I asked this question a while back but now I'm looking to implement an actual separation between my database access layer and the domain layer. I am also going to be working to move business logic into the domain where it belongs and out of the controller scripts.
I'm using Zend Framework which implements the Table Data Gateway and Row Data Gateway patterns for the data access layer, but it apparently fails to really define how to build a domain layer that is separate from the data access layer. I've considered using an Active Record pattern where the domain logic coexists with the data access logic, but I have the following situation that occurs at least once that I don't think Active Record will handle:
I have a single table "Person" which contains person_id and userType fields.
Each userType (admin, buyer, associate, supervisor) has specific business logic associated with it and all types inherit some basic functionality from a Person object.
I don't want to bloat the Row Data Gateway object with business logic that belongs specifically to just one type of user but I'm not certain how to construct the domain layer to represent the different types of users. For example, do I make a Person object that contains the PersonGateway object and then write wrapper functions that pass calls to the gateway object, or do I write the Person object to extend the PersonGateway object and then only implement the specific functions that I need?
Likewise, I would typically think that this is (in part) a factory problem where I need a factory method that will instantiate the correct sub-class based on userType. Is that still the best method here with Zend Framework's Zend_Db class?
Any suggestions or links to tutorials that talk about how to properly create a domain model on top of Zend_Db would be greatly appreciated.
Domain Models extend nothing. They're just plain classes you use to encapsulate business logic. They may use data access objects, so there may be a protected instance of a row data gateway object inside the class. A Row object usually represents an instance of the domain more closely than a Table object. Besides, you can always get the Table object with the Row's getTable() method.
Typically DM classes have an interface with methods corresponding to higher-level operations you can do with that class. But you don't necessarily want to surface all data access operations.
class Person {
// Zend_Db_Table_Row object
protected $data;
public function subscribeToService(Service $service) { ... }
public function sendMailTo(Person $recipient) { ... }
public function changePassword($newPassword) { ... }
}
I also blogged about this subject last spring, and wrote about it on the ZF mailing list recently.
As far as tutorials and resources, try http://domaindrivendesign.org/
Related
One of the things that I don't fully understand about the MVC model is loading multiple data tables from the model.
Let's say we have a Model called user.
public class User{
protected $username;
protected $email;
protected $id;
public function setUsername($name, $id) {
$db->set->name($name)
$db->set->id($id)
}
public function getUsernameById($id) {
return $db->get->name($id);
}
As far as I've learned, the model is equivalent to a single row in the database (called user).
Now I have written two functions. The first is for setting the username and the other is for retrieving the username through an ID.
Suppose you want to retrieve all users. Let's say through the getAllUsers() method. This method does not fit on the Model as it is not a single object.
Now I understand that for example, you can call this method (function) in your controller itself. But where can you define this method (function)? Since you don't do this on the model.
I also like to hear it if I am wrong :)
Basically your definition is correct:
Model objects hold data and define the logic for manipulating that
data. For example, a Student object in the Basic sample application is
a model object. It holds data describing facts about the object like
the first and last name of the student and has methods that can access
and change this data (getters & setters). Model objects are not
directly displayed. They often are reusable, distributed, persistent
and portable to a variety of platforms.
(from: http://best-practice-software-engineering.ifs.tuwien.ac.at/patterns/mvc.html)
So where to put the getAllUsers() function in case you do not just put it into the model?
Well first of all when we are talking about the term "model" we do not always mean model. In MVC the model (M) contains all of the business logic. This means it contains the domain model which is the model you are meaning in this question. But it also contains the service layer.
The service layer usually consists of various classes which provide functionalities like for instance getAllUsers() or getAllUsersWhichOwnAGreenTruck().
Here is a great graphic demonstrating this:
(Source: Domain Model and Service Layer patterns in P of EAA)
In modern applications e.g. built with the Laravel PHP Framework you usually generate the database as well as the model and the database mapping/communication automatically (Example Eloquent ORM: https://laravel.com/docs/5.8/eloquent). But you start implementing complex application logic within the service layer.
I'm a bit confused on how to perform insert and update statements using MVC.
