Say I have a class Product {string name; double price;...}
So if I want to implement a seasonal or a discounted product, I can add declare a isPromoted/isSeasonal or I can create a new class promotedProduct : Product {...}, seasonalProduct : Product{} and much more.
I have been told that beginners tend to misuse inheritance and how it is evil. So I am not sure if I am considered misusing in this case. Or maybe they are both wrong and using interface is better etc.
Will products simply be on promotion or not - ie - no actual promotion details (different rates, dates etc) - if do you'll want your first solution. Otherwise you'll want a relation to a Promotions class which would store such details.
Hope this helps.
Note: this is a general programming answer having never used Java personally.
One good rule of thumb is to separate what changes from what doesn't.
You cannot change type at runtime but you probably want to be able to promote/demote a product easily.
One option is to store state directly on the Product as fields (but this will get ugly if they accumulate, isLowInStock etc).
Probably better os treat Product as an entity and separate the isPromoted/isSeasonal state by:
Use the State pattern to encapsulate the state (you can tie particular behaviour to state changes with Strategy)
Externalize the state and make service calls
Use #1 to guard against future change (i.e. more fields). #2 is probably overkill.
Please don't use double to represent Price - see this question or google.
Do seasonal or discounted products add attributes? Do they have different behavior? That's what drives inheritance. I suspect there's no real difference in behaviour and no real difference in the attributes they carry.
Hence, I'd look at a discount simply as an attribute that every product has: any product might go an sale and get discounted, so I'd have a property indicating the current discount percentage or amount (which probably defaults to zero, or perhaps it's nullable. Ditto for seasonal: should that simply be a boolean flag?
Related
I have been struggling to find information on how a resource that contains generated values is modified. Below is a real world example:
Let's say we have 2 endpoints:
/categories and /products.
A category is used to contain various parameters that define any product belonging to it. For example, based on a category a product expiration date might be calculated, or some other properties might or might not be attached to a product.
Let's say we create a new product by sending a POST request to /products and among other fields we include the category ID property. Based on the category set a server creates and stores a new product along with various properties generated (expiration date, delivery policies) etc.
Now the problem arises when needing to modify (PATCH/ PUT) the mentioned product.
How are generated values edited? We can for example change a delivery policy, but then the product will contain a field that doesn't match what its attached category describes. Likewise, it might be very handy to modify its generated expiration date, however yet again that can create confusion about why a category says it should expire in 3 days but the product is set to expire in 20 days.
Another solution would be to make all these properties read-only and only allow regenerating them by changing the category, just like at creation.
However that poses 2 problems:
The biggest one being that a different category might not contain the same policy layout. For example, one category might enable generating GPS coordinates to ease the delivery, the other category does not. If we change the category, what do we do with these valuable properties already present? Do we drop them for the sake of clarity?
Another issue is limited flexibility. There might be cases when a property needs to be changed but the category needs to remain the same.
I think these questions are met and answered in probably every single REST API development and probably I am just missing something very simple and obvious. Could you help me understand the right way of going about this?
Thank you very much.
I think these questions are met and answered in probably every single REST API development and probably I am just missing something very simple and obvious. Could you help me understand the right way of going about this?
You write code to ensure that all of the invariants hold for the server's copy of the resource.
That can mean either (a) inspecting the body of the request, and returning a client error if the body doesn't satisfy the constraints you need to maintain, or (b) changing your resource in a way that doesn't exactly match the request you've received.
In the second case, you need to have a little bit of care with the response metadata, so that you don't imply that the representation of the request has been adopted "as is".
The code you are writing here is part of the origin server's implementation, deliberately hidden by the HTTP facade you present. The general purpose components in the middle don't care about those details; they just want you to use messaging semantics consistent with the HTTP (and related) specifications.
Can I construct a value object in the event handler or should I pass the parameters to the aggregate to construct the value object itself? Seller is the aggregate and offer is the value object. Will it be better for the aggregate to pass the value object in the event?
public async Task HandleAsync(OfferCreatedEvent domainEvent)
{
var seller = await this.sellerRepository.GetByIdAsync(domainEvent.SellerId);
var offer = new Offer(domainEvent.BuyerId, domainEvent.ProductId, seller.Id);
seller.AddOffer(offer);
}
should I pass the parameters to the aggregate to construct the value object itself?
You should probably default to passing the assembled value object to the domain entity / root entity.
The supporting argument is that we want to avoid polluting our domain logic with plumbing concerns. Expressed another way, new is not a domain concept, so we'd like that expression to live "somewhere else".
Note: that by passing the value to the domain logic, you protect that logic from changes to the construction of the values; for instance, how much code has to change if you later discover that there should be a fourth constructor argument?
That said, I'd consider this to be a guideline - in cases where you discover that violating the guideline offers significant benefits, you should violate the guideline without guilt.
