Disclaimer: I know that DDD/CQRS might not be a perfect choice for below use case, but it was working great so far for other tasks in the project and I want to stick with it for learning purposes (eg. learn the hard way where not to use it ;).
Let's assume I'm building a Blogging Platform and have 3 types of posts: LifestylePost, RecipePost and ReviewPost. They have the same abstract base class PostBase which is an Aggregate Root and all of them share some properties like Author or implement methods like Publish, Delete etc. which change their Status and verify whether the state is valid. Each created post starts as a Draft which barely requires any validation, but switching to Published verifies nearly all properties.
Implementing commands/handlers like PublishPost or DeletePost was a breeze, but the problem starts when I'm thinking about CreatePost or UpdatePost. These are some dilemmas I'm currently facing:
Create/update commands/handlers for each post types or one for all
Having them separately for each post types gives me more control over the logic, allows to tailor each command model precisely, but makes my API more complex for the client. With the second option I can resolve the post type by some discriminator in the command and check whether all necessary properties for that particular type were provided.
Create empty post draft first or already filled with the data
When a user creates a new post (draft with an initial state) I can already call the API and add an empty post to the database, which would then be updated or I can wait until user inputs any data and clicks save. It's basically a matter of the arguments in the constructors of the posts.
Handling post updates in general
This is the point where I'm having the biggest issues now. Since there are quite many properties that could or could not be changed by the user, it's hard to split them to particular methods inside the Aggregate Root different than just Update with huge amount of nullable arguments where null would mean that the property has not been provided by the client and therefore not changed. Also adding Status property here would mean that the proper method for validating the state would have to be resolved. This solution somehow doesn't feel like a proper DDD design...
Which decisions would you make in each points and why?
Create/update commands/handlers for each post types or one for all
Depends on whether the difference in post types has influence over the API or not. If your clients have differentiated interactions with your API depending on the post type, then I would have specific commands for each type. If your API is post type agnostic, then this is internal business concerns. You should then ensure that domain implementation is sufficiently different from type to type, because polymorphic domain is a lot more work than a simple "category" property.
Create empty post draft first or already filled with the data
Depends on whether knowing a user has started working on a draft has any value for your system. If it's like a personal blog, probably not. If you're writing software for a newspaper, then it could be insightful to anyone. Ask your domain experts.
Handling post updates in general
This is an interface design question rather than a business logic one in my opinion. If your users want to do a lot of small changes, you should consider providing PATCH routes in your API, with a standard like JsonPatch. Depending on your implementation technology, you could benefit from libraries that does the object patching for you, saving a lot of code writing.
Is there really a difference in behaviour between the post types? Don't they all have Draft(), Publish(), Cancel() behaviours?
Given that inheritance means X is a Y, then you're basically saying they are all the same. This feels to me like a single Post aggregate with the possibility of a "PostType" value somewhere that might be part of an invariant (e.g. if you had a business rule that says "Review Posts cannot be published until a cooling-off period has elapsed).
This would mean a single set of application services to invoke those methods (and validate the invariants they implement).
Related
I am implementing interaction between bounded contexts and I found out that it "somehow" cripples open closed principle and I am not sure, whether it is natural consequence of designing BCs and common tradeoff to consider or my design failure.
Consider Shop BC where you can create Order out of Cart with some items. Created order consists of OrderItems, each of them containing one of various types of ItemSpecification Value Object Interface like ProductSpecification or FooServiceSpecification, protecting invariants and containing some data. When Order is created, asynchronous event that can be listened to by any other BC is emitted.
That asynchronous event is created out of Order and is represented as (serialized) OrderCreatedEvent object, containing OrderDTO object, all placed in Core namespace which is shared with every BC so that any BC can depend on Core, but not the other way. All good so far, almost:
That OrderItemDTO must contain interface ItemSpecificationDTO, which need be implemented for every type of specification. My ItemSpecification VO (like any other VO/Entity in Order) has toCoreDTO() method to pragmatically achieve easy translation and it also makes relatively hard to implement new ItemSpecification and forget to implement according DTO. That's probably okay.
But what about other BC's which listen to that Event? In every BC this Event needs to be translated in it's AntiCorruption Layer and that BC may be interested only in some types of ItemSpecificationDTO and translate them to various Value Objects, important for that specific BC.
As Uncle Bob says about OCP in a wit:
You should be able to extend the behavior of a system without having
to modify that system.
But when I implement new type of ItemSpecification, for every BC which may be interested in this new type I need to specifically translate that new type from CoreDTO (okay I could write some abstraction for translating in each BC so I would still be just adding code without modyfying anything like adding if($x instanceof X)). But still, by adding new type of ItemSpecification I need to make appropriate extensions (and maybe even modify something because we don't live in ideal world) in other BCs.
