KiokuDB(3)

NAME
KiokuDB - Object Graph storage engine

TUTORIAL
If you're new to KiokuDB check out KiokuDB::Tutorial.

SYNOPSIS
    use KiokuDB;

    # use a DSN
    my $d = KiokuDB->connect( $dsn, %args );

    # or manually instantiate a backend
    my $d = KiokuDB->new(
        backend => KiokuDB::Backend::Files->new(
            dir        => "/tmp/foo",
            serializer => "yaml",
        ),
    );

    # create a scope object
    my $s = $d->new_scope;

    # takes a snapshot of $some_object
    my $uuid = $d->store($some_object);

    # or with a custom ID:
    $d->store( $id => $some_object ); # $id can be any string

    # retrieve by ID
    my $some_object = $d->lookup($uuid);

    # some backends (like DBI) support simple searchs
    $d->search({ name => "foo" });

    # others use GIN queries (DBI supports both)
    $d->search($gin_query);

DESCRIPTION
KiokuDB is a Moose based frontend to various data stores, somewhere in between Tangram and Pixie.

Its purpose is to provide persistence for "regular" objects with as little effort as possible, without sacrificing control over how persistence is actually done, especially for harder to serialize objects.

KiokuDB is also non-invasive: it does not use ties, "AUTOLOAD", proxy objects, "sv_magic" or any other type of trickery.

Many features important for proper Perl space semantics are supported, including shared data, circular structures, weak references, tied structures, etc.

KiokuDB is meant to solve two related persistence problems:

Transparent persistence
    Store arbitrary objects without changing their class definitions or worrying about schema details, and without needing to conform to the limitations of a relational model.

Interoperability
    Persisting arbitrary objects in a way that is compatible with existing data/code (for example interoprating with another app using CouchDB with JSPON semantics).

FUNDAMENTAL CONCEPTS
In order to use any persistence framework it is important to understand what it does and how it does it.

Systems like Tangram or DBIx::Class generally require explicit meta data and use a schema, which makes them fairly predictable.

When using transparent systems like KiokuDB or Pixie it is more important to understand what's going on behind the scenes in order to avoid surprises and limitations.

An architectural overview is available on the website: <http://www.iinteractive.com/kiokudb/arch.html>

The process is explained here and in the various component documentation in more detail.

  Collapsing
When an object is stored using KiokuDB it's collapsed into an KiokDB::Entry.

An entry is a simplified representation of the object, allowing the data to be saved in formats as simple as JSON.

References to other objects are converted to symbolic references in the entry, so objects can be saved independently of each other.

The entries are given to the KiokuDB::Backend for actual storage.

Collapsing is explained in detail in KiokuDB::Collapser. The way an entry is created varies with the object's class.

  Linking
When objects are loaded, entries are retrieved from the backend using their UIDs.

When a UID is already loaded (in the live object set of a KiokuDB instance, see KiokuDB::LiveObjects) the live object is used. This way references to shared objects are shared in memory regardless of the order the objects were stored or loaded.

This process is explained in detail in KiokuDB::Linker.

ROOT SET MEMBERSHIP
Any object that is passed to "store" or "insert" directly is implicitly considered a member of the root set.

This flag implies that the object is an identified resource and should not be garbage collected with any of the proposed garbage collection schemes.

The root flag may be modified explicitly:

    $kiokudb->set_root(@objects); # or unset_root

    $kiokudb->update(@objects);

Lastly, root set membership may also be specified explicitly by the typemap.

A root set member must be explicitly removed using "delete" or by removing it from the root set. Only non-members of the root set will be purged with any garbage collection scheme.

TRANSACTIONS
On supporting backends the "txn_do" method will execute a block and commit the transaction at its end.

Nesting of "txn_do" blocks is always supported, though rolling back a nested transaction may produce different results on different backends.

If the backend does not support transactions "txn_do" simply executes the code block normally.

CONCURRENCY
Most transactional backends are also concurrent.

KiokuDB::Backend::BDB and KiokuDB::Backend::CouchDB default to serializable transaction isolation and do not suffer from deadlocks, but serialization errors may occur, aborting the transaction (in which case the transaction should be tried again).