Will it be better for the aggregate to pass the value object in the event?
Maybe? Let's try a little bit of refactoring....
// WARNING: untested code ahead
public async Task HandleAsync(OfferCreatedEvent domainEvent)
{
var seller = await this.sellerRepository.GetByIdAsync(domainEvent.SellerId);
Handle(domainEvent, seller);
}
static Handle(OfferCreatedEvent domainEvent, Seller seller)
{
var offer = new Offer(domainEvent.BuyerId, domainEvent.ProductId, seller.Id);
seller.AddOffer(offer);
}
Note the shift - where HandleAsync needs to be aware of async/await constructs, Handle is just a single threaded procedure that manipulates two local memory references. What that procedure does is copy information from the OfferCreatedEvent to the Seller entity.
The fact that Handle here can be static, and has no dependencies on the async shell, suggests that it could be moved to another place; another hint being that the implementation of Handle requires a dependency (Offer) that is absent from HandleAsync.
Now, within Handle, what we are "really" doing is copying information from OfferCreatedEvent to Seller. We might reasonably choose:
seller.AddOffer(domainEvent);
seller.AddOffer(domainEvent.offer());
seller.AddOffer(new Offer(domainEvent));
seller.AddOffer(new Offer(domainEvent.BuyerId, domainEvent.ProductId, seller.Id));
seller.AddOffer(domainEvent.BuyerId, domainEvent.ProductId, seller.Id);
These are all "fine" in the sense that we can get the machine to do the right thing using any of them. The tradeoffs are largely related to where we want to work with the information in detail, and where we prefer to work with the information as an abstraction.
In the common case, I would expect that we'd use abstractions for our domain logic (therefore: Seller.AddOffer(Offer)) and keep the details of how the information is copied "somewhere else".
The OfferCreatedEvent -> Offer function can sensibly live in a number of different places, depending on which parts of the design we think are most stable, how much generality we can justify, and so on.
Sometimes, you have to do a bit of war gaming: which design is going to be easiest to adapt if the most likely requirements change happens?
I would also advocate for passing an already assembled value object to the aggregate in this situation. In addition to the reasons already mentioned by #VoiceOfUnreason, this also fits more naturally with the domain language. Also, when reading code and method APIs you can then focus on domain concepts (like an offer) without being distracted by details until you really need to know them.
This becomes even more important if you would need to pass in more then one value object (or entity). Rather passing in all the values required for construction as parameters not only makes the API more resilient to refactoring but also burdens the reader with more details.
The seller is receiving an offer.
Assuming this is what is meant here, fits better than something like the following:
The seller receives some buyer id, product id, etc.
This most probably would not be found in conversations using the ubiquitous language. In my opinion code should be as readable as possible and express the behaviour and business logic as close to human language as possible. Because you compile code for machines to execute it but the way you write it is for humans to easily understand it.
Note: I would even consider using factory methods on value objects in certain cases to unburden the client code of knowing what else might be needed to assemble a valid value object, for instance, if there are different valid constellations and ways of constructing the same value objects where some values need reasonable default values or values are chosen by the value object itself. In more complex situations a separate factory might even make sense.
Some of the Users in my database will also be Practitioners.
This could be represented by either:
an is_practitioner flag in the User table
a separate Practitioner table with a user_id column
It isn't clear to me which approach is better.
Advantages of flag:
fewer tables
only one id per user (hence no possibility of confusion, and also no confusion in which id to use in other tables)
flexibility (I don't have to decide whether fields are Practitioner-only or not)
possible speed advantage for finding User-level information for a practitioner (e.g. e-mail address)
Advantages of new table:
no nulls in the User table
clearer as to what information pertains to practitioners only
speed advantage for finding practitioners
In my case specifically, at the moment, practitioner-related information is generally one-to-many (such as the locations they can work in, or the shifts they can work, etc). I would not be at all surprised if it turns I need to store simple attributes for practitioners (i.e., one-to-one).
Questions
Are there any other considerations?
Is either approach superior?
You might want to consider the fact that, someone who is a practitioner today, is something else tomorrow. (And, by that I don't mean, not being a practitioner). Say, a consultant, an author or whatever are the variants in your subject domain, and you might want to keep track of his latest status in the Users table. So it might make sense to have a ProfType field, (Type of Professional practice) or equivalent. This way, you have all the advantages of having a flag, you could keep it as a string field and leave it as a blank string, or fill it with other Prof.Type codes as your requirements grow.
You mention, having a new table, has the advantage for finding practitioners. No, you are better off with a WHERE clause on the users table for that.
Your last paragraph(one-to-many), however, may tilt the whole choice in favour of a separate table. You might also want to consider, likely number of records, likely growth, criticality of complicated queries etc.
