I am new to DDD, and I am trying to figure out a way to update aggregate by using a PUT verb.
If all properties in the aggregate have private setters, then it's obvious I need to have set of functionality for every business requirement. For an example
supportTicket.Resolve();
It's clear for me that I can achieve this with an endpoint such as /api/tickets/5/resolve, but what if i want to provide a way to update whole ticket atomically?
As an example, user can make a PUT request to /api/tickets/5 with a following body
{"status" : "RESOLVED", "Title":"Some crazy title"}
Do I need to do something like this in the ApplicationSercvice
if(DTO.Status != null && dto.Status == "RESOLVED")
supportTicket.Resolve();
if(DTO.Title != null)
supportTicket.setNewTitle(DTO.title);
If that's the case and changing ticket title has some business logic to prevent changing it if the ticket is resolved, should I consider some kind of prioritizing when updating aggregate, or I am looking at this entirely wrong?
Domain Driven Design for RESTful Systems -- Jim Webber
what if i want to provide a way to update whole ticket atomically?
If you want to update the whole ticket atomically, ditch aggregates; aggregates are the wrong tool in your box if what you really want is a key value store with CRUD semantics.
Aggregates only make sense when their are business rules for the domain to enforce. Don't build a tractor when all you need is a shovel.
As an example, user can make a PUT request to /api/tickets/5
That's going to make a mess. In a CRUD implementation, replacing the current state of a resource by sending it a representation of a new state is appropriate. But that doesn't really fit for aggregates at all, because the state of the aggregate is not under the control of you, the client/publisher.
The more appropriate idiom is to publish a message onto a bus, which when handled by the domain will have the side effect of achieving the changes you want.
PUT /api/tickets/5/messages/{messageId}
NOW your application service looks at the message, and sends commands to the aggregate
if(DTO.Status != null && dto.Status == "RESOLVED")
supportTicket.Resolve();
if(DTO.Title != null)
supportTicket.setNewTitle(DTO.title);
This is OK, but in practice its much more common to make the message explicit about what is to be done.
{ "messageType" : "ResolveWithNewTitle"
, "status" : "RESOLVED"
, "Title":"Some crazy title"
}
or even...
[
{ "messageType" : "ChangeTitle"
, "Title" : "Some crazy title"
}
, { "messageType" : "ResolveTicket"
}
]
Basically, you want to give the app enough context that it can do real message validation.
let's say I had aggregates which encapsulated needed business logic, but besides that there is a new demand for atomic update functionality and I am trying to understand a best way to deal with this.
So the right way to deal with this is first to deal with it on the domain level -- sit down with your domain experts, make sure that everybody understands the requirement and how to express it in the ubiquitous language, etc.
Implement any new methods that you need in the aggregate root.
Once you have the use case correctly supported in the domain, then you can start worrying about your resources following the previous pattern - the resource just takes the incoming request, and invokes the appropriate commands.
Is changing the Title a requirement of Resolving a ticket? If not, they should not be the same action in DDD. You wouldn't want to not resolve the ticket if the new name was invalid, and you wouldn't want to not change the name if the ticket was not resolvable.
Make 2 calls to perform the 2 separate actions. This also allows for flexibility such as, the Title can be changed immediately, but perhaps "resolving" the ticket will kick off some complex and time consuming (asyncronous) work flow before the ticket is actually resolved. Perhaps it needs to have a manager sign off? You don't want the call to change "title" tied up in that mix.
If needs be, create something to orchestrate multiple commands as per #VoiceOfUnreason's comment.
Wherever possible, keep things separate, and code to use cases as opposed to minimizing interacitons with entities.
You're probably right. But it's probably wiser to encapsulate such logic inside the ticket it self, by making a "change()" method, receiving a changeCommandModel (or something like this), so you can define the business rules inside your domain object.
if (DTO.Status != null && dto.Status == "RESOLVED")
supportTicket.Resolve(DTO.title);
I will change the underlying method to take title as parameter, this clarify the resolve action. That second if and validation you want in the domain method. It's really preference, more importantly is the message and I agree with #VoiceOfUnreason second option.
Does anyone have any suggestions on a good methodology for enforcing record-level authorization while maintaining separation of concerns within an ASP.Net MVC web site?
With PrincipalPermission, you can decorate a method with:
PrincipalPermission(SecurityAction.Demand, Role = "GroupLeader")
to require that any visitor to that page be a member of the fixed role "GroupLeader".
Alternatively, you could decorate a method with:
[ClaimsPrincipalPermission(SecurityAction.Demand, Operation = "Manage", Resource = "Group")]
to require that any visitor to that page be allowed generically to manage a group.
