Skrz\Meta

Why?

At Skrz.cz, we work heavily with many different input/output formats and data sources (databases). E.g. data from partners come in as XML feeds; internally our micro-service architecture encodes data into JSON as wire format; data can come from MySQL, Redis, and Elasticsearch databases, and also has to be put in there.

However, in our PHP code base we want single object model that we could also share between projects. This need came mainly from micro services' protocols that got quite wild - nobody really knew what services sent to each other.

Serialization/deserialization had to be fast, therefore we created concept of so-called meta classes. A meta class is an object's companion class that handles object's serialization/deserialization from/into many different formats. Every class has exactly one meta class, in which methods from different modules are combined - modules can use each others methods (e.g. JsonModule uses methods generated by PhpModule).

Usage

Have simple value object:

namespace Skrz\API;

class Category
{
    /** @var string */
    public $name;

    /** @var string */
    public $slug;

    /** @var Category */
    public $parentCategory;

}

You would like to serialize object into JSON. What you might do is to create method toJson:

public function toJson()
{
    return json_encode(array(
        "name" => $this->name,
        "slug" => $this->slug,
        "parentCategory" => $this->parentCategory ? $this->parentCategory->toJson() : null
    ));
}

Creating such method for every value object that gets sent over wire is tedious and error-prone. So you generate meta class that implements such methods.

Meta classes are generated according to meta spec. A meta spec is a class extending Skrz\Meta|AbstractMetaSpec:

namespace Skrz;

use Skrz\Meta\AbstractMetaSpec;
use Skrz\Meta\JSON\JsonModule;
use Skrz\Meta\PHP\PhpModule;

class ApiMetaSpec extends AbstractMetaSpec
{

    protected function configure()
    {
        $this->match("Skrz\\API\\*")
            ->addModule(new PhpModule())
            ->addModule(new JsonModule());
    }

}

Method configure() initializes spec with matchers and modules. A matcher is a set of classes that satisfy certain criteria (e.g. namespace, class name). A module is generator that takes class matched by the matcher and generates module-specific methods in the meta class. ApiMetaSpec creates meta classes for every class directly in Skrz\API namespace (it does not include classes in sub-namespaces, e.g. Skrz\API\Meta). The meta classes are generated from PHP and JSON modules (Skrz\Meta\BaseModule providing basic functionality of a meta class is added automatically).

To actually generate classes, you have supply some files to spec to process:

use Symfony\Component\Finder\Finder;

$files = array_map(function (\SplFileInfo $file) {
    return $file->getPathname();
}, iterator_to_array(
    (new Finder())
        ->in(__DIR__ . "/API")
        ->name("*.php")
        ->notName("*Meta*")
        ->files()
));

$spec = new ApiMetaSpec();
$spec->processFiles($files);

Similar code should be part of your build process (or in development part of Grunt watch task etc.).

By default, spec generates meta class in Meta sub-namespace with Meta suffix (e.g. Skrz\API\Category -> Skrz\API\Meta\CategoryMeta) and stores it inside Meta sub-directory of original class's directory.

After the meta classes has been generated, usage is quite simple:

use Skrz\API\Category;
use Skrz\API\Meta\CategoryMeta;


$parentCategory = new Category();
$parentCategory->name = "The parent category";
$parentCategory->slug = "parent-category";

$childCategory = new Category();
$childCategory->name = "The child category";
$childCategory->slug = "child-category";
$childCategory->parentCategory = $parentCategory;


var_export(CategoryMeta::toArray($childCategory));
// array(
//     "name" => "The child category",
//     "slug" => "child-category",
//     "parentCategory" => array(
//         "name" => "The parent category",
//         "slug" => "parent-category",
//         "parentCategory" => null,
//     ),
// )


echo CategoryMeta::toJsonString($childCategory);
// {"name":"The child category","slug":"child-category","parentCategory":{"name":"The parent category","slug":"parent-category","parentCategory":null}}


$someCategory = CategoryMeta::fromJson(array(
    "name" => "Some category",
    "ufo" => 42, // unknown fields are ignores
));

var_export($someCategory instanceof Category);
// TRUE

var_export($someCategory->name === "Some category);
// TRUE

Annotations

Skrz\Meta uses Doctrine annotation parser. Annotations can change mappings. Also Skrz\Meta offers so called groups - different sources can offer different field names, however, they map onto same object.

@PhpArrayOffset

@PhpArrayOffset annotation can be used to change name of outputted keys in arrays generated by toArray and inputs to fromArray:

namespace Skrz\API;

use Skrz\Meta\PHP\PhpArrayOffset;

class Category
{
    /**
     * @var string
     * @PhpArrayOffset("THE_NAME")
     * @PhpArrayOffset("name", group="javascript")
     */
    protected $name;

    /**
     * @var string
     * @PhpArrayOffset("THE_SLUG")
     * @PhpArrayOffset("slug", group="javascript")
     */
    protected $slug;

    public function getName() { return $this->name; }

    public function getSlug() { return $this->slug; }

}

// ...

use Skrz\API\Meta\CategoryMeta;

$category = CategoryMeta::fromArray(array(
    "THE_NAME" => "My category name",
    "THE_SLUG" => "category",
    "name" => "Different name" // name is not an unknown field, so it is ignored
));

var_export($category->getName());
// "My category name"

var_export($category->getSlug());
// "category"

var_export(CategoryMeta::toArray($category, "javascript"));
// array(
//     "name" => "My category name",
//     "slug" => "category",
// )

@JsonProperty

namespace Skrz\API;

use Skrz\Meta\PHP\PhpArrayOffset;
use Skrz\Meta\JSON\JsonProperty;

class Category
{
    /**
     * @var string
     * @PhpArrayOffset("THE_NAME")
     * @JsonProperty("NAME")
     */
    protected $name;

    /**
     * @var string
     * @PhpArrayOffset("THE_SLUG")
     * @JsonProperty("sLuG")
     */
    protected $slug;

    public function getName() { return $this->name; }

    public function getSlug() { return $this->slug; }

}

// ...

use Skrz\API\Meta\CategoryMeta;

$category = CategoryMeta::fromArray(array(
    "THE_NAME" => "My category name",
    "THE_SLUG" => "category",
));

var_export(CategoryMeta::toJson($category));
// (object) array(
//     "NAME" => "My category name",
//     "sLuG" => "category",
// )

Known limitations

• private properties cannot be hydrated. Hydration of private properties would require using reflection, or using unserialize() hack, which is contrary to requirement of being fast. Therefore meta classes compilation will fail if there is a private property. If you need a private property, mark it using @Transient annotation and it will be ignored.

• There can be at most 31/63 groups in one meta class. Group name is encoded using bit in integer type. PHP integer is platform dependent and always signed, therefore there can be at most 31/63 groups depending on platform the PHP's running on.

TODO

• handling recursive structures
• YAML - just like JSON
• XML serialization/deserialization (model annotations according to javax.xml.bind.annotation)
  • DOMNode
  • SimpleXMLElement
  • XMLReader
  • XMLWriter

License

The MIT license. See LICENSE file.