And I do not know how to think about that. Is that downside of whole DDD approach? Or maybe feature indeed, because that hunting for what, where and how needs to be further extended in other BCs, is driven by domain needs instead of technical concerns? It seems right. In the end, I'm trying to do domain driven design :-) But it seems to me somehow dangerous too. I am afraid that one day we can forget to update some other BC and something bad happens. But that is probably because I play big part of domain expert role too, under which that "fear" should probably belong. Is my problem just sitting on two chairs or did I got something wrong? :-)
There are a lot of interesting details about this topic, but I would concentrate here on one specific aspect of bounded contexts.
That is that they are bounded for a reason. As in, there should be a boundary between the models/understanding of these contexts. Two contexts, even if they are related should have a different view on the system, even on the data that might be partly shared.
It seems to me that your "bounded contexts" want to work on the same model. You even created a "core" model which everyone can see and apparently must be able to understand. If this is the case, I'd argue that you lost the benefits of having different contexts, and you are just creating one big application, with one model.
To correct this problem, I think you would need to get rid of any central/core models, and work with "local" (bounded) models in the different contexts/services. When you need to communicate with other components, you need to define a protocol for those two, dictated by either or both parties.
For example a Shopping Cart might need to know the product-id for the backend system to create the order there. But the backend system doesn't need to know the model the shopping cart uses to know what the order is about (in its own model).
In the scala and scalajs library Diode, I have used but not entirely understood the PotAction class and only recently discovered the AsyncAction class, both of which seem to be favored in situations involving, well, asynchronous requests. While I understand that, I don't entirely understand the design decisions and the naming choices, which seem to suggest a more narrow use case.
Specifically, both AsyncAction and PotAction require an initialModel and a next, as though both are modeling an asynchronous request for some kind of refreshable, updateable content rather than a command in the sense of CQRS. I have a somewhat-related question open regarding synchronous actions on form inputs by the way.
I have a few specific use cases in mind. I'd like to know a sketch (not asking for implementation, just the concept) of how you use something like PotAction in conjunction with any of:
Username/password authentication in a conventional flow
OpenAuth-style authentication with a third-party involved and a redirect
Token or cookie authentication behind the scenes
Server-side validation of form inputs
Submission of a command for a remote shell
All of these seem to be a bit different in nature to what I've seen using PotAction but I really want to use it because it has already been helpful when I am, say, rendering something based on the current state of the Pot.
Historically speaking, PotAction came first and then at a later time AsyncAction was generalized out of it (to support PotMap and PotVector), which may explain their relationship a bit. Both provide abstraction and state handling for processing async actions that retrieve remote data. So they were created for a very specific (and common) use case.
I wouldn't, however, use them for authentication as that is typically something you do even before your application is loaded, or any data requested from the server.
Form validation is usually a synchronous thing, you don't do it in the background while user is doing something else, so again Async/PotAction are not a very good match nor provide much added value.
Finally for the remote command use case PotAction might be a good fit, assuming you want to show the results of the command to the user when they are ready. Perhaps PotStream would be even better, depending on whether the command is producing a steady stream of data or just a single message.
In most cases you should use the various Pot structures for what they were meant for, that is, fetching and updating remote data, and maybe apply some of the ideas or internal models (such as the retry mechanism) to other request types.
All the Pot stuff was separated from Diode core into its own module to emphasize that they are just convenient helpers for working with Diode. Developers should feel free to create their own helpers (and contribute back to Diode!) for new use cases.
I'm trying to make an architectural decision and I'm worried that I'm missing something big about URL routing / mapping when it comes to designing a basic REST API / framework.
Creating routing classes and such that is typically seen in REST API frameworks, that require one to manually map a URL to a class, and a class method (action), kind of seems like a failure to encapsulate the problem. When this can all be determed by parsing the URL dynamically and having an automatic router or front page controller.
GET https://api.example.com/companies/
Collection resource that gets a list of all companies.
GET https://api.example.com/companies/1
Fetches a single company by ID.
Which all seems to follow the template:https://api.example.com/<controller>/<parameter>/
Benefit 1: URL Decoupling and Abstraction
I assume one of the on paper benefits of having a typical routing class, is that you can decouple or abstract a URL from a resource / physical class. So you could have arbitrary URL's like GET https://api.example.com/poo/ instead of GET https://api.example.com/companies/ that fetches all the companies if you felt like it.
But in almost every example and use-case I've seen, the desire is to have a URL that matches the desired controller, action and parameters, 1 : 1.
Another possible benefit, is that collection resources within a resource, or nested resources, might be easier to achieve with URL mapping and typical routers. For example:
GET https://api.example.com/companies/1/users/
OR
GET https://api.example.com/companies/1/users/1/
Could be quite challenging to come up with a paradigm that can dynamically parse this to know what controller to call in order to get the data, what parameters to use, and where to use them. But I think I have come up with a standard way that could make this work dynamically.
Whereas manually mapping this would be easy.
I could just re-route GET https://api.example.com/companies/1/users/ to the users controller instead of the companies controller, bypassing it, and just set the parameter "1" to be the company id for the WHERE clause.
