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  • Flexible Decorator Pattern?

    - by Omar Kooheji
    I was looking for a pattern to model something I'm thinking of doing in a personal project and I was wondering if a modified version of the decorator patter would work. Basicly I'm thinking of creating a game where the characters attributes are modified by what items they have equiped. The way that the decorator stacks it's modifications is perfect for this, however I've never seen a decorator that allows you to drop intermediate decorators, which is what would happen when items are unequiped. Does anyone have experience using the decorator pattern in this way? Or am I barking up the wrong tree? Clarification To explain "Intermediate decorators" if for example my base class is coffe which is decorated with milk which is decorated with sugar (using the example in Head first design patterns) milk would be an intermediate decorator as it decorates the base coffee, and is decorated by the sugar. Yet More Clarification :) The idea is that items change stats, I'd agree that I am shoehorning the decorator into this. I'll look into the state bag. essentially I want a single point of call for the statistics and for them to go up/down when items are equiped/unequiped. I could just apply the modifiers to the characters stats on equiping and roll them back when unequiping. Or whenever a stat is asked for iterate through all the items and calculate the stat. I'm just looking for feedback here, I'm aware that I might be using a chainsaw where scissors would be more appropriate...

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  • Generics with constraints hierarchy

    - by devoured elysium
    I am currently facing a very disturbing problem: interface IStateSpace<Position, Value> where Position : IPosition // <-- Problem starts here where Value : IValue // <-- and here as I don't { // know how to get away this // circular dependency! // Notice how I should be // defining generics parameters // here but I can't! Value GetStateAt(Position position); void SetStateAt(Position position, State state); } As you'll down here, both IPosition, IValue and IState depend on each other. How am I supposed to get away with this? I can't think of any other design that will circumvent this circular dependency and still describes exactly what I want to do! interface IState<StateSpace, Value> where StateSpace : IStateSpace where Value : IValue { StateSpace StateSpace { get; }; Value Value { get; set; } } interface IPosition { } interface IValue<State> where State : IState { State State { get; } } Basically I have a state space IStateSpace that has states IState inside. Their position in the state space is given by an IPosition. Each state then has one (or more) values IValue. I am simplifying the hierarchy, as it's a bit more complex than described. The idea of having this hierarchy defined with generics is to allow for different implementations of the same concepts (an IStateSpace will be implemented both as a matrix as an graph, etc). Would can I get away with this? How do you generally solve this kind of problems? Which kind of designs are used in these cases? Thanks

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  • Getting error while connecting to facebook

    - by Bakhtiyor
    I have downloaded php-sdk for using facebook in php. I also created facebook application in the facebook web page in order to get API key and secret key. When I run example.php and try to connect to facebook it shows me the following error: Configuration errors: To fix this error, please set your Connect URL in the application settings editor. Once it has been set, users will be redirected to that URL instead of this page after logging in. I am using this php-sdk in the localhost. I tried to assign localhost as Connect URL but facebook doesn't accept it. I have read about Cross Domain Communication Channel and think this is what I need. But I don't know how to use it. Anyone can help me to solve this problem please? Update Actually what I need is following. I have Web Application and need to connect to facebook and search for users in facebook who has specific(user will specify this) Likes and Interests. Any idea about solving my problem?

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  • [Smalltalk] Store List of Instruction

    - by Luciano Lorenti
    Hi all, I have a design Problem. i have a Drawer class wich invokes a serie of methods of a kind-of-brush class and i have a predefined shapes which i want to draw. Each shape uses a list of instance methods from the drawer. I can have more than 1 brush object. I want to add custom shapes on runtime in the drawer instance, especifying the list of methods of the new shape. i've created a class method for every predefined shape that returns a BlockClosure with the instruccions. Obviously i have to give to each BlockClosure the brush object as parameter. I imagine a collection with all the BlockClosures in each instance of the Drawer Class. Maybe i can inherit a SequenceableCollection and make a instruccion collection. Each element of the collection it's a instruction and i give the brush object when i instance this new collection. I really don't know the best way to store these steps. (Maybe a shared variable?)

