Domain model

The main goal of a domain model is refactoring the logic around object manipulation by splitting it to independent layers. Each subsequent layer wraps the previous one creating an “onion” structure, thus realizing a design pattern called “Decorator.” The main feature of domain model is to use a composition instead of inheritance or basic decoration while building an architecture. This provides flexibility and transparency of an internal organization for a developer, because he does not know what layers are used and works with a domain model object as with a common object.

Inner architecture

Each layer defines its own operations’ implementation through a special proxy class. At first, operations are performed on the upper layer, then they successively pass the control to the underlying layers.

The nesting of layers can be specified explicitly using a programmer interface Gateway or implicitly using helper classes. Nesting may also depend on various conditions, skipping or adding additional layers during domain object creation.

Proxies

The layer behavior is described in special proxy classes that must provide exactly the same interface as the original class does. In addition, each proxy class has a field base indicating a lower layer object that is an instance of another proxy or original class.

To access the rest of the fields, you can use special proxy properties or universal methods set_property and get_property.

In addition, the proxy class must have an __init__ format method:

def __init__(self, base, helper_class=None, helper_kwargs=None, **kwargs)

where base corresponds to the underlying object layer, proxy_class and proxy_kwargs are optional and are used to create a helper class. Thus, to access a meth1 method from the underlying layer, it is enough to call it on the base object:

def meth1(*args, **kwargs):
        …
        self.base.meth1(*args, **kwargs)
        …

To get access to the domain object field, it is recommended to use properties that are created by an auxiliary function:

def _create_property_proxy(attr):
    def get_attr(self):
        return getattr(self.base, attr)

    def set_attr(self, value):
        return setattr(self.base, attr, value)

    def del_attr(self):
        return delattr(self.base, attr)

    return property(get_attr, set_attr, del_attr)

So, the reference to the underlying layer field prop1 looks like:

class Proxy(object):
        …
        prop1 = _create_property_proxy('prop1')
        …

If the number of layers is big, it is reasonable to create a common parent proxy class that provides further control transfer. This facilitates the writing of specific layers if they do not provide a particular implementation of some operation.

Gateway

gateway is a mechanism to explicitly specify a composition of the domain model layers. It defines an interface to retrieve the domain model object based on the proxy classes described above.

Example of the gateway implementation

This example defines three classes:

• Base is the main class that sets an interface for all the proxy classes.
• LoggerProxy class implements additional logic associated with the logging of messages from the print_msg method.
• ValidatorProxy class implements an optional check that helps to determine whether all the parameters in the sum_numbers method are positive.

class Base(object):
    ""Base class in domain model."""
    msg = "Hello Domain"

    def print_msg(self):
        print(self.msg)

    def sum_numbers(self, *args):
        return sum(args)

class LoggerProxy(object):
    """"Class extends functionality by writing message to log."""
    def __init__(self, base, logg):
        self.base = base
        self.logg = logg

    # Proxy to provide implicit access to inner layer.
    msg = _create_property_proxy('msg')

    def print_msg(self):
        # Write message to log and then pass the control to inner layer.
        self.logg.write("Message %s has been written to the log") % self.msg
        self.base.print_msg()

    def sum_numbers(self, *args):
        # Nothing to do here. Just pass the control to the next layer.
        return self.base.sum_numbers(*args)

class ValidatorProxy(object):
    """Class validates that input parameters are correct."""
    def __init__(self, base):
        self.base = base

    msg = _create_property_proxy('msg')

    def print_msg(self):
        # There are no checks.
        self.base.print_msg()

    def sum_numbers(self, *args):
        # Validate input numbers and pass them further.
        for arg in args:
            if arg <= 0:
                return "Only positive numbers are supported."
        return self.base.sum_numbers(*args)

Thus, the gateway method for the above example may look like:

def gateway(logg, only_positive=True):
    base = Base()
    logger = LoggerProxy(base, logg)
    if only_positive:
        return ValidatorProxy(logger)
    return logger

domain_object = gateway(sys.stdout, only_positive=True)

It is important to consider that the order of the layers matters. And even if layers are logically independent from each other, rearranging them in different order may lead to another result.

Helpers

Helper objects are used for an implicit nesting assignment that is based on a specification described in an auxiliary method (similar to gateway). This approach may be helpful when using a simple factory for generating objects. Such a way is more flexible as it allows specifying the wrappers dynamically.

The helper class is unique for all the proxy classes and it has the following form:

class Helper(object):
    def __init__(self, proxy_class=None, proxy_kwargs=None):
        self.proxy_class = proxy_class
        self.proxy_kwargs = proxy_kwargs or {}

    def proxy(self, obj):
        """Wrap an object."""
        if obj is None or self.proxy_class is None:
            return obj
        return self.proxy_class(obj, **self.proxy_kwargs)

    def unproxy(self, obj):
        """Return object from inner layer."""
        if obj is None or self.proxy_class is None:
            return obj
        return obj.base

Example of a simple factory implementation

Here is a code of a simple factory for generating objects from the previous example. It specifies a BaseFactory class with a generate method and related proxy classes:

class BaseFactory(object):
    """Simple factory to generate an object."""
    def generate(self):
        return Base()

class LoggerFactory(object):
    """Proxy class to add logging functionality."""
    def __init__(self, base, logg, proxy_class=None, proxy_kwargs=None):
        self.helper = Helper(proxy_class, proxy_kwargs)
        self.base = base
        self.logg = logg

    def generate(self):
        return self.helper.proxy(self.base.generate())

class ValidatorFactory(object):
    """Proxy class to add validation."""
    def __init__(self, base, only_positive=True, proxy_class=None, proxy_kwargs=None):
        self.helper = Helper(proxy_class, proxy_kwargs)
        self.base = base
        self.only_positive = only_positive

    def generate(self):
        if self.only_positive:
            # Wrap in ValidatorProxy if required.
            return self.helper.proxy(self.base.generate())
        return self.base.generate()

Further, BaseFactory and related proxy classes are combined together:

def create_factory(logg, only_positive=True):
    base_factory = BaseFactory()
    logger_factory = LoggerFactory(base_factory, logg,
                                   proxy_class=LoggerProxy,
                                   proxy_kwargs=dict(logg=logg))
    validator_factory = ValidatorFactory(logger_factory, only_positive,
                                         proxy_class = ValidatorProxy)
    return validator_factory

Ultimately, to generate a domain object, you create and run a factory method generate which implicitly creates a composite object. This method is based on specifications that are set forth in the proxy class.

factory = create_factory(sys.stdout, only_positive=False)
domain_object = factory.generate()

Why do you need a domain if you can use decorators?

In the above examples, to implement the planned logic, it is quite possible to use standard Python language techniques such as decorators. However, to implement more complicated operations, the domain model is reasonable and justified.

In general, the domain is useful when:

• there are more than three layers. In such case, the domain model usage facilitates the understanding and supporting of the code;
• wrapping must be implemented depending on some conditions, including dynamic wrapping;
• there is a requirement to wrap objects implicitly by helpers.