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• What is a best practice tier structure of a Java EE 6/7 application?

  - by James Drinkard

  I was attempting to find a best practice for modeling the tiers in a Java EE application yesterday and couldn't come up with anything current. In the past, say java 1.4, it was four tiers: Presentation Tier Web Tier Business Logic Tier DAL (Data Access Layer ) which I always considered a tier and not a layer. After working with Web Services and SOA I thought to add in a services tier, but that may fall under 3. the business logic tier. I did searches for quite a while and reading articles. It seems like Domain Driven Design is becoming more popular, but I couldn't find a diagram on it's tier structure. Anyone have ideas or diagrams on what the proper tier structure is for newer Java EE applications or is it really the same, but more items are ranked under the four I've mentioned?

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• Customizing configuration with Dependency Injection

  - by mathieu

  I'm designing a small application infrastructure library, aiming to simplify development of ASP.NET MVC based applications. Main goal is to enforce convention over configuration. Hovewer, I still want to make some parts "configurable" by developpers. I'm leaning towards the following design: public interface IConfiguration { SomeType SomeValue; } // this one won't get registered in container protected class DefaultConfiguration : IConfiguration { public SomeType SomeValue { get { return SomeType.Default; } } } // declared inside 3rd party library, will get registered in container protected class CustomConfiguration : IConfiguration { public SomeType SomeValue { get { return SomeType.Custom; } } } And the "service" class : public class Service { private IConfiguration conf = new DefaultConfiguration(); // optional dependency, if found, will be set to CustomConfiguration by DI container public IConfiguration Conf { get { return conf; } set { conf = value; } } public void Configure() { DoSomethingWith( Conf ); } } There, the "configuration" part is clearly a dependency of the service class, but it this an "overuse" of DI ?

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• How does a search functionality fit in DDD with CQRS?

  - by Songo

  In Vaughn Vernon's book Implementing domain driven design and the accompanying sample application I found that he implemented a CQRS approach to the iddd_collaboration bounded context. He presents the following classes in the application service layer: CalendarApplicationService.java CalendarEntryApplicationService.java CalendarEntryQueryService.java CalendarQueryService.java I'm interested to know if an application will have a search page that feature numerous drop downs and check boxes with a smart text box to match different search patterns; How will you structure all that search logic? In a command service or a query service? Taking a look at the CalendarQueryService.java I can see that it has 2 methods for a huge query, but no logic at all to mix and match any search filters for example. I've heard that the application layer shouldn't have any business logic, so where will I construct my dynamic query? or maybe just clutter everything in the Query service?

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• Why is nesting or piggybacking errors within errors bad in general?

  - by dietbuddha

  Why is nesting or piggybacking errors within errors bad in general? To me it seems bad intuitively, but I'm suspicious in that I cannot adequately articulate why it is bad. This may be because it is not in general bad and that it is only bad in specific instances. Why is it detrimental to design error/exception handling in such a way. The specific instance is that of a REST service. There is a desire by some to use http errors (specifically the 500 response) as a way to indicate any problem with specific instances of a resource. An example of an instance resource in this case would be: http://server/ticket/80 # instance http://server/ticket # not an instance So this is the behavior that is being proposed. If ticket 80 does not exist return a http response code of 500. Within the body of the error return the "real" error as an additional error code and description. If the ticket resource doesn't exist return a response code of 404.

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• Is it bad practice to pass instances through several layers?

  - by Puckl

  In my program design, I often come to the point where I have to pass object instances through several classes. For example, if I have a controller that loads an audio file, and then passes it to a player, and the player passes it to the playerRunnable, which passes it again somewhere else etc. It looks kind of bad, but I don´t know how to avoid it. Or is it OK to do this? EDIT: Maybe the player example is not the best because I could load the file later, but in other cases that does not work.

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• Earliest use of Comments as Semantically Meaningful Things in a Program?

