We're developing a comprehensive domain model encompassing 7(!) models/bounded contexts spanning several teams. We are yet to decide whether each one of the BCs is entirely disconnected from the others (being orchestrated by a layer above) or whether they are going to communicate via domain-events.
The application under development is for all purposes a SWT/Swing single-threaded application, so no fancy distributed mumbo jumbo between the different BCs is needed.
Yet, a big question remains: how to integrate all those different models? Should it be the Application Layer to undertake the task? If yes, and since in some (hopefully, few) cases the wiring and order ends up being complex, isn't the Application Layer the wrong place to do that?
For instance, consider the use of case of assembling a very complex synthetically created human (AssembleHumanoid). We have bounded contexts relating to the circulatory system, to the bone structure, the nervous system, ventilation system, coordination, immunological and mental systems and still the sensor system (lol, this was just all made up as you might imagine).
Wiring up all that stuff in the Application Layer feels kinda wrong. The obvious solution seems to be to create a 2nd Domain Layer just for orchestration matters. I've looked up but Vernon's Implementing Domain-Driven Design doesn't directly touch the issue (although he gets near # p531, "Composing Multiple Bounded Contexts").
What are your thoughts on the matter?
I'm right now tackling the same questions as you. My role in my project is architect and we have identified 5 BC's. But we are one team and intend to develop theses BC's within one large application. So our BC's are modules within a larger insurance application where each BC speaks its own ubiquitous language (Treaty, Reinsurance, security, medical risk assessment, premium).
But I have given this a lot of thoughts and I think we'll send updates to other BC through Domain Events. Our client is a MVC site that will consume our service layer. But My intention is that application layer have that kind of granularity so it will manage to perform the main task for the client without letting the client MVC project to coordination to other BC's.
We uses some shared Kernel between BC's but not for communicating. We do use DDD integration pattern where we have reference to other BC through Value Objects. We also have som BC to act like Factory, for example Security BC are creating different user roles for other BC's.
But when it comes to execution of a use case that actually need to to some final task in other BC's , Domain Event comes to rescue.
Related
I am studying Object Oriented Design and am using usecases with actors and scenario''s to plan out the application i am trying to build. No specific language yet, just the theory at the moment.
I have come to the point where i have identified and written out the use cases for the users, administrator, owner, etc and also the external systems like the feed generator.
but i have come to realise that my application actually consists of multiple smaller apps. like a data gathering application and a analysis application.
Can/should i use the data gathering and analysis app as an actor in the overall application too?
I can write specific use cases for them, with scenarios etc.
Typically, no.
Actor is an entity that sits outside of the system and produces some action. It gets to the system boundaries, but then all interactions between system components are modeled not as usecases, but as i.e. dynamic diagrams or sequence diagrams.
For the record, I think this approach is flawed and doesn't really help you in building applications. I personally prefer thinking about components and their interactions directly, without forcing the idea of architecture to fit a particular modeling scheme.
Background:
I'm thinking about web application organisation. I will separate front (web site for browser) from back (API): 2 apps, 2 repository, 2 hosting. Front will call API for almost everything.
So, if I have two separate domain services with my API (example: learning context and booking context) with no direct link between them, should I build 2 API (with 2 repository, 2 build process, etc.) ? Is it a good practice to build n APIs for n needs or one "big" API ? I'm speaking about a substantial web app with trafic.
(I hope this question will not be closed as not constructive... I think it's a real question for a concrete case, sorry if not. This question and some other about architecture were not closed so there is hope for mine)
It all depends on the application you are working on, its business needs, priorities you have and so on. Generally you have several options:
Stay with one monolithic application
Stay with one monolithic application but decouple domain model across separate modules/bundles/libraries
Create distributed architecture (like Service Oriented Architecture (SOA) or Event Driven Architecture (EDA))
One monolithic application
It's the easiest and the cheapest way to develop application on its beginning stage. You don't have to worry about complex architecture, complex deployment and development process. It also works better if there are no many developers around.
Once the application is growing up, this model begins to be problematic. You can't deploy modules separately, the app is more exposed to anti-patterns, spaghetti code/design (especially when a lot people working on it). QA process takes more and more time, which may make it unusable on CI basis. Introducing approaches like Continuous Integration/Delivery/Deployment is also much much harder.
Within this approach you have one repo/build process for all your APIs,
One monolithic application but decouple domain model
Within this approach you still have one big platform, but you connect logically separate modules on 3rd party basis. For example you may extract one module and create a library from it.
