David Muir Sharnoff > OOPS > OOPS

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NAME ^

 OOPS - Object Oriented Persistent Store

SYNOPSIS ^

 use OOPS;

 transaction(sub {
        $oops = new OOPS 
                dbi_dsn => $DBI_DSN, 
                username => $username, 
                password => $password,
                table_prefix => "MY",
                readonly => 0;

        $oops->commit;

        $oops->{my_1st_bit_of_data} = 'a scalar';
        $oops->{my_2nd_bit_of_data} = { A => 'hash' };
        $oops->{my_3rd_bit_of_data} = [ qw(An Array) ];
        $oops->{my_4rd_bit_of_data} = \'a reference to a scalar, ref, hash, or array' ];

        my $old_value = $oops->{multiple}{level}{dereference};

        my $object = $oops->load_object($integer_object_id);

        my $dbh = $oops->dbh();

        $oops->workaround27555($reference);

        my $was_virtual = $oops->virtual_object(\%{$oops->{some}{hash}, [ $new_value ]);

 });

 my $ref = getref(%hash, 'key')

DESCRIPTION ^

The goal of OOPS is to make perl objects easily and transparently persistent. OOPS handles deeply nested and cross-linked objects -- even object hierarchies that are too large to fit in memory and (with a hint) individual hash tables that are too large for memory. Objects will be demand-loaded into memory as they are accessed. All changes to your object hierarchy will be saved with a single commit().

Full transactional consistency is the only operational mode. Either all of your changes are saved or none of them are. While your program runs, you will see a consistent view of the data: no other running transactions will change the data you see. If another transaction changes data that you are using then at least one of the transactions must abort. OOPS will die() to abort the transaction.

OOPS maps all perl objects to the same RDBMS schema. No advance schema definition is required on the part of the user of OOPS. The name of the package (OOPS) comes from the realization that perl's data model is much more complicated than I initially understood. Internally, the RDBMS schema uses four tables: a table of objects, a table of attributes (keys and values), a table of large attributes that are too big to fit in normal (VARCHAR) columns, and a table of counters.

At this time, OOPS is expecting a web-like work flow:

 create OOPS instance

 access some objects

 modify some objects

 commit

 exit

If you need more than one transaction in a program, create more than one OOPS instance.

To make your data persistent, make a reference to your data from the OOPS object. To later retrieve your data, simply access it through the OOPS object.

EXAMPLE PROGRAM ^

 use OOPS;

 transaction(sub {
        my $oops = new OOPS 
                dbi_dsn => 'DBI:mysql:database=MY-DATABASE-NAME;host=localhost', 
                username => 'MY-USERNAME', 
                password => 'MY-PASSWORD',
                table_prefix => "MY-TABLE-PREFIX";

        my $p = $oops->{pages}{"/some/path"};

        $p->{next} = $oops->{pages}{"/some/other/path"};
        $p->{jpgs} = [ read_file("x.jpg"), read_file("y.jpg") ];

        $oops->commit;
 });

 exit;

SUPPORTED DATA TYPES ^

Perl blessed and unblessed HASHes, REFs, SCALARs, and ARRAYs are supported. Currently, HASH keys may not be longer than 255 characters. Class names may not be more than 128 characters long. References to hash keys and array elements are supported.

At the current time, large ARRAYs are not efficient. Use HASHes instead if this matters to you. References to array elements and hash values are not efficient.

Large HASHes are supported by only loading keys as tehy are accessed.

HASHes, array elements, and REFs are implemented with tie(). ARRAYs are not currently tie()d because of bugs in perl. Multiple references to the same scalar are supported. References to array elements and hash values are supported. Persistent data is reference counted and cycles must be manually broken to assure de-allocation. A persistent data garbage collector OOPS::GC is supplied if you cannot guarantee to break your own cycles.