I tried to draw two scenarios, with some notes inside the image. It's really only a draft just to help you to "see" the various entities. May be you already done something like it: in this case do not consider my answer please. As Whirl stated in his last paragraph, you should consider other things too.
Personally I would go for a separate table - as long as you can already identify some extra data that make sense only for a Practitioner (e.g.: full professional title, University, Hospital or any other Entity the Practitioner is associated with).
So in case in the future you discover more data that make sense only for the Practitioner and/or identify another distinct "subtype" of User (e.g. Intern) you can just add fields to the Practitioner subtable, or a new Table for the Intern.
It might be advantageous to use a User Type field as suggested by #Whirl Mind above.
I think that this is just one example of having to identify different type of Objects in your DB, and for that I refer to one of my previous answers here: Designing SQL database to represent OO class hierarchy
Is it better to have a method like this:
#1 void AddPerson(Person p);
or
#2 void AddPerson(int id, string lastName, string firstName);
Explain.
The one issue I noticed is I cannot point my web browser to AddPerson and test for #1, but I can for #2.
In addition to what ahus1 said, creating a separate Person.xsd XML schema is almost always a good idea, because this way you could import this Person.xsd into other XML schemas such as Invoice.xsd, creating a common language in order to describe your business.
Also, it makes it more simple to orchestrate services, imagine that you might have a given customer atribute into Invoice that is actually from Person Type. This way you could assign the customer value to other Person elements in just one step instead of having to copy all Person attributes in each step.
You may find at Canonical Schema SOA Pattern, good information about the benefits of this approach.
if you pass a person instead of the different elements, it will be easier for you when you enhance the Person in the future for example with a date of birth: your method names stay the same, you just enhance your domain model fill or read the date of birth where you need it.
Also you will find that as soon as you get a nested objects (if you want to add i.e. an address), a Person instead of the elements will give a uniform way for your methods.
Best regards,
Alexander.
I am writing a Product requirements specification. In this document I must describe the ways that the user can interact with the system in a very high level. Several of these operations are "Create-Read-Update-Delete" on some objects.
The question is, when writing use cases for these operations, what is the right way to do so? Can I write only one Use Case called "Manage Object x" and then have these operations as included Use Cases? Or do I have to create one use case per operation, per object? The problem I see with the last approach is that I would be writing quite a few pages that I feel do not really contribute to the understanding of the problem.
What is the best practice?
The original concept for use cases was that they, like actors, and class definitions, and -- frankly everything -- enjoy inheritance, as well as <<uses>> and <<extends>> relationships.
A Use Case superclass ("CRUD") makes sense. A lot of use cases are trivial extensions to "CRUD" with an entity type plugged into the use case.
A few use cases will be interesting extensions to "CRUD" with variant processing scenarios for -- maybe -- a fancy search as part of Retrieve, or a multi-step process for Create or Update, or a complex confirmation for Delete.
Feel free to use inheritance to simplify and normalize your use cases. If you use a UML tool, you'll notice that Use Cases have an "inheritance" arrow available to them.
The answer really depends on how complex the interactions are and how many variations are possible from object to object. There are two real reasons why I suggest that you develop specific use cases for each CRUD
(a) If you really are only doing a high-level summary of the interaction then the overhead is very small
(b) I've found it useful to specify a set of generic Use Cases for modifying 'Resources' and then extending / overriding particular steps for particular objects. Obviously the common behaviour is captured in the generic 'Resource' use cases.
As your understanding of the domain develops (i.e. as business users dump more requirements on you), you are more likely to add to the CRUD rather than remove it.
It makes sense to distinguish between workflow cases and resource/object lifecycles.
They interact but they are not the same; it makes sense to specify them both.
Use case scenarios or more extended workflow specifications typically describe how a case may proceed through the system's workflow. This will typically include interaction with various different resources. These interactions can often be characterized as C,R,U or D.
Resource lifecycles provide the process model of what may happen to a particular (type of) resource (object). They are often trivial "flower" models that say: any of C,R,U,D may happen to this resource in any order, so they are not very interesting by themselves.
The link between the two is that steps from the workflow and from the lifecycles coincide.
I feel representation - as long as it makes sense and is readable - does not matter. Conforming to the UML spec in all details is especially irrelevant.
What does matter, that you spec clearly states the operations and operation types the implementaton requires.
C: What form of insert operations exists. Can you insert rows not fully populated? Can you insert rows without an ID? Can you retrieve the ID last inserted? Can you cancel an insert selectively? What happens on duplicate keys or constraints failure? Is there a REPLACE INTO equivalent?
R: By what fields can you select? Can you do arbitrary grouping, orders? Can you create aggregate fields, aliases? How can you retrieve embedded (has many etc.) data? How do you specify depth of recursion, limits?
U, D: see R + C