  - by Alan Storm

  In certain corners of the PHP meta-programming world, it's become fashionable to use PHPDoc comments as a mechanism for providing semantically meaningful information to a program. That is, other code will parse the doc blocks and do something significant with the information encoded in those comments. Doctrine's annotations and code generation are an example of this. What's the earliest (or some early) use of this technique? I have vague memories of some early java Design by Contract implementations doing similar things, but I'm not sure of those folks were inventing the technique, or if they got it from somewhere. Mainly asking so I can provide some historical context for PHP developers who haven't come across the technique before, and are distrustful of it because it seems a little crazy pants.

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• Subscribe/Publish Model in Web-based Application (c#) - Best Practices for Event Handlers

  - by KingOfHypocrites

  I was recently exposed to a desktop application that uses an publish/subscribe model to handle commands, events, etc. I can't seem to find any good examples of using this in a web application, so I wonder if I am off base in trying to use this for web based development (on the server side)? I'm using asp.net c#. My main question in regards to the design is: When using a publish/subscribe model, is it better to have generic commands/events that pass no parameters and then have the subscribers look at static context objects that contain the data relevant to the event? Or is it better to create custom arguments for every event that contain data related to the event? The whole concept of a global container seems so convenient but at the same time seems to break encapsulation. Any thoughts or best practices from anyone who has implemented this type of model in a web based application? Even suggestions on this model out of the scope of my question are appreciated.

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• How to change the state of a singleton in runtime

  - by user34401

  Consider I am going to write a simple file based logger AppLogger to be used in my apps, ideally it should be a singleton so I can call it via public class AppLogger { public static String file = ".."; public void logToFile() { // Write to file } public static log(String s) { AppLogger.getInstance().logToFile(s); } } And to use it AppLogger::log("This is a log statement"); The problem is, what is the best time I should provide the value of file since it is a just a singleton? Or how to refactor the above code (or skip using singleton) so I can customize the log file path? (Assume I don't need to write to multiple at the same time) p.s. I know I can use library e.g. log4j, but consider it is just a design question, how to refactor the code above?

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• Distinguishing between sets of status reports

  - by user1769486

  I am working on an internal database monitoring system and am at a point where I sort of hit the wall in terms of application design. Basically I have an extensible plugin architecture where I shall have an OK, a warning or an error upon running a db verification. My first question whether it is sufficient to have only one status reported with an optional status message or provide the ability to have more than one returned (with attached messages) and then calculate an aggregated overall status. In particular in the latter case my second issue would be how to distinguish between two verification reports with the same status code (as it can come from different triggers). I would need to do this to see whether some change happened between the current and last verification. I could simply have string comparisons of the attached status messages mentioned above but that does not seem very reliable.

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• When can a design pattern make your software worse?

  - by magol

  When can a design pattern make your software worse? I have seen a program where they used the facade pattern between the GUI and logic. They considered that no objects may be transported over this, so only primitive types were used which made it difficult to code.

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• Is Abstract Factory Pattern implemented correctly for given scenario.... ???

  - by Amit

  First thing... I am novice to pattern world, so correct me if wrong anywhere Scenario: There are multiple companies providing multiple products of diff size so there are 3 entities i.e. Companies, Their Product and size of product I have implement Abstract Pattern on this i.e. so that I will create instance of IProductFactory interface to get desired product... Is below implementation of Abstract Factory Pattern correct ??? If not then please correct the approach + Also tell me if any other pattern can be used for such scenario Thanks in advance... public enum Companies { Samsung = 0, LG = 1, Philips = 2, Sony = 3 } public enum Product { PlasmaTv = 0, DVD = 1 } public enum ProductSize { FortyTwoInch, FiftyFiveInch } interface IProductFactory { IPhilips GetPhilipsProduct(); ISony GetSonyProduct(); } interface ISony { string CreateProducts(Product product, ProductSize size); } interface IPhilips { string CreateProducts(Product product, ProductSize size); } class ProductFactory : IProductFactory { public IPhilips GetPhilipsProduct() { return new Philips(); } public ISony GetSonyProduct() { return new Sony(); } } class Philips : IPhilips { #region IPhilips Members public string CreateProducts(Product product, ProductSize size) {// I have ingnore size for now.... string output = string.Empty; if (product == Product.PlasmaTv) { output = "Plasma TV Created !!!"; } else if (product == Product.DVD) { output = "DVD Created !!!"; } return output; } #endregion } class Sony : ISony {// I have ingnore size for now.... #region ISony Members public string CreateProducts(Product product, ProductSize size) { string output = string.Empty; if (product == Product.PlasmaTv) { output = "Plasma TV Created !!!"; } else if (product == Product.DVD) { output = "DVD Created !!!"; } return output; } #endregion } IProductFactory prodFactory = new ProductFactory(); IPhilips philipsObj = prodFactory.GetPhilipsProduct(); MessageBox.Show(philipsObj.CreateProducts(Product.DVD, ProductSize.FortyTwoInch)); or //ISony sonyObj = prodFactory.GetSonyProduct(); //MessageBox.Show(sonyObj.CreateProducts(Product.DVD, ProductSize.FortyTwoInch));

