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• Advice on designing a robust program to handle a large library of meta-information & programs

  - by Sam Bryant

  So this might be overly vague, but here it is anyway I'm not really looking for a specific answer, but rather general design principles or direction towards resources that deal with problems like this. It's one of my first large-scale applications, and I would like to do it right. Brief Explanation My basic problem is that I have to write an application that handles a large library of meta-data, can easily modify the meta-data on-the-fly, is robust with respect to crashing, and is very efficient. (Sorta like the design parameters of iTunes, although sometimes iTunes performs more poorly than I would like). If you don't want to read the details, you can skip the rest Long Explanation Specifically I am writing a program that creates a library of image files and meta-data about these files. There is a list of tags that may or may not apply to each image. The program needs to be able to add new images, new tags, assign tags to images, and detect duplicate images, all while operating. The program contains an image Viewer which has tagging operations. The idea is that if a given image A is viewed while the library has tags T1, T2, and T3, then that image will have boolean flags for each of those tags (depending on whether the user tagged that image while it was open in the Viewer). However, prior to being viewed in the Viewer, image A would have no value for tags T1, T2, and T3. Instead it would have a "dirty" flag indicating that it is unknown whether or not A has these tags or not. The program can introduce new tags at any time (which would automatically set all images to "dirty" with respect to this new tag) This program must be fast. It must be easily able to pull up a list of images with or without a certain tag as well as images which are "dirty" with respect to a tag. It has to be crash-safe, in that if it suddenly crashes, all of the tagging information done in that session is not lost (though perhaps it's okay to loose some of it) Finally, it has to work with a lot of images (10,000) I am a fairly experienced programmer, but I have never tried to write a program with such demanding needs and I have never worked with databases. With respect to the meta-data storage, there seem to be a few design choices: Choice 1: Invidual meta-data vs centralized meta-data Individual Meta-Data: have a separate meta-data file for each image. This way, as soon as you change the meta-data for an image, it can be written to the hard disk, without having to rewrite the information for all of the other images. Centralized Meta-Data: Have a single file to hold the meta-data for every file. This would probably require meta-data writes in intervals as opposed to after every change. The benefit here is that you could keep a centralized list of all images with a given tag, ect, making the task of pulling up all images with a given tag very efficient

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• How To Deal With Terrible Design Decisions

  - by splatto

  I'm a consultant at one company. There is another consultant who is a year older than me and has been here 3 months longer than I have, and a full time developer. The full-time developer is great. My concern is that I see the consultant making absolutely terrible design decisions. For example, M:M relationships are being stored in the database as a comma-delimited string rather than using a conjunction table to hold the relationships. For example, consider two tables, Car and Property: Car records: Camry Volvo Mercedes Property records: Spare Tire Satellite Radio Ipod Support Standard Rather than making a table CarProperties to represent this, he has made a "Property" attribute on the Car table whose data looks like "1,3,7,13,19,25," I hate how this decision and others are affecting the quality of my code. We have butted heads over this design three times in the past two months since I've been here. He asked me why my suggestion was better, and I responded that our database would be eliminating redundant data by converting to a higher normal form. I explained that this design flaw in particular is discussed and discouraged in entry level college programs, and he responded with a shot at me saying that these comma-separated-value database properties are taught when you do your masters (which neither of us have). Needless to say, he became very upset and demanded I apologize for criticizing his work, which I did in the interest of not wanting to be the consultant to create office drama. Our project manager is focused on delivering a product ASAP and is a very strong personality - Suggesting to him at this point that we spend some time to do this right will set him off. There is a strong likelihood that both of our contracts will be extended to work on a second project coming up. How will I be able to exert dominant influence over the design of the system and the data model to ensure that such terrible mistakes are not repeated in the next project? A glimpse at the dynamics: I can be a strong personality if I don't measure myself. The other consultant is not a strong personality, is a poor communicator, is quite stubborn and thinks he is better than everyone else. The project manager is an extremely strong personality who is focused on releasing tomorrow's product yesterday. The full-time developer is very laid back and easy going, a very effective communicator, but is someone who will accept bad design if it means not rocking the boat. Code reviews or anything else that takes "time" will be out of the question - there is no way our PM will be sold on such a thing by anybody.