Thanks to that you are able to introduce separate processes (QA, dev) for different libraries but you still have to deploy whole application at once. It also helps you avoid anti-patterns, but it may be hard to keep backward compatibility across libraries within the application lifespan.
Regarding your question, in this way you have separate API, dev process and repository for each "type of actions" as long as you move its domain logic to separate library.
Distributed architecture (SOA / EDA)
SOA has a lot profits. You can introduce completely different processes for each service: dev, QA, deploying. You can deploy just one service at once. You also can use different technologies for different purposes. QA process gets more reliable as it involves smaller projects. You can version communication (API) between services which makes them even more independent. Moreover you have better ability to scale horizontally.
On the other hand complexity of the high level architecture grows. You have much more different components you have to take care: authentication / authorisation between services, security, service discovering, distributed transactions etc. If your application is data driven (separate frontend which use APIs for consuming data) and particular services don't need to communicate to each other - it may be not as much complicate (but such assumption is IMO quite risky, sooner or letter you will need to communicate them).
In that approach you have separate API, with separate repositories and separate processes for each "type of actions" (which I understand ss separate domain model / services).
As I wrote on the beginning the way you choose depends on the application and its needs. Anyway, back to your original question, my suggestion is to keep APIs as separate as you can. Even if you have one monolithic application you should be able to version APIs separately and keep their domain logic separate. Separating repositories and/or processes depends on the approach you choose (eg. among these I mentioned before).
If I missed your point, please describe in more detailed way what answer do you expect.
Best!
We are implementing numerous services in our company and running into versioning issues with data contracts. One of the problems we have is that our data contract are also used as the model of the actual application behind the service. I was wondering what approach others have taken in this kind of situation or just service versioning in general. I am aware of the microsoft best practices guide but wanted to see if anybody has any other ideas on how to version.
The first rule of Services, Business Object != Message Object. Basicly, never expose your business objects as data contracts. Or as I like to say, you can't fax a cat. You can send a facsimile of a cat, but you can't send a cat over the wire. Here's a great picture to remind you: http://www.humorhound.com/2009/04/demotivational-poster-youre-doing-it-wrong/
In more modern terms, it is really the MVVM pattern. The view of the model that the domain layer uses is not built for a client, so you have to create a separate model and view for the other layers. Yes it seems like a lot more work, but in the end it is a much easier and better way to build service oriented applications. Versioning is just one of the ways that it makes life easier. The other important thing is that you tend to build models that are geared around how it is going to be used, and you wind up with more explict code (less crazy branching).
The way that we have implemented this is to build a facade layer on top of the business layer.
The facade layer talks to the rest of the world using the objects defined in the data contracts.
The facade layer maps the objects to internal objects before sending the data into the business layer.
This isolates the internal functionality of your system from the objects used in the data contracts.
Using VB.NET 2008
I want to know what is 3 Tier Architecture for windows application?
Can any one give a example of How to make a code for Inserting, Deleting, Updating in a database using 3 tier architecture.
Note am not asking a real code. Just give me a example.
From Multitier architecture
Three-tier'[2] is a client-server
architecture in which the user
interface, functional process logic
("business rules"), computer data
storage and data access are developed
and maintained as independent modules,
most often on separate platforms.
These days, a normal 3 tier application consists of a user interface written in Javascript, CSS and HTML which runs in the browser, a business rules layer which runs in a web server, and could indeed be built in VB.NET, and a storage layer which runs on a database server written in SQL and stored procedures.
Now it would be possible to do a user interface layer in VB.NET as a Windows application which then calls the business rules layer on the web server using a web services interface. This would give you more flexibility than the browser, and would not require learning as many APIs, however it is not common. It can really only be done in an enterprise situation.
This article has a simple VB.NET application that is a Windows GUI app, which call Google's web services API to do searches and to check spelling. That is a good example of a user interface layer. Then check this article for and exmple of a web service developed in VB.NET. This corresponds to the business rules layer, and in a real 3-tier application, it would be based around a database such as SQL server. If you were to use Access then it would not be a real 3-tier application. The database needs to be run on its own server and accessed across the network in order to be considered a tier.
The advantage of a 3-tier application is that you can scale each layer separately, and because each layer is simpler, scaling is also simpler. The DBAs can scale up to a database cluster, the business rules layer can scale up with a load-balancer and multiple servers, and the user-interface just gets replicated across as many clients as you need.