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• What is the best approach to solve a factory method problem which has to be an instance?

  - by Iago

  I have to add new funcionality in a web service legacy project and I'm thinking what is the best approach for a concrete situation. The web service is simple: It receives a XML file, unmarshalling, generates response's objects, marshalling and finally it sends the response as a XML file. For every XML files received, the web service always responds with the same XML structure. What I have to do is to generate a different XML file according to the XML received. So I have a controller class which has all marshalling/unmarshalling operations, but this controller class has to be an instance. Depending on XML received I need some marshalling methods or others. Trying to make few changes on legacy source, what is the best approach? My first approach was to do a factory method pattern with the controller class, but this class has to be an instance. I want to keep, as far as it goes, this structure: classController.doMarshalling(); I think this one is a bit smelly: if(XMLReceived.isTypeOne()) classController.doMarshallingOne(); else if(XMLReceived.isTypeTwo()) classController.doMarshallingTwo(); else if(XMLReceived.isTypeThree()) classController.doMarshallingThree(); else if ... I hope my question is well understood

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• Handling Types for Real and Complex Matrices in a BLAS Wrapper

  - by mga

  I come from a C background and I'm now learning OOP with C++. As an exercise (so please don't just say "this already exists"), I want to implement a wrapper for BLAS that will let the user write matrix algebra in an intuitive way (e.g. similar to MATLAB) e.g.: A = B*C*D.Inverse() + E.Transpose(); My problem is how to go about dealing with real (R) and complex (C) matrices, because of C++'s "curse" of letting you do the same thing in N different ways. I do have a clear idea of what it should look like to the user: s/he should be able to define the two separately, but operations would return a type depending on the types of the operands (R*R = R, C*C = C, R*C = C*R = C). Additionally R can be cast into C and vice versa (just by setting the imaginary parts to 0). I have considered the following options: As a real number is a special case of a complex number, inherit CMatrix from RMatrix. I quickly dismissed this as the two would have to return different types for the same getter function. Inherit RMatrix and CMatrix from Matrix. However, I can't really think of any common code that would go into Matrix (because of the different return types). Templates. Declare Matrix<T> and declare the getter function as T Get(int i, int j), and operator functions as Matrix *(Matrix RHS). Then specialize Matrix<double> and Matrix<complex>, and overload the functions. Then I couldn't really see what I would gain with templates, so why not just define RMatrix and CMatrix separately from each other, and then overload functions as necessary? Although this last option makes sense to me, there's an annoying voice inside my head saying this is not elegant, because the two are clearly related. Perhaps I'm missing an appropriate design pattern? So I guess what I'm looking for is either absolution for doing this, or advice on how to do better.

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• Best practice to collect information from child objects