Benefit 1.1: No Ties to Physical Paths
An addendum to benefit 1, would be that a developer could completely change the URL scheme and folder structure, without affecting the API, because everything is mapped abstractly. If I choose to move files, folders, classes, or rename them, it should just be a matter of changing the mapping / routing.
But still don't really get this benefit either, because even if you had to move your entire API to another location, a trivial change in .htaccess with fix this immediately.
So this:
GET https://api.example.com/companies/
TO
GET https://api.example.com/v1/companies/
Would not impact code, even in the slightest. Even with a dynamic router.
Benefit 2: Control Over What Functionality is Exposed
Another benefit I imagine a typical router class gives you, over a dynamic router that just interprets and parses the URL, is control over exactly what functionality you want to expose to the API consumer. If you just do everything dynamically, you're kind of dropping your pants, automatically giving your consumer access to the entire system.
I see this as a possible benefit for the dynamic router, as you wouldn't then have to manually define and map all your routes to resources. It's all there, automatically. To solve the exposure problem, I would probably do the opposite by defining a blacklist of what functionality the API consumer shouldn't be allowed to use. I might be more time effective, defining a blacklist, then defining each and every usable resource with mapping. Then again, it's riskier too I suppose. You could even do a whitelist... which is similar to a typical router, but you wouldn't need any extended logic at all. It's just a list of URL's that the system would check, before passing the URL to the dynamic router. Or it could just be a private property of the dynamic router class.
Benefit 3: When HTTP Methods Don't Quite Fit the Bill
One case where I see a typical routers shining, is where you need to execute an action, that conflicts with an existing resource. Let me explain.
Say you want to authenticate a user, by running the login function within your user class. But now, you can't execute POST https://api.example.com/users/ with credentials, because that is reserved for adding a new user. Instead, you need to somehow run the login method in your user class. You don't want to use POST https://api.example.com/users/login/ either, because then you're using verbs other than the HTTP methods. However, with a typical router, you can just map this directly, as said before. Easy.
url => "https://api.example.com/tenant/"
Controller => "users"
Action => "login"
Params => "api_key, api_secret"
But, once again, I see an plausible alternative. I could just create another controller, called login or tenant, that instantiates my user controller, and runs the login function. So a consumer could just POST https://api.example.com/tenant/, with credentials, and blam. Authentication.
Although, to get this alternative to work, I would have to physically create another controller, when with a URL mapper, I wouldn't need to. But this seperation of concerns, functionality and resources, is quite nice too. But, maybe that's the main trade off, would you rather just define a URL route, or have to create new classes for each nuance you encounter?
What am I not seeing, or understanding? Am I missing a core concept here and just ignorant? Are there more benefits to having typical URL mapping and routing classes and functionality, that I'm just not aware of, or have I pretty much got this?
A lot of the benefits to routing you describe are correct, and some of what you say about physical mappings is also true. I'd like to throw in some experience / practical information that colored my opinion on routers over the last few years.
first of all, the dynamic parsing of url works well (most of the time) when you architect your application according to the MVC design pattern. For example, I once built a very large application using Kohana, which is a hierarchical MVC framework, which allows you to extend controllers and models for the sake of making nested urls. In general, this makes a lot of sense. But there were a lot of times where it simply didn't make much sense to go build a whole class and model system around the need for one-off URLs to make the application more functional. But there are also times where MVC is not the design pattern you're using, and thus it is not the defining feature of your API, and routing is beautiful in this scenario. One could easily see this issue at work by playing with frameworks that have a lot of structural freedom, such as Slim framework or Express.js.
more often than people think, a fully functional API will have an element of RPC-ness to it in addition to the primarily RESTful endpoints available for consumption. And not only do those additional functionalities make more sense as a consumer when they're decorating existing resource method mappings. This tends to happen after you've built out most of your application and covered most of your bases, and then you realize that there are a couple little features you'd like to add in relation to a resource that isn't doesn't cleanly fit into the CREATE / READ / UPDATE / DELETE categories. you'll know it when you see it.
it really can not be understated, it is much safer to not go hacking on the actual structure of the controllers and models, adding, removing, changing, things for the sole purpose of adding an endpoint that isn't inherently following the same rules of the other controller methods (API endpoints).
another very important thing is that your API endpoints are actually more maleable than we often realize. What I mean is, you can be OK with the structure of your endpoints on monday, and then on friday,you get this task sent down from above saying you need to change all of these API calls to be of some other structure, and thats fine. but if you have a large application, this requires a very, very significant amount of file renaming, class renaming, linkages, and all sorts of very breakable code when the framework you're using has strict rules for class naming, file naming, physical file path structure and stuff like that...just imagine, changing a class name to make it work with the new structure, and now you've got to go hunt down every line of code that instantiated the old class, and change it. Furthermore, in that scenario, it could be said that the problem is that your code is tightly coupled with the url structure of your API, and that is not very maintainable should your url needs change.
Either way, you really ought to decide whats best for the particular application. but the more you use routers, the more you'll see why they're so useful.
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).