SUPPORTED PLATFORMS ^

Support for the following RDBMSs are is included:

PostgreSQL

OOPS 0.2005 has been tested with PostgreSQL version 9.1.3 on Linux. OOPS 0.2002 has been tested with PostgreSQL version 8.1.8 on Linux. OOPS 0.1004 was tested with PostgreSQL version 8.1.4 on Linux. OOPS 0.1003 was tested with 7.4.2 and 7.3.5 on Linux and 7.3.5 on pre-release DragonflyBSD. Somewhere between 7.4.2 and 8.1.4 PostgreSQL got more strict about TEXT versus BYTEA types. OOPS 1.003 was using TEXT where it should have been using BYTEA. Be sure to upgrade OOPS to 0.1004 or later *before* upgrading PostgreSQL to 8.x or later.

Biggest issue: PostgreSQL runs the test suite more slowly than mysql or SQLite.

On rare occaision PostgresSQL will fail with an ERROR: duplicate key violates unique constraint error. If this happens inside a call to transaction() it will automatically restart. You only need to worry about this if you are not using transaction().

mysql

OOPS 0.2005 has been tested with mysql 5.1.46 on Linux. OOPS 0.2003 has been tested with mysql 5.0.45 on Linux. OOPS 0.2002 has been tested with mysql 5.0.22 on Linux. OOPS 0.1004 has been tested with mysql 4.1.15 on Linux. OOPS 0.1003 was tested with mysql 4.0.16 and 4.0.18 using InnoDB tables.

The OOPS schema for mysql does not support large (>256 byte) hash keys.

Mysql locking doesn't work correctly. To get correct behavior, access to the database must be exclusive http://bugs.mysql.com/?id=28029. This problem is fixed in 5.0.45.

SQLite2

DBD::SQLite versions 0.x use SQLite 2.x. DBD::SQLite2 is the continuation of DBD::SQLite version 0.x codebase and uses SQLite 2.x.

DBD::SQLite2 is not 8-bit clean with respect to \0. OOPS uses a DBD::SQLite2 feature to translate binary nulls. A side-effect is that backslash will be doubled \ -> \\ in DBD::SQLite2 (the extras will be removed transparently by DBD::SQLite2.

OOPS does not use the counters table with DBD::SQLite2.

SQLite

DBD::SQLite versions 1.x use SQLite 3.x.

DBD::SQLite versions 1.x leak file descriptors and will eventually die if you create use and destroy too many OOPS instances. DBD::SQLite versions 1.x cannot pass the regression suite and may not be reliable in production use. Use DBD::SQLite2 instead.

OOPS does not use the counters table with DBD::SQLite.

SQLite is not 8-bit clean with respect to \0. OOPS uses a DBD::SQLite feature to translate binary nulls. A side-effect is that backslash will be doubled \ -> \\ in DBD::SQLite (the extras will be removed transparently by DBD::SQLite. As of 0.2002, OOPS doesn't make use of the newer (version 1.x) DBD::SQLite features for handling nulls.

To work around the problem with the leaked file descriptors, on systems that support /proc, file descriptors in excess of $OOPS::sqlite_v3::enable_fd_leak_workaround (500 by default) that match the sqlite database file will be closed when new OOPS instances are created. This sometimes helps. Set $OOPS::sqlite_v3::enable_fd_leak_workaround to 0 to disable.

Related bug reports: http://rt.cpan.org/Ticket/Display.html?id=20286, http://rt.cpan.org/Ticket/Display.html?id=20288.

Perl versions 5.8.2 through 5.12.1 are supported. Prior to 5.8.2, it wasn't possible to untie scalars from within the a tied scalar access method. An ugly workaround is possible if there is enough interest.

OOPS 0.2002 and 0.1004 have been tested on Linux 2.6.15 (Ubuntu 6.06LTS - Dapper Drake). OOPS 0.1003 was tested on Linux 2.4.23 (Debian unstable as of April '04); on FreeBSD 4.9; and on DragonflyBSD prerelease.

As far as performance goes, mysql and SQLite are both about twice as fast as PostgreSQL for applications that only have one transaction at at time. SQLite and mysql are particularly slow when there are multiple transactions as their lock granularity is the entire database.

Each RDBMS uses a somewhat different schema.