However, neither of these really address the scenario in which a user might have permission to edit some groups, but not others. As far as I can tell, even if I implement a custom ClaimsAuthorizationManager, there is no way to get access to the method parameters in order to do conditional authorization.
Additionally, both of the methods above just throw a SecurityException if the user doesn't have access rather than allowing a method to gracefully redirect the user to a page that explains what happened and what they might do to gain the necessary authorization.
Obviously, I also realize that I could just code authorization logic into the method itself and redirect accordingly, but I would rather separate that out, if possible, and keep the method code clean to only handle actually processing the request and also to be able to work within a framework that can be more generically applied.
So, is there an "out-of-the-box" way to handle this scenario, or do I have to implement a custom IAuthorizationFilter? I think I could handle both the record-level authorization and the graceful redirect, but without any parameters at the method level, it would be essentially a massive if ... else if statement.
The most straight forward way to handle this situation is creating a property on your entities to store the "owner" or "creator" of the entity. Then, when you query objects, you filter by this. For example:
public class Foo
{
public int Id { get; set; }
...
public string Creator { get; set; }
}
Then:
public ActionResult FooDetails(int id)
{
var foo = db.Foos.SingleOrDefault(m => m.Id == id && m.Creator == User.Identity.Name);
if (foo == null)
{
return new HttpNotFoundResult();
}
return View(foo);
}
It's not really appropriate to handle this via an authorization filter because you have to select the row to determine the user's permissions. Then, you must select the row again inside the action to send it to the view. Also, you'd have to do some acrobatics to get the filter to be able to know how it should select what from where so that you can even check the permissions in the first place. In the action, you're already selecting the entity, so just condition it based on whether the user "owns" it at the same time.
Danger ... Danger Dr. Smith... Philosophical post ahead
The purpose of this post is to determine if placing the validation logic outside of my domain entities (aggregate root actually) is actually granting me more flexibility or it's kamikaze code
Basically I want to know if there is a better way to validate my domain entities. This is how I am planning to do it but I would like your opinion
The first approach I considered was:
class Customer : EntityBase<Customer>
{
public void ChangeEmail(string email)
{
if(string.IsNullOrWhitespace(email)) throw new DomainException(“...”);
if(!email.IsEmail()) throw new DomainException();
if(email.Contains(“#mailinator.com”)) throw new DomainException();
}
}
I actually do not like this validation because even when I am encapsulating the validation logic in the correct entity, this is violating the Open/Close principle (Open for extension but Close for modification) and I have found that violating this principle, code maintenance becomes a real pain when the application grows up in complexity. Why? Because domain rules change more often than we would like to admit, and if the rules are hidden and embedded in an entity like this, they are hard to test, hard to read, hard to maintain but the real reason why I do not like this approach is: if the validation rules change, I have to come and edit my domain entity. This has been a really simple example but in RL the validation could be more complex
So following the philosophy of Udi Dahan, making roles explicit, and the recommendation from Eric Evans in the blue book, the next try was to implement the specification pattern, something like this
class EmailDomainIsAllowedSpecification : IDomainSpecification<Customer>
{
private INotAllowedEmailDomainsResolver invalidEmailDomainsResolver;
public bool IsSatisfiedBy(Customer customer)
{
return !this.invalidEmailDomainsResolver.GetInvalidEmailDomains().Contains(customer.Email);
}
}
But then I realize that in order to follow this approach I had to mutate my entities first in order to pass the value being valdiated, in this case the email, but mutating them would cause my domain events being fired which I wouldn’t like to happen until the new email is valid
So after considering these approaches, I came out with this one, since I am going to implement a CQRS architecture:
class EmailDomainIsAllowedValidator : IDomainInvariantValidator<Customer, ChangeEmailCommand>
{
public void IsValid(Customer entity, ChangeEmailCommand command)
{
if(!command.Email.HasValidDomain()) throw new DomainException(“...”);
}
}
Well that’s the main idea, the entity is passed to the validator in case we need some value from the entity to perform the validation, the command contains the data coming from the user and since the validators are considered injectable objects they could have external dependencies injected if the validation requires it.
Now the dilemma, I am happy with a design like this because my validation is encapsulated in individual objects which brings many advantages: easy unit test, easy to maintain, domain invariants are explicitly expressed using the Ubiquitous Language, easy to extend, validation logic is centralized and validators can be used together to enforce complex domain rules. And even when I know I am placing the validation of my entities outside of them (You could argue a code smell - Anemic Domain) but I think the trade-off is acceptable
But there is one thing that I have not figured out how to implement it in a clean way. How should I use this components...
