       This is because it's critical that when a request is complete the
       database handle is left in the same state it was when first connected.

       An exception is made for attributes with names starting ""private_"":
       They can be set after connect() but the change is only applied locally.

       You can't change statement handle attributes after prepare()

       You can't change statment handle attributes after prepare.

       An exception is made for attributes with names starting ""private_"":
       They can be set after prepare() but the change is only applied locally.

       You can't use transactions

       AutoCommit only. Transactions aren't supported.

       (In theory transactions could be supported when using a transport that
       maintains a connection, like "stream" does. If you're interested in
       this please get in touch via dbi-devATperl.org)

       You can't call driver-private sth methods

       But that's rarely needed anyway.

GENERAL CAVEATS
       A few important things to keep in mind when using DBD::Gofer:

       Temporary tables, locks, and other per-connection persistent state

       You shouldn't expect any per-session state to persist between requests.
       This includes locks and temporary tables.

       Because the server-side may execute your requests via a different
       database connections, you can't rely on any per-connection persistent
       state, such as temporary tables, being available from one request to
       the next.

       This is an easy trap to fall into. A good way to check for this is to
       test your code with a Gofer policy package that sets the
       "connect_method" policy to 'connect' to force a new connection for each
       request. The "pedantic" policy does this.

       Driver-private Database Handle Attributes

       Some driver-private dbh attributes may not be available if the driver
       has not implemented the private_attribute_info() method (added in DBI
       1.54).

       Driver-private Statement Handle Attributes

       Driver-private sth attributes can be set in the prepare() call. TODO

       Some driver-private dbh attributes may not be available if the driver
       has not implemented the private_attribute_info() method (added in DBI
       1.54).

       Multiple Resultsets

       Multiple resultsets are supported only if the driver supports the
       more_results() method (an exception is made for DBD::Sybase).

       Statement activity that also updates dbh attributes

       Some drivers may update one or more dbh attributes after performing
       activity on a child sth.  For example, DBD::mysql provides
       $dbh->{mysql_insertid} in addition to $sth->{mysql_insertid}. Currently
       mysql_insertid is supported via a hack but a more general mechanism is
       needed for other drivers to use.

       Methods that report an error always return undef

       With DBD::Gofer, a method that sets an error always return an undef or
       empty list.  That shouldn't be a problem in practice because the DBI
       doesn't define any methods that return meaningful values while also
       reporting an error.

       Subclassing only applies to client-side

       The RootClass and DbTypeSubclass attributes are not passed to the Gofer
       server.

CAVEATS FOR SPECIFIC METHODS
       last_insert_id

       To enable use of last_insert_id you need to indicate to DBD::Gofer that
       you'd like to use it.  You do that my adding a "go_last_insert_id_args"
       attribute to the do() or prepare() method calls. For example:

           $dbh->do($sql, { go_last_insert_id_args => [...] });

       or

           $sth = $dbh->prepare($sql, { go_last_insert_id_args => [...] });

       The array reference should contains the args that you want passed to
       the last_insert_id() method.

       execute_for_fetch

       The array methods bind_param_array() and execute_array() are supported.
       When execute_array() is called the data is serialized and executed in a
       single round-trip to the Gofer server. This makes it very fast, but
       requires enough memory to store all the serialized data.

       The execute_for_fetch() method currently isn't optimised, it uses the
       DBI fallback behaviour of executing each tuple individually.  (It could
       be implemented as a wrapper for execute_array() - patches welcome.)

TRANSPORTS
       DBD::Gofer doesn't concern itself with transporting requests and
       responses to and fro.  For that it uses special Gofer transport
       modules.

       Gofer transport modules usually come in pairs: one for the 'client'
       DBD::Gofer driver to use and one for the remote 'server' end. They have
       very similar names:

           DBD::Gofer::Transport::<foo>
           DBI::Gofer::Transport::<foo>

       Sometimes the transports on the DBD and DBI sides may have different
       names. For example DBD::Gofer::Transport::http is typically used with
       DBI::Gofer::Transport::mod_perl (DBD::Gofer::Transport::http and
       DBI::Gofer::Transport::mod_perl modules are part of the GoferTransport-
       http distribution).

       Bundled Transports

       Several transport modules are provided with DBD::Gofer:

       null

       The null transport is the simplest of them all. It doesn't actually
       transport the request anywhere.  It just serializes (freezes) the
       request into a string, then thaws it back into a data structure before
       passing it to DBI::Gofer::Execute to execute. The same freeze and thaw
       is applied to the results.

