I come from a mostly n-tier background, and I'm trying to move more towards a DDD architecture. I'm trying to find best practices for designing the application service, and after a few searches, am still left with a few questions. Granted, I know I can't be the first person to ask these questions, so if you know where these are answered, just point me the way and I'll happily close this.
Here are my main questions:
How "open" should your signatures be? For example, is it better to be more rigid with your signatures and use simple types as parameters when possible, or is it better to use objects (messages?) that can later be modified without breaking the signature?
If you want to expose variations of a signature, for example, a UserSearch method that returns a list of users based on various (and sometimes optional) search criteria, is it better to:
A. Use overloads
B. Use optional (or nullable) parameters
C. Break each scenario into its own unique method
D. Use messages
I know that some of these answers are subjective, and also depend on what all will be calling your application service. But I'm just trying to get a general direction of things to consider and other best practices at this point.
Thanks in advance.
Good questions. Thinking about the API is obviously important.
1) How open would depend for me would depend on who the consumers are. If this application service is only being used within the context of your own solution and/or team then I think it's fine to have specific messages (or rather their interfaces) or Dtos (data transfer objects). Though if as easy then keeping to simple types is best in my book and definitely better if being consumed by others. If they don't suffice then interfaced messages that provide only just enough. Again if you are going to be distributed to different platforms then simple messages of simple types is not a bad way to go.
2) Why not have a SearchCriteria object as a paramater? It could be a SearchCriteria message of simple types if you are looking at this as a start of a messaging bus.
As you say, your question is a little open but I'd be interested to hear more as it sounds like you are asking the right questions at least.
Jerad, those are tough questions to answer generally, as you noted.
My personal preference is to use primitives in method signatures where possible. If I need to pass 3+ primitives to a method, I define custom data transfer objects.
The thinking being: if multiple values are being passed together, it's likely they represent a concept in your problem space, and thus should become an object. For example, if you are passing X and Y coordinates to a method, I'd recommend creating a Point class or struct that represents that concept.
The only time I'd end up with variations on a signature, it would be to provide convenience methods that provide default values for method parameters. To continue the above example, a Draw method might not require a Point, in which case I'd use (0,0).
So, I'd answer #1 with "not very open" and #2 with A.
I hope that helps.
Related
I watched some videos, read some blogs about it. SO has many questions and answers on that subject but I can not find anywhere exact answer for my question.
Almost every question and answer has a lack of usage context.
I have one middle sized, asp.net-mvc, monolith application which is running in one process on IIS. I want to (refactor and) go all the way with DDD (and CQRS without separated storage for reads and writes for now) but for me it looks like impossible mission without breaking some rules/guides/etc.
Bounded Context
For example I have more than one BCs. Each should not cross their boundaries which means should not share their storage. Right?
It is not possible if you use the whole known (everywhere scattered over the web) solution to work with NHibernate session and UoW.
Aggregate Root
Only one AR should be modified in one transaction. When others ARs are involved should introduce eventual consistency (if I remember those are Eric Evans words).
I try to do it but it is not easy in app like that. Pub/Sub not working as desired because if event is published then all subscribers are take their action within one transaction (NSB/MT does that way).
If event handlers wants to modify others ARs they should be executed in separated transactions, right?
Is it possible to deal with it in monolith application (application where whole code works in one process)?
It is not possible if you use the whole known (everywhere scattered
over the web) solution
[...]
if event is published then all subscribers are take their action
within one transaction
I think you're setting yourself useless and harmful constraints by trying to stick to some "state of the art".
Migrating an entire application to DDD + CQRS is a massive undertaking. Some areas of it don't have well-documented beaten paths yet and you'll probably have a fair bit of exploration to do. My best advice would be to stay at a reasonable distance from "the way people do things". Both in traditional ASP.Net web apps because mainstream practices often don't match the way DDD+CQRS works, and in CQRS itself because the case studies out there are few and far between and most probably very domain specific, with a tendency to advocate the use of heavy tools which may not make sense in your context.
You may need to think out of the box, adopt things incrementally and refrain from goldplating everything. You'll be better off starting with very simple implementations that suit your needs exactly than throwing a ton of tools and frameworks at your codebase.
For instance, do you really need a service bus or could a simple Observer pattern suffice ?
Regarding NHibernate, most implementations out there use a (single) Session Per Request approach, but just because it's the most popular doesn't mean it's the only one. Have you tried using multiple ISessions (one for each BC) available at a more programmable level, such as per-action, or managed entirely manually ? Conversely, have you considered sharing a database between Bounded Contexts at first and see for yourself if that's bad or not ?
I've asked a couple of questions around this subject recently, and I think I'm managing to narrow down what I need to do.
I am attempting to create some "metrics" (quotes because these should not be confused with metrics relating to the performance of the application; these are metrics that are generated based on application data) in a Rails app; essentially I would like to be able to use something similar to the following in my view:
#metric(#customer,'total_profit','01-01-2011','31-12-2011').result
This would give the total profit for the given customer for 2011.