FUNCTIONS ^

transaction($funcref, @args)

transaction() is a wrapper for a complete transaction. Transactions that fail due to deadlock with other processes will be re-run automatically.

The first parameter is a reference to a function. Any additional parameters will be passed as parameters to that function. The return value of transaction() is the return value of &$funcref().

It is not necessary to use the transaction() method. Beware that nearly any operation on persistent date (even read operations) can cause deadlock.

Any use of persistent data can trigger a deadlock. The transaction() function catches this and retries automatically up to $OOPS::transaction_maxtries times (15 times unless you change it). If you don't use transaction() you might want to catch the exceptions that transaction() catches. To do this, you can regex match $@ against $OOPS::transfailrx.

Basically, transaction is a slightly fancier version of the following:

 sub transaction
 {
        my ($sub, @args) = @_;
        for (0..15) {
                eval {
                        &$sub(@args);
                }
                last unless $@;
                next if $@ =~ /$OOPS::transfailrx/;
                die $@;
        }
 }

The important thing to notice is that your code will be called over and over until the transaction succeeds. This means you should write your code so that it doesn't have any external side effects until after it calls commit().

The following package variables are available during transactions:

$OOPS::transaction_tries

The number of attempts at this transaction. Starts with 1.

$OOPS::transaction_maxtries

The maximum number of attempts that will be made.

$OOPS::transaction_failure_sleep

The minimum amount of time to sleep() if the transaction fails.

$OOPS::transaction_failure_maxsleep

The maximum amount of time to sleep() if the transaction fails.

@OOPS::transaction_rollback

A list of functions to call in the event of a transaction rollback (code refs).

getref(%hash, $key)

References to tied hash keys are buggy in all perls through 5.8.7 (and beyond?). Use getref(%hash, $key) to create your reference to a tied hash key. See: http://rt.perl.org/rt3/Ticket/Display.html?id=27555 and http://rt.perl.org/rt3/Ticket/Display.html?id=29224.

 $ref = getref(%hash, $key);

Alternatively, use $oops->workaround27555($ref).

Getref() and workaround27555() work around all the perl bugs with tied hash key references. Failure to use them may result in unexpected and inconsistent results.

walk_hash(%hash, $stride, [ $previous_last_key ])

If you have a large virtual hash, you may want to be able to iterate over the keys in the hash as a bunch of transactions rather than as one transaction.

Walk_hash() can be called on any HASH. If it isn't an OOPS virtual hash, it will return keys in batches just like if it had been called on an OOPS virtual hash.

Here is an example of how to use it:

 my $stride = 20;
 my $done = 0;
 my $lastkey;
 while (not $done) {
        transaction(sub {
                my $oops = OOPS->new(...);
                my @things = walk_hash(%{$oops->{my_table}}, $stride, $lastkey);
                for my $thing (@things) {
                        # do stuff
                }
                $lastkey = $things[$#things];
                $done = 1 unless @things == $stride;
        });
 }

Here is another example of how to use it (with OOPS::TxHash):

 use OOPS::TxHash;

 my $stride = 20;

 my %real_todo = ( stuff => undef );
 while (%real_todo) {
        transaction(sub {
                my $th = tie my %todo, 'OOPS::TxHash', \%real_todo or die;
                my $oops = OOPS->new(...);

                if (exists $todo{stuff}) {
                        my @keys = walk_hash(%{$oops->{stuff}}, $stride, $todo{stuff});

                        for my $k (@keys) {
                                # work on $oops->{stuff}{$k}
                        }

                        $todo{stuff} = $keys[$#keys];
                        delete $todo{stuff} unless @keys == $stride;
                }