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  • What is the equivalent of .NET events in Ruby?

    - by Gishu
    The problem is very simple. An object needs to notify some events that might be of interest to observers. When I sat to validate a design that I cooked up now in Ruby just to validate it.. I find myself stumped as to how to implement the object events. In .Net this would be a one-liner.. .Net also does handler method signature verification,etc. e.g. // Object with events public delegate void HandlerSignature(int a); public event HandlerSignature MyEvent; public event HandlerSignature AnotherCriticalEvent; // Client MyObject.MyEvent += new HandlerSignature(MyHandlerMethod); // MyHandlerMethod has same signature as delegate Is there an EventDispatcher module or something that I am missing that I can strap on to a Ruby class ? Hoping for an answer that plays along with Ruby's principle of least surprise. An event would be the name of the event plus a queue of [observer, methodName] objects that need to be invoked when the event takes place.

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  • Can the Diamond Problem be really solved?

    - by Mecki
    A typical problem in OO programming is the diamond problem. I have parent class A with two sub-classes B and C. A has an abstract method, B and C implement it. Now I have a sub-class D, that inherits of B and C. The diamond problem is now, what implementation shall D use, the one of B or the one of C? People claim Java knows no diamond problem. I can only have multiple inheritance with interfaces and since they have no implementation, I have no diamond problem. Is this really true? I don't think so. See below: [removed vehicle example] Is a diamond problem always the cause of bad class design and something neither programmer nor compiler needs to solve, because it shouldn't exist in the first place? Update: Maybe my example was poorly chosen. See this image Of course you can make Person virtual in C++ and thus you will only have one instance of person in memory, but the real problem persists IMHO. How would you implement getDepartment() for GradTeachingFellow? Consider, he might be student in one department and teach in another one. So you can either return one department or the other one; there is no perfect solution to the problem and the fact that no implementation might be inherited (e.g. Student and Teacher could both be interfaces) doesn't seem to solve the problem to me.

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  • Explaining the need to avoid horizontal scroll

    - by Bradley Herman
    I need help explaining to my boss why her design is poor on a client's website. She has no knowledge of the web, and it can be difficult as a web developer working with a woman who is a graphic designer (not even a web designer really). On a current site she has designed, an image bar "needs" to be ~1200px according to her, though it isn't necessary with the content. A quick sketch to illustrate what's going on: As you see, the banner spills out past the 960px of the content and as wide as 1200px. This creates a horizontal scroll when all the content is viewable within the 960px wide viewport. I need to make this an <img and not a CSS background because it's a jQuery slideshow that fades from image to image. I think this is a big problem because a lot of people are going to get a horizontal scroll bar imposed in their browser when they're still able to see all the relevant content. She thinks no one will notice and it'll be fine; I think it's very bad practice and confusing to the end user. How do I explain the problem to her?

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  • Naming convention for non-virtual and abstract methods

    - by eagle
    I frequently find myself creating classes which use this form (A): abstract class Animal { public void Walk() { // TODO: do something before walking // custom logic implemented by each subclass WalkInternal(); // TODO: do something after walking } protected abstract void WalkInternal(); } class Dog : Animal { protected override void WalkInternal() { // TODO: walk with 4 legs } } class Bird : Animal { protected override void WalkInternal() { // TODO: walk with 2 legs } } Rather than this form (B): abstract class Animal { public abstract void Walk(); } class Dog : Animal { public override void Walk() { // TODO: do something before walking // custom logic implemented by each subclass // TODO: walk with 4 legs // TODO: do something after walking } } class Bird : Animal { public override void Walk() { // TODO: do something before walking // custom logic implemented by each subclass // TODO: walk with 2 legs // TODO: do something after walking } } As you can see, the nice thing about form A is that every time you implement a subclass, you don't need to remember to include the initialization and finalization logic. This is much less error prone than form B. What's a standard convention for naming these methods? I like naming the public method Walk since then I can call Dog.Walk() which looks better than something like Dog.WalkExternal(). However, I don't like my solution of adding the suffix "Internal" for the protected method. I'm looking for a more standardized name. Btw, is there a name for this design pattern?

