Alternatively, is basic entity validation considered a specification(s)?
In general, is it better to keep basic entity validation (name cannot be null or empty, date must be greater than xxx) in the actual entity, or outside of it in a specification?
If in a specification, what would that look like? Would you have a spec for each field, or wrap it all up in one EntityIsValid type spec?
It seems to me that once people have learned a little about DDD, they pick up the Specification pattern and look to apply it everywhere. That is really the Golden Hammer anti-pattern.
The way I see a place for the Specification pattern, and the way I understood Domain-Driven Design, is that it is a design pattern you can choose to apply when you need to vary a business rule independently of an Entity.
Remember that DDD is an iterative approach, so you don't have to get it 'right' in the first take. I would start out with putting basic validation inside Entities. This fits well with the basic idea about OOD because it lets the object that represents a concept know about the valid ranges of data.
In most cases, you shouldn't even need explicit validation because Entities should be designed so that constraints are represented as invariants, making it impossible to create an instance that violates a constraint.
If you have a rule that says that Name cannot be null or empty, you can actively enforce it directly in your Entity:
public class MyEntity
{
private string name;
public MyEntity(string name)
{
if(string.IsNullOrEmpty(name))
{
throw new ArgumentException();
}
this.name = name;
}
public string Name
{
get { return this.name; }
set
{
if(string.IsNullOrEmpty(value))
{
throw new ArgumentException();
}
this.name = value;
}
}
}
The rule that name cannot be null is now an invariant for the class: it is now impossible for the MyEntity class to get into a state where that rule is broken.
If later on you discover that the rule is more complex, or shared between many different concepts, you can always extract it into a Specification.
Entities have both data and behavior, so letting your entities ensure their invariants is the way to go IMHO. Else, you might end up with an anemic domain model [Fowler].
If your context allows you to enforce the rules in the setters as Mark Seemann suggests, it would be great since you don't have all the "IsValid" and/or "BrokenRules" logic in your model.
I've been in two contexts where we found ourselves needing the aforementioned solution though:
A classic response/request web solution where the web page displays all the broken rules of an entity upon failing save.
The model is read from a database which is updated externally (hence it's not impossible for the entity to be invalid despite the setter logic, unless you let your ORM use the setters, but the whole point for us was to find out about the validity).
Related
Is there a place for value objects in an event sourced domain model?
Lets define a value object as an object with immutable state that guards its invariants and has no particular identifier.
An event sourced domain model in this context is a domain that is entirely or partially event sourced, meaning that its current state can be derived from applying all events that have occurred in the past. Events themselves are considered immutable, even over time.
Debate has taken place about the validity of using value objects within events - this question goes slightly further: Do value objects have a place in event sourced domains at all?
The (potential) problem with using value objects is that it becomes rather tricky to alter the domain in such a way that invariants are tightened.
An example of this scenario would be to have a Username value object, with the sole constraint that the name must be anywhere between 2 and 16 characters.
While this has been working well for some time, the business decides to only allow usernames of at least 5 characters.
A migration period begins and users with names of less than 5 characters are asked to update their names.
Lets say the process was successful, correction events are applied and everyone is happy.
We tighten the constraints on our Username value object to require at least 5 characters.
For a while everyone is happy, but then we discover a problem with the snapshots and replay all events.
We now face an exception from our Username object: by loading the historic data, we're breaking an invariant of our domain.
The rules of a value objects apply retroactively - does this make them inherently unsuitable for event sourcing? Would it be worth applying versioning of value objects? Is there a simpler way of avoiding such problems?
I would say, that at the moment you redefined what Username means, and you don't migrate historical data somehow, you've essentially created 2 different Username meanings.
Because there are 2 different meanings of the word, you have to make it explicit in the code somehow. "Versioning" is one way, although I wouldn't use such a generic solution, there are different modeling options.
You could make it explicit that the history of a "username" is just that, a history. So for example create a HistoricUsername, which is the event-sourced object, even a value object if you want. And create a Username which is at all times the username with the most current rules, which is not persisted at all, but created from a HistoricUsername if it can.
Some people suggest sometimes to extract the "rules" from the object, and re-apply it later. That way the object itself is valid at all times and you can ask it to validate itself against rules that might change. I don't really prefer these kinds of solutions, but it's an option, and the Username would still be a value-object.
