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  • Replacing objects, handling clones, dealing with write logs

    - by Alix
    Hi everyone, I'm dealing with a problem I can't figure out how to solve, and I'd love to hear some suggestions. [NOTE: I realise I'm asking several questions; however, answers need to take into account all of the issues, so I cannot split this into several questions] Here's the deal: I'm implementing a system that underlies user applications and that protect shared objects from concurrent accesses. The application programmer (whose application will run on top of my system) defines such shared objects like this: public class MyAtomicObject { // These are just examples of fields you may want to have in your class. public virtual int x { get; set; } public virtual List<int> list { get; set; } public virtual MyClassA objA { get; set; } public virtual MyClassB objB { get; set; } } As you can see they declare the fields of their class as auto-generated properties (auto-generated means they don't need to implement get and set). This is so that I can go in and extend their class and implement each get and set myself in order to handle possible concurrent accesses, etc. This is all well and good, but now it starts to get ugly: the application threads run transactions, like this: The thread signals it's starting a transaction. This means we now need to monitor its accesses to the fields of the atomic objects. The thread runs its code, possibly accessing fields for reading or writing. If there are accesses for writing, we'll hide them from the other transactions (other threads), and only make them visible in step 3. This is because the transaction may fail and have to roll back (undo) its updates, and in that case we don't want other threads to see its "dirty" data. The thread signals it wants to commit the transaction. If the commit is successful, the updates it made will now become visible to everyone else. Otherwise, the transaction will abort, the updates will remain invisible, and no one will ever know the transaction was there. So basically the concept of transaction is a series of accesses that appear to have happened atomically, that is, all at the same time, in the same instant, which would be the moment of successful commit. (This is as opposed to its updates becoming visible as it makes them) In order to hide the write accesses in step 2, I clone the accessed field (let's say it's the field list) and put it in the transaction's write log. After that, any time the transaction accesses list, it will actually be accessing the clone in its write log, and not the global copy everyone else sees. Like this, any changes it makes will be done to the (invisible) clone, not to the global copy. If in step 3 the commit is successful, the transaction should replace the global copy with the updated list it has in its write log, and then the changes become visible for everyone else at once. It would be something like this: myAtomicObject.list = updatedCloneOfListInTheWriteLog; Problem #1: possible references to the list. Let's say someone puts a reference to the global list in a dictionary. When I do... myAtomicObject.list = updatedCloneOfListInTheWriteLog; ...I'm just replacing the reference in the field list, but not the real object (I'm not overwriting the data), so in the dictionary we'll still have a reference to the old version of the list. A possible solution would be to overwrite the data (in the case of a list, empty the global list and add all the elements of the clone). More generically, I would need to copy the fields of one list to the other. I can do this with reflection, but that's not very pretty. Is there any other way to do it? Problem #2: even if problem #1 is solved, I still have a similar problem with the clone: the application programmer doesn't know I'm giving him a clone and not the global copy. What if he puts the clone in a dictionary? Then at commit there will be some references to the global copy and some to the clone, when in truth they should all point to the same object. I thought about providing a wrapper object that contains both the cloned list and a pointer to the global copy, but the programmer doesn't know about this wrapper, so they're not going to use the pointer at all. The wrapper would be like this: public class Wrapper<T> : T { // This would be the pointer to the global copy. The local data is contained in whatever fields the wrapper inherits from T. private T thisPtr; } I do need this wrapper for comparisons: if I have a dictionary that has an entry with the global copy as key, if I look it up with the clone, like this: dictionary[updatedCloneOfListInTheWriteLog] I need it to return the entry, that is, to think that updatedCloneOfListInTheWriteLog and the global copy are the same thing. For this, I can just override Equals, GetHashCode, operator== and operator!=, no problem. However I still don't know how to solve the case in which the programmer unknowingly inserts a reference to the clone in a dictionary. Problem #3: the wrapper must extend the class of the object it wraps (if it's wrapping MyClassA, it must extend MyClassA) so that it's accepted wherever an object of that class (MyClass) would be accepted. However, that class (MyClassA) may be final. This is pretty horrible :$. Any suggestions? I don't need to use a wrapper, anything you can think of is fine. What I cannot change is the write log (I need to have a write log) and the fact that the programmer doesn't know about the clone. I hope I've made some sense. Feel free to ask for more info if something needs some clearing up. Thanks so much!

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  • How do you fix "Too many open files" problem in Hudson?

