I started writing graphic dsl in eclipse by using Epsilon. However the problem is there is that epsilon doesn't have support for writing specific behavior for dsls. For the reference, i'm writing a dsl for data flow diagrams, where i'd need to implement system for decomposition, meaning i'd have to fetch all of external entities, interfaces and connections that are related to the process i'm decomposing it by and transfer them to lower level of decomposition. Does anyone know if there's a way to deal with this problem within epsilon framework, or is there any other framework that would allow me to make dsl out of metamodel with addition of defining a behavior for the dsl.
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Recently I've been introduced to the Command Dispatcher Pattern which could help the commands to be decoupled from the command handlers in our project that's based on the Domain-Driven Design approach and CQRS pattern.
Anyway, I'm confused it with the Mediator design pattern.
Robert Harvey has already answered a question about the Command Dispatcher pattern as following:
A Command Dispatcher is an object that links the Action-Request with
the appropriate Action-Handler. It's purpose is to decouple the
command operation from the sending and receiving objects so that
neither has knowledge of the other.
According to the Wikipedia, The mediator pattern is described as:
With the mediator pattern, communication between objects is
encapsulated within a mediator object. Objects no longer communicate
directly with each other, but instead communicate through the
mediator. This reduces the dependencies between communicating objects,
thereby reducing coupling.
So, as my understanding both of them are separating the command form the commander which allow us to decouple from the caller.
I've seen some projects on Github that are using the Command Dispatcher Pattern to invoke the desired handler for the requested command while the other ones are using mediator pattern to dispatch the messages. (E.g. in most of the DotNet projects, the MediatR library is used to satisfy that).
However, I'd like to know what are the differences and benefits of using one pattern than another in our project that is based on the DDD approach and CQRS pattern?
The Command Dispatcher and Mediator patterns (as well as the Event Aggregator Pattern) share similarities in that they introduce a mediating component to decouple direct communication between components. While each can be used to achieve use cases for which the other pattern was conceived, they are each concrete patterns which differ in their original targeted problems as well as the level to which they are suited for each need.
The Command Dispatcher Pattern is primarily a convention-over-configuration approach typically used for facilitating UI layer calls into an Application Layer using discrete types to handle commands and queries as opposed to a more traditional Application Service design. When representing the queries and commands that might typically be represented in a course-grained service (e.g. OrderService) as discrete components (e.g. CreateOrderCommand, GetOrderQuery, etc.), this can result in quite a bit of noise in UI-level components such as ASP.Net MVC Controllers where a constructor might otherwise need to inject a series of discrete interfaces, only one of which would typically be needed for each user request (e.g. Controller Action). Introducing a dispatcher greatly reduces the amount of code needed in implementing components such as ASP.Net MVC Controllers since the only dependency that need be injected is the dispatcher. While not necessarily a primary motivation of the pattern, it also introduces the ability to uniformly apply other patterns such as Pipes and Filters, and provides a seam where command handler implementations can be determined at run time. The MediatR library is actually an implementation of this pattern.
The Mediator Pattern concerns the creation of mediating components which encapsulate domain-specific orchestration logic that would otherwise require coupling between components. That is to say, the mediating component in this case isn't just a dumb dispatcher ("Hey, anybody know how to handle an XYZRequest?"), but is purpose-built to follow a specific set of operations that need to occur when a given operation happens, potentially across multiple components. The example given in the GoF Design Patterns book is a UI component with many interconnected elements such that changes to one need to effect changes to a number of other components and vice-versa (e.g. typing into a text field causes changes to a drop-down and multiple check boxes and radio buttons, while selecting entries within the dropdown effect changes to what's in the text field, check boxes, and radio buttons, etc.). With the provided solution, a mediating component contains logic to know exactly which components need to get updated, and how, when each of the other components change.
So, the Mediator Pattern would be used when you need a component custom-built to facilitate how a number of other components interact where normal coupling would otherwise negatively affect maintainability whereas the Command Dispatcher Pattern is simply used as a dumb function router to decouple the caller from the called function.
Mediator pattern is more low level and generic in its pure concept. Mediator pattern does not define the kind of communication or the kind of message you use. In Command Dispatcher you are in a upper layer (contextually and conceptually) in which the kind of communication and message is already defined.
You should be able to implement a Command Dispatcher patter with a Mediator pattern (ergo with MediatR) as foundation.
I created an algorithm that calculates certain things. This can be considered as the model. The algorithm is implemented in a fully functional way, so it uses immutable classes only.
Now using this model, I would like to develop a GUI layer on the top of it. However I do not know anything about the best-practises of building GUI in Scala. I intend to use ScalaFX.
My problem is the following: in ScalaFX (similarly to JavaFX) you can bind values from the GUI to object properties. This clearly violates the functional paradigm, but seems very convenient.
This would require rewriting my classes to use bindable properties which would feel like the tail wagging the dog — the model would depend on the GUI.
