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 AT 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~Xrn 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 AT 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, 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 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 v5.16.3 2014-06-10 DBD::Gofer(3)
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