KiokuDB::Tutorial - Getting started with KiokuDB


The easiest way to install KiokuDB and a number of backends is Task::KiokuDB.

KiokuDB depends on Moose and a few other modules out of the box, but no specific storage module.

KiokuDB is a frontend to several backends, much like DBI uses DBDs to connect to actual databases.

For development and testing you can use the KiokuDB::Backend::Hash backend, which is an in memory store, but for production KiokuDB::Backend::BDB is the recommended backend.

See below for instructions on getting KiokuDB::Backend::BDB installed.


A KiokuDB directory is the object that contains all the common functionality regardless of the backend.

The simplest directory ready for use can be created like this:

    my $dir = KiokuDB->new(
        backend => KiokuDB::Backend::Hash->new

We will revisit other more interesting backend configuration later in this document, but for now this will do.


Let's start with this simple class:

    package Person;
    use Moose;

    has name => (
        isa => "Str",
        is  => "rw",

We can instantiate it:

    my $obj = Person->new( name => "Homer Simpson" );

and insert the object to the database as follows:

    my $scope = $dir->new_scope;

    my $homer_id = $dir->store($obj);

This is very trivial use of KiokuDB, but it illustrates a few important things.

First, no schema is necessary. KiokuDB can use Moose to introspect your object without needing to predefine anything like tables.

Second, every object in the database has an ID. If you don't choose an ID for an object, KiokuDB will assign a UUID instead. The ID is like a primary key in a relational database. If you want to choose an ID for your object, you can do something like:

    $dir->store( homer => $obj );

and $obj's ID will be homer. If you don't provide an ID a UUID will be assigned automatically.

Third, all KiokuDB operations need to be performed within a scope. The scope does not apply to a simple example like the above, but becomes necessary once weak references are used. We will look into that in more detail later.


So now that Homer has been inserted into the database, we can fetch him out of there using the ID we got from store.

    my $homer = $dir->lookup($homer_id);

Assuming that $scope and $obj are still in scope, $homer and $obj will actually be the same reference:

    refaddr($homer) == refaddr($obj)

This is because KiokuDB tracks which objects are "live" in the live object set (KiokuDB::LiveObjects).

If $obj and $scope are no longer in scope you'd need to create a new scope, and then fetch the object from the database again:

    my $scope = $dir->new_scope;

    my $homer = $dir->lookup($homer_id);

In this case since the original instance of Homer is no longer live, but has been garbage collected by Perl, KiokuDB will fetch it from the backend.


Let's extend the Person class to hold some more interesting data than just a name:

    package Person;

    has spouse => (
        isa => __PACKAGE__,
        is  => "rw",
        weak_ref => 1,

This new spouse attribute will hold a reference to another person object.

Let's first create and insert another object:

    my $marge_id = $dir->store(
        Person->new( name => "Marge Simpson" ),

Now that we have both objects in the database, lets link them together:

        my $scope = $dir->new_scope;

        my ( $marge, $homer ) = $dir->lookup( $marge_id, $homer_id );


        $dir->store( $marge, $homer );

Now we have created a persistent object graph, that is several objects which point to each other.

The reason spouse was had the weak_ref option was so that this circular structure will not leak.

When then objects are updated in the database KiokuDB sees that their spouse attribute contains references, and this relationship will be encoded using their unique ID in storage.

To load the graph, we can do something like this:

        my $scope = $dir->new_scope;

        my $homer = $dir->lookup($homer_id);

        print $homer->spouse->name; # Marge Simpson

        my $scope = $dir->new_scope;

        my $marge = $dir->lookup($marge_id);

        print $marge->spouse->name; # Homer Simpson

        refaddr($marge) == refaddr($marge->spouse->spouse); # true

When KiokuDB is loading the initial object, all the objects the object depends on will also be loaded. The spouse attribute contains a reference to another object (by ID), and this link is resolved at inflation time.

The purpose of new_scope

This is where new_scope becomes important. As objects are inflated from the database, they are pushed onto the live object scope, in order to increase their reference count.

If this was not done, by the time $homer was returned from lookup his spouse attribute would have been cleared because there is no other reference to Marge.

If, on the other hand the circular structure was not weak it would have to be broken manually, which is very error prone.

By using this idiom:

        my $scope = $dir->new_scope;

        # do all KiokuDB work in here

You are ensuring that the objects live at least as long as is necessary.

In a web application context usually you create one new scope per request.

While scopes can nest, this is not a requirement.

