DBI::Roadmap - Planned Enhancements for the DBI

Tim Bunce - 12th November 2004

This document gives a high level overview of the future of the Perl DBI module.

The DBI module is the standard database interface for Perl applications. It is used worldwide in countless applications, in every kind of business, and on platforms from clustered super-computers to PDAs. Database interface drivers are available for all common databases and many not-so-common ones.

The planned enhancements cover testing, performance, high availability and load balancing, batch statements, Unicode, database portability, and more.

Addressing these issues together, in coordinated way, will help ensure maximum future functionality with minimal disruptive (incompatible) upgrades.

Broad categories of changes are outlined here along with some rationale, but implementation details and minor planned enhancements are omitted. More details can be found in: <http://svn.perl.org/modules/dbi/trunk/ToDo>

These are grouped into categories and are not listed in any particular order.

The DBI has always treated performance as a priority. Some parts of the implementation, however, remain unoptimized, especially in relation to threads.

* When the DBI is used with a Perl built with thread support enabled (such as for Apache mod_perl 2, and some common Linux distributions) it runs significantly slower. There are two reasons for this and both can be fixed but require non-trivial changes to both the DBI and drivers.

* Connection pooling in a threaded application, such as mod_perl, is difficult because DBI handles cannot be passed between threads. An alternative mechanism for passing connections between threads has been defined, and an experimental connection pool module implemented using it, but development has stalled.

* The majority of DBI handle creation code is implemented in Perl. Moving most of this to C will speed up handle creation significantly.

* The popular fetchrow_hashref() method is many times slower than fetchrow_arrayref(). It has to get the names of the columns, then create and load a new hash each time. A CW$h->{FetchHashReuse} attribute would allow the same hash to be reused each time making fetchrow_hashref() about the same speed as fetchrow_arrayref().

* Support for asynchronous (non-blocking) DBI method calls would enable applications to continue processing in parallel with database activity. This is also relevant for GUI and other event-driven applications. The DBI needs to define a standard interface for this so drivers can implement it in a portable way, where possible.

These changes would significantly enhance the performance of the DBI and many applications which use the DBI.

The DBI has a test suite. Every driver has a test suite. Each is limited in its scope. The driver test suite is testing for behavior that the driver author thinks the DBI specifies, but may be subtly incorrect. These test suites are poorly maintained because the return on investment for a single driver is too low to provide sufficient incentive.

A common test suite that can be reused by all the drivers is needed. It would:

* Improve the quality of the DBI and drivers.

* Ensure all drivers conform to the DBI specification. Easing the porting of applications between databases, and the implementation of database independent modules layered over the DBI.

* Improve the DBI specification by clarifying unclear issues in order to implement test cases.

* Encourage expansion of the test suite as driver authors and others will be motivated by the greater benefits of their contributions.

* Detect and record optional functionality that a driver has not yet implemented.

* Improve the testing of DBI subclassing, DBI::PurePerl and the various transparent drivers, such as DBD::Proxy and DBD::Multiplex, by automatically running the test suite through them.

These changes would improve the quality of all applications using the DBI.

* The DBD::Multiplex driver provides a framework to enable a wide range of dynamic functionality, including support for high-availability, failover, load-balancing, caching, and access to distributed data. It is currently being enhanced but development has stalled.

* The DBD::Proxy module is complex and relatively inefficient because it's trying to be a complete proxy for most DBI method calls. For many applications a simpler proxy architecture that operates with a single round-trip to the server would be simpler, faster, and more flexible.

New proxy client and server classes are needed, which could be subclassed to support specific client to server transport mechanisms (such as HTTP and Spread::Queue). Apart from the efficiency gains, this would also enable the use of a load-balanced pool of stateless servers for greater scalability and reliability.

* The DBI currently offers no support for distributed transactions. The most useful elements of the standard XA distributed transaction interface standard could be included in the DBI specification. Drivers for databases which support distributed transactions could then be extended to support it.

These changes would enable new kinds of DBI applications for critical environments.

Use of Unicode with the DBI is growing rapidly. The DBI should do more to help drivers support Unicode and help applications work with drivers that don't yet support Unicode directly.

* Define expected behavior for fetching data and binding parameters.

* Provide interfaces to support Unicode issues for XS and pure Perl drivers and applications.

* Provide functions for applications to help diagnose inconsistencies between byte string contents and setting of the SvUTF8 flag.

These changes would smooth the transition to Unicode for many applications and drivers.

Batch statements are a sequence of SQL statements, or a stored procedure containing a sequence of SQL statements, which can be executed as a whole.

Currently the DBI has no standard interface for dealing with multiple results from batch statements. After considerable discussion, an interface design has been agreed upon with driver authors, but has not yet been implemented.

These changes would enable greater application portability between databases, and greater performance for databases that directly support batch statements.

* The methods of the DBI API are installed dynamically when the DBI is loaded. The data structure used to define the methods and their dispatch behavior should be made part of the DBI API. This would enable more flexible and correct behavior by modules subclassing the DBI and by dynamic drivers such as DBD::Proxy and DBD::Multiplex.

* Handle attribute information should also be made available, for the same reasons.

* Currently is it not possible to discover all the child statement handles that belong to a database handle (or all database handles that belong to a driver handle). This makes certain tasks more difficult, especially some debugging scenarios. A cache of weak references to child handles would solve the problem without creating reference loops.

* It is often useful to know which handle attributes have been changed since the handle was created (e.g., in mod_perl where a handle needs to be reset or cloned). This will become more important as developers start exploring use of the newly added CW$h1->swap_inner_handle($h2) method.