On the other hand, I could have an independent GUI layer. In this case I would need proxy objects to bind to and I would have to create my model objects based on these proxy objects. This would feel more idiomatic but implies a lot of code duplication and extra work. My model and the proxy objects would have to be kept in sync and I would have to take care of copying the attributes.
What is a good way of doing this? A GUI is always full of mutability so functional programming does not feel right here. Nevertheless I love Scala so I would like to keep using it for the GUI, too.
Despite the extra effort, take the second approach. Create small mutable "view" instances for each of your model. Bind the views to the widgets and install observers or hooks that update the view proxies based on changes in your model. Don't let the GUI API dictate how your concurrency approach and model should look like.
I believe there are a few open source libraries around that provide a more functional and/or reactive abstraction layer to the plain Scala-Swing or Scala-FX.
Complexity of the interface is another factor affecting coupling. The
more complex each interface is, the higher will be the degree of
coupling
The above quoted sentence is from A concise introduction to Software Engineering ,Chapter 6: Design
I don't know how the interface can be used in communication between modules of component diagram ?
Interfaces are implemented (exposed) by the components.
Other components reference this interfaces and use them to create objects, invoke methods, etc. This relationship between a component and the interface it uses is called dependency.
The more interfaces you have in your system, the complexer your system is.
Actually, the factor that influences the complexity much more than pure interface count is the nature and the structure of just mentioned dependencies. A solid system architecture has low number of dependencies, organized in a clear, clean and simple manner.
Especially dangerous are so called circular dependencies. They should be fully avoided.
A simple example:
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I am trying to make the leap from functional programs for "hello world" equivalents to more real-world applications.
As I come from a Java world and have been exposed to all it's design patterns, my modeling process is still very Java oriented (e.g. I think in terms of *Managers, *Factory, *ClientFactory, *Handler etc.)
Changing my thought process in one shot, will be hard so I was hoping to get some pointers on how the following scenarios (described in a OO way) would be modeled in a functional language.
Examples in a functional language like Clojure/Haskell (or perhaps a hybrid like Scala) would be helpful.
Stateless Request handlers
E.g. is a Servlet. It is essentially a request handler with methods like doGet, doPost. How would one model such a class in a functional language?
Orchestrator classes
Such classes don't do anything by themselves, but just orchestrate the whole process or workflow. They offer multiple entry point APIs.
E.g. A OrderOrchestrator orchestrates a multiple step workflow starting with payment instrument validation, shopping cart management, payment, shipment initiation etc.
They might maintain some internal state of their own that is used by the different steps like payment, shipment etc.
ClientFactory pattern
Let's say you have written a client that for a LogService that is used by your client to log traffic data about their services. The client logs the data in S3 under buckets and accounts managed by you and you provide additional services like reporting and analytics on this data.
You don't want your customer to worry about providing the configuration information like AWS account info etc and hence you provide a ClientFactory that instantiates the appropriate client object based on whether this is for testing or production purposes without requiring the customer to provide any configuration. E.g. LogServiceClientFactory.getProdInstance() or LogServiceClientFactory.getTestInstance().
How is such a client modeled in a functional language?
Builder Pattern and other Fluent API designs
Client libraries often provide Builders to create objects with complex configuration. Sometimes APIs are also fluent to make it easy to create. An example of Fluent API is Mockito APIs : Mockito.when(A.get()).thenReturn(a) IIRC this is internally implemented by returning progressively restrictive Builders to allow the developer to write this code.
Is this a parallel to this in the functional programming world?
Datastore instances
Let's say that your codebase uses data stored in a ActiveUserRegistry from multiple places. You want only 1 instance of this registry to exist and have the entire code base access this registry. So you provide a ActiveUserRegistry.getInstance() that guarantees that all the code base accesses the instance (Assume that the instance is thread-safe etc.)
How is this managed in a functional setting? Do we have to make sure the same instance is passed around in the entire codebase?
Below is something to get started:
Stateless Request handlers
Clojure: Protocols
Haskell: Type classes
Orchestrator classes
State monad
ClientFactory pattern
LogServiceClientFactory is a Module and getProdInstance and getTestInstance being the functions in the module.
Builder Pattern and other Fluent API designs
Function composition
Datastore instances
Clojure: Function that uses an atom (to store and use the single instance)
Haskell: TVar,MVar
I'm not vary familiar with the many of these Java-style structures, but I'll take a stab at answering:
Stateless Request handlers
These exist in the functional world as well. Functions can fill this role easily, even with something as simple as a function from requests to responses. The Play Framework uses something more powerful, specifically a function from the Request to an Iteratee (type (RequestHeader) ⇒ Iteratee[Array[Byte], SimpleResult]). The Iteratee is an entity that can progressively consume input (Array[Byte]) as it is received and eventually produce the response (SimpleResult) to give back to the client. The request handler function is stateless and can be reused. The Iteratee is also stateless - the result of feeding it each chunk is actually to get a new Iteratee back, which is then fed the next chunk. (I'm oversimplifying really, it uses Futures, is entirely non-blocking, and has effective error handling - worth looking at to get a feel of the power and simplicity that functional-style code can bring to this problem).