You are free to create as many or as few scopes as you like, as long as there is at least one, but note that child scopes refer to their parents to ensure that all objects that were already live at the time that a scope is created are still alive


More complex relationships (not necessarily 1 to 1) are fairly easy to model with Set::Object.

Let's extend the Person class to add such a relationship:

    package Person;

    has children => (
        does => "KiokuDB::Set",
        is   => "rw",

KiokuDB::Set objects are KiokuDB specific wrappers for Set::Object.

    my @kids = map { Person->new( name => $_ ) } qw(maggie lisa bart);

    use KiokuDB::Util qw(set);

    my $set = set(@kids);



The set convenience function creates a new KiokuDB::Set::Transient object. A transient set is one which started its life in memory space.

The weak_set convenience function also exists, creating a transient set with Set::Object::Weak used internally to help avoid circular structures (for instance if setting a parent attribute in our example).

The set object behaves pretty much like a normal Set::Object:

    my @kids = $dir->lookup($homer_id)->children->members;

The main difference is that sets coming from the database are deferred by default, that is the objects in @kids are not loaded until they are actually needed.

This allows large object graphs to exist in the database, while only being partially loaded, without breaking the encapsulation of user objects. This behavior is implemented in KiokuDB::Set::Deferred and KiokuDB::Set::Loaded.

This set object is optimized to make most operations defer loading. For instance, if you intersect two deferred sets, only the members of the intersection set will need to be loaded.


Storing an object with KiokuDB involves passing it to KiokuDB::Collapser, the object that "flattens" objects into KiokuDB::Entry before the entries are inserted into the backend.

The collapser uses a KiokuDB::TypeMap object that tells it how objects of each type should be collapsed.

During retrieval of objects the same typemap is used to reinflate objects back into working objects.

Trying to store an object that is not in the typemap is an error. The reason behind this is that many objects depend on runtime states (for instance DBI handles need a socket, objects based on XS modules have an internal pointer as an integer), and even though the majority of objects are safe to serialize, even a small bit of unreported fragility is usually enough to create large, hard to debug problems.

An exception to this rule is Moose based objects, because they have sufficient meta information available through Moose's powerful reflection support in order to be safely serialized.

Additionally, the standard backends provide a default typemap for common objects (DateTime, Path::Class, etc), which by default is merged with any custom typemap you pass to KiokuDB.

So, in order to actually get KiokuDB to store things like Class::Accessor based objects, you can do something like this:

    my $dir = KiokuDB->new(
        backend => $backend,
        typemap => KiokuDB::TypeMap->new(
            entries => {
                "My::Object" => KiokuDB::TypeMap::Entry::Naive->new,

KiokuDB::TypeMap::Entry::Naive is a type map entry that performs naive collapsing of the object, by simply walking it recursively.

When the collapser encounters an object it will ask KiokuDB::TypeMap::Resolver for a collapsing routine based on the class of the object.

This lookup is typically performed by ref $object, not using inheritence, because a typemap entry that is safe to use with a superclass isn't necessarily safe to use with a subclass. If you do want inherited entries, specify isa_entries:

        isa_entries => {
            "My::Object" => KiokuDB::TypeMap::Entry::Naive->new,

If no normal (ref keyed) entry is found for an object, the isa entries are searched for a superclass of that object. Subclass entries are tried before superclass entries. The result of this lookup is cached, so it only happens once per class.

Typemap Entries

If you want to do custom serialization hooks, you can specify hooks to collapse your object:

        collapse => sub {
            my $object = shift;


            return @some_args;
        expand => sub {
            my ( $class, @some_args ) = @_;


            return $object;

These hooks are called as methods on the object to be collapsed.

For instance the Path::Class related typemap ISA entry is:

    'Path::Class::Entity' => KiokuDB::TypeMap::Entry::Callback->new(
        intrinsic => 1,
        collapse  => "stringify",
        expand    => "new",

The intrinsic flag is discussed in the next section.

Another option for typemap entries is KiokuDB::TypeMap::Entry::Passthrough, which is appropriate when you know the backend's serialization can handle that data type natively.

For example, if your object has a Storable hook which you know is appropriate (e.g. contains no sub objects that need to be collapsible) and your backend uses KiokuDB::Backend::Serialize::Storable. DateTime is an example of a class with such storable hopes:

    'DateTime' => KiokuDB::Backend::Entry::Passthrough->new( intrinsic => 1 )

Intrinsic vs. First Class

In KiokuDB every object is normally assigned an ID, and if the object is shared by several objects this relationship will be preserved.

However, for some objects this is not the desired behavior. These are objects that represent values, like DateTime, Path::Class entries, URI objects, etc.