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  • Users in database server or database tables

    - by Batcat
    Hi all, I came across an interesting issue about client server application design. We have this browser based management application where it has many users using the system. So obvisously within that application we have an user management module within it. I have always thought having an user table in the database to keep all the login details was good enough. However, a senior developer said user management should be done in the database server layer if not then is poorly designed. What he meant was, if a user wants to use the application then a user should be created in the user table AND in the database server as a user account as well. So if I have 50 users using my applications, then I should have 50 database server user logins. I personally think having just one user account in the database server for this database was enough. Just grant this user with the allowed privileges to operate all the necessary operation need by the application. The users that are interacting with the application should have their user accounts created and managed within the database table as they are more related to the application layer. I don't see and agree there is need to create a database server user account for every user created for the application in the user table. A single database server user should be enough to handle all the query sent by the application. Really hope to hear some suggestions / opinions and whether I'm missing something? performance or security issues? Thank you very much.

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  • How can I place validating constraints on my method input parameters?

    - by rcampbell
    Here is the typical way of accomplishing this goal: public void myContractualMethod(final String x, final Set<String> y) { if ((x == null) || (x.isEmpty())) { throw new IllegalArgumentException("x cannot be null or empty"); } if (y == null) { throw new IllegalArgumentException("y cannot be null"); } // Now I can actually start writing purposeful // code to accomplish the goal of this method I think this solution is ugly. Your methods quickly fill up with boilerplate code checking the valid input parameters contract, obscuring the heart of the method. Here's what I'd like to have: public void myContractualMethod(@NotNull @NotEmpty final String x, @NotNull final Set<String> y) { // Now I have a clean method body that isn't obscured by // contract checking If those annotations look like JSR 303/Bean Validation Spec, it's because I borrowed them. Unfortunitely they don't seem to work this way; they are intended for annotating instance variables, then running the object through a validator. Which of the many Java design-by-contract frameworks provide the closest functionality to my "like to have" example? The exceptions that get thrown should be runtime exceptions (like IllegalArgumentExceptions) so encapsulation isn't broken.

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  • How to wrap a thirdparty library COM class for notifications in a C++ project

    - by geocoin
    A thirdparty vendor has provided a COM interface as an external API to their product, and for sending instructions to it, it's fine with the mechanism of generating class wrappers with visual studio by importing from the dll, then creating instances of the created classes, calling their CreateDispatch(), using the access functions and releasing when done. For return notifications, I'd normally create an EventsListener class derived from IDispatch using the Invoke function to handle events returning from the interface. This vendor has created an Events lass which I have to wrap and expose, then explicitly tell the installation where to look. all the example are given is C# where it's really easy, but I'm struggling on how to do it in C++ in the C# example, the interop dll provided is simply added as a reference and derived into a class like so: using System; using System.Runtime.InteropServices; using System.Windows.Forms; using System.Text; using <THIER INTEROP LIB> namespace some.example.namespace { [ComVisible(true)] public class EventViewer : IEvents //where IEvents is their events class { public void OnEvent(EventID EventID, object pData) //overridden function { //event handled here } } } In broad terms I assume that I must create a COM interface, since they require a ProgID from me to instantiate, but how do I derive that's been wrapped by the import and then expose the created class to COM I'm just not sure where to even start, as all the tutorials I've seen so far talk in terms of creating brand new classes not wrapping a third party one

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  • Explaining to boss why we need to avoid horizontal scroll

    - by Bradley Herman
    I need help explaining to my boss why her design is poor on a clients website. She has no knowledge of web and it can be difficult as a web developer working with a woman who is a graphic designer (not even a web designer really). On a current site she has designed, an image bar "needs" to be like 1200px according to her, though it isn't necessary with the content. I'll show a quick sketch to illustrate what's going on: http://imgur.com/MNGOT.jpg As you see, the banner spills out past the 960px of the content and as wide as 1200px. This creates a horizontal scroll when all the content is viewable within the 960px wide viewport. I need to make this an img and not a css background because it's a jquery slideshow that fades from image to image. I think this is a big problem because a lot of people are going to get a horizontal scroll bar imposed in their browser when they're still able to see all the relevant content. How do I help her explain it. She thinks no one will notice and it'll be fine, I think it's very bad practice and confusing to the end user. Any help?