Is it ok to create an instance of an object in your controller and pass it to your service to save it or do you pass data to your service and handle everything else in there?
Insert
In my controller something like:
$userservice->insert("myname","mypassword");
In my UserService:
function insert($username,$password){
$user = ORM::for_table('user')->create();
$user->username= $username;
$user->password= $password;
$user->save();
}
Update
In my controller something like:
$userservice->update("myname","mypassword",1);
In my UserService:
function insert($username,$password,$id){
$user = ORM::for_table('user')->find($id);
$user->username= $username;
$user->password= $password;
$user->save();
}
Is this good practice?
Because I see a lot of these answers where for example a user is being created in the controller and passed to a repository to save it:
Proper Repository Pattern Design in PHP?
But I don't like the idea of creating a user in the controller...
Controllers belongs to application layer and controlls only activity. In your example the activities are Create- and Update for an existing or for a new User. These operations belongs to the Domain Layer, which contains services. Thus services encapsulate the domain as a gatekeeper and provides operations for resolving domain like a facade.
Is it ok to create an instance of an object in your controller and pass it to your service to save it or do you pass data to your service and handle everything else in there?
The service should provide a method to pass a ValueObject. ValueObjects are better to encapsulate lot of data (Property values for User). Inside the service, the ValueObject should be delegated to Filter and Validator. If validation didn't fail the ValueObject will be delegated to a DataMapper. The DataMapper will map the properties of ValueObject to a data-model for the UserRepository (ORM). Repositories often need another model of data, e.g. Objects versus storage mediums based on RDBMS like MySQL.
This approach should be strict to seperate the concerns between layers to improve maintainabilty and interchangeabilty. Services should be thin and acts as a delegator to Domain Objects (Filter, Validator, etc.), for example see Service Layer Pattern.
So, where should be a value object created?
I would prefer that the service provides a method for this: getEntityPrototype() by using the prototype pattern.
Be careful with naming. ValueObject is an object which have no identity. Entity is an object with identity (here id of User). For an existing User you will have have a method like getUserById($id), which should return an UserEntity. If User does not exist for given id, it should return a NullObject. To create a new User getEntityPrototype() will return an UserEntity which have no identity yet, so you will call it ValueObject or better Prototype of Entity. After setting properties (e.g. by a FormObject) and persisting this object is a real entity. In a Factory for this service you can set the EntityPrototype.
What you should think about in this case is if the classes have only one responsibility.
Controller decides about the flow of the action. If there's a need for registering a user then it registers him, but it should not define how to do it, but ask a service to complete this task and get the result.
On the other hand you should have some kind of UserManager which updates, creates and fetches users - is this single responsibility? Kinda, yes - it's managing them in a broad sense.
There's a slight problem you have with your methods' names though. You should have registerUser not insert since it's way easier to tell what it actually does.
You should Pass Data to Model. MVC is all about dividing tasks Controller - Handles Application Flow, Model - Contains all the business login Database etc and View - here you decide how to show. Basically the UI part is stored here
So the Controller should send data to Model and model decides what to do with the data. The advantage of coding this way is that in future if you want to change something in the code you know where to look, or if you ask a designer to redesign your website you only have to give him the VIEW part of code . If the designer does something that caused an error , correcting that wont take that much time. If you follow MVC properly Adding,Updating or Maintaining functionality wont be a problem
So I've been studying the role of the repository pattern as a means of decoupling the persistence layer from my models in an MVC framework. Prior to this, I might have my UserModel calling active record methods directly in order to store/retrieve domain objects.
Here's a sketch of what I'm thinking in regards to the call stack in a request that should create a new User:
Here are my questions:
Is this a correct implementation of the repository pattern?
I understand that the controller should take the user's information from the request and pass it into the model. How does that usually happen? Should the controller create a User object and then pass that into the model? I sure as heck don't wanna just pass in an array of values into the model--nor do I want to pass in 15 arguments to the model method that creates a user.