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• Object model design: collections on classes

  - by Luke Puplett

  Hi all, Consider Train.Passengers, what type would you use for Passengers where passengers are not supposed to be added or removed by the consuming code? I'm using .NET Framework, so this discussion would suit .NET, but it could apply to a number of modern languages/frameworks. In the .NET Framework, the List is not supposed to be publicly exposed. There's Collection and ICollection and guidance, which I tend to agree with, is to return the closest concrete type down the inheritance tree, so that'd be Collection since it is already an ICollection. But Collection has read/write semantics and so possibly it should be a ReadOnlyCollection, but its arguably common sense not to alter the contents of a collection that you don't have intimate knowledge about so is it necessary? And it requires extra work internally and can be a pain with (de)serialization. At the extreme ends I could just return Person[] (since LINQ now provides much of the benefits that previously would have been afforded by a more specified collection) or even build a strongly-typed PersonCollection or ReadOnlyPersonCollection! What do you do? Thanks for your time. Luke

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• A design pattern for data binding an object (with subclasses) to asp.net user control

  - by Rohith Nair

  I have an abstract class called Address and I am deriving three classes ; HomeAddress, Work Address, NextOfKin address. My idea is to bind this to a usercontrol and based on the type of Address it should bind properly to the ASP.NET user control. My idea is the user control doesn't know which address it is going to present and based on the type it will parse accordingly. How can I design such a setup, based on the fact that, the user control can take any type of address and bind accordingly. I know of one method like :- Declare class objects for all the three types (Home,Work,NextOfKin). Declare an enum to hold these types and based on the type of this enum passed to user control, instantiate the appropriate object based on setter injection. As a part of my generic design, I just created a class structure like this :- I know I am missing a lot of pieces in design. Can anybody give me an idea of how to approach this in proper way.

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• How to design database having multiple interrelated entities

  - by Sharath Chandra

  I am designing a new system which is more of a help system for core applications in banks or healthcare sector. Given the nature of the system this is not a heavy transaction oriented system but more of read intensive. Now within this application I have multiple entities which are related to each other. For e.g. Assume the following entities in the system User Training Regulations Now each of these entities have M:N Relationship with each other. Assuming the usage of a standard RDBMS, the design may involve many relationship tables each containing the relationships one other entity ("User_Training", "User_Regulations", "Training_Regulations"). This design is limiting since I have more than 3 entities in the system and maintaining the relationship graph is difficult this way. The most frequently used operation is "given an entity get me all the related entities" . I need to design the database where this operation is relatively inexpensive. What are the different recommendations for modelling this kind of database.

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• Design for object with optional and modifiable attributtes?

  - by Ikuzen

  I've been using the Builder pattern to create objects with a large number of attributes, where most of them are optional. But up until now, I've defined them as final, as recommended by Joshua Block and other authors, and haven't needed to change their values. I am wondering what should I do though if I need a class with a substantial number of optional but non-final (mutable) attributes? My Builder pattern code looks like this: public class Example { //All possible parameters (optional or not) private final int param1; private final int param2; //Builder class public static class Builder { private final int param1; //Required parameters private int param2 = 0; //Optional parameters - initialized to default //Builder constructor public Builder (int param1) { this.param1 = param1; } //Setter-like methods for optional parameters public Builder param2(int value) { param2 = value; return this; } //build() method public Example build() { return new Example(this); } } //Private constructor private Example(Builder builder) { param1 = builder.param1; param2 = builder.param2; } } Can I just remove the final keyword from the declaration to be able to access the attributes externally (through normal setters, for example)? Or is there a creational pattern that allows optional but non-final attributes that would be better suited in this case?