Since they will be injected, they won’t fit naturally inside my domain entities, so basically I see two options:
Pass the validators to each method of my entity
Validate my objects externally (from the command handler)
I am not happy with the option 1 so I would explain how I would do it with the option 2
class ChangeEmailCommandHandler : ICommandHandler<ChangeEmailCommand>
{
// here I would get the validators required for this command injected
private IEnumerable<IDomainInvariantValidator> validators;
public void Execute(ChangeEmailCommand command)
{
using (var t = this.unitOfWork.BeginTransaction())
{
var customer = this.unitOfWork.Get<Customer>(command.CustomerId);
// here I would validate them, something like this
this.validators.ForEach(x =. x.IsValid(customer, command));
// here I know the command is valid
// the call to ChangeEmail will fire domain events as needed
customer.ChangeEmail(command.Email);
t.Commit();
}
}
}
Well this is it. Can you give me your thoughts about this or share your experiences with Domain entities validation
EDIT
I think it is not clear from my question, but the real problem is: Hiding the domain rules has serious implications in the future maintainability of the application, and also domain rules change often during the life-cycle of the app. Hence implementing them with this in mind would let us extend them easily. Now imagine in the future a rules engine is implemented, if the rules are encapsulated outside of the domain entities, this change would be easier to implement
I am aware that placing the validation outside of my entities breaks the encapsulation as #jgauffin mentioned in his answer, but I think that the benefits of placing the validation in individual objects is much more substantial than just keeping the encapsulation of an entity. Now I think the encapsulation makes more sense in a traditional n-tier architecture because the entities were used in several places of the domain layer, but in a CQRS architecture, when a command arrives, there will be a command handler accessing an aggregate root and performing operations against the aggregate root only creating a perfect window to place the validation.
I'd like to make a small comparison between the advantages to place validation inside an entity vs placing it in individual objects
Validation in Individual objects
Pro. Easy to write
Pro. Easy to test
Pro. It's explicitly expressed
Pro. It becomes part of the Domain design, expressed with the current Ubiquitous Language
Pro. Since it's now part of the design, it can be modeled using UML diagrams
Pro. Extremely easy to maintain
Pro. Makes my entities and the validation logic loosely coupled
Pro. Easy to extend
Pro. Following the SRP
Pro. Following the Open/Close principle
Pro. Not breaking the law of Demeter (mmm)?
Pro. I'is centralized
Pro. It could be reusable
Pro. If required, external dependencies can be easily injected
Pro. If using a plug-in model, new validators can be added just by dropping the new assemblies without the need to re-compile the whole application
Pro. Implementing a rules engine would be easier
Con. Breaking encapsulation
Con. If encapsulation is mandatory, we would have to pass the individual validators to the entity (aggregate) method
Validation encapsulated inside the entity
Pro. Encapsulated?
Pro. Reusable?
I would love to read your thoughts about this
I agree with a number of the concepts presented in other responses, but I put them together in my code.
First, I agree that using Value Objects for values that include behavior is a great way to encapsulate common business rules and an e-mail address is a perfect candidate. However, I tend to limit this to rules that are constant and will not change frequently. I'm sure you are looking for a more general approach and e-mail is just an example, so I won't focus on that one use-case.
The key to my approach is recognizing that validation serves different purposes at different locations in an application. Put simply, validate only what is required to ensure that the current operation can execute without unexpected/unintended results. That leads to the question what validation should occur where?
In your example, I would ask myself if the domain entity really cares that the e-mail address conforms to some pattern and other rules or do we simply care that 'email' cannot be null or blank when ChangeEmail is called? If the latter, than a simple check to ensure a value is present is all that is needed in the ChangeEmail method.
In CQRS, all changes that modify the state of the application occur as commands with the implementation in command handlers (as you've shown). I will typically place any 'hooks' into business rules, etc. that validate that the operation MAY be performed in the command handler. I actually follow your approach of injecting validators into the command handler which allows me to extend/replace the rule set without making changes to the handler. These 'dynamic' rules allow me to define the business rules, such as what constitutes a valid e-mail address, before I change the state of the entity - further ensuring it does not go into an invalid state. But 'invalidity' in this case is defined by the business logic and, as you pointed out, is highly volitile.
Having come up through the CSLA ranks, I found this change difficult to adopt because it does seem to break encapsulation. But, I agrue that encapsulation is not broken if you take a step back and ask what role validation truly serves in the model.
I've found these nuances to be very important in keeping my head clear on this subject. There is validation to prevent bad data (eg missing arguments, null values, empty strings, etc) that belongs in the method itself and there is validation to ensure the business rules are enforced. In the case of the former, if the Customer must have an e-mail address, then the only rule I need to be concerned about to prevent my domain object from becoming invalid is to ensure that an e-mail address has been provided to the ChangeEmail method. The other rules are higher level concerns regarding the validity of the value itself and really have no affect on the validity of the domain entity itself.