In order for this pattern to really work, it looks like to me I would need to have a domain object that is just a simple data structure with no behavior and then if I'm using an ORM, I would have an ORM object which will describe how the object is persisted. Initially I resisted this because it feels like duplicate code, but if I'm really separating persistence from the business logic, this would be needed right? For example, what if I went with an in-memory store? I would no longer use the ORM object.
Am I thinking correctly here? Is this acceptable. Please help me connect the dots in my head.
1. Is this a correct implementation of the repository pattern?
I'm not sure where you been doing that research, but you have got it wrong.
Repositories as for separating the domain objects from data mappers.
There no such thing as "models". Model in MVC design pattern is one of the to main layers: presentation layer and model layer.
And the repository pattern is incompatible with active record (anti)pattern, which combines domain and storage logic in single instance, thus causing a major SRP violation.
To use a real world example for, when and how to use a repository here is an example:
You are creating some document management tool, where said documents can come from several sources (for example: local SQL database, SOAP service and cache). In this situation you create a repository, which deals with the "routing" of storage. It is the part of application, that decides which data mapper to use for storing/retrieving each document.
The goal of repository is to separate the domain logic from the interaction with storage. For the system, that was described above, a repository would also let add new data sources, without need to rewrite large amounts of code (if any). You could just add another type of mapper for the document.
2. Should the controller create a User object and then pass that into the model?
To begin with, controller itself should not create anything. Instead your controller should use a factory for acquiring instance of the object that you need. This factory can be provided to the controller through constructor or some other method. This is called: Dependency Injection (to learn more about it, watch this lecture).
Also, as noted above, model is a layer, not any specific class or object. The responsibility of controller is to alter the state of model layer (by passing data to it). You could interact with domain objects and mappers (or repositories) directly in the controller, but it would mean leaking some of the business logic in the controller. It is recommended to instead use services, which then manipulates said domain objects and storage related structures.
As for the issue with 10+ parameter, that you would require for creation of new user account, let's assume you have action with following footprint:
public function postUser( Request $request )
{
....
}
If the action gets called with specific Request instance, you have two options how to deal with large amount of parameters:
Wrap the instance in a decorator, which would let you call a single method for forming the data from request in a specific array. Then you pass this array to the service(s).
Form the array inside the controller's action and pass it, where th data is required.
The former solution is more suited for large scale applications, where such formation of data would be required repeatedly though-out the code. But in a small/medium projects the second option is the common-sense approach.
Thing is, the job of the controller is to take the user's input, and distribute it to the model layer and current view. And formation of such array fits right-in with this mandate.
3. (..) main object that is just a simple data structure with no behavior and then (..)
No. Domain object is not "simple data". It is where most of the domain business logic resides in the application.
And forget about magical ORMs. First step for implementing a repository is to separate the domain and storage logic. Domain object handles the validation and business rules, mapper deals with persistence and data integrity (small example here).
Another thing that you must realize is that repositories for web application do not really interact with in-memory persistence (apart from cache). Instead your repository would be juggling mappers for different data sources.
Should the controller create a User object and then pass that into the model?
I'm not sure what you mean by "pass that into the model" -- the User object is the model. "Controller" and "model" represent different layers in the design, they are not specific objects, and there shouldn't be a separate UserModel object as you mentioned.
The repository interface itself is generally considered part of the model, though the domain objects shouldn't be saving themselves -- this should be done in the controller.
Your controller's job would then be to interpret the request and create a User object, then use the repository to save the user:
$user = new User(...); // based on Request
$repository->save($user);
it looks like to me I would need to have a domain object that is just a simple data structure with no behavior
This is not true, you can and should encapsulate behaviour in your domain objects. As for how persistence is actually implemented, a good ORM should take care of most of the details and you shouldn't have to create additional classes by hand.
The last few days, I have extensively read books and web pages about OOP and MVC in PHP, so that I can become a better programmer. I've come upon a little problem in my understanding of MVC:
Where do I put a mysql_query?
Should I put it in the controller and call a method on a model that returns data based on the provided query? Or should I put it in the model itself? Are both of the options I'm providing total garbage?
Materials on the subject of MVC
You could have listed the books you were reading, because most (if not all) php books, which touch on MVC, are wrong.
