KiokuDB::Tutorial(3)

NAME
KiokuDB::Tutorial - Getting started with KiokuDB

INSTALLATION
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 or KiokuDB::Backend::DBI are the recommended backends.

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

CREATING A DIRECTORY
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.

You can also use DSN strings to connect to the various backends:

    KiokuDB->connect("hash");

    KiokuDB->connect("dbi:SQLite:dbname=foo", create => 1);

    KiokuDB->connect("bdb:dir=foo", create => 1);

Or use a configuration file

    KiokuDB->connect("/path/to/my_db.yml");
    KiokuDB->connect("/path/to/dir");

With a configuration file like this:

    backend:
      class: KiokuDB::Backend::DBI
      dsn: dbi:SQLite:dbname=/tmp/test.db
      create: 1

USING THE DBI BACKEND
During this tutorial we will be using the DBI backend for two reasons. The first is DBI's ubiquity - almost everyone has used and knows how to install and use it. The second is the possibility of easily looking behind the scenes, to more clearly demonstrate what KiokuDB is doing.

That said, the examples will work with all backends exactly the same.

The $dir variable used below is created like this:

    my $dir = KiokuDB->connect(
        "dbi:SQLite:dbname=kiokudb_tutorial.db",
        create => 1,
    );

Note that if you are connecting with a username and password you must specify these as named arguments:

    my $dir = KiokuDB->connect(
        $dsn,
        user     => $user,
        password => $password,
    );

INSERTING OBJECTS
Let's start by defining a simple class using Moose:

    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.

LOADING OBJECTS
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.

WHAT WAS STORED
Let's peek into the database momentarily. Launch the SQL command line tool to your database:

    % sqlite3 kiokudb_tutorial.db
    SQLite version 3.4.0
    Enter ".help" for instructions
    sqlite>

The database schema has two tables, "entries" and "gin_index":

    sqlite> .tables
    entries    gin_index

"gin_index" is used for more complex queries, and we'll get back to it at the end of the tutorial.

Let's have a closer look at "entries":

    sqlite> .schema entries
    CREATE TABLE entries (
      id varchar NOT NULL,
      data blob NOT NULL,
      class varchar,
      root boolean NOT NULL,
      tied char(1),
      PRIMARY KEY (id)
    );

The main columns are "id" and "data". In KiokuDB every object has an ID which serves as a primary key and a BLOB of data associated with it.

Since the default serializer for the DBI backend is KiokuDB::Serializer::JSON, we can peek at the data.

First we'll set "sqlite"'s output mode to "line". This is easier to read with large columns:

    sqlite> .mode line

And select the data from the table:

    sqlite> select id, data from entries;
       id = 201C5B55-E759-492F-8F20-A529C7C02C8B
     data = {"__CLASS__":"Person","data":{"name":"Homer Simpson"},"id":"201C5B55-E759-492F-8F20-A529C7C02C8B","root":true}

As you can see the "name" attribute is stored under the "data" key inside the blob, as is the object's class.

The "data" column contains all of the data necessary to recreate the object.

All the other columns are used solely for lookups. Later on we'll show how to create more search columns.

When using KiokuDB::Backend::BDB the on-disk format is actually a hash of "id" to "data".

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

    package Person;

    has spouse => (
        isa => "Person",
        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, let's link them together:

    {
        my $scope = $dir->new_scope;

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

        $marge->spouse($homer);
        $homer->spouse($marge);

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

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

The reason "spouse" 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.

REFERENCES IN THE DATABASE
Now that we have an object graph in the database let's have another look at what's inside.

    sqlite> select id, data from entries;
       id = 201C5B55-E759-492F-8F20-A529C7C02C8B
     data = {"__CLASS__":"Person","data":{"name":"Homer Simpson","spouse":{"$ref":"05A8D61C-6139-4F51-A748-101010CC8B02.data"}},"id":"201C5B55-E759-492F-8F20-A529C7C02C8B","root":true}

       id = 05A8D61C-6139-4F51-A748-101010CC8B02
     data = {"__CLASS__":"Person","data":{"name":"Marge Simpson","spouse":{"$ref":"201C5B55-E759-492F-8F20-A529C7C02C8B.data"}},"id":"05A8D61C-6139-4F51-A748-101010CC8B02","root":true}

You'll notice the "spouse" field has a JSON object with a $ref field inside it holding the UUID of the target object.