       The null transport is the best way to test if your application will
       work with Gofer.  Just set the DBI_AUTOPROXY environment variable to
       ""dbi:Gofer:transport=null;policy=pedantic"" (see "Using DBI_AUTOPROXY"
       below) and run your application, or ideally its test suite, as usual.

       It doesn't take any parameters.

       pipeone

       The pipeone transport launches a subprocess for each request. It passes
       in the request and reads the response.

       The fact that a new subprocess is started for each request ensures that
       the server side is truly stateless. While this does make the transport
       very slow, it is useful as a way to test that your application doesn't
       depend on per-connection state, such as temporary tables, persisting
       between requests.

       It's also useful both as a proof of concept and as a base class for the
       stream driver.

       stream

       The stream driver also launches a subprocess and writes requests and
       reads responses, like the pipeone transport.  In this case, however,
       the subprocess is expected to handle more that one request. (Though it
       will be automitically restarted if it exits.)

       This is the first transport that is truly useful because it can launch
       the subprocess on a remote machine using "ssh". This means you can now
       use DBD::Gofer to easily access any databases that's accessible from
       any system you can login to.  You also get all the benefits of ssh,
       including encryption and optional compression.

       See "Using DBI_AUTOPROXY" below for an example.

       Other Transports

       Implementing a Gofer transport is very simple, and more transports are
       very welcome.  Just take a look at any existing transports that are
       similar to your needs.

       http

       See the GoferTransport-http distribution on CPAN:
       http://search.cpan.org/dist/GoferTransport-http/

       Gearman

       I know Ask Bjorn Hansen has implemented a transport for the "gearman"
       distributed job system, though it's not on CPAN at the time of writing
       this.

CONNECTING
       Simply prefix your existing DSN with
       ""dbi:Gofer:transport=$transport;dsn="" where $transport is the name of
       the Gofer transport you want to use (see "TRANSPORTS").  The
       "transport" and "dsn" attributes must be specified and the "dsn"
       attributes must be last.

       Other attributes can be specified in the DSN to configure DBD::Gofer
       and/or the Gofer transport module being used. The main attributes after
       "transport", are "url" and "policy". These and other attributes are
       described below.

       Using DBI_AUTOPROXY

       The simplest way to try out DBD::Gofer is to set the DBI_AUTOPROXY
       environment variable.  In this case you don't include the "dsn=" part.
       For example:

           export DBI_AUTOPROXY="dbi:Gofer:transport=null"

       or, for a more useful example, try:

           export DBI_AUTOPROXY="dbi:Gofer:transport=stream;url=ssh:userATexample.com"

       Connection Attributes

       These attributes can be specified in the DSN. They can also be passed
       in the \%attr parameter of the DBI connect method by adding a ""go_""
       prefix to the name.

       transport

       Specifies the Gofer transport class to use. Required. See "TRANSPORTS"
       above.

       If the value does not include "::" then ""DBD::Gofer::Transport::"" is
       prefixed.

       The transport object can be accessed via $h->{go_transport}.

       dsn

       Specifies the DSN for the remote side to connect to. Required, and must
       be last.

       url

       Used to tell the transport where to connect to. The exact form of the
       value depends on the transport used.

       policy

       Specifies the policy to use. See "CONFIGURING BEHAVIOUR POLICY".

       If the value does not include "::" then ""DBD::Gofer::Policy"" is
       prefixed.

       The policy object can be accessed via $h->{go_policy}.

       timeout

       Specifies a timeout, in seconds, to use when waiting for responses from
       the server side.

       retry_limit

       Specifies the number of times a failed request will be retried. Default
       is 0.

       retry_hook

       Specifies a code reference to be called to decide if a failed request
       should be retried.  The code reference is called like this:

         $transport = $h->{go_transport};
         $retry = $transport->go_retry_hook->($request, $response, $transport);

       If it returns true then the request will be retried, upto the
       "retry_limit".  If it returns a false but defined value then the
       request will not be retried.  If it returns undef then the default
       behaviour will be used, as if "retry_hook" had not been specified.

       The default behaviour is to retry requests where
       $request->is_idempotent is true, or the error message matches "/induced
       by DBI_GOFER_RANDOM/".

       cache

       Specifies that client-side caching should be performed.  The value is
       the name of a cache class to use.

       Any class implementing get($key) and set($key, $value) methods can be
       used.  That includes a great many powerful caching classes on CPAN,
       including the Cache and Cache::Cache distributions.