This has been the source of a lot of 'discussions' with fellow developers but when most take a broader view and investigate the role validation really serves, they tend to see the light.
Finally, there is also a place for UI validation (and by UI I mean whatever serves as the interface to the application be it a screen, service endpoint or whatever). I find it perfectly reasonably to duplicate some of the logic in the UI to provide better interactivity for the user. But it is because this validation serves that single purpose why I allow such duplication. However, using injected validator/specification objects promotes reuse in this way without the negative implications of having these rules defined in multiple locations.
Not sure if that helps or not...
I wouldn't suggest trowing big pieces of code into your domain for validation. We eliminated most of our awkward placed validations by seeing them as a smell of missing concepts in our domain. In your sample code you write I see validation for an e-mail address. A Customer doesn't have anything to do with email validation.
Why not make an ValueObject called Email that does this validation at construct?
My experience is that awkward placed validations are hints to missed concepts in your domain. You can catch them in Validator objects, but I prefer value object because you make the related concept part of your domain.
I am at the beginning of a project and I am going to implement my validation outside my domain entities. My domain entities will contain logic to protect any invariants (such as missing arguments, null values, empty strings, collections, etc). But the actual business rules will live in validator classes. I am of the mindset of #SonOfPirate...
I am using FluentValidation that will essentially give me bunch of validators that act on my domain entities: aka, the specification pattern. Also, in accordance with the patterns described in Eric's blue book, I can construct the validators with any data they may need to perform the validations (be it from the database or another repository or service). I would also have the option to inject any dependencies here too. I can also compose and reuse these validators (e.g. an address validator can be reused in both an Employee validator and Company validator). I have a Validator factory that acts as a "service locator":
public class ParticipantService : IParticipantService
{
public void Save(Participant participant)
{
IValidator<Participant> validator = _validatorFactory.GetValidator<Participant>();
var results = validator.Validate(participant);
//if the participant is valid, register the participant with the unit of work
if (results.IsValid)
{
if (participant.IsNew)
{
_unitOfWork.RegisterNew<Participant>(participant);
}
else if (participant.HasChanged)
{
_unitOfWork.RegisterDirty<Participant>(participant);
}
}
else
{
_unitOfWork.RollBack();
//do some thing here to indicate the errors:generate an exception (or fault) that contains the validation errors. Or return the results
}
}
}
And the validator would contain code, something like this:
public class ParticipantValidator : AbstractValidator<Participant>
{
public ParticipantValidator(DateTime today, int ageLimit, List<string> validCompanyCodes, /*any other stuff you need*/)
{...}
public void BuildRules()
{
RuleFor(participant => participant.DateOfBirth)
.NotNull()
.LessThan(m_today.AddYears(m_ageLimit*-1))
.WithMessage(string.Format("Participant must be older than {0} years of age.", m_ageLimit));
RuleFor(participant => participant.Address)
.NotNull()
.SetValidator(new AddressValidator());
RuleFor(participant => participant.Email)
.NotEmpty()
.EmailAddress();
...
}
}
We have to support more than one type of presentation: websites, winforms and bulk loading of data via services. Under pinning all these are a set of services that expose the functionality of the system in a single and consistent way. We do not use Entity Framework or ORM for reasons that I will not bore you with.
Here is why I like this approach:
The business rules that are contained in the validators are totally unit testable.
I can compose more complex rules from simpler rules
I can use the validators in more than one location in my system (we support websites and Winforms, and services that expose functionality), so if there is a slightly different rule required for a use case in a service that differs from the websites, then I can handle that.
All the vaildation is expressed in one location and I can choose how / where to inject and compose this.
You put validation in the wrong place.
You should use ValueObjects for such things.
Watch this presentation http://www.infoq.com/presentations/Value-Objects-Dan-Bergh-Johnsson
It will also teach you about Data as Centers of Gravity.
There also a sample of how to reuse data validation, like for example using static validation methods ala Email.IsValid(string)
I would not call a class which inherits from EntityBase my domain model since it couples it to your persistence layer. But that's just my opinion.
I would not move the email validation logic from the Customer to anything else to follow the Open/Closed principle. To me, following open/closed would mean that you have the following hierarchy:
public class User
{
// some basic validation
public virtual void ChangeEmail(string email);
}
public class Employee : User
{
// validates internal email
public override void ChangeEmail(string email);
}
public class Customer : User
{
// validate external email addresses.
public override void ChangeEmail(string email);
}
You suggestions moves the control from the domain model to an arbitrary class, hence breaking the encapsulation. I would rather refactor my class (Customer) to comply to the new business rules than doing that.
Use domain events to trigger other parts of the system to get a more loosely coupled architecture, but don't use commands/events to violate the encapsulation.