If you want to become a better developer, i would recommend for you to start with article by Marting Fowler - GUI Architectures. Followed by book from same author - "Patterns of Enterprise Application Architecture". Then the next step would be for you to research SOLID principles and understand how to write code which follows Law of Demeter. This should cover the basics =]
Can I use MVC with PHP ?
Not really. At least not the classical MVC as it was defined for Smalltalk.
Instead in PHP you have 4 other patterns which aim for the same goal: MVC Model2, MVP, MVVM and HMVC. Again, I am too lazy to write about differences one more time, so I'll just link to an old comment of mine.
What is Model ?
First thing you must understand is that Model in MVC is not a class or an object. It is a layer which contains multitude of classes. Basically model layer is all of the layers combined (though, the second layer there should be called "Domain Object Layer", because it contains "Domain Model Objects"). If you care to read quick summary on what is contained in each part of Model layer, you can try reading this old comment (skip to "side note" section).
The image is taken from Service Layer article on Fowler's site.
What does the Controllers do ?
Controller has one major responsibilities in MVC (I'm gonna talk about Model2 implementation here):
Execute commands on structures from model layer (services or domain objects), which change the state of said structures.
It usually have a secondary responsibility: to bind (or otherwise pass) structures from Model layer to the View, but it becomes a questionable practice, if you follow SRP
Where do I put SQL related code ?
The storage and retrieval of information is handled at the Data Source Layer, and is usually implemented as DataMapper (do not confuse with ORMs, which abuse that name).
Here is how a simplified use of it would look like:
$mapper = $this->mapperFactory->build(Model\Mappers\User::class);
$user = $this->entityFactory->build(Model\Entities\User::class);
$user->setId(42);
$mapper->fetch($user);
if ($user->isBanned() && $user->hasBannExpired()){
$user->setStatus(Model\Mappers\User::STATUS_ACTIVE);
}
$mapper->store($user);
As you see, at no point the Domain Object is even aware, that the information from it was stored. And neither it cases about where you put the data. It could be stored in MySQL or PostgreSQL or some noSQL database. Or maybe pushed to remote REST API. Or maybe the mapper was a mock for testing. All you would need to do, to replace the mapper, is provide this method with different factory.
Also, please see these related posts:
understanding MVC Views in PHP
testable Controllers with dependencies
how should services communicate between each other?
MVC for advanced PHP developers
Model and Entity Classes represents the data and the logic of an application, what many calls business logic. Usually, it’s responsible for:
Storing, deleting, updating the application data. Generally it includes the database operations, but implementing the same operations invoking external web services or APIs is not an unusual at all.
encapsulating the application logic. This is the layer that
should implement all the logic of the application
Here is the MVC Sequence Diagram which shows the flow during a http request:
In this case Model is the best place to implement the code realted to access database.
The model contains the domain objects or data structures that represent the application's state. [wikipedia]. So the model would be the place to make the database call.
In the 'classic' (lack of a better word atm) MVC pattern the view would get the current state from the model.
Don't make the mistake by saying that the model is for accessing the database. It's more than just accessing the database.
For one, don't use mysql_query() and family; they're being deprecated, so consider also learning about PDO and/or mysqli.
The model takes care of data handling; it provides an interface to the controller by which it retrieves and/or stores information. So this would be a primary place where database actions take place.
Update
To answer a question asked by the OP in the comments: "one generic model for the whole db or a model for each table/action?"
Models are meant to abstract away individual tables (although there are models that exclusively handle a single table); for instance, instead of asking for all articles and then query the usernames for the authors you would have one function like this:
function getArticles()
{
// query article table and join with user table to get username
}
How many models you will create largely depends on how big the project is and how inter-related the data is. If you can identify independent groups of data, it's likely that you'd create a model for each group; but this is no hard & fast rule.
Data manipulation can be part of the same model, unless you want a clear separation between read-only and write-only models (I wouldn't know of a situation that warrants this, but who knows).
To go even further, your model should not contain the database access code. This belongs to another layer outside the Model/View/Controller: this is called the persistence layer, which can be implemented using an Object-Relational Mapper such as the popular Doctrine 2 for PHP.
