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

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	separate 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	statement 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-dev@perl.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 sth 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 automatically 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 BjA,rn 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:user@example.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, up to 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 instantiated 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 above.

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 piggyback 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 v5.32.0			  2020-01-26			 DBD::Gofer(3)

NAME | SYNOPSIS | DESCRIPTION | CONSTRAINTS | GENERAL CAVEATS | CAVEATS FOR SPECIFIC METHODS | TRANSPORTS | CONNECTING | CONFIGURING BEHAVIOUR POLICY | AUTHOR | LICENCE AND COPYRIGHT | ACKNOWLEDGEMENTS | SEE ALSO | Caveats for specific drivers | TODO

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