  - by Markus

  I'm regularly seeing the following pattern: public abstract class BaseItem { BaseItem[] children; // ... public void DoSomethingWithStuff() { StuffCollection collection = new StuffCollection(); foreach(child c : children) c.AddRequiredStuff(collection); // do something with the collection ... } public abstract void AddRequiredStuff(StuffCollection collection); } public class ConcreteItem : BaseItem { // ... public override void AddRequiredStuff(StuffCollection collection) { Stuff stuff; // ... collection.Add(stuff); } } Where I would use something like this, for better information hiding: public abstract class BaseItem { BaseItem[] children; // ... public void DoSomethingWithStuff() { StuffCollection collection = new StuffCollection(); foreach(child c : children) collection.AddRange(c.RequiredStuff()); // do something with the collection ... } public abstract StuffCollection RequiredStuff(); } public class ConcreteItem : BaseItem { // ... public override StuffCollection RequiredStuff() { StuffCollection stuffCollection; Stuff stuff; // ... stuffCollection.Add(stuff); return stuffCollection; } } What are pros and cons of each solution? For me, giving the implementation access to parent's information is some how disconcerting. On the other hand, initializing a new list, just to collect the items is a useless overhead ... What is the better design? How would it change, if DoSomethingWithStuff wouldn't be part of BaseItem but a third class? PS: there might be missing semicolons, or typos; sorry for that! The above code is not meant to be executed, but just for illustration.

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• Best approach for tracking dependent state

  - by Pace

  Let's pretend I work on a project tracking application. The application is a database backed, server hosted, web application. In this application there are Projects which have many Activities which have many Tasks. A Task has two date fields an originalDueDate and a projectedDueDate. In addition, there are dynamic fields on the Activities and the Projects which indicate whether the Activity or Project is behind schedule based on the projected due dates of the child tasks and various other variables such as remaining buffer time, etc. There are a number of things that can cause the projectedDueDate to change. For example, an employee working on the project may (via a server request) enter in a shipping delay. Alternatively, a site may (via a server request) enter in an unexpected closure. When any of these things occur I need to not only update the projectedDueDate of the Task but also trigger the corresponding Project and Activity to update as well. What is the best way to do this? I've thought of the observer pattern but I don't keep a single copy of all these objects in memory. When a request comes in, I query the Task in from the database, at that point there is no associated Activity in memory that would be a listener. I could remove the ability to query for Tasks and force the application to query first by Project, then by Activity (in context of Project), then by task (in context of Activity) adding the observer relationships at each step but I'm not sure if that is the best way. I could setup a database event listening system so when a Task modified event is dispatched I have a handler which queries for the Activity at that point. I could simply setup a two-way relationship between Task and Activity so that the Task knows about the parent Activity and when the Task updates his state the Task grabs his parent and updates state. Right now I'm stuck considering all the options and am wondering if any single approach (doesn't have to be a listed approach) is jumping out at others as the best approach.

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• dynamic behavior of factory class

  - by manu1001

  I have a factory class that serves out a bunch of properties. Now, the properties might come either from a database or from a properties file. This is what I've come up with. public class Factory { private static final INSTANCE = new Factory(source); private Factory(DbSource source) { // read from db, save properties } private Factory(FileSource source) { // read from file, save properties } // getInstance() and getProperties() here } What's a clean way of switching between these behaviors based on the environment. I want to avoid having to recompile the class each time.

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• QuestionOrAnswer model?

  - by Mark

  My site has Listings. Users can ask Questions about listings, and the author of the listing can respond with an Answer. However, the Answer might need clarification, so I've made them recursive (you can "answer" an answer). So how do I set up the database? The way I have it now looks like this (in Django-style models): class QuestionOrAnswer(Model): user = ForeignKey(User, related_name='questions') listing = ForeignKey(Listing, related_name='questions') parent = models.ForeignKey('self', null=True, blank=True, related_name='children') message = TextField() But what bugs me is that listing is now an attribute of the answers as well (it doesn't need to be). What happens if the database gets mangled and an answer belongs to a different listing than its parent question? That just doesn't make any sense. We can separate it with polymorphism: QuestionOrAnswer user message created updated Question(QuestionOrAnswer) shipment Answer(QuestionOrAnswer) parent = ForeignKey(QuestionOrAnswer) And that ought to work, but now ever question and answer is split into 2 tables. Is it worth this overhead for clearly defined models?

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• Help naming a class that has a single public method called Execute()

  - by devoured elysium

  I have designed the following class that should work kind of like a method (usually the user will just run Execute()): public abstract class ??? { protected bool hasFailed = false; protected bool hasRun = false; public bool HasFailed { get { return hasFailed; } } public bool HasRun { get { return hasRun; } } private void Restart() { hasFailed = false; hasRun = false; } public bool Execute() { ExecuteImplementation(); bool returnValue = hasFailed; Restart(); return returnValue; } protected abstract void ExecuteImplementation(); } My question is: how should I name this class? Runnable? Method(sounds awkward)?