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  • How can I load txt file from internet into my jsf app?

    - by Elena
    Hi all! It's me again) I have another problem. I want to load file (for example - txt) from web. I tried to use the next code in my managed bean: public void run() { try { URL url = new URL(this.filename); URLConnection connection = url.openConnection(); bufferedReader = new BufferedReader(new InputStreamReader(connection.getInputStream())); if (bufferedReader == null) { return; } System.out.println("wwwwwwwwwwwwwwwwwwwww"); String str = bufferedReader.readLine(); System.out.println("qqqqqqqqqqqqqqqqqqqqqqq = " + str); while (bufferedReader.readLine() != null) { System.out.println("---- " + bufferedReader.readLine()); } } catch(MalformedURLException mue) { System.out.println("MalformedURLException in run() method"); mue.printStackTrace(); } catch(IOException ioe) { System.out.println("IOException in run() method"); ioe.printStackTrace(); } finally { try { bufferedReader.close(); } catch(IOException ioe) { System.out.println("UOException wile closing BufferedReader"); ioe.printStackTrace(); } } } public String doFileUpdate() { String str = FacesContext.getCurrentInstance().getExternalContext().getRequestServletPath(); System.out.println("111111111111111111111 str = " + str); str = "http://narod.ru/disk/20957166000/test.txt.html";//"http://localhost:8080/sfront/files/test.html"; System.out.println("222222222222222222222 str = " + str); FileUpdater fileUpdater = new FileUpdater(str); fileUpdater.run(); return null; } But the BufferedReader returns the html code of the current page, where i am trying to call managed bean's method. It's very strange thing - I have googled and none have had this problem. Maybe I do something wrong, maybe there us a simplest way to load file into web (jsf) app not using net API. Any ideas? Thanks very much for help! With best wishes)

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  • Facebook and retrieving a users "wall"

    - by Neurofluxation
    I have been given the dubious task of working with the Facebook API. I have managed to get quite far with all the little bits and pieces (bringing in "fan pages" and "friend lists"). However, I cannot seem to import a users wall into my App using SOLELY Javascript. I have this code to bring in the users friends: var widget_div = document.getElementById("profile_pics"); FB.ensureInit(function () { FB.Facebook.get_sessionState().waitUntilReady(function() { FB.Facebook.apiClient.friends_get(null, function(result) { var markup = ""; var num_friends = result ? Math.min(100, result.length) : 0; if (num_friends > 0) { for (var i=0; i<num_friends; i++) { markup += "<div align='left' class='commented' style='background-color: #fffbcd; border: 1px solid #9d9b80; padding: 0px 10px 0px 0px; margin-bottom: 5px; width: 75%; height: 50px; font-size: 16px;'><fb:profile-pic size='square' uid='"+result[i]+"' facebook-logo='true'></fb:profile-pic><div style='float: right; padding-top: 15px;'><fb:name uid='"+result[i]+"'></fb:name></div></div>"; } } widget_div.innerHTML = markup; FB.XFBML.Host.parseDomElement(widget_div); }); }); }); /*******YOUR FRIENDS******/ FB.XFBML.Host.parseDomTree(); Any idea whether I can change this to retrieve the Walls? Thanks in advance you great people! ^_^

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  • notify listener inside or outside inner synchronization

    - by Jary Zeels
    Hello all, I am struggling with a decision. I am writing a thread-safe library/API. Listeners can be registered, so the client is notified when something interesting happens. Which of the two implementations is most common? class MyModule { protected Listener listener; protected void somethingHappens() { synchronized(this) { ... do useful stuff ... listener.notify(); } } } or class MyModule { protected Listener listener; protected void somethingHappens() { Listener l = null; synchronized(this) { ... do useful stuff ... l = listener; } l.notify(); } } In the first implementation, the listener is notified inside the synchronization. In the second implementation, this is done outside the synchronization. I feel that the second one is advised, as it makes less room for potential deadlocks. But I am having trouble to convince myself. A downside of the second imlementation is that the client might receive 'incorrect' notifications, which happens if it accessed the module prior to the l.notify() statement. thanks a lot

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  • How do I use a modalViewController Identically in Two Controllers?