Exceptions
I just noticed that you throw DomainException. That's a way to generic exception. Why don't you use the argument exceptions or the FormatException? They describe the error much better. And don't forget to include context information helping you to prevent the exception in the future.
Update
Placing the logic outside the class is asking for trouble imho. How do you control which validation rule is used? One part of the code might use SomeVeryOldRule when validating while another using NewAndVeryStrictRule. It might not be on purpose, but it can and will happen when the code base grows.
It sounds like you have already decided to ignore one of the OOP fundamentals (encapsulation). Go ahead and use a generic / external validation framework, but don't say that I didn't warn you ;)
Update2
Thanks for your patience and your answers, and that's the reason why I posted this question, I feel the same an entity should be responsible to guarantee it's in a valid state (and I have done it in previous projects) but the benefits of placing it in individual objects is huge and like I posted there's even a way to use individual objects and keep the encapsulation but personally I am not so happy with design but on the other hand it is not out of the table, consider this ChangeEmail(IEnumerable> validators, string email) I have not thought in detail the imple. though
That allows the programmer to specify any rules, it may or may not be the currently correct business rules. The developer could just write
customer.ChangeEmail(new IValidator<Customer>[] { new NonValidatingRule<Customer>() }, "notAnEmail")
which accepts everything. And the rules have to be specified in every single place where ChangeEmail is being called.
If you want to use a rule engine, create a singleton proxy:
public class Validator
{
IValidatorEngine _engine;
public static void Assign(IValidatorEngine engine)
{
_engine = engine;
}
public static IValidatorEngine Current { get { return _engine; } }
}
.. and use it from within the domain model methods like
public class Customer
{
public void ChangeEmail(string email)
{
var rules = Validator.GetRulesFor<Customer>("ChangeEmail");
rules.Validate(email);
// valid
}
}
The problem with that solution is that it will become a maintenance nightmare since the rule dependencies are hidden. You can never tell if all rules have been specified and working unless you test every domain model method and each rule scenario for every method.
The solution is more flexible but will imho take a lot more time to implement than to refactor the method who's business rules got changed.
I cannot say what I did is the perfect thing to do for I am still struggling with this problem myself and fighting one fight at a time. But I have been doing so far the following thing :
I have basic classes for encapsulating validation :
public interface ISpecification<TEntity> where TEntity : class, IAggregate
{
bool IsSatisfiedBy(TEntity entity);
}
internal class AndSpecification<TEntity> : ISpecification<TEntity> where TEntity: class, IAggregate
{
private ISpecification<TEntity> Spec1;
private ISpecification<TEntity> Spec2;
internal AndSpecification(ISpecification<TEntity> s1, ISpecification<TEntity> s2)
{
Spec1 = s1;
Spec2 = s2;
}
public bool IsSatisfiedBy(TEntity candidate)
{
return Spec1.IsSatisfiedBy(candidate) && Spec2.IsSatisfiedBy(candidate);
}
}
internal class OrSpecification<TEntity> : ISpecification<TEntity> where TEntity : class, IAggregate
{
private ISpecification<TEntity> Spec1;
private ISpecification<TEntity> Spec2;
internal OrSpecification(ISpecification<TEntity> s1, ISpecification<TEntity> s2)
{
Spec1 = s1;
Spec2 = s2;
}
public bool IsSatisfiedBy(TEntity candidate)
{
return Spec1.IsSatisfiedBy(candidate) || Spec2.IsSatisfiedBy(candidate);
}
}
internal class NotSpecification<TEntity> : ISpecification<TEntity> where TEntity : class, IAggregate
{
private ISpecification<TEntity> Wrapped;
internal NotSpecification(ISpecification<TEntity> x)
{
Wrapped = x;
}
public bool IsSatisfiedBy(TEntity candidate)
{
return !Wrapped.IsSatisfiedBy(candidate);
}
}
public static class SpecsExtensionMethods
{
public static ISpecification<TEntity> And<TEntity>(this ISpecification<TEntity> s1, ISpecification<TEntity> s2) where TEntity : class, IAggregate
{
return new AndSpecification<TEntity>(s1, s2);
}
public static ISpecification<TEntity> Or<TEntity>(this ISpecification<TEntity> s1, ISpecification<TEntity> s2) where TEntity : class, IAggregate
{
return new OrSpecification<TEntity>(s1, s2);
}
public static ISpecification<TEntity> Not<TEntity>(this ISpecification<TEntity> s) where TEntity : class, IAggregate
{
return new NotSpecification<TEntity>(s);
}
}
and to use it, I do the following :
command handler :
public class MyCommandHandler : CommandHandler<MyCommand>
{
public override CommandValidation Execute(MyCommand cmd)
{
Contract.Requires<ArgumentNullException>(cmd != null);
var existingAR= Repository.GetById<MyAggregate>(cmd.Id);
if (existingIntervento.IsNull())
throw new HandlerForDomainEventNotFoundException();
existingIntervento.DoStuff(cmd.Id
, cmd.Date
...