This way, you never touch any (my)SQL code. The persistence layer takes care of this for you.
I really advise you to have a look at a Doctrine tutorial, this is a really professional way to create your applications.
Instead of working with raw data loaded from the database, you create objects that hold your data, and the behavior associated with it.
For example, you might have a User class, such as:
class User
{
protected $id;
protected $name;
protected $privileges;
public function setName($name) { ... }
public function getName() { ... }
public function addPrivilege(Privilege $privilege) { ... }
public function getPrivileges() { ... }
}
You controller will only interact with objects:
class UserController
{
public function testAction()
{
// ...
$user = $em->getRepository('User')->find(123); // load User with id 123
$user->setName('John'); // work with your objects,
echo $user->getName(); // and don't worry about the db!
$em->flush(); // persist your changes
}
}
Behind the scenes, the ORM takes care of all the low-level work of issuing a SELECT query, instantiating your object, detecting modifications to your object, and issuing the necessary UPDATE statement!
I am fairly new to using separate layers for the business logic (Domain) and database access logic, but in the course of working things out I've come across a problem to which I still feel I haven't found a great solution.
Clarification My existing solution uses Data Mappers to deal with the database interactions directly. However, as I've further investigated this issue many people have suggested that the Domain layer should not directly communicate with nor contain the Data Mappers that actually perform the database interaction. This is why I placed the Repository objects between the Domain and the necessary Data Mappers but this doesn't feel quite natural or correct. So the real question is what layer naturally exists to handle communication between the Domain and the Data Mappers? Any examples of how to structure it would be appreciated.
For example:
How do I properly handle retrieving a collection of domain objects within the context of another domain object?
How do I force the insertion of a single domain object or collection of objects based on an action performed against another object. The case I'm facing currently is that when a Person is attached to a Campaign, then I need to insert all of the Events that need to be executed for that Person for that Campaign.
There is a distinction between a domain model and the implementation of it. Just because your model shows a relationship Person ---> Campaign ---> Event does not mean that you have to implement it in this way. IOW, your model shows your analysis and design in an object-oriented way, yet you implement that model in OOP which is limited in how well it can replicate that model in code.
Consider the following.
A Person is not defined by its ownership of a Campaign, so campaign can be left out of its knowledge responsibities. On the other hand, a Campaign is defined by the Events that occur as part of its execution, so it is fair to have a collection of events within a campaign. The point that I am making is that each class should have just enough behaviour and knowledge to make it whole.
As for communication between the domain and the persistence layers, consider them as two very distinct systems that are not concerned with the other. All each of them knows is what its responsiblities are and what announcements it makes. For example, the persistence layer knows how to persist data passed to it and to announce that data have been saved. However, the persistence layer does not necessarily need to understand the domain objects. Similarly, the domain layer understands Person, Campaign, and Event but knows nothing about persistence.
The implication of the above is that the domain layer needs to be a whole by itself and should not be dependent on the persistence layer for its data. However, it still needs to be supplied with data to perform its responsibilities. That data can come from either the user interface or the database and is passed to it via a third-party that knows about both domain and persistence layers.
So, in code (pseudo-C#)...
namespace DomainLayer
{
interface IDomainListener
{
void PersonCreated(Person person);
}
class Person
{
private string name;
public Person(string name)
{
this.name = name;
}
public string Name
{
get { return name; }
}
}
class Domain
{
private IDomainListener listener;
public Domain(IDomainListener listener) {
this.listener = listener;
}
public void CreatePerson(string name) {
Person person = new Person(name);
listener.PersonCreated(person);
}
}
}
namespace PersistenceLayer
{
interface IPersistenceListener
{
void PersonDataSaved(int id, object data);
}
class Persistence
{
private IPersistenceListener listener;
public Persistence(IPersistenceListener listener)
{
this.listener = listener;
}
public void SaveData(object data)
{
int id = ...; // save data and return identifier
listener.DataSaved(id, data);
}
}
}
namespace MyApplication
{
class MyController : IDomainListener, IPersistenceListener
{
public void CreatePersonButton_Clicked()
{
Domain domain = new Domain(this);
domain.CreatePerson(NameTextbox.Text);
}
public void PersonCreated(Person person)
{
Persistence persistence = new Persistence(this);
persistence.SavePersonData(person.Name);
}
public void DataSaved(int id, object data)
{
// display data on UI
}
}
}
As you can see, the namespaces represent the different tiers. The XYZListener interfaces define the announcements that are made by the XYZ tier. Any other tiers that are interested in these announcements and will respond to them need to implement these interfaces, as does our MyApplication tier.