So the problem is not really that value-objects don't fit into event-sourcing, it's just that the modeling has to be more accurate.
Do value objects have a place in event sourced domains at all?
Yes.
Is there a simpler way of avoiding such problems?
"Don't do that."
The problem you are describing is really one about messaging - if we make backwards incompatible changes to our messages, then things break.
(More precisely, you have a "Username" message, and you are trying to re-use that message with a new set of constraints that reject some previously valid uses of the message).
The answer is that you don't introduce backwards incompatible changes - instead, introduce new names that match the new requirements, and deprecated the old ones.
Which is to say, adding support for new messages, and removing support for the old messages, become two separately managed options.
Greg Young's book Versioning in an Event Sourced System dedicates some chapters to this idea. Also, Rich Hickey ends up touching on these important ideas in most of his talks -- I'd suggest starting from Spec-ulation.
The "value object", meaning that the type that the current implementation of the domain model uses to move the information around, is a separate concern from the messages. The data structures we use in memory don't need to be coupled to our serialization formats.
The representation of the information on the wire is distinct from the representation of information in memory, and that in turn is distinct from the abstractions that manipulate the information in memory.
The challenging thing is that, at the beginning of a project, you have the least amount of information about when the different representations are going to diverge.
We've solved this in a slightly different way. By separating the public API of our value objects from the internal (domain only) API, we are able to evolve one without affecting the other.
For example:
public class Username
{
private readonly string value;
// Domain-only (internal) constructor.
// Does not enforce constriants and can only be called within the domain.
internal Username(string value)
{
this.value = value;
}
// Public factory method.
// Enforces business constraints. Used by consumers of the domain (application layer etc.)
// to create new instances of the value object.
public static Username Create(string value)
{
// Business constraints. These will evolve and grow over time.
if (value == null)
{
// throw exception etc.
}
if (value.Length < 2)
{
// throw exception etc.
}
return new Username(value);
}
}
Consumers of the domain must use the static Create method to create a new instance of the value object. This factory method contains all of our business constraints and prevents an instance being created in an invalid state.
Inside the domain, classes have access to the internal (constraint-less) constructor. Since this does not enforce any business constraints, an instance of the value object can always be created in this way (regardless of its value). By using this constructor when replaying events we can ensure that historical data will always succeed.
The benefits of this design are:
A single class is used to represent the domain concept (no need for multiple classes, versioning etc.).
Business rules are free to evolve over time.
Historical data always works. A Username from a year ago is still a user name, even if our rules have changed.
Although already answered I do find this an interesting situation.
I agree with others that the event data should be record-based and, therefore, nothing more than a data container that may be used to reconstitute the aggregate.
That being said when the rules change so does the domain. A major portion of domain-driven design is to capture as much of the domain (rules/structure) as is required. If this is the case should the changes in the rules not also be kept?
For instance, if we have a Username Value Object and it starts out with the 2 to 16 characters rules then that is coded as such:
public class Username
{
public string Value { get; }
public Username(string value)
{
if (value.Length < 2 || value.Length > 16)
{
throw new DomainException("Username must be between 2 and 16 characters");
}
Value = value;
}
}
Now we get to 1 March 2018 and the rule changes. We can keep the rule around:
public class Username
{
public string Value { get; }
public Username(string value, DateTime registrationDate)
{
if (registrationDate < new Date(2018, 3, 1) &&
(value.Length < 2 || value.Length > 16))
{
throw new DomainException("Username must be between 2 and 16 characters");
}
if (registrationDate >= new Date(2018, 3, 1) &&
(value.Length < 5 || value.Length > 16))
{
throw new DomainException("Username must be between 5 and 16 characters");
}
Value = value;
}
}
That is the basic idea. In this way we keep our "old" rules around as well. This may become quite a hassle but I don't have enough experience to say. Changing our rules retroactively may introduce some pretty tricky situation so I guess one would need to evaluate this on a case-by-case basis.
Just a thought.
I was reading through strategy pattern and was trying to implement it but I have got stuck at deciding the strategy implementation which I feel violates the open-closed principle.
In strategy pattern we code to interface and based on client interaction we will pass in the strategy implementation.