);
Repository.Save(existingIntervento, cmd.GetCommitId());
return existingIntervento.CommandValidationMessages;
}
the aggregate :
public void DoStuff(Guid id, DateTime dateX,DateTime start, DateTime end, ...)
{
var is_date_valid = new Is_dateX_valid(dateX);
var has_start_date_greater_than_end_date = new Has_start_date_greater_than_end_date(start, end);
ISpecification<MyAggregate> specs = is_date_valid .And(has_start_date_greater_than_end_date );
if (specs.IsSatisfiedBy(this))
{
var evt = new AgregateStuffed()
{
Id = id
, DateX = dateX
, End = end
, Start = start
, ...
};
RaiseEvent(evt);
}
}
the specification is now embedded in these two classes :
public class Is_dateX_valid : ISpecification<MyAggregate>
{
private readonly DateTime _dateX;
public Is_data_consuntivazione_valid(DateTime dateX)
{
Contract.Requires<ArgumentNullException>(dateX== DateTime.MinValue);
_dateX= dateX;
}
public bool IsSatisfiedBy(MyAggregate i)
{
if (_dateX> DateTime.Now)
{
i.CommandValidationMessages.Add(new ValidationMessage("datex greater than now"));
return false;
}
return true;
}
}
public class Has_start_date_greater_than_end_date : ISpecification<MyAggregate>
{
private readonly DateTime _start;
private readonly DateTime _end;
public Has_start_date_greater_than_end_date(DateTime start, DateTime end)
{
Contract.Requires<ArgumentNullException>(start == DateTime.MinValue);
Contract.Requires<ArgumentNullException>(start == DateTime.MinValue);
_start = start;
_end = end;
}
public bool IsSatisfiedBy(MyAggregate i)
{
if (_start > _end)
{
i.CommandValidationMessages.Add(new ValidationMessage(start date greater then end date"));
return false;
}
return true;
}
}
This allows me to reuse some validations for different aggregate and it is easy to test. If you see any flows in it. I would be real happy to discuss it.
yours,
From my OO experience (I am not a DDD expert) moving your code from the entity to a higher abstraction level (into a command handler) will cause code duplication. This is because every time a command handler gets an email address, it has to instantiate email validation rules. This kind of code will rot after a while, and it will smell very badly. In the current example it might not, if you don't have another command which changes the email address, but in other situations it surely will...
If you don't want to move the rules back to a lower abstraction level, like the entity or an email value object, then I strongly suggest you to reduce the pain by grouping the rules. So in your email example the following 3 rules:
if(string.IsNullOrWhitespace(email)) throw new DomainException(“...”);
if(!email.IsEmail()) throw new DomainException();
if(email.Contains(“#mailinator.com”)) throw new DomainException();
can be part of an EmailValidationRule group which you can reuse easier.
From my point of view there is no explicit answer to the question where to put the validation logic. It can be part of every object depending on the abstraction level. In you current case the formal checking of the email address can be part of an EmailValueObject and the mailinator rule can be part of a higher abstraction level concept in which you state that your user cannot have an email address pointing on that domain. So for example if somebody wants to contact with your user without registration, then you can check her email against formal validation, but you don't have to check her email against the mailinator rule. And so on...
So I completely agree with #pjvds who claimed that this kind of awkward placed validation is a sign of a bad design. I don't think you will have any gain by breaking encapsulation, but it's your choice and it will be your pain.
The validation in your example is validation of a value object, not an entity (or aggregate root).
I would separate the validation into distinct areas.
Validate internal characteristics of the Email value object internally.
I adhere to the rule that aggregates should never be in an invalid state. I extend this principal to value objects where practical.
Use createNew() to instantiate an email from user input. This forces it to be valid according to your current rules (the "user#email.com" format, for example).