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• Should I use a modified singleton design pattern that only allows one reference to its instance?

  - by Graham

  Hi, I have a class that would normally just generate factory objects, however this class should only used once throughout the program in once specifix place. What is the best design pattern to use in this instance? I throught that having a modified singleton design which only allows one reference to instance throughout the program would be the correct way to go. So only the first call to getInstance() returns the factory library. Is this a good or bad idea? Have I missed out another fundermental design pattern for solving this problem? Thanks for your help.

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• How to properly implement the Strategy pattern in a web MVC framework?

  - by jboxer

  In my Django app, I have a model (lets call it Foo) with a field called "type". I'd like to use Foo.type to indicate what type the specific instance of Foo is (possible choices are "Number", "Date", "Single Line of Text", "Multiple Lines of Text", and a few others). There are two things I'd like the "type" field to end up affecting; the way a value is converted from its normal type to text (for example, in "Date", it may be str(the_date.isoformat())), and the way a value is converted from text to the specified type (in "Date", it may be datetime.date.fromtimestamp(the_text)). To me, this seems like the Strategy pattern (I may be completely wrong, and feel free to correct me if I am). My question is, what's the proper way to code this in a web MVC framework? In a client-side app, I'd create a Type class with abstract methods "serialize()" and "unserialize()", override those methods in subclasses of Type (such as NumberType and DateType), and dynamically set the "type" field of a newly-instantiated Foo to the appropriate Type subclass at runtime. In a web framework, it's not quite as straightforward for me. Right now, the way that makes the most sense is to define Foo.type as a Small Integer field and define a limited set of choices (0 = "Number", 1 = "Date", 2 = "Single Line of Text", etc.) in the code. Then, when a Foo object is instantiated, use a Factory method to look at the value of the instance's "type" field and plug in the correct Type subclass (as described in the paragraph above). Foo would also have serialize() and unserialize() methods, which would delegate directly to the plugged-in Type subclass. How does this design sound? I've never run into this issue before, so I'd really like to know if other people have, and how they've solved it.

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• Attempting to extract a pattern within a string

  - by Brian

  I'm attempting to extract a given pattern within a text file, however, the results are not 100% what I want. Here's my code: import java.util.regex.Matcher; import java.util.regex.Pattern; public class ParseText1 { public static void main(String[] args) { String content = "<p>Yada yada yada <code> foo ddd</code>yada yada ...\n" + "more here <2004-08-24> bar<Bob Joe> etc etc\n" + "more here again <2004-09-24> bar<Bob Joe> <Fred Kej> etc etc\n" + "more here again <2004-08-24> bar<Bob Joe><Fred Kej> etc etc\n" + "and still more <2004-08-21><2004-08-21> baz <John Doe> and now <code>the end</code> </p>\n"; Pattern p = Pattern .compile("<[1234567890]{4}-[1234567890]{2}-[1234567890]{2}>.*?<[^%0-9/]*>", Pattern.MULTILINE); Matcher m = p.matcher(content); // print all the matches that we find while (m.find()) { System.out.println(m.group()); } } } The output I'm getting is: <2004-08-24> bar<Bob Joe> <2004-09-24> bar<Bob Joe> <Fred Kej> <2004-08-24> bar<Bob Joe><Fred Kej> <2004-08-21><2004-08-21> baz <John Doe> and now <code> The output I want is: <2004-08-24> bar<Bob Joe> <2004-08-24> bar<Bob Joe> <2004-08-24> bar<Bob Joe> <2004-08-21> baz <John Doe> In short, the sequence of "date", "text (or blank)", and "name" must be extracted. Everything else should be avoided. For example the tag "Fred Kej" did not have any "date" tag before it, therefore, it should be flagged as invalid. Also, as a side question, is there a way to store or track the text snippets that were skipped/rejected as were the valid texts. Thanks, Brian

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• Minimize useless tweaking of a numeric app

