I want to develop a data-driven WPF application, which uses WCF to connect to the server-side, which itself uses NHibernate to persist data. For examle there is a domain-object called "Customer" and there is also a flattened (with Automapper) "CustomerDTO" which is returned by a WCF-operation called "GetCustomer(int customerId)".
I don't know where I should make data-validation and how I should handle client-side updating, so that one could modify single or multiple properties on the client by editing a form and finally clicking "save"...
Could you please provide me with some common patterns in such a situation or any best-pratice examples, which target real LOB-applications (n-tiered pattern, multiple layers, etc.)
Sounds like a good fit for Self Tracking Entities. Be aware that STE's are hot off the press and still have a bit more maturing to do. The approach used with them should answer your common patterns question.
Related
I really like all the boilerplate code Spring Data Rest writes for you, but I'd rather have just a 'regular?' REST server without all the HATEOAS stuff. The main reason is that I use Dojo Toolkit on the client side, and all of its widgets and stores are set up such that the json returned is just a straight array of items, without all the links and things like that. Does anyone know how to configure this with java config so that I get all the mvc code written for me, but without all the HATEOAS stuff?
After reading Oliver's comment (which I agree with) and you still want to remove HATEOAS from spring boot.
Add this above the declaration of the class containing your main method:
#SpringBootApplication(exclude = RepositoryRestMvcAutoConfiguration.class)
As pointed out by Zack in the comments, you also need to create a controller which exposes the required REST methods (findAll, save, findById, etc).
So you want REST without the things that make up REST? :) I think trying to alter (read: dumb down) a RESTful server to satisfy a poorly designed client library is a bad start to begin with. But here's the rationale for why hypermedia elements are necessary for this kind of tooling (besides the probably familiar general rationale).
Exposing domain objects to the web has always been seen critically by most of the REST community. Mostly for the reason that the boundaries of a domain object are not necessarily the boundaries you want to give your resources. However, frameworks providing scaffolding functionality (Rails, Grails etc.) have become hugely popular in the last couple of years. So Spring Data REST is trying to address that space but at the same time be a good citizen in terms of restfulness.
So if you start with a plain data model in the first place (objects without to many relationships), only want to read them, there's in fact no need for something like Spring Data REST. The Spring controller you need to write is roughly 10 lines of code on top of a Spring Data repository. When things get more challenging the story gets becomes more intersting:
How do you write a client without hard coding URIs (if it did, it wasn't particularly restful)?
How do you handle relationships between resources? How do you let clients create them, update them etc.?
How does the client discover which query resources are available? How does it find out about the parameters to pass etc.?
If your answers to these questions is: "My client doesn't need that / is not capable of doing that.", then Spring Data REST is probably the wrong library to begin with. What you're basically building is JSON over HTTP, but nothing really restful then. This is totally fine if it serves your purpose, but shoehorning a library with clear design constraints into something arbitrary different (albeit apparently similar) that effectively wants to ignore exactly these design aspects is the wrong approach in the first place.
I am part of a studentproject and we are to develop a product for a company using Java EE. As "lead architect" in the project I am responsible for composing a good design which should be flexible for further extensions.
Background info: We are to develop a website with a drag and drop GUI with possibilites to connect data sources with data manipulations to perform on that specific data. The GUI should be generic and possible to integrate with upcoming products. This means that we cannot code to an implementation in the presentation layer. Instead we will use an interface to define what kind of data manipulations that are possible for all kinds of products. However, each product might also sport product specific data manipulations (thus extending the interface with more methods).
The problem I have with the scenario above is that I dont see how we could pass on these "product specific data manipulations" to the GUI and say that, in addition to the generic interface, we also possess these data manipulation actions...
Now I had a discussion with some of the more experienced programmers from the company and they told me that there is a common solution to this problem - more specifically known as the "Observer pattern". They draw something like [1] on the whiteboard and explained that it would be possible to "register" to a third party (getApplicationContext) that in turn could convey our product specific interface. This is a common problem to get rid of those nasty circular dependencies, they explained.
I have now had a look on the observer pattern and how it works and I still dont really get how I am supposed to solve the design problem. Could someone possibly try to explain how it would turn out in my specific scenario? I have no real problem understanding how it works with "subjects" and "observers".