                $oops->commit;
                $th->commit;
        });
 }

Here is yet another example (with a wrapper function):

 walk_eval(20, 
        sub { OOPS->new(...) },
        sub { my $oops = shift; $oops->{my_table} },
        sub { 
                my ($oops, $thing) = @_;
                # do stuff
        },
        allatonce => 0
 );

 sub walk_eval
 {
        my ($stride, $get_oops, $get_hash, $agent, %opts) = @_;
        my $done = 0;
        my $last = undef;
        $stride ||= 100;
        while (! $done) {
                transaction(sub {
                        my $oops = &$get_oops();
                        my $hash = &$get_hash($oops);
                        my @items = walk_hash(%$hash, $stride, $last);
                        if ($opts{allatonce}) {
                                &$agent($oops, @items);
                        } else {
                                for my $item (@items) {
                                        &$agent($oops, $item);
                                }
                        }
                        $oops->commit();
                        $last = $items[$#items];
                        $done = 1 unless @items == $stride;
                });
        }
 }

PUBLIC CLASS METHODS ^

OOPS->new(%parameters)

Creates a OOPS object instance. More than one object instance is allowed at the same time. Instances are unaware of each other. Making a reference from a persistent object in one instance to a persistent object in another instance will cause recursive copying from one instance to the other. (Untested).

The %parameters are documented in the next section.

OOPS->initial_setup(%parameters)

Drops and recreates the database tables. Don't use it too often :-) The regression suite drops and re-creates the tables many times.

The %parameters are documented in the next section.

PARAMETERS FOR NEW ^

The new() and initial_setup() methods take a hash specification for their behavior. Here are the parameters allowed:

dbi_dsn / $ENV{OOPS_DSN} / $ENV{OOPS_DRIVER} / $ENV{DBI_DSN} / $ENV{DBI_DRIVER

Many ways to specify the DSN for DBI: as an argument; as an OOPS environment variable; as a DBI environment variable. Use at least one :-) See the DBI documentation for more details.

user / $ENV{OOPS_USER} / $ENV{DBI_USER}

Three ways to specify the user for DBI: as an argument; as an OOPS environment variable; as a DBI environment variable. Not required for all databases.

password / $ENV{OOPS_PASS} / $ENV{DBI_PASS}

Three ways to specify the password for DBI: as an argument; as an OOPS environment variable; as a DBI environment variable. Not required for all databases.

readonly

If true, commit() will be disabled and the transaction isolation level will be set to READ COMMITTED instead of SERIALIZABLE for the databases where this matters (mysql, PostgreSQL). This should be used if you plan to scan across large amounts of data because otherwise everything you read will end up locked. This feature is still experimental (not covered by the test suite).

less_caching

If true, iterating over the keys of a virtual hash (see virtual_object) will not cache the key values in memory. If your virtual hash is too big to fit in memory, this is a very good idea. Alternatively, you can use the clear_cache method (below). This feature is still experimental (not covered by the test suite).

table_prefix / $ENV{OOPS_PREFIX}

OOPS allows a prefix to be supplied for it's internal table names. If you set a prefix of FOO_ then it will use a FOO_object table instead of an object table. This can be set as an argument to new() or it can be set with the environment variable $OOPS_PREFIX. This allows multiple separate object spaces to exist within the same backend SQL database. It's intended use is to support testing vs. production environments but it could also be used to segregate object spaces that don't link to each other.

The current tables and indexes are:

object

Each row is an object.

attribute

Each row is an attribute value in an object.

big

Each row is a large attribute value.

counters

Counters for things like object ids.

temp

A temporary table.

group_index

Object grouping index.

value_index

Lookup by attribute value.

auto_upgrade / $ENV{OOPS_UPGRADE}

The relational schema for OOPS is not same for all versions of OOPS. If this is set, then an older schema will be upgraded to the current schema. If this is not set, then OOPS will use the older schema unchanged. In most cases, this means that OOPS will use a historical version of itself rather than the current version. See the SCHEMA VERSIONS section of this document.

auto_initialize / $ENV{OOPS_INIT}

The DBMS must be initialized before it can be used: tables created, a few rows inserted, etc. This can be handled by calling initial_setup() or it can be handled automatically by new() if auto_initialize or $OOPS_AUTO_INIT is set.

default_synchronous / $ENV{OOPS_SYNC}

With SQLite, an additional parameter to OOPS->new() is recognized: default_synchronous. Possible values are:

FULL

Sync() all transactions to disk before returning.