KiokuDB::Backend::Files provides good concurrency support but will only detect deadlocks on platforms which return "EDEADLK" from "flock". Directory::Transactional may provide alternative mechanisms in the future.

Concurrency support in KiokuDB::Backend::DBI depends on the database. SQLite defaults to serializable transaction isolation out of the box, wheras MySQL and PostgreSQL default to read committed.

Depending on your application read committed isolation may be sufficient, but due to the graph structure nature of the data repeatable reads or serializable level isolation is highly reccomended. Read committed isolation generally works well when each row in the database is more or less independent of others, and various constraints ensure integrity. Unfortunately this is not the case with the graph layout.

To enable stronger isolation guarantees see "Transactions" in KiokuDB::Backend::DBI for per-database pointers.

ATTRIBUTES
KiokuDB uses a number of delegates which do the actual work.

Of these only "backend" is required, the rest have default definitions.

Additional attributes that are not commonly used are listed in "INTERNAL ATTRIBUTES".

backend
    This attribute is required.

    This must be an object that does KiokuDB::Backend.

    The backend handles storage and retrieval of entries.

typemap
    This is an instance KiokuDB::TypeMap.

    The typemap contains entries which control how KiokuDB::Collapser and KiokuDB::Linker handle different types of objects.

allow_classes
    An array references of extra classes to allow.

    Objects blessed into these classes will be collapsed using KiokuDB::TypeMap::Entry:Naive.

allow_bases
    An array references of extra base classes to allow.

    Objects derived from these classes will be collapsed using KiokuDB::TypeMap::Entry:Naive.

allow_class_builders
    If true adds KiokuDB::TypeMap::ClassBuilders to the merged typemap.

    It's possible to provide a hash reference of options to give to "new" in KiokuDB::TypeMap::ClassBuilders.

check_class_versions
    Controls whether or not the class versions of objects are checked on load.

    Defaults to true.

class_version_table
    A table of classes and versions that is passed to the default typemap entry for Moose/Class::MOP objects.

    When a class version has changed between the time that an object was stored and the time it's being retrieved, the data must be converted.

    See KiokuDB::TypeMap::Entry::MOP for more details.

METHODS

connect $dsn, %args
    DWIM wrapper for "new".

    $dsn represents some sort of backend (much like DBI dsns map to DBDs).

    An example DSN is:

        my $dir = KiokuDB->connect("bdb:dir=path/to/data/");

    The backend moniker name is extracted by splitting on the colon. The rest of the string is passed to "new_from_dsn", which is documented in more detail in KiokuDB::Backend.

    Typically DSN arguments are separated by ";", with "=" separating keys and values. Arguments with no value are assumed to denote boolean truth (e.g. "jspon:dir=foo;pretty" means "dir => "foo", pretty => 1"). However, a backend may override the default parsing, so this is not guaranteed.

    Extra arguments are passed both to the backend constructor, and the "KiokuDB" constructor.

    Note that if you need a typemap you still need to pass it in:

        KiokuDB->connect( $dsn, typemap => $typemap );

    The DSN can also be a valid JSON string taking one of the following forms:

        dsn => '["dbi:SQLite:foo",{"schema":"MyApp::DB"}]'

        dsn => '{"dsn":"dbi:SQLite:foo","schema":"MyApp::DB"}'

    This allows more complicated arguments to be specified accurately, or arbitrary options to be specified when the backend has nonstandard DSN parsing (for instance KiokuDB::Backend::DBI simply passes the string to DBI, so this is necessary in order to specify options on the command line).

configure $config_file, %args
    TODO

new %args
    Creates a new directory object.

    See "ATTRIBUTES"

new_scope
    Creates a new object scope. Handled by "live_objects".

    The object scope artificially bumps up the reference count of objects to ensure that they live at least as long as the scope does.

    This ensures that weak references aren't deleted prematurely, and the object graph doesn't get corrupted without needing to create circular structures and cleaning up leaks manually.

lookup @ids
    Fetches the objects for the specified IDs from the live object set or from storage.

store @objects
store %objects
store_nonroot @objects
store_nonroot %objects
    Recursively collapses @objects and inserts or updates the entries.