Here is an UML diagram of the design where we are using a reference of the specific product. This is what is undesirable and something we would like to get around.
(maybe I got this all wrong...)
I am sorry but I cant change the picture to the correct one as I am a new user... Here is a link to an updated UML diagram:
It seems what you are looking for is the Model View Controller design pattern. The Observer pattern is just a part of this design pattern. There is a short description for doing this with Java Servlets and JavaServer Pages from Java EE on the wikipedia article.
I am working on a packaged product that is supposed to cater to multiple clients with varying requirements (to a certain degree) and as such should be built in a manner to be flexible enough to be customizable by each specific client. The kind of customization we are talking about here is that different client's may have differing attributes for some of the key business objects. Also, they could have differing business logic tied in with their additional attributes as well
As an very simplistic example: Consider "Automobile" to be a business entity in the system and as such has 4 key attributes i.e. VehicleNumber, YearOfManufacture, Price and Colour.
It is possible that one of the clients using the system adds 2 more attributes to Automobile namely ChassisNumber and EngineCapacity. This client needs some business logic associated with these fields to validate that the same chassisNumber doesnt exist in the system when a new Automobile gets added.
Another client just needs one additional attribute called SaleDate. SaleDate has its own business logic check which validates if the vehicle doesnt exist in some police records as a stolen vehicle when the sale date is entered
Most of my experience has been in mostly making enterprise apps for a single client and I am really struggling to see how I could handle a business entity whose attributes are dynamic and also has a capacity for having dynamic business logic as well in an object oriented paradigm
Key Issues
Are there any general OO principles/patterns that would help me in tackling this kind of design?
I am sure people who have worked on generic / packaged products would have faced similar scenarios in most of them. Any advice / pointers / general guidance is also appreciated.
My technology is .NET 3.5/ C# and the project has a layered architecture with a business layer that consists of business entities that encompass their business logic
This is one of our biggest challenges, as we have multiple clients that all use the same code base, but have widely varying needs. Let me share our evolution story with you:
Our company started out with a single client, and as we began to get other clients, you'd start seeing things like this in the code:
if(clientName == "ABC") {
// do it the way ABC client likes
} else {
// do it the way most clients like.
}
Eventually we got wise to the fact that this makes really ugly and unmanageable code. If another client wanted theirs to behave like ABC's in one place and CBA's in another place, we were stuck. So instead, we turned to a .properties file with a bunch of configuration points.
if((bool)configProps.get("LastNameFirst")) {
// output the last name first
} else {
// output the first name first
}
This was an improvement, but still very clunky. "Magic strings" abounded. There was no real organization or documentation around the various properties. Many of the properties depended on other properties and wouldn't do anything (or would even break something!) if not used in the right combinations. Much (possibly even most) of our time in some iterations was spent fixing bugs that arose because we had "fixed" something for one client that broke another client's configuration. When we got a new client, we would just start with the properties file of another client that had the configuration "most like" the one this client wanted, and then try to tweak things until they looked right.
We tried using various techniques to get these configuration points to be less clunky, but only made moderate progress:
if(userDisplayConfigBean.showLastNameFirst())) {
// output the last name first
} else {
// output the first name first
}
There were a few projects to get these configurations under control. One involved writing an XML-based view engine so that we could better customize the displays for each client.
<client name="ABC">
<field name="last_name" />
<field name="first_name" />
</client>
Another project involved writing a configuration management system to consolidate our configuration code, enforce that each configuration point was well documented, allow super users to change the configuration values at run-time, and allow the code to validate each change to avoid getting an invalid combination of configuration values.
These various changes definitely made life a lot easier with each new client, but most of them failed to address the root of our problems. The change that really benefited us most was when we stopped looking at our product as a series of fixes to make something work for one more client, and we started looking at our product as a "product." When a client asked for a new feature, we started to carefully consider questions like:
How many other clients would be able to use this feature, either now or in the future?
Can it be implemented in a way that doesn't make our code less manageable?
Could we implement a different feature that what they are asking for, which would still meet their needs while being more suited to reuse by other clients?