Now if we have bunch of strategies so we need to decide using conditions which strategy the client chooses something like
IStrategy str;
if(stragety1) {
str = new Strategy1()
} else if (stragety2) {
str = new Strategy2()
} and so on..
str.run()
Now as per open-closed principle the above is open to extension but it is not closed to modification
If I need to add another strategy(extension) in future I do need to alter this code.
is there a way where this could be avoided or it is how we need to implement strategy pattern ?
1) You must separate selecting/creating a concrete strategy from its uses. I. e. use function selectStrategy, pass it as (constructor) parameter, etc.
2) There is no way to fully avoid conditional creation, but you can hide it (e. g. using some dictionary for mapping state=>strategy) and/or shift it into another level of the application. The last approach is very powerful and flexible, but depends on the task. In some cases you may put selecting/creating on the same level that uses it. In other cases you may even end up with delegation selecting/creating to the highest/lowest level.
2.1) You can use the Registry pattern and kinda avoid modification of "core" object when adding new strategy's.
This is indeed not closed to modification, but that is due to the way you initialize. You are using a value (enum?) to determine which Strategy subclass should be used. As #bpjoshi points out their comment, this is more of a Factory pattern.
Wikipedia discusses how a Strategy pattern can support the Open/Closed Principle, instead of hampering it.
In that example, they use a Car class with a Brake Strategy. Some cars brake with ABS, some don't. Different Car subclasses and instances can be given different Strategies for braking.
To get your code closed for modification, you need to select the Strategies differently. You want to select the Strategy in the place where new behavior or subclass is defined. You'd have to refactor your code so that the specific Strategy subclass is applied at the point where the code is extended.
I think, there is misunderstanding about Closed for Modifications.
In 1988, Mayer said:
Software that works should when possible not be changed when your application is extended with new functionality.
and Rober C. Matrin said:
This definition is obviously dated.
Think about that very carefully. If the behaviors of all the modules in your system could be extended, without modifying them, then you could add new features to that system without modifying any old code. The features would be added solely by writing new code.
https://8thlight.com/blog/uncle-bob/2014/05/12/TheOpenClosedPrinciple.html
Adding some new codes without modifying old codes do not conflict with Open-Closed Principle.
I think the decision you are referring to should be the responsibility of a factory class. The following is some example code:
public interface ISalary
{
decimal Calculate();
}
public class ManagerSalary : ISalary
{
public decimal Calculate()
{
return 0;
}
}
public class AdminSalary : ISalary
{
public decimal Calculate()
{
return 0;
}
}
public class Employee
{
private ISalary salary;
public Employee(ISalary salary)
{
this.salary = salary;
}
public string Name { get; set; }
public decimal CalculateSalary()
{
return this.salary.Calculate();
}
}
The Employee class uses the Strategy pattern and follows the Open/Closed principle, i.e. it is open to new strategy types (ISalary implementations) through injection via the constructor, but closed to modification.
The piece that is missing is the code that creates the Employee objects, something like:
public enum EmployeeType
{
Manager,
Admin
}
public class EmployeeFactory
{
public Employee CreateEmployee(EmployeeType type)
{
if (type == EmployeeType.Manager)
return new Employee(new ManagerSalary());
else if (type == EmployeeType.Admin)
return new Employee(new AdminSalary());
etc
}
}
This is a very simple factory pattern. There are better ways to do this but this is the simplest way to explain the concept.
In trying to centralize how items are added, or removed from my business entity classes, I have moved to the model where all lists are only exposed as ReadOnlyCollections and I provide Add and Remove methods to manipulate the objects in the list.
Here is an example:
public class Course
{
public string Name{get; set;}
}
public class Student
{
private List<Course>_courses = new List<Course>();
public string Name{get; set;}
public ReadOnlyCollection<Course> Courses {
get{ return _courses.AsReadOnly();}
}
public void Add(Course course)
{
if (course != null && _courses.Count <= 3)
{
_courses.Add(course);
}
}
public bool Remove(Course course)
{
bool removed = false;
if (course != null && _courses.Count <= 3)
{
removed = _courses.Remove(course);
}
return removed;
}
}
Part of my objective in doing the above is to not end up with an Anemic data-model (an anti-pattern) and also avoid having the logic that adds and removes courses all over the place.