       You can use ""cache=1"" is a shortcut for
       ""cache=DBI::Util::CacheMemory"".  See DBI::Util::CacheMemory for a
       description of this simple fast default cache.

       The cache object can be accessed via $h->go_cache. For example:

           $dbh->go_cache->clear; # free up memory being used by the cache

       The cache keys are the frozen (serialized) requests, and the values are
       the frozen responses.

       The default behaviour is to only use the cache for requests where
       $request->is_idempotent is true (i.e., the dbh has the ReadOnly
       attribute set or the SQL statement is obviously a SELECT without a FOR
       UPDATE clause.)

       For even more control you can use the "go_cache" attribute to pass in
       an instanciated cache object. Individual methods, including prepare(),
       can also specify alternative caches via the "go_cache" attribute. For
       example, to specify no caching for a particular query, you could use

           $sth = $dbh->prepare( $sql, { go_cache => 0 } );

       This can be used to implement different caching policies for different
       statements.

       It's interesting to note that DBD::Gofer can be used to add client-side
       caching to any (gofer compatible) application, with no code changes and
       no need for a gofer server.  Just set the DBI_AUTOPROXY environment
       variable like this:

           DBI_AUTOPROXY='dbi:Gofer:transport=null;cache=1'

CONFIGURING BEHAVIOUR POLICY
       DBD::Gofer supports a 'policy' mechanism that allows you to fine-tune
       the number of round-trips to the Gofer server.  The policies are
       grouped into classes (which may be subclassed) and referenced by the
       name of the class.

       The DBD::Gofer::Policy::Base class is the base class for all the policy
       packages and describes all the available policies.

       Three policy packages are supplied with DBD::Gofer:

       DBD::Gofer::Policy::pedantic is most 'transparent' but slowest because
       it makes more  round-trips to the Gofer server.

       DBD::Gofer::Policy::classic is a reasonable compromise - it's the
       default policy.

       DBD::Gofer::Policy::rush is fastest, but may require code changes in
       your applications.

       Generally the default "classic" policy is fine. When first testing an
       existing application with Gofer it is a good idea to start with the
       "pedantic" policy first and then switch to "classic" or a custom
       policy, for final testing.

AUTHOR
       Tim Bunce, <http://www.tim.bunce.name>;

LICENCE AND COPYRIGHT
       Copyright (c) 2007, Tim Bunce, Ireland. All rights reserved.

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

ACKNOWLEDGEMENTS
       The development of DBD::Gofer and related modules was sponsored by
       Shopzilla.com (<http://Shopzilla.com>;), where I currently work.

SEE ALSO
       DBI::Gofer::Request, DBI::Gofer::Response, DBI::Gofer::Execute.

       DBI::Gofer::Transport::Base, DBD::Gofer::Policy::Base.

       DBI

Caveats for specific drivers
       This section aims to record issues to be aware of when using Gofer with
       specific drivers.  It usually only documents issues that are not
       natural consequences of the limitations of the Gofer approach - as
       documented avove.

TODO
       This is just a random brain dump... (There's more in the source of the
       Changes file, not the pod)

       Document policy mechanism

       Add mechanism for transports to list config params and for Gofer to
       apply any that match (and warn if any left over?)

       Driver-private sth attributes - set via prepare() - change DBI spec

       add hooks into transport base class for checking & updating a result
       set cache
          ie via a standard cache interface such as:
          http://search.cpan.org/~robm/Cache-FastMmap/FastMmap.pm
          http://search.cpan.org/~bradfitz/Cache-Memcached/lib/Cache/Memcached.pm
          http://search.cpan.org/~dclinton/Cache-Cache/
          http://search.cpan.org/~cleishman/Cache/ Also caching instructions
       could be passed through the httpd transport layer in such a way that
       appropriate http cache headers are added to the results so that web
       caches (squid etc) could be used to implement the caching.  (MUST
       require the use of GET rather than POST requests.)

       Rework handling of installed_methods to not piggback on dbh_attributes?

       Perhaps support transactions for transports where it's possible (ie
       null and stream)?  Would make stream transport (ie ssh) more useful to
       more people.

       Make sth_result_attr more like dbh_attributes (using '*' etc)

       Add @val = FETCH_many(@names) to DBI in C and use in Gofer/Execute?

       Implement _new_sth in C.



perl v5.10.0                      2008-06-16                   DBD::Gofer(3pm)