Use createExisting() to instantiate an email from persistent storage. This performs no validation, which is important - you don't want an exception to be thrown for a stored email that was valid yesterday but invalid today.
class Email
{
private String value_;
// Error codes
const Error E_LENGTH = "An email address must be at least 3 characters long.";
const Error E_FORMAT = "An email address must be in the 'user#email.com' format.";
// Private constructor, forcing the use of factory functions
private Email(String value)
{
this.value_ = value;
}
// Factory functions
static public Email createNew(String value)
{
validateLength(value, E_LENGTH);
validateFormat(value, E_FORMAT);
}
static public Email createExisting(String value)
{
return new Email(value);
}
// Static validation methods
static public void validateLength(String value, Error error = E_LENGTH)
{
if (value.length() < 3)
{
throw new DomainException(error);
}
}
static public void validateFormat(String value, Error error = E_FORMAT)
{
if (/* regular expression fails */)
{
throw new DomainException(error);
}
}
}
Validate "external" characteristics of the Email value object externally, e.g., in a service.
class EmailDnsValidator implements IEmailValidator
{
const E_MX_MISSING = "The domain of your email address does not have an MX record.";
private DnsProvider dnsProvider_;
EmailDnsValidator(DnsProvider dnsProvider)
{
dnsProvider_ = dnsProvider;
}
public void validate(String value, Error error = E_MX_MISSING)
{
if (!dnsProvider_.hasMxRecord(/* domain part of email address */))
{
throw new DomainException(error);
}
}
}
class EmailDomainBlacklistValidator implements IEmailValidator
{
const Error E_DOMAIN_FORBIDDEN = "The domain of your email address is blacklisted.";
public void validate(String value, Error error = E_DOMAIN_FORBIDDEN)
{
if (/* domain of value is on the blacklist */))
{
throw new DomainException(error);
}
}
}
Advantages:
Use of the createNew() and createExisting() factory functions allow control over internal validation.
It is possible to "opt out" of certain validation routines, e.g., skip the length check, using the validation methods directly.
It is also possible to "opt out" of external validation (DNS MX records and domain blacklisting). E.g., a project I worked on initially validated the existance of MX records for a domain, but eventually removed this because of the number of customers using "dynamic IP" type solutions.
It is easy to query your persistent store for email addresses that do not fit the current validation rules, but running a simple query and treating each email as "new" rather than "existing" - if an exception is thrown, there's a problem. From there you can issue, for example, a FlagCustomerAsHavingABadEmail command, using the exception error message as guidance for the user when they see the message.
Allowing the programmer to supply the error code provides flexibility. For example, when sending a UpdateEmailAddress command, the error of "Your email address must be at least 3 characters long" is self explanatory. However, when updating multiple email addresses (home and work), the above error message does not indicate WHICH email was wrong. Supplying the error code/message allows you to provide richer feedback to the end user.
I wrote a blog post on this topic a while back. The premise of the post was that there are different types of validation. I called them Superficial Validation and Domain Based Command Validation.
This simple version is this. Validating things like 'is it a number' or 'email address' are more often than not just superficial. These can be done before the command reaches the domain entities.
However, where the validation is more tied to the domain then it's right place is in the domain. For example, maybe you have some rules about the weight and type of cargo a certain lorry can take. This sounds much more like domain logic.
Then you have the hybrid types. Things like set based validation. These need to happen before the command is issued or injected into the domain (try to avoid that if at all possible - limiting dependencies is a good thing).
Anyway, you can read the full post here: How To Validate Commands in a CQRS Application
I'm still experimenting with this concept but you can try Decorators. If you use SimpleInjector you can easily inject your own validation classes that run ahead of your command handler. Then the command can assume it is valid if it got that far. However, This means all validation should be done on the command and not the entities. The entities won't go into an invalid state. But each command must implement its own validation fully so similar commands may have duplication of rules but you could either abstract common rules to share or treat different commands as truly separate.
You can use a message based solution with Domain Events as explained here.
Exceptions are not the right method for all validation errors, is not said that a not valid entity is an exceptional case.
If the validation is not trivial, the logic to validate the aggregate can be executed directly on the server and while you are trying to set new input you can raise a Domain Event to tell to the user (or the application that is using your domain) why the input is not correct.
I need to organize some simple security in a class depends on value of the enum.
All that I can figure out is using attribute on a method and then run check then if it fails throw an exception.
Sample:
[ModulePermission(PermissonFlags.Create)]
public void CreateNew()
{
CheckPermission();
System.Windows.Forms.MessageBox.Show("Created!");
}
protected void CheckPermission()
{
var method = new System.Diagnostics.StackTrace().GetFrame(1).GetMethod();
if (!flags.HasFlag(method.GetCustomAttributes(true).Cast<ModulePermissionAttribute>().First().Flags))
{
throw new ApplicationException("Access denied");
}
}
is there more elegant or simple way to do this, like just to trigger an event when method run?
Why not just use standard Code Access Security instead of reimplementing the attribute handling and stack walking?
I think that if you read through the linked documentation, you'll see that what you have is nowhere close to what is needed to achieve actual security. Thankfully, this hard problem has already been solved...