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• design a model for a system of dependent variables

  - by dbaseman

  I'm dealing with a modeling system (financial) that has dozens of variables. Some of the variables are independent, and function as inputs to the system; most of them are calculated from other variables (independent and calculated) in the system. What I'm looking for is a clean, elegant way to: define the function of each dependent variable in the system trigger a re-calculation, whenever a variable changes, of the variables that depend on it A naive way to do this would be to write a single class that implements INotifyPropertyChanged, and uses a massive case statement that lists out all the variable names x1, x2, ... xn on which others depend, and, whenever a variable xi changes, triggers a recalculation of each of that variable's dependencies. I feel that this naive approach is flawed, and that there must be a cleaner way. I started down the path of defining a CalculationManager<TModel> class, which would be used (in a simple example) something like as follows: public class Model : INotifyPropertyChanged { private CalculationManager<Model> _calculationManager = new CalculationManager<Model>(); // each setter triggers a "PropertyChanged" event public double? Height { get; set; } public double? Weight { get; set; } public double? BMI { get; set; } public Model() { _calculationManager.DefineDependency<double?>( forProperty: model => model.BMI, usingCalculation: (height, weight) => weight / Math.Pow(height, 2), withInputs: model => model.Height, model.Weight); } // INotifyPropertyChanged implementation here } I won't reproduce CalculationManager<TModel> here, but the basic idea is that it sets up a dependency map, listens for PropertyChanged events, and updates dependent properties as needed. I still feel that I'm missing something major here, and that this isn't the right approach: the (mis)use of INotifyPropertyChanged seems to me like a code smell the withInputs parameter is defined as params Expression<Func<TModel, T>>[] args, which means that the argument list of usingCalculation is not checked at compile time the argument list (weight, height) is redundantly defined in both usingCalculation and withInputs I am sure that this kind of system of dependent variables must be common in computational mathematics, physics, finance, and other fields. Does someone know of an established set of ideas that deal with what I'm grasping at here? Would this be a suitable application for a functional language like F#? Edit More context: The model currently exists in an Excel spreadsheet, and is being migrated to a C# application. It is run on-demand, and the variables can be modified by the user from the application's UI. Its purpose is to retrieve variables that the business is interested in, given current inputs from the markets, and model parameters set by the business.

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• What OO Design to use ( is there a Design Pattern )?

  - by Blundell

  I have two objects that represent a 'Bar/Club' ( a place where you drink/socialise). In one scenario I need the bar name, address, distance, slogon In another scenario I need the bar name, address, website url, logo So I've got two objects representing the same thing but with different fields. I like to use immutable objects, so all the fields are set from the constructor. One option is to have two constructors and null the other fields i.e: class Bar { private final String name; private final Distance distance; private final Url url; public Bar(String name, Distance distance){ this.name = name; this.distance = distance; this.url = null; } public Bar(String name, Url url){ this.name = name; this.distance = null; this.url = url; } // getters } I don't like this as you would have to null check when you use the getters In my real example the first scenario has 3 fields and the second scenario has about 10, so it would be a real pain having two constructors, the amount of fields I would have to declare null and then when the object are in use you wouldn't know which Bar you where using and so what fields would be null and what wouldn't. What other options do I have? Two classes called BarPreview and Bar? Some type of inheritance / interface? Something else that is awesome?

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• Representing complex object dependencies

  - by max

  I have several classes with a reasonably complex (but acyclic) dependency graph. All the dependencies are of the form: class X instance contains an attribute of class Y. All such attributes are set during initialization and never changed again. Each class' constructor has just a couple parameters, and each object knows the proper parameters to pass to the constructors of the objects it contains. class Outer is at the top of the dependency hierarchy, i.e., no class depends on it. Currently, the UI layer only creates an Outer instance; the parameters for Outer constructor are derived from the user input. Of course, Outer in the process of initialization, creates the objects it needs, which in turn create the objects they need, and so on. The new development is that the a user who knows the dependency graph may want to reach deep into it, and set the values of some of the arguments passed to constructors of the inner classes (essentially overriding the values used currently). How should I change the design to support this? I could keep the current approach where all the inner classes are created by the classes that need them. In this case, the information about "user overrides" would need to be passed to Outer class' constructor in some complex user_overrides structure. Perhaps user_overrides could be the full logical representation of the dependency graph, with the overrides attached to the appropriate edges. Outer class would pass user_overrides to every object it creates, and they would do the same. Each object, before initializing lower level objects, will find its location in that graph and check if the user requested an override to any of the constructor arguments. Alternatively, I could rewrite all the objects' constructors to take as parameters the full objects they require. Thus, the creation of all the inner objects would be moved outside the whole hierarchy, into a new controller layer that lies between Outer and UI layer. The controller layer would essentially traverse the dependency graph from the bottom, creating all the objects as it goes. The controller layer would have to ask the higher-level objects for parameter values for the lower-level objects whenever the relevant parameter isn't provided by the user. Neither approach looks terribly simple. Is there any other approach? Has this problem come up enough in the past to have a pattern that I can read about? I'm using Python, but I don't think it matters much at the design level.