    - by Theory
    I'm using the Three20 TTMessageController in my app. I've figured out how to use it, adding on a bunch of other stuff (including TTMessageControllerDelegate methods and ABPeoplePickerNavigationControllerDelegate methods). It works great for me, after a bit of a struggle to figure it out. The trouble I'm having now is a design issue: I want to use it identically in two different places, including with the same delegate methods. My current approach is that I've put all the code into a single class inheriting from NSObject, called ComposerProxy, and I'm just having the two controllers that use it use the proxy, like so: ComposerProxy *proxy = [[ComposerProxy alloc] initWithController:this]; [proxy go]; The go method constructs the TTMessageController, configures it, adds it to a UINavigationController, and presents it: [self.controller presentModalViewController: navController animated: YES]; This works great, as I have all my code nicely encapsulated in ComposerProxy and I need only the above two lines anywhere I want to use it. The downside, though, is that I can't dealloc the proxy variable without getting crashes. I can't autorelease it, either: same problem. So I'm wondering if my proxy approach is a poor one. How does one normally encapsulate a bunch of behaviors like this without requiring a lot of duplicate code in the classes that use it? Do I need to add a delegate class to my ComposerProxy and make the controller responsible for dismissing the modal view controller in a hypothetical composerDidFinish method or some such? Many TIA!

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  • How to have a policy class implement a virtual function?

    - by dehmann
    I'm trying to design a policy-based class, where a certain interface is implemented by the policy itself, so the class derives from the policy, which itself is a template (I got this kind of thinking from Alexandrescu's book): #include <iostream> #include <vector> class TestInterface { public: virtual void test() = 0; }; class TestImpl1 { public: void test() {std::cerr << "Impl1" << std::endl;} }; template<class TestPolicy> class Foo : public TestInterface, TestPolicy { }; Then, in the main() function, I call test() on (potentially) various different objects that all implement the same interface: int main() { std::vector<TestInterface*> foos; foos.push_back(new Foo<TestImpl1>()); foos[0]->test(); delete foos[0]; return 0; } It doesn't compile, though, because the following virtual functions are pure within ‘Foo<TestImpl1>’: virtual void TestInterface::test() I thought TestInterface::test() is implemented because we derive from TestImpl1?

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  • How to access device settings on a Sony Ericsson mobile phone?

    - by TheRHCP
    Edited on April 29th Hello everyone, I recently bought a Sony Ericsson mobile phone which embeds Symbian S60 and I would like to add a missing feature myself. In fact I cannot actually disable Internet connection in an easy way when roaming, which cost me a lot of money last time I moved away ... So I would like to develop a little application that would just replace the actual Internet configuration with a fake configuration to avoid auto-connections. So what I would like to know is how can I access programmatically to my phone settings? I believe that this is possible but I do not really have a clue where to start. I know that Sony Ericsson provides a SDK to run Java applications on its customised JVM but Symbian is also providing a SDK to develop applications for S60 devices in many languages. The real questions is which SDK will provide an API able to access phone settings. This is not well documented so I am asking this question with the hope that someone here already had experience with development for Sony Ericsson/Symbian devices. Thanks. EDIT: It seems that I was totally wrong concerning my phone. This not based on any Symbian OS at all. This is pure Sony Ericsson so the only solution would be to look if Sony Ericsson extended J2ME functionality in their own JVM. I am gonna investigate on this.