I don't know if is it the right way to use it, but I often use 3-tier int the following way:
One big Solution, with the name of the project
One dll project wich has the conection with the DB, using LINQ or whathever. Validating only the required fields of the DB
Another DLL project, wich has a reference to the project that conects to DB, and validate all data using the bussiness rules. Sometimes you may want a repositorium class wich has methods that can be used from the GUI layer
Finally, the GUI layer that can be HTML or WINForms, wich references to the bussines layer and calls all the appropiates methods, passing the data transparently and waiting for validation on the bussines rules.
You can comunicate with each layer using bool methods that returns true if everything is good, and personalized exceptions for each of the possible errors, and catch them on the upper layer.
I'll give you the gist of it. Real crash course.
You have three tiers:
DAL - Data Access Layer
BRL - Business Rule Layer
Presentation - The Forms, and such.
In the DAL, you configure how your application connects to databases, how it recieves datasets, etc. etc. Everything that has to do with data access.
In the BRL, you lay down how your program is going to handle the data it recieves from the DAL. Methods, and other things go in here.
And in the Presentation area, you simply make things purty and instantiate things from the BRL. The presentation area never has to touch the DAL, and that's the beauty of the 3tier layout. You can work on different areas and not step on other peoples toes.
I find the best way to understand it is to look at an example. If you go here:
http://www.codeproject.com/KB/vb/N-Tier_Application_VB.aspx
You can download an example and read the walk through for creating a very basic 3 Tier App in VB.Net. It's a little old in that it's a Visual Studio 2003 project but it should be easy enough to follow the upgrade wizard and get it up and running to check it out.
I'd like to give a brief overview about this style of programming and maybe I'll explain it in more details next time.
First of all, the 3-Tire concept engages dividing your program or application you are designing into 3 layers, the first layer is for manipulating the database in the operation called CRUD which stands for {Create,Read,Update,Delete} data from your database, using any kind of Databases : for example Oracle,SQLserver,MySql etc. That means you can connect your application with any type of databases without specifying a connection String to Only one database and we'll get more details about this next time.
The Second layer is Business layer which includes user data verification and other similar operations in which you process your business rules and the core of program,
The Third and Last layer is the Presentation layer which relates to User input and UI User Interfaces {The different forms for Input}
Frankly you can divide your Solution {Program,Application,Website} to sub-Programs to avoid data loss,organize your work, and to divide the development of your application among a team members.
in my point of view this is a great thing should be learnt in development, and as I was told by the grapeVine, if you want to enrich your knowledge and experience then you should be acknowledged about this important subject.
In my SOA architecture, I have several WCF services.
All of my services need to access the database.
Should I create a specialized WCF service in charge of all the database access ?
Or is it ok if each of my services have their own database access ?
In one version, I have just one Entity layer instanced in one service, and all the other services depend on this service.
In the other one the Entity layer is duplicated in each of my services.
The main drawback of the first version is the coupling induced.
The drawback of the other version is the layer duplication, and maybe SOA bad practice ?
So, what do so think good people of Stack Overflow ?
Just my personal opinion, if you create a service for all database access then multiple services depend on ONE service which sort of defeats the point of SOA (i.e. Services are autonomous), as you have articulated. When you talk of layer duplication, if each service has its own data to deal with, is it really duplication. I realize that you probably have the same means of interacting with your relational databases or back from the OOA days you had a common class library that encapsulated data access for you. This is one of those things I struggle with myself, but I see no problem in each service having its own data layer. In fact, in Michele Bustamante's book (Chapter 1 - Page 8) - she actually depicts this and adds "Services encapsulate business components and data access". If you notice each service has a separate DALC layer. This is a good question.
It sounds as if you have several services but a single database.
If this is correct you do not really have a pure SOA architecture since the services are not independant. (There is nothing wrong with not having a pure SOA architecture, it can often be the correct choice)
Adding an extra WCF layer would just complicate and slow down your solution.
I would recommend that you create a single data access dll which contains all data access and is referenced by each WCF service. That way you do not have any duplication of code. Since you have a single database, any change in the database/datalayer would require a redeployment of all services in any case.
Why not just use a dependency injection framework, and, if they are currently using the same database, then just allow them to share the same code, and if these were in the same project then they would all use the same dll.
That way, later, if you need to put in some code that you don't want the others to share, you can make changes and just create a new DAO layer.
If there is a certain singleton that all will use, then you can just inject that in when you inject in the dao layer.
But, this will require that they use the same DI framework controller.
The real win that SOA brings is that it reduces the number of linkages between applications.
In the past I've worked with organizations who have done it a many different ways. Some data layers are integrated, and some are abstracted.
The way I've seen it most successfully done is when you create generic data-layer services for each app/database and you create the higher level services based on your newly created data layer.