When data is loaded KiokuDB queues up references to unloaded objects and then loads them in order to materialize the memory resident object graph.

If you're curious about why the data is represented this way, this format is called "JSPON", or JavaScript Persistent Object Notation (<http://www.jspon.org/>). When using KiokuDB::Backend::Storable the KiokuDB::Entry and KiokuDB::Reference objects are serialized with their storable hooks instead.

OBJECT SETS
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);

    $homer->children($set);

    $dir->store($homer);

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.

THE TYPEMAP
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 inheritance, 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":

    KiokuDB::TypeMap->new(
        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:

    KiokuDB::TypeMap::Entry::Callback->new(
        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));

    $obj_1->file($path);

    $obj_2->file($path);

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

While the following is true when the data is being inserted, it will no longer be true when $obj_1 and $obj_2 are loaded from the database:

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

This is because both $obj_1 and $obj_2 each got its own copy of $path.

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.

  The Default Typemap
Each backend comes with a default typemap, with some built in entries for common CPAN modules' objects. KiokuDB::TypeMap::Default contains more details.

SIMPLE SEARCHES
Most backends support an inefficient but convenient simple search, which scans the entries and matches fields.

If you want to make use of this API we suggest using KiokuDB::Backend::DBI since simple searching is implemented using an SQL where clause, which is much more efficient (you do have to set up the column manually though).

Calling the "search" method with a hash reference as the only argument invokes the simple search functionality, returning a Data::Stream::Bulk with the results:

    my $stream = $dir->search({ name => "Homer Simpson" });

    while ( my $block = $stream->next ) {
        foreach my $object ( @$block ) {
            # $object->name eq "Homer Simpson"
       }
    }

This exact API is intentionally still underdefined. In the future it will be compatible with DBIx::Class 0.09's syntax.

  DBI SEARCH COLUMNS
In order to make use of the simple search API we need to configure columns for our DBI backend.

Let's create a 'name' column to search by:

    my $dir = KiokuDB->connect(
        "dbi:SQLite:dbname=foo",
        columns => [
            # specify extra columns for the 'entries' table
            # in the same format you pass to DBIC's add_columns

            name => {
                data_type => "varchar",
                is_nullable => 1, # probably important
            },
        ],
    );

You can either alter the schema manually, or use "kioku dump" to back up your data, delete the database, connect with "create => 1" and then use "kioku load".

To populate this column we'll need to load Homer and update him:

    {
        my $s = $dir->new_scope;
        $dir->update( $dir->lookup( $homer_id ) );
    }

And this is what it looks in the database:

       id = 201C5B55-E759-492F-8F20-A529C7C02C8B
     name = Homer Simpson

GETTING STARTED WITH BDB
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.

KiokuDB::Backend::DBI is newer and not as tested, but also supports transactions and Search::GIN based queries. It performs quite well too, but isn't as fast as KiokuDB::Backend::BDB.

  Installing KiokuDB::Backend::BDB
KiokuDB::Backend::BDB needs the BerkeleyDB module, and a recent version of Berkeley DB itself, which can be found here: <http://www.oracle.com/technology/software/products/berkeley-db/db/index.html>.

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 require the "create" argument to be true, but it doesn't hurt.

This "connect" call is equivalent to the above:

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

TRANSACTIONS
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 {
        $dir->store($obj);
    });

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");
        $dir->store($obj);

        die "an error";
    });

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

Transactions will nest properly, and with most backends they generally increase write performance as well.

QUERIES
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::Callback to do custom indexing of our objects:

    my $dir = KiokuDB->new(
        backend => KiokuDB::Backend::BDB::GIN->new(
            extract => Search::GIN::Extract::Callback->new(
                extract => sub {
                    my ( $obj, $extractor, @args ) = @_;

                    if ( $obj->isa("Person") ) {
                        return {
                            type => "user",
                            name => $obj->name,
                        };
                    }

                    return;
                },
            ),
        ),
    );

    $dir->store( @random_objects );

To look up the objects, we use the a manual key lookup query:

    my $query = Search::GIN::Query::Manual->new(
        values => {
            type => "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 person: ", $person->name;
        }
    }

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

    my @people = $stream->all;

Search::GIN is very much in its infancy, and is very under documented. However it does work for simple searches such as this and contains pre canned solutions like Search::GIN::Extract::Class.

In short, it works today, but watch this space for new developments.