Serialization and injection
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Serialization is a form of persistence: serialized data survives the process and the RAM where it was created and can be reconstituted inside different processes and machines that live in a different time or place. Sometimes serialization is a poor form of persistence in fact, one that confuses the boundary between the different schemas the data can fit in.
However, what I found useful in the last years of development is to institute a strict separation: serialize Value Objects, Entities, and everything that represents the state of the application. Meanwhile, use Dependency Injection over services that are part of a larger object graph and never serialize this second kind of objects.
In the discussion that follows, I make the assumption that serialization and deserialization occur on the same machine (e.g. like for web-oriented sessions.) The problem with serialization, which work transparently most of the time, is the need to serialize service objects instead of limiting the procedure to data structures. How can you store such objects?
Some options to solve this problems are really not options. Serialization by itself will fail because of the staleness of the references contained in these objects.
For example, in PHP trying to serialize a database connections composed by a Repository or DAO object will rightly fail with an exception. Whenever an object represents a resource of the current machine, it cannot usually be serialized except in the case when the only resource involved is RAM.
If the resource is disk space or other running processes such as a database daemon, the reconstitution of the object in another place and time will fail and it's best to just stop the developer immediately during storage.
Some solutions to the problem try to avoid the staleness problem by serializing objects without their resources, and make them regrab a new version of them on deserialization.
In PHP for example, this can be done with the __sleep() and __wakeup() magic methods, called automatically during serialization and deserializaton respectively. This deserialization mechanism introduces a dependency from the serialized Entity to external services: such a dependency is already in place when building the object the first time (passing the XService in the constructor) but it is aggravated when deserializing (depending on a XServiceFactory instead of just an XService).
An improvement, from the dependencies point of view, is to reattach collaborators to deserialized objects like you would for other persistence-related tasks. For example, EntityRepository can inject the missing pieces of Entity every time its find() method is called.
However, there is still another option, which is the most resilient from the modelling point of view and not only that of dependency management: injecting non-serializable collaborators through the stack.
Objects can collaborate even without keeping field references to each other, and injecting dependencies as parameters move the dependency starting point from the server to the client object (which may or may not be desirable). What is most important is that Entities are relieved of having to manage external references in any context, not only that of persistence and in particular serialization.
The metaphor for the 3rd option
Misko Hevery likes to say: have you ever seen a credit card able to charge itself? If a CreditCard is an Entity in your domain, it would be very strange to keeping a wire attached to your wallet wherever you go.
With the first option, you have the card spring a wire when it is taken out of the wallet, like in horror movies. This intelligent cable tries as its best to attach to the nearest Point of Sale (a bad case of bluetooth I think).
With Repositories in mind, you're not dealing with automated wires anymore, but you're still attaching cables between cards and fixed devices.
In reality, cards collaborate with the PoS in a fast process that does not last more than a few seconds. Actually, sometimes they don't touch it at all, as in all Internet-based purchases.
Keeping services around to deal with external dependencies does not mean the API of your Domain Model has to be biased towards service objects:
pos.charge(creditCard); // can equivalently be: creditCard.chargeOn(pos);
This is a form of Double Dispatch since there are two objects collaborating and you can dispatch (send messages) to both, being polimorphic by substituting both objects. The sequence of calls is:
client -> creditCard -> pos
The client object still looks at CreditCard as a behaviorally complete object, but it is clear which dependency is necessary to run each use case (CreditCard method). You can persist a CreditCard easily and send it over the wire to caches or databases. When it comes the time to charge, it is the client that has to bring forward a service able to connect to a bank.
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