These changes would simplify and improve the stability of many advanced uses of the DBI.

The DBI can be extended in three main dimensions: subclassing the DBI, subclassing a driver, and callback hooks. Each has different pros and cons, each is applicable in different situations, and all need enhancing.

* Subclassing the DBI is functional but not well defined and some key elements are incomplete, particularly the DbTypeSubclass mechanism (that automatically subclasses to a class tree according to the type of database being used). It also needs more thorough testing.

* Subclassing a driver is undocumented, poorly tested and very probably incomplete. However it's a powerful way to embed certain kinds of functionality 'below' applications while avoiding some of the side-effects of subclassing the DBI (especially in relation to error handling).

* Callbacks are currently limited to error handling (the HandleError and HandleSetError attributes). Providing callback hooks for more events, such as a row being fetched, would enable utility modules, for example, to modify the behavior of a handle independent of any subclassing in use.

These changes would enable cleaner and more powerful integration between applications, layered modules, and the DBI.

* Enabling DBI trace output at a high level of detail causes a large volume of output, much of it probably unrelated to the problem being investigated. Trace output should be controlled by the new named-topic mechanism instead of just the trace level.

* Calls to XS functions (such as many DBI and driver methods) don't normally appear in the call stack. Optionally enabling that would enable more useful diagnostics to be produced.

* Integration with the Perl debugger would make it simpler to perform actions on a per-handle basis (such as breakpoint on execute, breakpoint on error).

These changes would enable more rapid application development and fault finding.

* The DBI has not yet addressed the issue of portability among SQL dialects. This is the main hurdle limiting database portability for DBI applications.

The goal is not to fully parse the SQL and rewrite it in a different dialect. That's well beyond the scope of the DBI and should be left to layered modules. A simple token rewriting mechanism for five comment styles, two quoting styles, four placeholder styles, plus the ODBC {foo ...} escape syntax, is sufficient to significantly raise the level of SQL portability.

* Another problem area is date/time formatting. Since version 1.41 the DBI has defined a way to express that dates should be fetched in SQL standard date format (YYYY-MM-DD). This is one example of the more general case where bind_col() needs to be called with particular attributes on all columns of a particular type.

A mechanism is needed whereby an application can specify default bind_col() attributes to be applied automatically for each column type. With a single step, all DATE type columns, for example, can be set to be returned in the standard format.

These changes would enable greater database portability for applications and greater functionality for layered modules.

* Clarify current intellectual property status, including a review of past contributions to ensure the DBI is unemcumbered.

* Establish a procedure for vetting future contributions for any intellectual property issues.

These changes are important for companies taking a formal approach to assessing their risks in using Open Source software.

* Reduce the work needed to create new database interface drivers.

* Definition of an interface to support scrollable cursors.

The current DBI implementation in C code is unlikely to run on Perl 6. Perl 6 will target the Parrot virtual machine and so the internal architecture will be radically different from Perl 5.

One of the goals of the Parrot project is to be a platform for many dynamic languages (including Python, PHP, Ruby, etc) and to enable those languages to reuse each others modules. A database interface for Parrot is also a database interface for any and all languages that run on Parrot.

The Perl DBI would make an excellent base for a Parrot database interface because it has more functionality, and is more mature and extensible, than the database interfaces of the other dynamic languages.

I plan to better define the API between the DBI and the drivers and use that API as the primary API for the 'raw' Parrot database interface. This project is known a Parrot DBDI (for DataBase Driver Interface). The announcement can be read in <http://groups.google.com/groups?selm=20040127225639.GF38394@dansat.data-plan.com>

The bulk of the work will be translating the DBI C and Perl base class code into Parrot PIR, or a suitable language that generates PIR. The project stalled, due to Parrot not having key functionality at the time. That has been resolved but the project has not yet restarted.

Each language targeting Parrot would implement their own small 'thin' language-specific method dispatcher (a Perl6 DBI, Python DBI, PHP DBI etc) layered over the common Parrot DBDI interface and drivers.

The major benefit of the DBDI project is that a much wider community of developers share the same database drivers. There would be more developers maintaining less code so the benefits of the Open Source model are magnified.

The first priority is to make all the infrastructure changes that impact drivers and make an alpha release available for driver authors.

As far as possible, the changes will be implemented in a way that enables driver authors use the same code base for DBI v1 and DBI v2.

The main changes required by driver authors are:

* Code changes for PERL_NO_GET_CONTEXT, plus removing PERL_POLLUTE and DBIS

* Code changes in DBI/DBD interface (new way to create handles, new callbacks etc)

* Common test suite infrastructure (driver-specific test base class)

A small set of incompatible changes that may impact some applications will also be made in v2.0. See http://svn.perl.org/modules/dbi/trunk/ToDo

Once DBI v2.0 is available, the other enhancements can be implemented incrementally on the updated foundations. Priorities for those changes have not been set.

DBI v1 will continue to be maintained on a separate branch for bug fixes and any enhancements that ease the transition to DBI v2.

See <http://dbi.perl.org/contributing> for how you can help.

If your company has benefited from the DBI, please consider if it could make a donation to The Perl Foundation DBI Development fund at <http://dbi.perl.org/donate> to secure future development.

Alternatively, if your company would benefit from a specific new DBI feature, please consider sponsoring its development through my consulting company, Data Plan Services. Work is performed rapidly on a fixed-price payment-on-delivery basis. Contact me for details.

Using such targeted financing allows you to contribute to DBI development and rapidly get something specific and directly valuable to you in return.

My company also offers annual support contracts for the DBI, which provide another way to support the DBI and get something specific in return. Contact me for details.

Thank you.