  - by Potatoswatter

  I'm developing a numeric application (nonlinear optimizer), with a zillion knobs to tweak and rising. It's not my first foray into this domain, but this time there are even more variables in the code and I'm on a tight schedule. Don't want to waste time fiddling. Days or even months can potentially be wasted adjusting variables, recompiling, and reprocessing benchmark datasets. The resulting data is viewed and trouble spots are checked. The overall quality of the solution is reported by the program but the meaning of the report could change over time. (Numeric units for the report are one thing I'm trying to nail down.) One main problem is organizing result files to identify each with specific code changes. Note taking can be a pain, is there software to help with this? Are there agreed best practices to making this kind of development cycle reliably move forward? The solver package converges to its optimal solution with mechanical determination, but I'm all too familiar with the way an excess of design decisions can mire development.

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• Prioritize compiler functionality/tasks, when designing a new language

  - by Mahdi

  Well, the question should be so hard to ask and I expect couple of down votes, however, I'm really interested to have your ideas and recommendations. :) I've already made a very simple compiler, with a few and limited functionality. Now I'm getting more on it to make it more like a real-world compiler. I definitely need to start over 'cause I've much more experience and ideas in this area rather a few years ago. So, I want to know, right now, from the very first step again, which tasks/features for the new compiler should implement first and which tasks has lower priority rather than others? For example, I'd say, first I'd go to decide about the object-oriented structure for the new language, but you might say, hey, just go for a compiler that could define a variable, when you finished that, then start thinking about OOP designs ... I prefer to hear the pros and cons for your suggestions also. Actually I like to start from Bottom to Top, where I could add simplest tasks first, and later adding more complex ones, but I'm totally open for any new ideas, and really appreciate that. Also please consider that I'm thinking about the design concepts. Actually I expect answers like: Priority from Highest to Lowest: variables, because .... functions, because .... loops, because .... ... Not: define a syntax for your new language, and start parsing your source code ...

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• Struggling with the Single Responsibility Principle

  - by AngryBird

  Consider this example: I have a website. It allows users to make posts (can be anything) and add tags that describe the post. In the code, I have two classes that represent the post and tags. Lets call these classes Post and Tag. Post takes care of creating posts, deleting posts, updating posts, etc. Tag takes care of creating tags, deleting tags, updating tags, etc. There is one operation that is missing. The linking of tags to posts. I am struggling with who should do this operation. It could fit equally well in either class. On one hand, the Post class could have a function that takes a Tag as a parameter, and then stores it in a list of tags. On the other hand, the Tag class could have a function that takes a Post as a parameter and links the Tag to the Post. The above is just an example of my problem. I am actually running into this with multiple classes that are all similar. It could fit equally well in both. Short of actually putting the functionality in both classes, what conventions or design styles exist to help me solve this problem. I am assuming there has to be something short of just picking one? Maybe putting it in both classes is the correct answer?

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• From a DDD perspective is a report generating service a domain service or an infrastructure service?

  - by Songo

  Let assume we have the following service whose responsibility is to generate Excel reports: class ExcelReportService{ public String generateReport(String fileFormatFilePath, ResultSet data){ ReportFormat reportFormat = new ReportFormat(fileFormatFilePath); ExcelDataFormatterService excelDataFormatterService = new ExcelDataFormatterService(); FormattedData formattedData = excelDataFormatterService.format(data); ExcelFileService excelFileService = new ExcelFileService(); String reportPath= excelFileService.generateReport(reportFormat,formattedData); return reportPath; } } This is pseudo code for the service I want to design where: fileFormatFilePath: path to a configuration file where I'll keep the format of my excel file (headers, column widths, number of columns,..etc) data: the actual records returned from the database. This data can't be used directly coz I might need to make further calculations to the data before inserting them to the excel file. ReportFormat: Value object to hold the report format, has methods like getHeaders(), getColumnWidth(),...etc. ExcelDataFormatterService: a service to hold any logic that need to be applied to the data returned from the database before inserting it to the file. FormattedData: Value object the represents the formatted data to be inserted. ExcelFileService: a wrapper top the 3rd party library that generates the excel file. Now how do you determine whether a service is an infrastructure or domain service? I have the following 3 services here: ExcelReportService, ExcelDataFormatterService and ExcelFileService?

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