When implementing a feature, we would take the long view. Rather than creating a new database field that would only be used by one client, we might create a whole new table which could allow any client to define any number of custom fields. It would take more work up-front, but we could allow each client to customize their own product with a great degree of flexibility, without requiring a programmer to change any code.
That said, sometimes there are certain customizations that you can't really accomplish without investing an enormous effort in complex Rules engines and so forth. When you just need to make it work one way for one client and another way for another client, I've found that your best bet is to program to interfaces and leverage dependency injection. If you follow "SOLID" principles to make sure your code is written modularly with good "separation of concerns," etc., it isn't nearly as painful to change the implementation of a particular part of your code for a particular client:
public FirstLastNameGenerator : INameDisplayGenerator
{
IPersonRepository _personRepository;
public FirstLastNameGenerator(IPersonRepository personRepository)
{
_personRepository = personRepository;
}
public string GenerateDisplayNameForPerson(int personId)
{
Person person = _personRepository.GetById(personId);
return person.FirstName + " " + person.LastName;
}
}
public AbcModule : NinjectModule
{
public override void Load()
{
Rebind<INameDisplayGenerator>().To<FirstLastNameGenerator>();
}
}
This approach is enhanced by the other techniques I mentioned earlier. For example, I didn't write an AbcNameGenerator because maybe other clients will want similar behavior in their programs. But using this approach you can fairly easily define modules that override default settings for specific clients, in a way that is very flexible and extensible.
Because systems like this are inherently fragile, it is also important to focus heavily on automated testing: Unit tests for individual classes, integration tests to make sure (for example) that your injection bindings are all working correctly, and system tests to make sure everything works together without regressing.
PS: I use "we" throughout this story, even though I wasn't actually working at the company for much of its history.
PPS: Pardon the mixture of C# and Java.
That's a Dynamic Object Model or Adaptive Object Model you're building. And of course, when customers start adding behaviour and data, they are programming, so you need to have version control, tests, release, namespace/context and rights management for that.
A way of approaching this is to use a meta-layer, or reflection, or both. In addition you will need to provide a customisation application which will allow modification, by the users, of your business logic layer. Such a meta-layer does not really fit in your layered architecture - it is more like a layer orthoganal to your existing architecture, though the running application will probably need to refer to it, at least on initialisation. This type of facility is probably one of the fastest ways of screwing up the production application known to man, so you must:
Ensure that the access to this editor is limited to people with a high level of rights on the system (eg administrator).
Provide a sandbox area for the customer modifications to be tested before any changes they are testing are put on the production system.
An "OOPS" facility whereby they can revert their production system to either your provided initial default, or to the last revision before the change.
Your meta-layer must be very tightly specified so that the range of activities is closely defined - George Orwell's "What is not specifically allowed, is forbidden."
Your meta-layer will have objects in it such as Business Object, Method, Property and events such as Add Business Object, Call Method etc.
There is a wealth of information about meta-programming available on the web, but I would start with Pattern Languages of Program Design Vol 2 or any of the WWW resources related to, or emanating from Kent or Coplien.
We develop an SDK that does something like this. We chose COM for our core because we were far more comfortable with it than with low-level .NET, but no doubt you could do it all natively in .NET.
The basic architecture is something like this: Types are described in a COM type library. All types derive from a root type called Object. A COM DLL implements this root Object type and provides generic access to derived types' properties via IDispatch. This DLL is wrapped in a .NET PIA assembly because we anticipate that most developers will prefer to work in .NET. The Object type has a factory method to create objects of any type in the model.
Our product is at version 1 and we haven't implemented methods yet - in this version business logic must be coded into the client application. But our general vision is that methods will be written by the developer in his language of choice, compiled to .NET assemblies or COM DLLs (and maybe Java too) and exposed via IDispatch. Then the same IDispatch implementation in our root Object type can call them.
If you anticipate that most of the custom business logic will be validation (such as checking for duplicate chassis numbers) then you could implement some general events on your root Object type (assuming you did it something like the way we do.) Our Object type fires an event whenever a property is updated, and I suppose this could be augmented by a validation method that gets called automatically if one is defined.
It takes a lot of work to create a generic system like this, but the payoff is that application development on top of the SDK is very quick.