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• Where, in an object oriented system should you, if at all, choose (C-style) structs over classes?

  - by Anto

  C and most likely many other languages provide a struct keyword for creating structures (or something in a similar fashion). These are (at least in C), from a simplified point of view like classes, but without polymorphism, inheritance, methods, and so on. Think of an object-oriented (or multi paradigm) language with C-style structs. Where would you choose them over classes? Now, I don't believe they are to be used with OOP as classes seem to replace their purposes, but I wonder if there are situations where they could be preferred over classes in otherwise object-oriented programs and in what kind of situations. Are there such situations?

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• New design patterns/design strategies

  - by steven

  I've studied and implemented design patterns for a few years now, and I'm wondering. What are some of the newer design patterns (since the GOF)? Also, what should one, similar to myself, study [in the way of software design] next? Note: I've been using TDD, and UML for some time now. I'm curious about the newer paradigm shifts, and or newer design patterns.

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• Object-Oriented equivalent of LISP's progn function?

  - by Archer

  I'm currently writing a LISP parser that iterates through some AutoLISP code and does its best to make it a little easier to read (changing prefix notation to infix notation, changing setq assignments to "=" assignments, etc.) for those that aren't used to LISP code/only learned object oriented programming. While writing commands that LISP uses to add to a "library" of LISP commands, I came across the LISP command "progn". The only problem is that it looks like progn is simply executing code in a specific order and sometimes (not usually) assigning the last value to a variable. Am I incorrect in assuming that for translating progn to object-oriented understanding that I can simply forgo the progn function and print the statements that it contains? If not, what would be a good equivalent for progn in an object-oriented language?

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• Design Pattern Advice for Bluetooth App for Android

  - by Aimee Jones

  I’m looking for some advice on which patterns would apply to some of my work. I’m planning on doing a project as part of my college work and I need a bit of help. My main project is to make a basic Android bluetooth tracking system where the fixed locations of bluetooth dongles are mapped onto a map of a building. So my android app will regularly scan for nearby dongles and triangulate its location based on signal strength. The dongles location would be saved to a database along with their mac addresses to differentiate between them. The android phones location will then be sent to a server. This information will be used to show the phone’s location on a map of the building, or map of a route taken, on a website. My side project is to choose a suitable design pattern that could be implemented in this main project. I’m still a bit new to design patterns and am finding it hard to get my head around ones that may be suitable. I’ve heard maybe some that are aimed at web applications for the server side of things may be appropriate. My research so far is leading me to the following: Navigation Strategy Pattern Observer Pattern Command Pattern News Design Pattern Any advice would be a great help! Thanks

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• Authorization design-pattern / practice?