    This performs a full update of every reachable object from @objects, snapshotting everything.

    Strings found in the object list are assumed to be IDs for the following objects.

    The "nonroot" variant will not mark the objects as members of the root set (therefore they will be subject to garbage collection).

update @objects
    Performs a shallow update of @objects (referants are not updated).

    It is an error to update an object not in the database.

deep_update @objects
    Update @objects and all of the objects they reference. All references objects must already be in the database.

insert @objects
insert %objects
insert_nonroot @objects
insert_nonroot %objects
    Inserts objects to the database.

    It is an error to insert objects that are already in the database, all elements of @objects must be new, but their referants don't have to be.

    @objects will be collapsed recursively, but the collapsing stops at known objects, which will not be updated.

    The "nonroot" variant will not mark the objects as members of the root set (therefore they will be subject to garbage collection).

delete @objects_or_ids
    Deletes the specified objects from the store.

    Note that this can cause lookup errors if the object you are deleting is referred to by another object, because that link will be broken.

set_root @objects
unset_root @objects
    Modify the "root" flag on the associated entries.

    "update" must be called for the change to take effect.

txn_do $code, %args
txn_do %args
    Executes $code within the scope of a transaction.

    This requires that the backend supports transactions (KiokuDB::Backend::Role::TXN).

    Transactions may be nested.

    If the "scope" argument is true an implicit call to "new_scope" will be made, keeping the scope for the duration of the transaction.

search \%proto
search @args
    Searching requires a backend that supports querying.

    The "\%proto" form is currently unspecified but in the future should provide a simple but consistent way of looking up objects by attributes.

    The second form is backend specific querying, for instance Search::GIN::Query objects passed to KiokuDB::Backend::BDB::GIN or the generic GIN backend wrapper KiokuDB::GIN.

root_set
    Returns a Data::Stream::Bulk of all the root objects in the database.

all_objects
    Returns a Data::Stream::Bulk of all the objects in the database.

grep $filter
    Returns a Data::Stream::Bulk of the objects in "root_set" filtered by $filter.

scan $callback
    Iterates the root set calling $callback for each object.

object_to_id
objects_to_ids
id_to_object
ids_to_objects
    Delegates to KiokuDB::LiveObjects

directory
    Returns $self.

    This is used when setting up KiokuDB::Role::API delegation chains. Calling "directory" on any level of delegator will always return the real KiokuDB instance no matter how deep.

GLOBALS

$SERIAL_IDS
    If set at compile time, the default UUID generation role will use serial IDs, instead of UUIDs.

    This is useful for testing, since the same IDs will be issued each run, but is utterly broken in the face of concurrency.

INTERNAL ATTRIBUTES
These attributes are documented for completeness and should typically not be needed.

collapser
    KiokuDB::Collapser

    The collapser prepares objects for storage, by creating KiokDB::Entry objects to pass to the backend.

linker
    KiokuDB::Linker

    The linker links entries into functioning instances, loading necessary dependencies from the backend.

live_objects
    KiokuDB::LiveObjects

    The live object set keeps track of objects and entries for the linker and the resolver.

    It also creates scope objects that help ensure objects don't garbage collect too early ("new_scope" in KiokuDB::LiveObjects, KiokuDB::LiveObjects::Scope), and transaction scope objects used by "txn_do" (KiokuDB::LiveObjects::TXNScope).

typemap_resolver
    An instance of KiokuDB::TypeMap::Resolver. Handles actual lookup and compilation of typemap entries, using the user typemap.

SEE ALSO

  Prior Art on the CPAN

Pixie
DBM::Deep
OOPS
Tangram
DBIx::Class
    Polymorphic retrieval is possible with DBIx::Class::DynamicSubclass

Fey::ORM
MooseX::Storage

VERSION CONTROL
KiokuDB is maintained using Git. Information about the repository is available on <http://www.iinteractive.com/kiokudb/>

AUTHOR
Yuval Kogman <nothingmuch@woobling.org>

COPYRIGHT
    Copyright (c) 2008, 2009 Yuval Kogman, Infinity Interactive. All
    rights reserved This program is free software; you can redistribute
    it and/or modify it under the same terms as Perl itself.