I can, of course, create a metric model with a custom result method, but I am confused about the best way to go about creating the custom metrics (e.g. total_profit, total_revenue, etc.) in such a way that they are easily extensible so that custom metrics can be added on a per-user basis.
My initial thoughts were to attempt to store the formula for each custom metric in a structure with operand, operation and operation_type models, but this quickly got very messy and verbose, and was proving very hard to do in terms of adding each metric.
My thoughts now are that perhaps I could create a custom metrics helper method that would hold each of my metrics (thus I could just hard code each one, and pass variables to each method), but how extensible would this be? This option doesn't seem very rails-esque.
Can anyone suggest a better alternative for approaching this problem?
EDIT: The answer below is a good one in that it keeps things very simple - though i'm concerned it may be fraught with danger, as it uses eval (thus there is no prospect of ever using user code). Is there another option for doing this (my previous option where operands etc. were broken down into chunks used a combination of constantize and get_instance_variable - is there a way these could be used to make the execution of a string safer)?
This question was largely answered with some discussion here: Rails - Scalable calculation model.
For anyone who comes across this, the solution is essentially to ensure an operation always has two operands, but an operand can either be an attribute, or the result of a previous calculation (i.e. it can be a metric itself), and it is thus highly scalable. This avoids the need to eval anything, and thus avoids the potential security holes that this entails.
I'm starting to see Contexts everywhere I look. In ASP.NET MVC, there are ControllerContexts, RequestContexts, HttpContexts, FormContexts. In Entity Framework, you have ObjectContexts and DbContexts. Ninject has Ninject.Activation.IContext.
What the heck is a context?
Well, it's kind of a dependency-injection thing, that allows people to say 'Here is the environment you will operate in'. Generally, they provide, unsurprisingly, the "context" for whatever it is. I.e., some state. Perhaps the URL, perhaps some HTTP headers, whatever.
You are seeing a lot of them because ASP.NET is focused on testing and hence allows these items to be "swapped in", such that you can provide your own context implementations (i.e. define your own state), so that you can run tests appropriately and successfully.
If you're wondering what state is, well it's just various bits of data that are "given", by the environment. I.e. today it is cold in the office. This is part of the state. But perhaps I want to run my test when it is hot in the office, so I would be able to subclass OfficeContext and return the appropriate state for the appropriate method/etc.
IMO, Context denotes some (possibly mutable) state about some thing. Typically context would be cross-cutting layers and often carries domain neutral data across layers.
Context is information outside of the scope of the thing you're currently doing but which has implications that may be essential.
Imagine if someone asks you the meaning of the English word "fly". There are multiple definitions: the buzzing little inspects or the sustained act of gliding through air. In order to give the right answer you need the context which tells you which definition they're looking for - are they reading a book about Diptera or the Wright brothers?
Regarding computing, say you're implementing an HTTP handler. It might be able to generate a response without knowing anything else (Hello, World!) but it's more likely that it needs the context of the HTTP request information - what were the request parameters, acceptable encoding types, etc. so it can produce a meaningful response to the user agent.
I think of them as being like your environment variables and profile settings in a telnet/ssh session. They bundle together different settings to allow tools to perform differently based on the context (i.e. environment) they're run in.
IMO, it's just another argument. In my (limited) experience, I've seen it be the calling class. You have to know what you're doing to do what you're doing well. Context is what you're doing, what's happening/running.
The above answers are by and large quite good. The only thing I would add is that its most common usage is as a "glue" to lower layers of a system. Generally the system in question is some kind of "container" system, where your code is executed inside of a larger code base that hides a lot of execution details from you. The context is the abstracted interface to that larger system.
I wonder what sort of things you look for when you start working on an existing, but new to you, system? Let's say that the system is quite big (whatever it means to you).
Some of the things that were identified are:
Where is a particular subroutine or procedure invoked?
What are the arguments, results and predicates of a particular function?
How does the flow of control reach a particular location?
Where is a particular variable set, used or queried?
Where is a particular variable declared?
Where is a particular data object accessed, i.e. created, read, updated or deleted?
What are the inputs and outputs of a particular module?
But if you look for something more specific or any of the above questions is particularly important to you please share it with us :)
I'm particularly interested in something that could be extracted in dynamic analysis/execution.
I like to use a "use case" approach:
First, I ask myself "what's this software's purpose?": I try to identify how users are going to interact with the application;
Once I have some "use case", I try to understand what are the objects that are more involved and how they interact with other objects.
Once I did this, I draw a UML-type diagram that describe what I've just learned for further reference. What happens after depends on the task I've been assigned, i.e. modify the code, document the code etc.
There is the question of what motivation do I have for learning the new system:
Bug fix/minor enhancement - In this case, I may focus solely on that portion of the system that performs a specific function that needs to be altered. This is a way to break down a huge system but also is a way to identify if the issue is something I can fix or if it is something that I have to hand to the off-the-shelf company whose software we are using,e.g. a CRM, CMS, or ERP system can be a customized off-the-shelf system so there are many pieces to it.