Orchestrator classes
I'm not familiar with this pattern, so forgive me if this makes no sense. Having one giant mutable object that gets passed around is an anti-pattern. In functional code, there would be separate datatypes to represent the data that needs to passed between each stage of the process. These datatypes would be immutable.
As for things that organize other things, look at Akka and how one actor can monitor other actors underneath it, handling errors or restarting them as needed.
Builder Pattern and other Fluent API designs
Functional program has these and takes them to their logical conclusion. Functional code allows for very powerful DSLs. As for an example, check out a parser combinator library, either the one in the Scala standard library or one of the libraries for Haskell.
ClientFactory pattern and Datastore instances
I don't think this is any different in functional code. Either you have a singleton, or you do proper dependency injection. The Factory pattern is used in functional code as well, though first-class functions make many design patterns too trivial to be worth naming (from the GoF: Factory, Factory method, Command, and at least some instances of Strategy and Template can usually just be functions).
Have a look at Functional Programming Patterns in Scala and Clojure: http://pragprog.com/book/mbfpp/functional-programming-patterns-in-scala-and-clojure .
It should exactly have what you need.
As we know in OOP that interface provides a set of operations without implementation but
class is the opposite.
in Object oriented design ,we use uml the interface has a set of operations without implementation
and the class also has a set of operations without implementation(i know class has attributes in addition to its operations)?
so, what is the difference in UML?
As we know in OOP that interface provides a set of operations without implementation but class is the opposite.
Not quite true - abstract classes are classes that have one or more methods declared but not defined (in C++ and Java these are abstract methods). You can have a class defined with all its methods abstract - in which case there is close similarity with an interface.
One key idea in UML, though, is that an interface is a set of methods exposed to other classes or components. The purpose is to define a set of operations.
However, moving to programming, a method may be made abstract to aid development (e.g. by ensuring all subclasses have an implementation). This method might be purely internal to the class.
One last observation: the term interface and class in UML are not quite synonymous to interface and class in a language, say Java. For example, Java does not allow multiple class inheritance. Instead Java has the interface which allows a class to implement multiple types (not classes - a subtle difference)
EDIT
Quick note technical words:
Declare: Stating to the system that a variable or operation exists and its type or signature
Define: Same as declaring, but additionally providing a complete implementation of a variable or operation
Interface: A set of declarations of operations
Type: An object's interface(s) and other operations
Class: An object's class defines (not declares) how the object is implemented, including its internal state and the implementation of its operations
Define is to Declare as Class is to Type.
(see What is the difference between Type and Class?)
The purpose of interface is to define a set of operations but we are do the same for class also define a set of operations?
So the purpose of the interface is to declare (not define) a set of public operations that other objects want to use. A class (in UML) is the complete set of operations (public and private). A class (in Java, C++, etc.) additionally defines all non-abstract operations.
So the key is the intent: When other components of the system want to use a set of operations, use interface. When you're using UML to describe an implementation (of a component, algorithm, etc.) use class.
when I go to class that assumed to implement those operations I can't see any implementation for those operations as a diagram describe those operations or anything give a sign for implementation?
UML tool is for modelling and so deliberately avoids providing a place where you enter operation definitions - that is left for later. The idea is that you:
Define the model in UML
Use the UML tool to generate code in the target language
(And some allow you to import your code back into the tool to modify the model with any changes made during implementation. This is called "round-trip" modelling, something which the old TogetherJ product excelled at)
This deliberate gap (you might say deficiency) means that 'define' vs. 'declare' in UML is meaningless. Sorry.
Perhaps you've just seen models created for describing an overview, rather than modelling the system fully, but you can model the behaviour of a class's operations in most UML tools, and some tools also model the behaviour sufficiently that it can be executed .
The behaviour associated with an operation can be modelled using UML state machines, using UML action semantics or in several other ways. Quite often this is left out of the model - it is not always useful to go to that level of detail, so the implementations may just be hinted at in the documentation associated with the operation. But concrete classes in UML definitely have concrete behaviours associated with their operations, so the difference between UML and programming is that UML focuses on behaviour rather than implementation.
According to Wikipedia -
Unified Modeling Language (UML) is a standardized general-purpose
modeling language in the field of object-oriented software
engineering. The Unified Modeling Language includes a set of graphic
notation techniques to create visual models of object-oriented
software-intensive systems.
So, most important thing is UML is general-purpose and graphical. It is not only about classes and interfaces.
UML offers a standard way to visualize a system's architectural blueprints.
Software Construction Needs a Plan. Structure diagrams, Behavior diagrams, Interaction diagrams helps to Visualise In Multiple Dimensions and Levels of Detail which is
Appropriate For Both New and Legacy Systems.
Unified and Universal, Accommodates Parallel Development of Large Systems.
When I think of UML, one term which comes to mind is software quality. One thing that has plagued the software industry in recent year is poor software design. While the software industry has done fairly well for the last decade, the impact of globalization is changing the ways in which software is designed.