KiokuDB can be asked to collapse such objects intrinsicly, that is instead of creating a new KiokuDB::Entry with its own ID for the object, the object gets collapsed directly into its parent's structures.

This means that shared references that are collapsed intrinsically will be loaded back from the database as two distinct copies, so updates to one will not affect the other.

For instance, when we run the following code:

    use Path::Class;

    my $path = file(qw(path to foo));



    $dir->store( $obj_1, $obj_2 );


    refaddr($obj_1->file) == refaddr($obj_2->file)

holds true when inserting, but is no longer the case after $obj_1 and $obj_2 are reloaded from the database.

This behavior is usually more appropriate for objects that aren't mutated, but are instead cloned and replaced, and for which creating a first class entry in the backend with its own ID is undesired.


So far we've only made use of KiokuDB::Backend::Hash, so while our objects were serialized, they were not actually stored on disk.

The most mature backend for KiokuDB is KiokuDB::Backend::BDB. It performs very well, and supports many features, like Search::GIN integration to provide customized indexing of your objects and transactions.

Installing KiokuDB::Backend::BDB

KiokuDB::Backend::BDB needs the BerkeleyDB module, and a recent version of Berkeley DB itself, which can be found here:

BerkeleyDB (the library) normally installs into /usr/local/BerkeleyDB.4.7, while BerkeleyDB (the module) looks for it in /usr/local/BerkeleyDB, so adding a symbolic link should make installation easy.

Once you have BerkeleyDB installed, KiokuDB::Backend::BDB should install without problem and you can use it with KiokuDB.

Using KiokuDB::Backend::BDB

To use the BDB backend we must first create the storage. To do this the create flag must be passed:

    my $backend = KiokuDB::Backend::BDB->new(
        manager => {
            home   => Path::Class::Dir->new(qw(path to storage)),
            create => 1,

The BDB backend uses BerkeleyDB::Manager to do a lot of the BerkeleyDB gruntwork. The BerkeleyDB::Manager object will be instantiated using the arguments provided in the manager attribute.

Now that the storage is created we can make use of this backend, much like before:

    my $dir = KiokuDB->new( backend => $backend );

Subsequent opens will not required the create argument to be true, but it doesn't hurt.

As a convenience feature, KiokuDB provides the connect method. The above code could be written more concisely as:

    my $dir = KiokuDB->connect( "bdb:dir=path/to/storage", create => 1 );


Some backends (ones which do the KiokuDB::Backend::Role::TXN role) can be used with transactions.

If you are familiar with DBIx::Class this should be very familiar:

    $dir->txn_do(sub {

This will create a BerkeleyDB level transaction, and all changes to the database are committed if the block was executed cleanly.

If any error occurred the transaction will be rolled back, and the changes will not be visible to subsequent reads.

Note that KiokuDB does not touch live instances, so if you do something like

    $dir->txn_do(sub {
        my $scope = $dir->new_scope;
        $obj->name("Dancing Hippy");

the name attribute is not rolled back, it is simply the store operation that gets reverted.

Transactions will nest properly, and with BDB they generally increase write performance.


KiokuDB:Backend::BDB::GIN is a subclass of KiokuDB::Backend::BDB that provides Search::GIN integration.

Search::GIN is a framework to index and query objects, inspired by Postgres' internal GIN api. GIN stands for Generalized Inverted Indexes.

Using Search::GIN arbitrary search keys can be indexed for your objects, and these objects can then be looked up using queries.

For instance, one of the pre canned searches Search::GIN supports out of the box is class indexing. Let's use Search::GIN::Extract::Class to do class lookups, as is commonly expected of OODBMSs:

    my $dir = KiokuDB->new(
        backend => KiokuDB::Backend::BDB::GIN->new(
            extract => Search::GIN::Extract::Class->new,

    $dir->store( @random_objects );

To look up the objects, we use the corresponding query:

    my $query = Search::GIN::Query::Class->new(
        isa => "Person", # find any object that ->isa("Person")

    my $stream = $dir->search($query);

The result is Data::Stream::Bulk object that represents the search results. It can be iterated as follows:

    while ( my $block = $stream->next ) {
        foreach my $person ( @$block ) {
            print "found a preson: ", $person->name;

Or even more simply, if you don't mind loading the whole resultset into memory:

    my @people = $stream->all;

The way it works internally that Search::GIN::Extract::Class introspects the objects as they are being inserted to the database, and extracts all the keys that Search::GIN::Query::Class would need to later find these objects in the lookup. This process can be heavily customized and supports far more functionality than simple queries like the above. See Search::GIN for more details.