You say that your customers should be able to add custom properties and implement business logic themselves "without programming". If your system also implements data storage based on the types (ours does) then the customer could add properties without programming, by editing the model (we provide a GUI model editor.) You could even provide a generic user application that dynamically presents the appropriate data-entry controls depending on the types, so your customers could capture custom data without additional programming. (We provide a generic client application but it's more a developer tool than a viable end-user application.) I don't see how you could allow your customers to implement custom logic without programming... unless you want to provide some kind of drag-n-drop GUI workflow builder... surely a huge task.
We don't envisage business users doing any of this stuff. In our development model all customisation is done by a developer, but not necessarily an expensive one - part of our vision is to allow less experienced developers produce robust business applications.
Design a core model that acts as its own independent project
Here's a list of some possible basic requirements...
The core design would contain:
classes that work (and possibly be extended) in all of the subprojects.
more complex tools like database interactions (unless those are project specific)
a general configuration structure that should be considered standard across all projects
Then, all of the subsequent projects that are customized per client are considered extensions of this core project.
What you're describing is the basic purpose of any Framework. Namely, create a core set of functionality that can be set apart from the whole so you don't have to duplicate that development effort in every project you create. Ie, drop in a framework and half your work is done already.
You might say, "what about the SCM (Software Configuration Management)?"
How do you track revision history of all of the subprojects without including the core into the subproject repository?
Fortunately, this is an old problem. Many software projects, especially those in the the linux/open source world, make extensive use of external libraries and plugins.
In fact git has a command that's specifically used to import one project repository into another as a sub-repository (preserving all of the sub-repository's revision history etc). In fact, you can't modify the contents of the sub-repository because the project won't track it's history at all.
The command I'm talking about is called 'git submodule'.
You may ask, "what if I develop a really cool feature in one client's project that I'd like to use in all of my client's projects?".
Just add that feature to the core and run a 'git submodule sync' on all the other projects. The way git submodule works is, it points to a specific commit within the sub-repository's history tree. So, when that tree is changed upstream, you need to pull those changes back downstream to the projects where they're used.
The structure to implement such a thing would work like this. Lets say that you software is written specifically to manage a car dealership (inventory, sales, employees, customers, orders, etc...). You create a core module that covers all of these features because they are expected to be used in the software for all of your clients.
But, you have recently gained a new client who wants to be more tech savvy by adding online sales to their dealership. Of course, their website is designed by a separate team of web developers/designers and webmaster but they want a web API (Ie, service layer) to tap into the current infrastructure for their website.
What you'd do is create a project for the client, we'll call it WebDealersRUs and link the core submodule into the repository.
The hidden benefit of this is, once you start to look as a codebase as pluggable parts, you can start to design them from the start as modular pieces that are capable of being dropped in to a project with very little effort.
Consider the example above. Lets say that your client base is starting to see the merits of adding a web-front to increase sales. Just pull the web API out of the WebDealersRUs into its own repository and link it back in as a submodule. Then propagate to all of your clients that want it.
What you get is a major payoff with minimal effort.
Of course there will always be parts of every project that are client specific (branding, ect). That's why every client should have a separate repository containing their unique version of the software. But that doesn't mean that you can't pull parts out and generalize them to be reused in subsequent projects.
While I approach this issue from the macro level, it can be applied to smaller/more specific parts of the codebase. The key here is code that you wish to re-use needs to be genericized.
OOP comes into play here because: where the functionality is implemented in the core but extended in client's code you'll use a base class and inherit from it; where the functionality is expected to return a similar type of result but the implementations of that functionality may be wildly different across classes (Ie, there's no direct inheritance hierarchy) it's best to use an interface to enforce that relationship.
I know your question is general, not tied to a technology, but since you mention you actually work with .NET, I suggest you look at a new and very important technology piece that is part of .NET 4: the 'dynamic' type.
There is also a good article on CodeProject here: DynamicObjects – Duck-Typing in .NET.
It's probably worth to look at, because, if I have to implement the dynamic system you describe, I would certainly try to implement my entities based on the DynamicObject class and add custom properties and methods using the TryGetxxx methods. It also depends whether you are focused on compile time or runtime. Here is an interesting link here on SO: Dynamically adding members to a dynamic object on this subject.