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  • Add multiple entities to Javascript namespace from different files

    - by Brian M. Hunt
    Given a namespaces ns used in two different files: abc.js ns = ns || (function () { foo = function() { ... }; return { abc : foo }; }()); def.js // is this correct? ns = ns || {} ns.def = ns.def || (function () { defoo = function () { ... }; return { deFoo: defoo }; }()); Is this the proper way to add def to the ns to a namespace? In other words, how does one merge two contributions to a namespace in javascript? If abc.js comes before def.js I'd expect this to work. If def.js comes before abc.js I'd expect ns.abc to not exist because ns is defined at the time. It seems there ought to be a design pattern to eliminate any uncertainty of doing inclusions with the javascript namespace pattern. I'd appreciate thoughts and input on how best to go about this sort of 'inclusion'. Thanks for reading. Brian

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  • Advice on designing and building distributed application to track vehicles

    - by dario-g
    I'm working on application for tracking vehicles. There will be about 10k or more vehicles. Each will be sending ~250bytes in each minute. Data contains gps location and everything from CAN Bus (every data that we can read from vehicle computer and dashboard). Data are sent by GSM/GPRS (using UDP protocol). Estimated rows with this data per day is ~2000k. I see there 3 main blocks. 1. Multithreaded Socket Server (MSS) - I have it. MSS stores received data to the queue (using NServiceBus). 2. Rule Processor Server (RPS) - this is core of this system. This block is responsible for parsing received data, storing in the database, processing rules, sending messages to Notifier Server (this will be sending e-mails/sms texts). Rule example. As I said between received bytes there will be information about current speed. When speed will be above 120 then: show alert in web application for specified users, send e-mail, send sms text. (There can be more than one instance of RPS). 3. Web application - allows reporting and defining rules by users, monitoring alerts, etc. I'm looking for advice how to design communication between RPS and Web application. Some questions: - Should Web application and RPS have separated databases or one central database will be enough? I have one domain model in web application. If there will be one central database then can I use the same model (objects) on RPS? So, how to send changed rules to RPS? I try to decouple this blocks as much as possible. I'm planning to create different instance of application for each client (each client will have separated database). One client will be have 10k vehicles, others only 100 vehicles.

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  • facebook: can't send email from app to user

    - by flybywire
    I can't send email to my app users, even though I have the permissions. I am working with the java library, although I don't think it is related to that. long uid = ...; Collection<Long> uids = new ArrayList<Long>(); uids.add(uid); FacebookXmlRestClient client = new FacebookXmlRestClient(api, secret); boolean sendEmailPerm = client.users_hasAppPermission(Permission.EMAIL,uid); System.out.println("Can send email: "+ sendEmailPerm); Collection<String> sent = client.notifications_sendTextEmail(uids, "subject", "body"); System.out.println("Succesfully sent email to: "+sent); sent = client.notifications_sendFbmlEmail(uids, "subject", "body"); System.out.println("Succesfully sent email to: "+sent); I am trying both with fbml and text email. I can also obtain the user's proxied_email property but when I send email to that address with my regular mail client is doesn't arrive. The output is: Can send email: true Succesfully sent email to: [] Succesfully sent email to: []

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  • Google maps not showing satellite background - streets on white background.

    - by WooYek
    Custom map is broken on satellite view, does not show satellite imagery. Any ideas, what's wrong? Overlays are also broken - they're not transparent. Code: <div id="map_canvas" class="grid_8 omega" style="width:460px; height: 420px"></div> <script src="http://maps.google.com/maps?file=api&amp;v=3&amp;sensor=true&amp;key=my-key-is-here" type="text/javascript"></script> <script type="text/javascript"> function initialize() { if (GBrowserIsCompatible()) { var map = new GMap2(document.getElementById("map_canvas")); map.setCenter(new GLatLng(52.229676, 21.012229), 13); map.setMapType(G_HYBRID_MAP); map.setUIToDefault(); } var geocoder = new GClientGeocoder(); function showAddress(address) { geocoder.getLatLng(address, function(point) { if (!point) { alert('Nie mozna znalezc adresu: '+address); } else { map.setCenter(point, 13); var marker = new GMarker(point); map.addOverlay(marker); marker.openInfoWindowHtml(address); } } ); } showAddress('some address goes here') } $('body').ready(initialize); $('body').unload(GUnload); </script>