NORMAL

The default: sync() at critical moments only - protects against program failure, but not all power or OS failures.

OFF

Don't sync() at all and go really fast.

PUBLIC OBJECT METHODS ^

->commit()

Writes any changed objects back to the database and commits the transaction. Currently only one commit() call is allowed. Do not access your persistent data after commit() -- it may work but this is not covered well in the regression suite.

->virtual_object(\%hash [,$new_value])

Queries [and sets] the load-virtual flag on a persistent hash. Hashes that load virtual will do separate queries for each key rather than load the entire hash. This is a good thing if your has has lots of keys. This flag takes effect the next time the hash is loaded. The value is a perl boolean.

This may be handled automatically in the future.

->workaround27555($reference)

References to tied hash keys are buggy in all perls through 5.12.1 (and beyond?). Use workaround27555($reference) to register your new tied hash key references so that they can be transformed into references that actually work correctly.

 $ref = \%hash{$key};
 $oops->workaround27555($ref);

workaround27555() is harmless if called on other sorts of references so it is safe to use indiscriminately. See http://rt.perl.org/rt3/Ticket/Display.html?id=27555.

Alternatively, use getref(%hash, $key).

->dbh()

This returns the main DBI database handle used by OOPS. This function is provided for those who want to hand-write queries. Please note: no changes are written to the DBMS by OOPS until $oops-commit()>.

->load_object($id)

This will load a persistent object by number. It returns the object or undef if the object doesn't exists. This function is provided for those who want to hand-write queries.

->clear_cache()

This will clear out some of the cache that oops maintains. It is useful if you're iterating over the keys of a large virtual object and don't want to overflow memory. This feature is still experimental (not covered by the test suite).

->lock($ref)

Lock an object or a scalar. While this is never needed for transactional consistentcy within the OOPS data, it can be useful for transactions that have external side-effects like sending an email. In theory, if you pre-lock everything you might change, your transaction will not need to be rolled back due to deadlock. When you lock an object, it is the object record that is locked, not the contents. Locking \%hash won't stop another process from modifying $hash->{key}. Lock \$hash->{key} if you want that.

LIMITATIONS, ERRATA, DEVELOPMENT STATUS, BUGS ^

OOPS has been thoroughly tested. The regression suite is very well developed: there is twice as much code in the test suite as there is in the module itself. The suite does over 1.5million tests. I have so much confidence in my test suite, I'm offering a bounty on bugs!

Limitations

OOPS caches in memory all data that is pulled in during a transaction. For virtual_object hash'es only, there are ways to prevent absolutely everything from being cached (the less_caching contructor agrument and the clear_cache method) but for other objects you need to have enough memory to hold everything you touch.

You can only call ->commit once per OOPS instance. The cache is emptied when the OOPS instance is destroyed. This means that OOPS will be loading up its cache over and over.

Known bugs in OOPS

memory leaks

OOPS currently has memory leaks. This may or may not matter to your application. The rate of leakage varies depending on which RDBMS is used. SQLite seems to have the most significant problems. Most of the leaks are not in OOPS itself but in the modules it uses and thus are not easily addressed by changing OOPS.

delayed DESTROY

Additional references to the in-memory copies of persistent data are kept by OOPS. These extra references will prevent DESTROY methods from being called as soon as they otherwise would be. They'll usually be delayed until the OOPS object is itself DESTROYed.

other magic

Other perl magic attributes are not currently stored persistently. Many probably could be supported, but many could not. For example, taint does not work on tied hashes: http://rt.perl.org/rt3/Ticket/Display.html?id=6758.

unreferenced blessed scalars

When you bless a reference to a scalar value, the blessing is stored with the scalar, not the reference. The blessing remains even if there is no reference to the scalar. The following code prints true.

 my $x = 'foobar';
 my $y = \$x;
 bless $y, 'baz';
 $y = 7;
 $y = \$x;
 my $z = ref($y);
 print "true\n" if $z eq 'baz';

At the current time, OOPS does not store such blessings. OOPS does not remember blessings when there isn't a reference.

re-blessing the OOPS object

Reblessing the OOPS object is not supported.