Some background: the application I am working with is an Asp.net application, where the lists used to be exposed as a list previously, which resulted in all kinds of ways in which Courses were added to the Student (some places a check was made and others the check was not made).
But my question is: is the above a good idea?
Yes, this is a good approach, in my opinion you're not doing anything than decorating your list, and its better than implementing your own IList (as you save many lines of code, even though you lose the more elegant way to iterate through your Course objects).
You may consider receiving a validation strategy object, as in the future you might have a new requirement, for ex: a new kind of student that can have more than 3 courses, etc
I'd say this is a good idea when adding/removing needs to be controlled in the manner you suggest, such as for business rule validation. Otherwise, as you know from previous code, there's really no way to ensure that the validation is performed.
The balance that you'll probably want to reach, however, is when to do this and when not to. Doing this for every collection of every kind seems like overkill. However, if you don't do this and then later need to add this kind of gate-keeping code then it would be a breaking change for the class, which may or may not be a headache at the time.
I suppose another approach could be to have a custom descendant of IList<T> which has generic gate-keeping code for its Add() and Remove() methods which notifies the system of what's happening. Something like exposing an event which is raised before the internal logic of those methods is called. Then the Student class would supply a delegate or something (sorry for being vague, I'm very coded-out today) when instantiating _courses to apply business logic to the event and cancel the operation (throw an exception, I imagine) if the business validation fails.
That could be overkill as well, depending on the developer's disposition. But at least with something a little more engineered like this you get a single generic implementation for everything with the option to add/remove business validation as needed over time without breaking changes.
I've done that in the past and regretted it: a better option is to use different classes to read domain objects than the ones you use to modify them.
For example, use a behavior-rich Student domain class that jealously guards its ownership of courses - it shouldn't expose them at all if student is responsible for them - and a StudentDataTransferObject (or ViewModel) that provides a simple list of strings of courses (or a dictionary when you need IDs) for populating interfaces.
I had a discussion at work regarding "Inheritance in domain model is complicating developers life". I'm an OO programmer so I started to look for arguments that having inheritance in domain model will ease the developer life actually instead of having switches all over the place.
What I would like to see is this :
class Animal {
}
class Cat : Animal {
}
class Dog : Animal {
}
What the other colleague is saying is :
public enum AnimalType {
Unknown,
Cat,
Dog
}
public class Animal {
public AnimalType Type { get; set; }
}
How do I convince him (links are WELCOME ) that a class hierarchy would be better than having a enum property for this kind of situations?
Thanks!
Here is how I reason about it:
Only use inheritance if the role/type will never change.
e.g.
using inheritance for things like:
Fireman <- Employee <- Person is wrong.
as soon as Freddy the fireman changes job or becomes unemployed, you have to kill him and recreate a new object of the new type with all of the old relations attached to it.
So the naive solution to the above problem would be to give a JobTitle enum property to the person class.
This can be enough in some scenarios, e.g. if you don't need very complex behaviors associated with the role/type.
The more correct way would be to give the person class a list of roles.
Each role represents e.g an employment with a time span.
e.g.
freddy.Roles.Add(new Employement( employmentDate, jobTitle ));
or if that is overkill:
freddy.CurrentEmployment = new Employement( employmentDate, jobTitle );
This way , Freddy can become a developer w/o we having to kill him first.
However, all my ramblings still haven't answered if you should use an enum or type hierarchy for the jobtitle.
In pure in mem OO I'd say that it's more correct to use inheritance for the jobtitles here.
But if you are doing O/R mapping you might end up with a bit overcomplex data model behind the scenes if the mapper tries to map each sub type to a new table.
So in such cases, I often go for the enum approach if there is no real/complex behavior associated with the types.
I can live with a "if type == JobTitles.Fireman ..." if the usage is limited and it makes things easer or less complex.
e.g. the Entity Framework 4 designer for .NET can only map each sub type to a new table. and you might get an ugly model or alot of joins when you query your database w/o any real benefit.
However I do use inheritance if the type/role is static.
e.g. for Products.
you might have CD <- Product and Book <- Product.
Inheritance wins here because in this case you most likely have different state associated with the types.
CD might have a number of tracks property while a book might have number of pages property.
So in short, it depends ;-)
Also, at the end of the day you will most likely end up with a lot of switch statements either way.
