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Skrz\Meta

Different wire formats, different data sources, single object model

Requirements

Skrz\Meta requires PHP >= 5.4.0 and Symfony >= 2.7.0.

Installation

Add as Composer dependency:

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:

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

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:

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:

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:

Fields

• Fields represent set of symbolic field paths.
• They are composite (fields can have sub-fields).
• Fields can be supplied as $filter parameters in to*() methods.

Fields are inspired by:

• Facebook Graph API's ?fields=... query parameter
• Google Protocol Buffers' FieldMask (and its JSON serialization)

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:

@JsonProperty

@JsonProperty marks names of JSON properties. (Internally every group created by @JsonProperty creates PHP group prefixed by json: - PHP object is first mapped to array using json: group, then the array is serialized using json_encode().)

@XmlElement & @XmlElementWrapper & @XmlAttribute & @XmlValue

• Modelled after javax.xml.bind.annotation.
• Works with DOMDocument (for streaming or DOM-based XML APIs).

For more examples see classes in test/Skrz/Meta/Fixtures/XML and test/Skrz/Meta/XmlModuleTest.php.

@PhpDiscriminatorMap & @JsonDiscriminatorMap

@PhpDiscriminatorMap and @JsonDiscriminatorMap encapsulate inheritance.

@PhpDiscriminatorOffset & @JsonDiscriminatorProperty

@PhpDiscriminatorOffset and @JsonDiscriminatorProperty make subclasses differentiated using offset/property.

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

• YAML - just like JSON
• @XmlElementRef

License

The MIT license. See LICENSE file.