Not with an enum, but with strings - voila (enforced by the runtime, even in full-trust):
public static class PermissionFlags {
public const string Create = "Create";
}
[PrincipalPermission(SecurityAction.Demand, Role = PermissionFlags.Create)]
public void CreateNew() {
System.Windows.Forms.MessageBox.Show("Created!");
}
All you need to do now is to represent the user as a principal. This is done for you in ASP.NET, and there is a winform plugin (in VS2008 etc) to use ASP.NET for membership. It can be configured for vanilla winforms and WCF, too; at the most basic level, GenericPrincipal / GenericIdentity:
// during login...
string[] roles = { PermissionFlags.Create /* etc */ };
Thread.CurrentPrincipal = new GenericPrincipal(
new GenericIdentity("Fred"), // user
roles);
But you can write your own principal / identity models easily enough (deferred / cached access checks, for example).
You might want to look at doing this with something like PostSharp, which will give you a framework for applying the attributes so that you don't have to run the check in your method. This may, however, increase the complexity depending on how the currently active flags are accessed. You'd probably need some class to cache the current permissions for the current user.
You could take a look at Aspect Oriented Programming.
Check out Postsharp for instance, which will enable you to 'weave' some additional logic at compile time in the methods that you've decorated with your ModulePermission attribute.
By doing so, you will not have to call the 'CheckPermission' method anymore inside that 'secured' method, since that logic can be weaved by Postsharp.
(A while ago, I've been playing around with Postsharp: http://fgheysels.blogspot.com/2008/08/locking-system-with-aspect-oriented.html )
In a previous question one of the comments from Dr. Herbie on the accepted answer was that my method was performing two responsibilities..that of changing data and saving data.
What I'm trying to figure out is the best way to separate these concerns in my situation.
Carrying on with my example of having a Policy object which is retrieved via NHibernate....
The way I'm currently setting the policy to inactive is as follows:
Policy policy = new Policy();
policy.Status = Active;
policyManager.Inactivate(policy);
//method in PolicyManager which has data access and update responsibility
public void Inactivate(Policy policy)
{
policy.Status = Inactive;
Update(policy);
}
If I were to separate the responsibility of data access and data update what would be the best way to go about it?
Is it better to have the PolicyManager (which acts as the gateway to the dao) manage the state of the Policy object:
Policy policy = new Policy();
policy.Status = Active;
policyManager.Inactivate(policy);
policyManager.Update(policy);
//method in PolicyManager
public void Inactivate(Policy policy)
{
policy.Status = Inactive;
}
Or to have the Policy object maintain it's own state and then use the manager class to save the information to the database:
Policy policy = new Policy();
policy.Status = Active;
policy.Inactivate();
policyManager.Update(policy);
//method in Policy
public void Inactivate()
{
this.Status = Inactive;
}
What I would do:
Create a repository which saves and retrieves Policies. (PolicyRepository)
If you have complex logic that must be performed to activate / deactivate a policy, you could create a Service for that. If that service needs access to the database, then you can pass a PolicyRepository to it, if necessary.
If no complex logic is involved, and activating / deactivating a policy is just a matter of setting a flag to false or true, or if only members of the policy class are involved, then why is 'Activated' not a simple property of the Policy class which you can set to false / true ?
I would only create a service, if other objects are involved, or if DB access is required to activate or deactivate a policy.
As a continuation of my original comment :) ...
Currently your best bet is the third option, but if things get more complex you could go with the second, while adding facade methods to perform pre-specified sequences:
Policy policy = new Policy();
policy.Status = Active;
policyManager.InactivateAndUpdate(policy);
//methods in PolicyManager
public void Inactivate(Policy policy)
{
// possibly complex checks and validations might be put there in the future? ...
policy.Status = Inactive;
}
public void InactivateAndUpdate(Policy policy)
{
Inactivate(policy);
Update(policy);
}
The InactivateAndUpdate is a kind of facade method, which is just there to make the calling code a little neater, while still allowing the methods doing the actual work to be separate concerns (kind of breaks single responsibility for methods, but sometimes you just have to be pragmatic!). I deliberately name such methods in the style XandY to make them stand out as doing two things.
The InactivateAndUpdate method then frees you up to start implementing strategy patterns or splitting out the actual implementation methods as command objects for dynamic processing or whatever other architecture might become feasible in the future.
I would definitely go with the 3rd option for the reasons you mentioned:
the Policy object maintain it's own
state and then use the manager class
to save the information to the
database
Also take a look at the Repository Pattern. It might substitute your PolicyManager.
If the status is part of the state of the Policy class, then the Policy should also have the Inactivate method -- that's just basic encapsulation. Entangling multiple classes in a single responsibility is at least as bad as giving a single class multiple responsibilities.
Alternatively, the status could be considered metadata about the Policy, belonging not to the Policy but to the PolicyManager. In that case, though, the Policy shouldn't know its own status at all.