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  • VEMap and a GeoRSS feed(hosted separately)

    - by Alexis Abril
    The scenario is as follows: A WCF web service exists that outputs a valid GeoRSS feed. This lives in its own domain as a number of different applications have access to it. A web page(on a different site) has been created with an instance of a VEMap(Bing/Virtual Earth map object). Now, VEMap can accept an input feed in this format via the following: var layer = new VEShapeLayer(); var veLayerSpec = new VEShapeSourceSpecification(VEDataType.GeoRSS, "someurl", layer); map.ImportShapeLayerData(veLayerSpec, onComplete, true); onComplete is a callback function I'm using to replace the default pin graphic with something custom. The question is in regards to "someurl", which is a path to a local xml file containing the geographic information(georss simple format). I've realized this feed and the map must be hosted in the same domain, so I've created a generic handler that reads the remote feed and returns it in the same format. var veLayerSpec = new VEShapeSourceSpecification(VEDataType.GeoRSS, "/somelocalhandler.ashx", layer); When I do this, I get the VEMap error("z is null"). This is the same error one would receive when trying to access a remote feed. When I copy the feed into a local xml file(ie, "feed.xml") there is no error. The order of operations is currently: remote feed - local handler - VEMap import If I'm over complicating this procedure, let me know! I'm a bit new to the Bing Maps API and might have missed something. Any assistance is appreciated.

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  • SQL Concurrent test update question

    - by ptoinson
    Howdy Folks, I have a SQLServer 2008 database in which I have a table for Tags. A tag is just an id and a name. The definition of the tags table looks like: CREATE TABLE [dbo].[Tag]( [ID] [int] IDENTITY(1,1) NOT NULL, [Name] [varchar](255) NOT NULL CONSTRAINT [PK_Tag] PRIMARY KEY CLUSTERED ( [ID] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ) Name is also a unique index. further I have several processes adding data to this table at a pretty rapid rate. These processes use a stored proc that looks like: ALTER PROC [dbo].[lg_Tag_Insert] @Name varchar(255) AS DECLARE @ID int SET @ID = (select ID from Tag where Name=@Name ) if @ID is null begin INSERT Tag(Name) VALUES (@Name) RETURN SCOPE_IDENTITY() end else begin return @ID end My issues is that, other than being a novice at concurrent database design, there seems to be a race condition that is causing me to occasionally get an error that I'm trying to enter duplicate keys (Name) into the DB. The error is: Cannot insert duplicate key row in object 'dbo.Tag' with unique index 'IX_Tag_Name'. This makes sense, I'm just not sure how to fix this. If it where code I would know how to lock the right areas. SQLServer is quite a different beast. First question is what is the proper way to code this 'check, then update pattern'? It seems I need to get an exclusive lock on the row during the check, rather than a shared lock, but it's not clear to me the best way to do that. Any help in the right direction will be greatly appreciated. Thanks in advance.

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  • Modelling problem - Networked devices with commands

    - by Schneider
    I encountered a head scratching modelling problem today: We are modelling a physical control system composed of Devices and NetworkDevices. Any example of a Device is a TV. An example of a NetworkDevice is an IR transceiver with Ethernet connection. As you can see, to be able to control the TV over the internet we must connect the Device to the NetworkDevice. There is a one to many relationship between Device and NetworkDevice i.e. TV only has one NetworkDevice (the IR transceiver), but the IR transceiver may control many Devices (e.g. many TVs). So far no problem. The complicated bit is that every Device has a collection of Commands. The type of the Command (e.g IrCommand, SerialCommand - N.B. not currently modelled) depends on the type of NetworkDevice that the Device is connected to. In the current legacy system the Device has a collection of generic Commands (no typing) where fields are "interpreted" depending on the NetworkDevice type. How do I go about modelling this in OOP such that: You can only ever add a Command of the appropriate type, given the NetworkDevice the Device is attached to? If I change the NetworkDevice the Commands collection changes to the appropriate type Make it so the API is simple/elegant/intuitive to use

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