Two approaches is what I feel:
1) If different clients fall on to same domain (as Manufacturing/Finance) then it's better to design objects in such a way that BaseObject should have attributes which are very common and other's which could vary in between clients as key-value pairs. On top of it, try to implement rule engine like IBM ILog(http://www-01.ibm.com/software/integration/business-rule-management/rulesnet-family/about/).
2) Predictive Model Markup Language(http://en.wikipedia.org/wiki/PMML)
To be perfectly clear, I do not expect a solution to this problem. A big part of figuring this out is obviously solving the problem. However, I don't have a lot of experience with well architected n-tier applications and I don't want to end up with an unruly BLL.
At the moment of writing this, our business logic is largely a intermingled ball of twine. An intergalactic mess of dependencies with the same identical business logic being replicated more than once. My focus right now is to pull the business logic out of the thing we refer to as a data access layer, so that I can define well known events that can be subscribed to. I think I want to support an event driven/reactive programming model.
My hope is that there's certain attainable goals that tell me how to design these collection of classes in a manner well suited for business logic. If there are things that differentiate a good BLL from a bad BLL I'd like to hear more about them.
As a seasoned programmer but fairly modest architect I ask my fellow community members for advice.
Edit 1:
So the validation logic goes into the business objects, but that means that the business objects need to communicate validation error/logic back to the GUI. That get's me thinking of implementing business operations as objects rather than objects to provide a lot more metadata about the necessities of an operation. I'm not a big fan of code cloning.
Kind of a broad question. Separate your DB from your business logic (horrible term) with ORM tech (NHibernate perhaps?). That let's you stay in OO land mostly (obviously) and you can mostly ignore the DB side of things from an architectural point of view.
Moving on, I find Domain Driven Design (DDD) to be the most successful method for breaking a complex system into manageable chunks, and although it gets no respect I genuinely find UML - especially action and class diagrams - to be critically useful in understanding and communicating system design.
General advice: Interface everything, build your unit tests from the start, and learn to recognise and separate the reusable service components that can exist as subsystems. FWIW if there's a bunch of you working on this I'd also agree on and aggressively use stylecop from the get go :)
I have found some o fthe practices of Domain Driven Design to be excellent when it comes to splitting up complex business logic into more managable/testable chunks.
Have a look through the sample code from the following link:
http://dddpds.codeplex.com/
DDD focuses on your Domain layer or BLL if you like, I hope it helps.
We're just talking about this from an architecture standpoint, and what remains as the gist of it is "abstraction, abstraction, abstraction".
You could use EBC to design top-down and pass the interface definitions to the programmer teams. Using a methology like this (or any other visualisation technique) visualizing the dependencies prevents you from duplicating business logic anywhere in your project.
Hmm, I can tell you the technique we used for a rather large database-centered application. We had one class which managed the datalayer as you suggested which had suffix DL. We had a program which automatically generated this source file (which was quite convenient), though it also meant if we wanted to extend functionality, you needed to derive the class since upon regeneration of the source you'd overwrite it.
We had another file end with OBJ which simply defined the actual database row handled by the datalayer.
And last but not least, with a well-formed base class there was a file ending in BS (standing for business logic) as the only file not generated automatically defining event methods such as "New" and "Save" such that by calling the base, the default action was done. Therefore, any deviation from the norm could be handled in this file (including complete rewrites of default functionality if necessary).
You should create a single group of such files for each table and its children (or grandchildren) tables which derive from that master table. You'll also need a factory which contains the full names of all objects so that any object can be created via reflection. So to patch the program, you'd merely have to derive from the base functionality and update a line in the database so that the factory creates that object rather than the default.
Hope that helps, though I'll leave this a community wiki response so perhaps you can get some more feedback on this suggestion.
Have a look in this thread. May give you some thoughts.
How should my business logic interact with my data layer?
This guide from Microsoft could also be helpful.
Regarding "Edit 1" - I've encountered exactly that problem many times. I agree with you completely: there are multiple places where the same validation must occur.
The way I've resolved it in the past is to encapsulate the validation rules somehow. Metadata/XML, separate objects, whatever. Just make sure it's something that can be requested from the business objects, taken somewhere else and executed there. That way, you're writing the validation code once, and it can be executed by your business objects or UI objects, or possibly even by third-party consumers of your code.