When the "create button" is clicked, the controller creates the Domain facade object for the domain layer and registers itself as a listener. It then calls the CreatePerson method which instantiates a Person then announces that this has been done, passing the new instance. The controller responds to this announcement in the PersonCreated implementation where it spawns a facade of the persistence layer and registers itself as the listener again. It then calls the SaveData method whichannounces DataSaved when completed. The implementation of that method then displays the data on the UI.
As you can see, the domain layer and the persistence layer are each aware of only tmemselves and are not concerned with the responsibilities of the other. It is the application logic, manifested here as the controller, that wires the two together.
Back to your specific problem, you could have a method FindPerson on the persistence, which would announce PersonFound(int id). The response by the controller would be to call the persistence layer to retrieve data about campaign and events, then call the domain layer with that data to build the Person.
Sorry for the long answer...
Gabriel, this is called the "impedance matching problem." There are many solutions around, from heavyweight ones like J2EE entity beans to Ruby ActiveRecord to simply coding a hand connection.
Update
Okay, well, its hard to see exactly how to attack this without a lot more information, but here's the basic approach.
Any of these sorts of architectural issues are driven by non-functional requirements like performance; in addition, there is a correctness issue here, in that you want to make sure updates are done in the correct order. So, you're going to need to think about the workload, which is to say the pattern of usage in real-world application. With that in mind, you basically have a couple of issues: first, the base data types in your application may not map correctly to the data base (eg, what's a VARCHAR property represented as in your code?), and second your domain model may not map cleanly to your database model.
What you would like is to have the database and the dmain model work out so that one instance of a domain object is exactly a row of a table in your database model; in large-scale applications you can rarely do this because of either performance constraints or constraints imposed by a pre-existing database model.
Now, if you completely control your database model, it simplifies things somewhat, because then you can make your database model more closely resemble the domain. This might mean the database model is somewhat denormalized, but if so, you can (depending on your database) handle that with views, or just not have a completely normalized database. Normalization is a useful theoretical construct, but that doesn't mean you can't relax it in a real system.
If you don't completely control your database model, then you need a layer of objects that make the mapping. You've got a bunch of options to choose from in implementing that: you can build views or denormalized tables in the database, you can build intermediate objects, or you can do some of both, or even have several steps of both (ie, an intermediate object that accesses a denormalizaed table.)
At that point, though, you run into issues with "don't repeat yourself" and "do the simplest thing that will possibly work." Think about what is most likely to change? Your domain model? If you've got a strong domain model, that's less likely --- the business changes relatively rarely. The exact representation of data in the database? A little more common. Or, most commonly, the exact patterns of use (like discovering a need to handle concurrent updates.) So, when you think about that, what do you need to do to make it as easy as possible to deal with the most common changes.
I realize this isn't giving you very precise instructions, but I don't think we can offer precise instructions without knowing a whole lot about your applicaiton. But then I also kind of get the impression you're wondering about what the "right" way of handling this would be, while you are already working with something that more or less does the job. So, I'd end up by asking "what are you unhappy with now?" and "How would you like to solve that?"
Many systems employ an independent data layer to handle persistence to and from a database. There are several models for the organization of such a layer. Some use a sort of factory-like implementation, others employ a one-to-one mapping with one data layer class per domain class.
The model for the data layer often depends on style and preference. What is important is separating the persistence layer from the domain layer. I believe there are tools out there that will help you generate this layer, but my PHP knowledge is thin so I can't name any specifically for PHP.
I would look at the data abstraction layers used by PHPCake and Symfony.