DBD::Pg and ASCII NULL

DBD::Pg does not easily support ASCII NULL. OOPS has only partial support for ASCII NULL with PostgreSQL. Everthing works, but don't have ASCII NULL in your class names.

Circular references sometimes break

OOPS mostly allows you to make circular self-references:

 my $x;
 $x = \$x;

In some (rare) cases circular self-references sometimes break. Since I can't think on any good reason to make a circular self-reference, I'm not going to fix this bug.

Long hash keys

The database schema for OOPS does not support huge hash keys.

Bugs in perl that effect OOPS

References to hash keys

Persistent hashes are implemented with tie. There are bugs with perl's implementation of references to tied hash keys. These bugs will be triggered in several situations: creating a reference to tied hash key that doesn't exist yet; deleting a key that has a reference tied to it; assigning through a reference to a key that has multiple references.

All of the above can either be avoided or you can workaround them by either calling workaround27555($YOUR_REFERENCE) whenever you create a tied hash key reference or by using getref(%hash, $key) to create your reference.

The perl bugs are documented in: http://rt.perl.org/rt3/Ticket/Display.html?id=27555 and http://rt.perl.org/rt3/Ticket/Display.html?id=29224.

local and tie

local(%some_tied_hash) doesn't work right. Thus local(%some_persistent_hash) won't work right either: http://rt.perl.org/rt3/Ticket/Display.html?id=6017.

scalar(%hash)

Tied scalar(%hash) support was added in perl 5.8.3 and does not exist in 5.8.2.

Tied arrays don't work right

There are a couple of bugs with tied arrays that prevent OOPS from using them: http://rt.perl.org/rt3/Ticket/Display.html?id=22570 and http://rt.perl.org/rt3/Ticket/Display.html?id=22571. OOPS fully loads arrays into memory to work-around this problem. This isn't a big deal unless you've got big arrays.

This was fixed in perl 5.10, but OOPS does not yet take advantage of tied arrays.

SQLite and perl's malloc().

If SQLite is used with a perl that has been compiled to use perl's malloc(), it will report LOTS of Bad free() ignored (PERL_CORE) errors. It is not currently known if these errors are harmful beyond generating lots of output to STDERR. The default perl configuration on FreeBSD uses perl's malloc().

SCHEMA VERSIONS ^

As OOPS is developed, the database schema that OOPS uses changes.

OOPS notices if the version of the schema in the DBMS is different than the version the code currently supports. When this happens there are two possibilities: either OOPS will upgrade the schema to the current version or it will use an older version of OOPS to access the data. See the auto_upgrade notes in the PARAMETERS FOR NEW section.

This behavior allows the OOPS module to be upgraded without disrupting installed applications.

DATABASE ADMINISTRATION ^

Like Perl, OOPS uses reference counting. If you have circular data structures and you delete all you references to the circular data structure, you'll leak persistent memory.

Use OOPS::GC to mop up the leak.

Previous versions of OOPS have had bugs that caused the reference counts on the persistent data to become inaccurate. Use OOPS::Fsck to fix such problems. Do this before once, before using OOPS::GC.

FUTURE DIRECTIONS ^

OOPS isn't done. There are a bunch of things that I am considering adding to it. If any of these things is important to you, speak up so that I know there is interest...

fix the bugs

There are bugs listed in the DEVELOPMENT STATUS section that could be fixed. First up is fixing the memory leaks that are in OOPS itself.

code cleanup and general performance enhancements

The initial releases of OOPS concentrated on correct behavior and other aspects of the module were somewhat ignored. The code could be cleaned up a bunch.

perl-syntax SQL query translator
 SELECT Employee WHERE $Employee->{salary} > 5000

It's possible to translate perl-syntax queries into real SQL that can be used to query the object store.

better grouping

Objects are loaded in groups rather than individually. There is much room for improvement in choosing how groups are formed. This is largely undeveloped as yet.

caching

Many possibilities. A cache-invalidation daemon to note when objects have changed. Re-verification of touched data from the database. Ability to call commit() more than once.

weak references

Support for persistent weak references is possible.

external references to objects

Currently objects are reference counted internally. You must have a reference to something from an already existing object for it to continue to exist.

contracts

OOPS has to do a lot of scanning of objects to see if they've changed. Explicit notification of changes would improve performance.