Let's say you want to edit a "Product" , even if you use inheritance, you will probably have code like this:
if (product is Book)
Response.Redicted("~/EditBook.aspx?id" + product.id);
Because encoding the edit book url in the entity class would be plain ugly since it would force your business entites to know about your site structure etc.
Having an enum is like throwing a party for all those Open/Closed Principle is for suckers people.
It invites you to check if an animal is of a certain type and then apply custom logic for each type. And that can render horrible code, which makes it hard to continue building on your system.
Why?
Doing "if this type, do this, else do that" prevents good code.
Any time you introduce a new type, all those ifs get invalid if the new type is not handled. In larger systems, it's hard to find all those ifs, which will lead to bugs eventually.
A much better approach is to use small, well-defined feature interfaces (Interface segregation principle).
Then you will only have an if but no 'else' since all concretes can implement a specific feature.
Compare
if (animal is ICanFly flyer)
flyer.Sail();
to
// A bird and a fly are fundamentally different implementations
// but both can fly.
if (animal is Bird b)
b.Sail();
else if (animal is Fly f)
b.Sail();
See? the former one needs to be checked once while the latter has to be checked for every animal that can fly.
Enums are good when:
The set of values is fixed and never or very rarely changes.
You want to be able to represent a union of values (i.e. combining flags).
You don't need to attach other state to each value. (Java doesn't have this limitation.)
If you could solve your problem with a number, an enum is likely a good fit and more type safe. If you need any more flexibility than the above, then enums are likely not the right answer. Using polymorphic classes, you can:
Statically ensure that all type-specific behavior is handled. For example, if you need all animals to be able to Bark(), making Animal classes with an abstract Bark() method will let the compiler check for you that each subclass implements it. If you use an enum and a big switch, it won't ensure that you've handled every case.
You can add new cases (types of animals in your example). This can be done across source files, and even across package boundaries. With an enum, once you've declared it, it's frozen. Open-ended extension is one of the primary strengths of OOP.
It's important to note that your colleague's example is not in direct opposition to yours. If he wants an animal's type to be an exposed property (which is useful for some things), you can still do that without using an enum, using the type object pattern:
public abstract class AnimalType {
public static AnimalType Unknown { get; private set; }
public static AnimalType Cat { get; private set; }
public static AnimalType Dog { get; private set; }
static AnimalType() {
Unknown = new AnimalType("Unknown");
Cat = new AnimalType("Cat");
Dog = new AnimalType("Dog");
}
}
public class Animal {
public AnimalType Type { get; set; }
}
This gives you the convenience of an enum: you can do AnimalType.Cat and you can get the type of an animal. But it also gives you the flexibility of classes: you can add fields to AnimalType to store additional data with each type, add virtual methods, etc. More importantly, you can define new animal types by just creating new instances of AnimalType.
I'd urge you to reconsider: in an anemic domain model (per the comments above), cats don't behave differently than dogs, so there's no polymorphism. An animal's type really is just an attribute. It's hard to see what inheritance buys you there.
Most importantly OOPS means modeling reality. Inheritance gives you the opportunity to say Cat is an animal. Animal should not know if its a cat now shout it and then decide that it is suppose to Meow and not Bark, Encapsulation gets defeated there. Less code as now you do not have to do If else as you said.
Both solutions are right.
You should look which techniques applies better to you problem.
If your program uses few different objects, and doesn't add new classes, its better to stay with enumerations.
But if you program uses a lot of different objects (different classes), and may add new classes, in the future, better try the inheritance way.
I am working on a little pinball-game project for a hobby and am looking for a pattern to encapsulate constant variables.
I have a model, within which there are values which will be constant over the life of that model e.g. maximum speed/maximum gravity etc. Throughout the GUI and other areas these values are required in order to correctly validate input. Currently they are included either as references to a public static final, or just plain hard-coded. I'd like to encapsulate these "constant variables" in an object which can be injected into the model, and retrieved by the view/controller.
To clarify, the value of the "constant variables" may not necessarily be defined at compile-time, they could come from reading in a file; user input etc. What is known at compile time is which ones are needed. A way which may be easier to explain it is that whatever this encapsulation is, the values it provides are immutable.