There is one caveat: some validation rules are easy to encapsulate/transport; "last name is a required field" for example. However, some of your validation rules may be too complex and involve far too many objects to be easily encapsulated or described in metadata: "user can include that coupon only if they aren't an employee, and the order is placed on labor day weekend, and they have between 2 and 5 items of this particular type in their cart, unless they also have these other items in their cart, but only if the color is one of our 'premiere sale' colors, except blah blah blah...." - you know how business 'logic' is! ;)
In those cases, I usually just accept the fact that there will be some additional validation done only at the business layer, and ensure there's a way for those errors to be propagated back to the UI layer when they occur (you're going to need that communication channel anyway, to report back persistence-layer errors anyway).
I'm a student currently dabbling in a .Net n-tier app that uses Nhibernate+WCF+WPF.
One of the things that is done quite terribly is object graph serialisation, In fact it isn't done at all, currently associations are ignored and we are using DTOs everywhere.
As far as I can tell one method to proceed is to predefine which objects and collections should be loaded and serialised to go across the wire, thus being able to present some associations to the client, however this seems limited, inflexible and inconsistent (can you tell that I don't like this idea).
One option that occurred to me was to simply replace the NHProxies that lazy load collection on the client tier with a "disconnectedProxy" that would retrieve the associated stuff over the wire. This would mean that we'd have to expand our web service signature a little and do some hackery on our generated proxies but this seemed like a good T4/other code gen experiment.
As far as I can tell this seems to be a common stumbling block but after doing a lot of reading I haven't been able to figure out any good/generally accepted solutions. I'm looking for a bit of direction as much as any particular solution, but if there is an easy way to make the client "feel" connected please let me know.
You ask a very good question that unfortunately does not have a very clean answer. Even if you were able to get lazy loading to work over WCF (which we were able to do) you still would have issues using the proxy interceptor. Trust me on this one, you want POCO objects on the client tier!
What you really need to consider...what has been conceived as the industry standard approach to this problem from the research I have seen, is called persistence vs. usage or persistence ignorance. In other words, your object model and mappings represent your persistence domain but it does not match your ideal usage scenarios. You don't want to bring the whole database down to the client just to display a couple properties right??
It seems like such a simple problem but the solution is either very simple, or very complex. On one hand you can design your entities around your usage scenarios but then you end up with proliferation of your object domain making it difficult to maintain. On the other, you still want the rich object model relationships in order to write granular business logic.
To simplify this problem let’s examine the two main gaps we need to fill…between the database and the database/service layer and the service to client gap. NHibernate fills the first one just fine by providing an ORM to load data into your objects. It does a decent job, but in order to achieve great performance it needs to be tweaked using custom loading strategies. I digress…
The second gap, between the server and client, is where things get dicey. To simplify, imagine if you did not send any mapped entities over the wire to the client? Try creating a mechanism that exchanges business entities into DTO objects and likewise DTO objects into business entities. That way your client deals with only DTOs (POCO of course), and your business logic can maintain its rich structure. This allows you to leverage not only NHibernate’s lazy loading mechanism, but other benefits from the session such as L1 cache.
For brevity and intellectual property reasons I will not go into the design of said mechanism, but hopefully this is enough information to point you in the right direction. If you don’t care about performance or latency at all…just turn lazy loading off all together and work through the serialization issues.
It has been a while for me but the injection/disconnected proxies may not be as bad as it sounds. Since you are a student I am going to assume you have some time and want to muck around a bit.
If you want to inject your own custom serialization/deserialization logic you can use IDataContractSurrogate which can be applied using DataContractSerializerOperationBehavior. I have only done a few basic things with this but it may be worth looking into. By adding some fun logic (read: potentially hackish) at this layer you might be able to make it more connected.
Here is an MSDN post about someone who came to the same realization, DynamicProxy used by NHibernate makes it not possible to directly serialize NHibernate objects doing lazy loading.
If you are really determined to transport the object graph across the network and preserve lazy loading functionality. Take a look at some code I produced over here http://slagd.com/?page_id=6 . Basically it creates a fake session on the other side of the wire and allows the nhibernate proxies to retain their functionality. Not saying it's the right way to do things, but it might give you some ideas.