OOPS could call functions before saving and after loading to transform objects for a better or cleaner on-disk representation.

support for 'base' & accessor methods

This isn't something that I care about but maybe someone else does?

schema enforcement

Allow explicit schemas to be defined. Do not save objects that don't conform. Eg: Hash::Util::lock_keys().

RDBMS -> object mapping

Map existing RDBMS schemas into objects.

data viewer

Viewing large datasets of deep and cross-linked data is difficult. Perhaps a CGI-based or Tk-based data navigator would help.

support for tied data structures

It is possible to support storing tied data. The tied object is what would need to be persistent. This would only work on some kinds of ties.

support for other base types.

Right now, just HASH, SCALAR, REF, and ARRAY are supported. Regular expressions, file handles, I don't know it's possible to support code references.

on-line data migration

By doing double updates, OOPS could support live migration from one DBMS to another.

WRITING SQL QUERIES BY HAND ^

If you want to query your data, then until a translator is written, your only choice for making queries is to write them by hand. Using your data does not require a query: anything you've got a reference to will be loaded as you access it. Queries are for performing searches that don't have a perl-object index.

Each perl HASH, REF/SCALAR, or ARRAY has a row in the object table and multiple rows in the attribute and big tables.

Here are the columns you'll care about:

object

There is one row per perl object.

id

The object id.

class

The blessed class name (limited to 255 characters).

otype

The type of object:

'H'

A HASH.

'A'

An ARRAY.

'S'

A SCALAR or REF.

attribute

This is a table of key/value pairs. The keys correspond to perl hash keys and perl array indexes. The values correspond to perl hash values and array element values.

id

The object id.

pkey

The hash key or array index.

pval

he hash value or array value. Limited to 255 characters.

ptype

Flags the type of the value. Possible values are:

'0'

A normal value. Numeric or string.

'B'

An big value. pval will be a copy of the start of the value for the first N characters. The end of pval will be a MD5 checksum of the full big value.

'R'

A reference to another object.

big

This is a table of values that were too large for the normal columns. Even with databases that support wide columns, a separate big table is used so that you don't load large scalars unless you actually need the value.

id

The object id.

pkey

The hash key or array index.

pval

The hash value or array value. Limited to whatever the underlying database will support as it's largest blob.

fragno

Blob fragment number. This column only exists with SQLite. SQLite has a smallish maximum row size and so big values must be split into multiple rows.

REFs are are special. There are several types of REFs: references to scalar values; references to objects; secondary references to scalar values; references to scalar values that are part of another object (references to hash keys and references to array elements).

The representation of references is designed so that you don't need to care what sort of REF it is when you're doing a query.

The basic REF is a ref to a value inside another object. An example:

 OBJECT TABLE
 id             class           otype

 1              OOPS::NamedObj  H
 383            SCALAR          R
 384            SCALAR          R
 385            SCALAR          R
 386            REF             R
 400            HASH            H
 500            ARRAY           A

 ATTRIBUTE TABLE
 id             pkey            pval            ptype

 1              A500            500             R
 1              H400            400             R
 1              R383            383             R
 1              R384            384             R
 1              R385            385             R
 1              R386            386             R

 383            400             'a-key'         0

 384            384             'nopkey'        0
 384            'nopkey'        'a-value'       0

 385            384             'nopkey'        0

 386            386             'nopkey'        0
 386            'nopkey'        500             R

 400            'a-key'         'a-value        0
 400            'another-key'   'another-value' 0
 400            'A500'          500             R

 500            0               'a-value'       0
 500            1               'another-value' 0

HASH 1 is %$oops.

REF 383 is a reference to the key 'a-key' in object #400 (a HASH).