  - by Lawtonfogle

  On one end, you have users. On the other end, you have activities. I was wondering if there is a best practice to relate the two. The simplest way I can think of is to have every activity have a role, and assign every user every role they need. The problem is that this gets really messy in practice as soon as you go beyond a trivial system. A way I recently designed was to have users who have roles, and roles have privileges, and activities require some combinations of privileges. For the trivial case, this is more complex, but I think it will scale better. But after I implemented it, I felt like it was overkill for the system I had. Another option would be to have users, who have roles, and activities require you to have a certain role to perform with many activities sharing roles. A more complex variant of this would given activities many possible roles, which you only needed one of. And an even more complex variant would be to allow logical statements of role ownership to use an activity (i.e. Must have A and (B exclusive or C) and must not have D). I could continue to list more, but I think this already gives a picture. And many of these have trade offs. But in software design, there are oftentimes solutions, while perhaps not perfect in every possible case, are clearly top of the pack to an extent it isn't even considered opinion based (i.e. how to store passwords, plain text is worse, hashing better, hashing and salt even better, despite the increased complexity of each level) (i.e. 2, Smart UI designs for applications are bad, even if it is subjective as to what the best design is). So, is there a best practice for authorization design that is not purely opinion based/subjective?

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• Clean MVC design when there is viewer latency

  - by Tony Suffolk 66

  It isn't clear if this question has already been answered, so apologies in advance if this is a duplicate : I am implementing a game and trying to design around a clean MVC pattern - so my Control plane will implement the rules of the game (but not how the game is displayed), and the View plane implements how the game is displayed, and user iteraction - i.e. what game items or controls the user has activated. The challenge that I have is this : In my game the Control Plane can move game items more or less instaneously (The decision about what item to place where - and some of the initial consequences of that placement are reasonably trivial to calculate), but I want to design the Control Plane so that the View plane can display these movements either instaneously or using movement animations. The other complication is that player interaction must be locked out while those game items are moving (similar to chess - you can't attack an opposing piece as it moves past one of your pieces) So do I : Implement all the logic in the Control Plane asynchronously - and separate the descision making from the actions - so the Control plane decides piece 'A' needs to move to a given place - tells the view plane, and but does not implement the move in data until the view plane informs the control plane that the move/animation is complete. A lot of interlock points between the two layers. Implement all the control plane logic in one place - decisions and movement (keeping track of what moved where), and pass all the movements in one go to the View plane to do with what it will. Control Plane is almost fire and forget here. A hybrid of 1 & 2 - The control plane implements all the moves in a temporary data store - but maintains a second store which reflects what is actually visible to the viewer, based on calls and feedback from the View plane. All 3 are relatively easy to implement (target language is python), but having never done a clean MVC pattern with view latency before - I am not sure which design is best

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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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• 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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• 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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• Design Book–Dimensional or No Dimensional, that is..the question

  - by drsql

  So, it is right there in the title of the book “Relational Database Design” etc (the title is kinda long :)  But as I consider what to cover and, conversely, what not to cover, dimensional design inevitably pops up. So I am considering including it in the book. One thing I try to do is to cover topics to a level where you can start using it immediately, and I am not sure that I could get a deep enough coverage of the subject to do that. I don’t really feel like it has to be the definitive source...(read more)

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• Recommendations for books and training resources covering for Design for Programmers

  - by Jon Hopkins

  Off the back of one of the answers to this question (currently the second highest scoring), it made me think, what's the best way to get developers up to speed on good basic design principals. I'm not talking about making them into graphic designers but some developers almost take pride in ugly UIs, seeing them as unimportant next to the functionality. What primarily interested in are the graphic design elements rather than the usability aspects which is pretty well covered by books such as Don't Make Me Think. Use of white space, emphasis, font selection and a million other things I'm probably not even aware of. I know people are often seen as artistic or not artistic but surely the basics can be taught and someone has written a book covering this?

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• Design Pattern for Complex Data Modeling