I'm looking for a way to achieve this which:
has compile time type-safety (i.e. not mapping a string to variable at runtime)
avoids anything static (including enums, which can't be extended)
I know I could define an interface which has the methods such as:
public int getMaximumSpeed();
public int getMaximumGravity();
... and inject an instance of that into the model, and make it accessible in some way. However, this results in a lot of boilerplate code, which is pretty tedious to write/test etc (I am doing this for funsies :-)).
I am looking for a better way to do this, preferably something which has the benefits of being part of a shared vocabulary, as with design patterns.
Is there a better way to do this?
P.S. I've thought some more about this, and the best trade-off I could find would be to have something like:
public class Variables {
enum Variable {
MaxSpeed(100),
MaxGravity(10)
Variable(Object variableValue) {
// assign value to field, provide getter etc.
}
}
public Object getVariable(Variable v) { // look up enum and get member }
} // end of MyVariables
I could then do something like:
Model m = new Model(new Variables());
Advantages: the lookup of a variable is protected by having to be a member of the enum in order to compile, variables can be added with little extra code
Disadvantages: enums cannot be extended, brittleness (a recompile is needed to add a variable), variable values would have to be cast from Object (to Integer in this example), which again isn't type safe, though generics may be an option for that... somehow
Are you looking for the Singleton or, a variant, the Monostate? If not, how does that pattern fail your needs?
Of course, here's the mandatory disclaimer that Anything Global Is Evil.
UPDATE: I did some looking, because I've been having similar debates/issues. I stumbled across a list of "alternatives" to classic global/scope solutions. Thought I'd share.
Thanks for all the time spent by you guys trying to decipher what is a pretty weird question.
I think, in terms of design patterns, the closest that comes to what I'm describing is the factory pattern, where I have a factory of pseudo-constants. Technically it's not creating an instance each call, but rather always providing the same instance (in the sense of a Guice provider). But I can create several factories, which each can provide different psuedo-constants, and inject each into a different model, so the model's UI can validate input a lot more flexibly.
If anyone's interested I've came to the conclusion that an interface providing a method for each psuedo-constant is the way to go:
public interface IVariableProvider {
public int maxGravity();
public int maxSpeed();
// and everything else...
}
public class VariableProvider {
private final int maxGravity, maxSpeed...;
public VariableProvider(int maxGravity, int maxSpeed) {
// assign final fields
}
}
Then I can do:
Model firstModel = new Model(new VariableProvider(2, 10));
Model secondModel = new Model(new VariableProvider(10, 100));
I think as long as the interface doesn't provide a prohibitively large number of variable getters, it wins over some parameterised lookup (which will either be vulnerable at run-time, or will prohibit extension/polymorphism).
P.S. I realise some have been questioning what my problem is with static final values. I made the statement (with tongue in cheek) to a colleague that anything static is an inherently not object-oriented. So in my hobby I used that as the basis for a thought exercise where I try to remove anything static from the project (next I'll be trying to remove all 'if' statements ;-D). If I was on a deadline and I was satisfied public static final values wouldn't hamstring testing, I would have used them pretty quickly.
If you're just using java/IOC, why not just dependency-inject the values?
e.g. Spring inject the values via a map, specify the object as a singleton -
<property name="values">
<map>
<entry> <key><value>a1</value></key><value>b1</value></entry>
<entry> <key><value>a2</value></key><value>b3</value></entry>
</map>
</property>
your class is a singleton that holds an immutable copy of the map set in spring -
private Map<String, String> m;
public String getValue(String s)
{
return m.containsKey(s)?m.get(s):null;
}
public void setValues(Map m)
{
this.m=Collections.unmodifiableMap(m):
}
From what I can tell, you probably don't need to implement a pattern here -- you just need access to a set of constants, and it seems to me that's handled pretty well through the use of a publicly accessible static interface to them. Unless I'm missing something. :)
If you simply want to "objectify" the constants though, for some reason, than the Singleton pattern would probably be called for, if any; I know you mentioned in a comment that you don't mind creating multiple instances of this wrapper object, but in response I'd ask, then why even introduce the sort of confusion that could arise from having multiple instances at all? What practical benefit are you looking for that'd be satisfied with having the data in object form?
Now, if the values aren't constants, then that's different -- in that case, you probably do want a Singleton or Monostate. But if they really are constants, just wrap a set of enums or static constants in a class and be done! Keep-it-simple is as good a "pattern" as any.