REF 384 is a ref to scalar. It uses two rows to make writing queries easier.

REF 385 is a duplicate reference to a scalar value. It duplicates REF 384. In behavior, these two REFs should be identical even though they are represented differently in the database.

REF 386 is a ref to an object: #500 (an ARRAY).

HASH 400 is a normal hash.

ARRAY 500 is a normal hash.

This example data is what you would end up with after running code like:

 my $oops = new OOPS 
        dbi_dsn => 'DBI:mysql:database=MY-DATABASE-NAME;host=localhost', 
        username => 'MY-USERNAME', 
        password => 'MY-PASSWORD';

 $oops->{A500} = [ 'a-value', 'another-value' ];

 $oops->{H400} = { 
        'a-key' => 'a-value',
        'another-key' => 'another-value',
        'A500' => $oops->{A500},
 };

 $oops->{R383} = \$oops->{H400}{'a-key'};
 $oops->workaround27555($oops->{R383});

 $oops->{R384} = \'a-value';

 $oops->{R385} = $oops->{R384};

 $oops->{R386} = \$oops->{A500};

 $oops->commit;

SQL queries require a bunch of joins to link data structures together. Here are some examples.

SELECT Foobar WHERE $Foobar->{xyz} = 'abc'
 SELECT object.id
 FROM   object, attribute
 WHERE  object.class = 'Foobar'
 AND    object.id = attribute.id
 AND    object.otype = 'H'
 AND    attribute.pkey = 'xyz'
 AND    attribute.pval = 'abc'
 AND    attribute.ptype = '0'
SELECT Foobar WHERE ${$Foobar->{xyz}} = 'abc'

This example should show why an automatic translator would be a good idea...

 SELECT ohash.object
 FROM   object AS ohash,
        attribute AS ahash,
        object AS oref,
        attribute AS aref,
        attribute AS target
 WHERE  ohash.class = 'Foobar'
 AND    ohash.otype = 'H'
 AND    ahash.id = ohash.id
 AND    ahash.pkey = 'xyz'
 AND    ahash.ptype = 'R'
 AND    oref.id = ahash.pval
 AND    oref.otype = 'S'                # this is the outer ref
 AND    oref.id = aref.id               
 AND    aref.pval = target.pkey         # here's the reference indirection
 AND    target.pval = 'abc'
 AND    target.ptype = '0'

If you construct a query like these examples that return object id's, then use $object = $oops->load_object($id) to load them into memory.

I recommend that hand-written queries be read-only as there are additional columns that must be kept consistent. For example, the object table includes a reference count column to handle garbage collection of the persistent data.

RUNNING THE REGRESSION TEST SUITE ^

The regression test suite empties and re-creates the persistent store over and over again. To prevent the accidental erasure of production data, all of the tests require a special environment variable to be set $OOPSTEST_DSN. This variable replaces the normal $DBI_DSN or $OOPS_DSN. Correspondingly there is a $OOPSTEST_USER, $OOPSTEST_PASS, and $OOPSTEST_PREFIX.

Set these variables to something different than what you use for your production data!

Most of the tests take a long time to run and are disabled by default. If you can run the full suite in less than six hours please tell me about your configuration.

Beware mysql logging. On Debian unstable, the default configuration for mysql used to log every SQL statement. Running the test suite to completion will generate several gigabytes of log file. Running out of disk space will cause the tests to fail. On DragonflyBSD the default mysql configuration includes making replication master logs.

THE COMPETITION ^

There are a number of other modules that make perl objects persistent.

Other object persistence solutions either store data in blobs that cannot be querried or they they're a mapping from a relation database into perl data that does not allow arbitrary data to be stored.

http://poop.sourceforge.net/ has an overview of options.

EXAMPLE PROGRAMS ^

The following applications use OOPS: Qpsmtpd::Plugin::Quarantine

LICENSE ^

Copyright(C) 2004-2006 David Muir Sharnoff <muir@idiom.org> Copyright(C) 2012 Google, Inc

OOPS may be used, modifified and redistributed on the same terms as Perl itself.

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