  - by Aaron Hayman

  I'm developing a program that has a SQL database as a backing store. As a very broad description, the program itself allows a user to generate records in any number of user-defined tables and make connections between them. As for specs: Any record generated must be able to be connected to any other record in any other user table (excluding itself...the record, not the table). These "connections" are directional, and the list of connections a record has is user ordered. Moreover, a record must "know" of connections made from it to others as well as connections made to it from others. The connections are kind of the point of this program, so there is a strong possibility that the number of connections made is very high, especially if the user is using the software as intended. A record's field can also include aggregate information from it's connections (like obtaining average, sum, etc) that must be updated on change from another record it's connected to. To conserve memory, only relevant information must be loaded at any one time (can't load the entire database in memory at load and go from there). I cannot assume the backing store is local. Right now it is, but eventually this program will include syncing to a remote db. Neither the user tables, connections or records are known at design time as they are user generated. I've spent a lot of time trying to figure out how to design the backing store and the object model to best fit these specs. In my first design attempt on this, I had one object managing all a table's records and connections. I attempted this first because it kept the memory footprint smaller (records and connections were simple dicts), but maintaining aggregate and link information between tables became....onerous (ie...a huge spaghettified mess). Tracing dependencies using this method almost became impossible. Instead, I've settled on a distributed graph model where each record and connection is 'aware' of what's around it by managing it own data and connections to other records. Doing this increases my memory footprint but also let me create a faulting system so connections/records aren't loaded into memory until they're needed. It's also much easier to code: trace dependencies, eliminate cycling recursive updates, etc. My biggest problem is storing/loading the connections. I'm not happy with any of my current solutions/ideas so I wanted to ask and see if anybody else has any ideas of how this should be structured. Connections are fairly simple. They contain: fromRecordID, fromTableID, fromRecordOrder, toRecordID, toTableID, toRecordOrder. Here's what I've come up with so far: Store all the connections in one big table. If I do this, either I load all connections at once (one big db call) or make a call every time a user table is loaded. The big issue here: the size of the connections table has the potential to be huge, and I'm afraid it would slow things down. Store in separate tables all the outgoing connections for each user table. This is probably the worst idea I've had. Now my connections are 'spread out' over multiple tables (one for each user table), which means I have to make a separate DB called to each table (or make a huge join) just to find all the incoming connections for a particular user table. I've avoided making "one big ass table", but I'm not sure the cost is worth it. Store in separate tables all outgoing AND incoming connections for each user table (using a flag to distinguish between incoming vs outgoing). This is the idea I'm leaning towards, but it will essentially double the total DB storage for all the connections (as each connection will be stored in two tables). It also means I have to make sure connection information is kept in sync in both places. This is obviously not ideal but it does mean that when I load a user table, I only need to load one 'connection' table and have all the information I need. This also presents a separate problem, that of connection object creation. Since each user table has a list of all connections, there are two opportunities for a connection object to be made. However, connections objects (designed to facilitate communication between records) should only be created once. This means I'll have to devise a common caching/factory object to make sure only one connection object is made per connection. Does anybody have any ideas of a better way to do this? Once I've committed to a particular design pattern I'm pretty much stuck with it, so I want to make sure I've come up with the best one possible.

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• How do we keep dependent data structures up to date?

  - by Geo

  Suppose you have a parse tree, an abstract syntax tree, and a control flow graph, each one logically derived from the one before. In principle it is easy to construct each graph given the parse tree, but how can we manage the complexity of updating the graphs when the parse tree is modified? We know exactly how the tree has been modified, but how can the change be propagated to the other trees in a way that doesn't become difficult to manage? Naturally the dependent graph can be updated by simply reconstructing it from scratch every time the first graph changes, but then there would be no way of knowing the details of the changes in the dependent graph. I currently have four ways to attempt to solve this problem, but each one has difficulties. Nodes of the dependent tree each observe the relevant nodes of the original tree, updating themselves and the observer lists of original tree nodes as necessary. The conceptual complexity of this can become daunting. Each node of the original tree has a list of the dependent tree nodes that specifically depend upon it, and when the node changes it sets a flag on the dependent nodes to mark them as dirty, including the parents of the dependent nodes all the way down to the root. After each change we run an algorithm that is much like the algorithm for constructing the dependent graph from scratch, but it skips over any clean node and reconstructs each dirty node, keeping track of whether the reconstructed node is actually different from the dirty node. This can also get tricky. We can represent the logical connection between the original graph and the dependent graph as a data structure, like a list of constraints, perhaps designed using a declarative language. When the original graph changes we need only scan the list to discover which constraints are violated and how the dependent tree needs to change to correct the violation, all encoded as data. We can reconstruct the dependent graph from scratch as though there were no existing dependent graph, and then compare the existing graph and the new graph to discover how it has changed. I'm sure this is the easiest way because I know there are algorithms available for detecting differences, but they are all quite computationally expensive and in principle it seems unnecessary so I'm deliberately avoiding this option. What is the right way to deal with these sorts of problems? Surely there must be a design pattern that makes this whole thing almost easy. It would be nice to have a good solution for every problem of this general description. Does this class of problem have a name?

