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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.