Project work - This would be the other case and is where I'd probably try to build myself a view from 30,000 feet or so to know what are the high-level components and which areas of the system does the project impact. An example of this is where I'd join a company and work off of an existing code base but I don't have the luxury of having the small focus like in the previous case. Part of that view is to look for any patterns in the code in terms of naming conventions, project structure, etc. as this may be useful once I start changing some code in the system. I'd probably do some tracing through the system and try to see where are the uglier parts of the code. By uglier I mean those parts that are kludge-like and may have some spaghetti code as this was rushed when first written and is now being reworked heavily.
To my mind another way to view this is the question of whether I'm going to be spending days or weeks wrapping my head around a system like in the second case or should this be a case where it hopefully takes only a few hours, optimistically that is, to get my footing to make the necessary changes.
If you're writing something by yourself, whether to practice, solve a personal problem, or just for entertainment, is it ok, once in a while, to have a public field? Maybe?
Let me give you an analogy.
I come from a part of the world where English is not the primary language. But it’s necessary for all things in life.
During one of those usual days during my pre-teen years I said something very funny in English. Then my Dad said, “Son, think in English. Then you’ll get fluent”
I think it applies perfectly to this situation.
Think,try and question best practices in your playground. You will soon realize what’s best for what.Why are properties needed in the first place. Why should this be public? Why should I not call a virtual member from the constructor? Let me try using "new" modifier for a method call. What happens when I write 10 nested levels of if-then-else and try reading it again after 10 days. Why the heck should I use a factory pattern for a simple project. Et cetera.
And then you’ll realize without shooting at your foot, why design patterns are patterns...
I think it's reasonable if you're consciously throwing the code away afterwards. In particular, if you're experimenting with something completely different, taking shortcuts makes sense. Just don't let it lead to habits which cross over into "real" code.
Violating general principles is always "ok"! It is not an error to violate a principle but it is a trade off. The cost of not writing clean code will be higher the longer your software will survive. My take on this is: If in doubt make it clean!
Of course it's OK. It's your code, you can do whatever you want with it. Personally, I try to stick to good practice also in my private code, just to make it a natural habit so I don't have to think about it.
The short answer is yes, if you believe that you're gaining a lot by making things public instead of private with accessors you are welcome to do so. Consistency, I think, is the biggest thing to keep in mind. For instance, don't make some variables straight public, and some not. Do the same across the board if you break with best practices. It comes back to a trade-off. Almost no-one follows many of the IEEE specs for how Software Engineering should be executed and documented because the overhead is far too great, and it can get unmanageable. The same is true for personal, light-weight programming. It's okay to do something quick and dirty, just do not get used to it.
Public members are acceptable in the Data Transfer Object design patter:
Typically, the members in the Transfer Object are defined as public, thus eliminating the need for get and set methods.
One of the key advantages of OOP is for scaling and maintainability. By encapsulating code, one can hide the implementation. This means other programmers don't have to know the implementation, and can't change your object's internal state. If you language doesn't support properties, you end up with a lot of code which obfuscates and bloats your project. If the code doesn't need to be worked on by multiple programmers, you aren't producing a reusable component, and YOU are the maintenance programmer, then code in whatever manner allows you to get things done.
Does a maid need to make his/her own bed in the morning in order to practice properly making a bed?
Side note: it also depends on the language:
In Scala, according to the Uniform Access Principle, clients read and write field values as if they are publicly accessible, even though in some case they are actually calling methods. The maintainer of the class has the freedom to change the implementation without forcing users to make code changes.
Scala keeps field and method names in the same namespace.
Many languages, like Java, keep field and method names in separate namespaces.
However, these languages can’t support the uniform access principle as a result, unless they build in ad hoc support in their grammars or compilers.
So the real question is:
What service are you exposing (here by having a public field)?.
If the service (get/set a given type value) makes sense for your API, then the "shortcut" is legitimate.
As long as you encapsulate that field eventually, is it ok because you made the shortcut for the "right" reason (API and service exposure), versus the "wrong" reason (quick ad-hoc access).
A good unit test (thinking like the user of your API) can help you check if that field should be accessed directly or if it is only useful for internal development of other classes within your program.
Here's my take on it:
I'd advise avoiding public fields. They have a nasty habit of biting you later on because you can't control them. (The word you're looking for here is volatility.) Further, if you decide to change their internal implementation, you have to touch a lot more code.
Then again, that's what refactoring tools are for. If you have a decent refactoring tool, that's not nearly so difficult.
There is no silver bullet. I can't repeat this enough. If you have work to do, and you need to get it done in a hurry, writing one line of code instead of eight (as is the case in Visual Basic) is certainly faster.
Rules were meant to be broken. If a rule doesn't necessarily apply in your case, don't use it. Design patterns, coding guidelines, laws and best practices should not be treated as a straightjacket that requires you to needlessly complicate your code to the point where it is enormously complex and difficult to understand and maintain. Don't let someone force you into a practice just because it's popular or "standard" when it doesn't fit your requirements.
Again, this is a subjective opinion, and your mileage may vary.