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• Is OOP hard because it is not natural?

  - by zvrba

  One can often hear that OOP naturally corresponds to the way people think about the world. But I would strongly disagree with this statement: We (or at least I) conceptualize the world in terms of relationships between things we encounter, but the focus of OOP is designing individual classes and their hierarchies. Note that, in everyday life, relationships and actions exist mostly between objects that would have been instances of unrelated classes in OOP. Examples of such relationships are: "my screen is on top of the table"; "I (a human being) am sitting on a chair"; "a car is on the road"; "I am typing on the keyboard"; "the coffee machine boils water", "the text is shown in the terminal window." We think in terms of bivalent (sometimes trivalent, as, for example in, "I gave you flowers") verbs where the verb is the action (relation) that operates on two objects to produce some result/action. The focus is on action, and the two (or three) [grammatical] objects have equal importance. Contrast that with OOP where you first have to find one object (noun) and tell it to perform some action on another object. The way of thinking is shifted from actions/verbs operating on nouns to nouns operating on nouns -- it is as if everything is being said in passive or reflexive voice, e.g., "the text is being shown by the terminal window". Or maybe "the text draws itself on the terminal window". Not only is the focus shifted to nouns, but one of the nouns (let's call it grammatical subject) is given higher "importance" than the other (grammatical object). Thus one must decide whether one will say terminalWindow.show(someText) or someText.show(terminalWindow). But why burden people with such trivial decisions with no operational consequences when one really means show(terminalWindow, someText)? [Consequences are operationally insignificant -- in both cases the text is shown on the terminal window -- but can be very serious in the design of class hierarchies and a "wrong" choice can lead to convoluted and hard to maintain code.] I would therefore argue that the mainstream way of doing OOP (class-based, single-dispatch) is hard because it IS UNNATURAL and does not correspond to how humans think about the world. Generic methods from CLOS are closer to my way of thinking, but, alas, this is not widespread approach. Given these problems, how/why did it happen that the currently mainstream way of doing OOP became so popular? And what, if anything, can be done to dethrone it?

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• Design Application to "Actively" Invite Users (pretend they have privileges)

  - by user3086451

  I am designing an application where users message one another privately, and may send messages to any Entity in the database (an Entity may not have a user account yet, it is a professional database). I am not sure how to best design the database and the API to allow messaging unregistered users. The application should remain secure, and data only accessed by those with correct permissions. Messages sent to persons without user accounts serve as an invitation. The invited person should be able to view the message, act on it, and complete the user registration upon receiving an InviteMessage. In simple terms, I have: User misc user fields (email, pw, dateJoined) Entity (large professional dataset): personalDetails... user->User (may be null) UserMessage: sender->User recipient->User dateCreated messageContent, other fields..... InviteMessage: sender->User recipient->Entity expiringUrl inviteeEmail inviteePhone I plan to alert the user when selecting a recipient that is not registered yet, and inform that he may send the message as an invitation by providing email, phone where we can send the invitation. Invitations will have a unique, one-time-use URL, e.g. uuid.uuid4(). When accessed, the invitee will see the InviteMessage and details about completing his/her registration profile. When registration is complete, InviteMessage details to a new instance of UserMessage (to not lose their data), and assign it to the newly created User. The ability to interact with and invite persons who do not yet have accounts is a key feature of the application, and it seems better to separate the invitation from the private, app messages (easier to keep functionality separate, better if data model changes). Is this a reasonable, good design? If not, what would you suggest? Do you have any improvements? Am I correct to choose to create a separate endpoint for creating invitations via the API?

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