    - by Randyaa
    We use Hudson as a continuous integration system to execute automated builds (nightly and based on CVS polling) of a lot of our projects. Some projects poll CVS every 15 minutes, some others poll every 5 minutes and some poll every hour. Every few weeks we'll get a build that fails with the following output: FATAL: java.io.IOException: Too many open files java.io.IOException: java.io.IOException: Too many open files at java.lang.UNIXProcess.<init>(UNIXProcess.java:148) The next build always worked (with 0 changes) so we always chalked it up to 2 build jobs being run at the same time and happening to have too many files open during the process. This weekend we had a build fail Friday night (automatic nightly build) with the message and every other nightly build also failed. Somehow this triggered Hudson to continuously build every project which failed until the issue was resolved. This resulted in a build every 30 minutes or so of every project until sometime Saturday night when the issue magically disappeared.

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  • Several client waiting for the same event

    - by ff8mania
    I'm developing a communication API to be used by a lot of generic clients to communicate with a proprietary system. This proprietary system exposes an API, and I use a particular classes to send and wait messages from this system: obviously the system alert me that a message is ready using an event. The event is named OnMessageArrived. My idea is to expose a simple SendSyncMessage(message) method that helps the user/client to simply send a message and the method returns the response. The client: using ( Communicator c = new Communicator() ) { response = c.SendSync(message); } The communicator class is done in this way: public class Communicator : IDisposable { // Proprietary system object ExternalSystem c; String currentRespone; Guid currentGUID; private readonly ManualResetEvent _manualResetEvent; private ManualResetEvent _manualResetEvent2; String systemName = "system"; String ServerName = "server"; public Communicator() { _manualResetEvent = new ManualResetEvent(false); //This methods are from the proprietary system API c = SystemInstance.CreateInstance(); c.Connect(systemName , ServerName); } private void ConnectionStarter( object data ) { c.OnMessageArrivedEvent += c_OnMessageArrivedEvent; _manualResetEvent.WaitOne(); c.OnMessageArrivedEvent-= c_OnMessageArrivedEvent; } public String SendSync( String Message ) { Thread _internalThread = new Thread(ConnectionStarter); _internalThread.Start(c); _manualResetEvent2 = new ManualResetEvent(false); String toRet; int messageID; currentGUID = Guid.NewGuid(); c.SendMessage(Message, "Request", currentGUID.ToString()); _manualResetEvent2.WaitOne(); toRet = currentRespone; return toRet; } void c_OnMessageArrivedEvent( int Id, string root, string guid, int TimeOut, out int ReturnCode ) { if ( !guid.Equals(currentGUID.ToString()) ) { _manualResetEvent2.Set(); ReturnCode = 0; return; } object newMessage; c.FetchMessage(Id, 7, out newMessage); currentRespone = newMessage.ToString(); ReturnCode = 0; _manualResetEvent2.Set(); } } I'm really noob in using waithandle, but my idea was to create an instance that sends the message and waits for an event. As soon as the event arrived, checks if the message is the one I expect (checking the unique guid), otherwise continues to wait for the next event. This because could be (and usually is in this way) a lot of clients working concurrently, and I want them to work parallel. As I implemented my stuff, at the moment if I run client 1, client 2 and client 3, client 2 starts sending message as soon as client 1 has finished, and client 3 as client 2 has finished: not what I'm trying to do. Can you help me to fix my code and get my target? Thanks!

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  • XPath expression exception

    - by fftoolbar
    Hi I am not really sure if i am doing this right with XPath expression. I am trying to locate a text on DOM and this text is assingned to a variable. I have text stored in SQLite and i have retrievd the text and i am trying to locate it on the webpage which actually contains the text. so i ahve the following code: var searchText = dataset[x]['selectedText']; alert(dataset[x]['selectedText']); var res = googbar_frames[0].contentDocument.evaluate("//*[.=searchText]",googbar_frames[0].contentDocument.body,null,XPathResult.ANY_TYPE,null); alert(res.snapshotLength); And i get the following error. Error: Permission denied for <http://en.wikipedia.org> to call method XPathException.toString on <>. Error: Permission denied for <http://en.wikipedia.org> to call method XPathException.toString on <>. Have got the expression correct. I am trying to look for the text on DOM. Or am i going wrong somwehere? cheers

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  • c source code to remove subset transactions form text file

    - by user324887
    I have a file containing data as follows 10 20 30 40 70 20 30 70 30 40 10 20 29 70 80 90 20 30 40 40 45 65 10 20 80 45 65 20 I want to remove all subset transaction from this file. output file should be like follows 10 20 30 40 70 29 70 80 90 20 30 40 40 45 65 10 20 80 Where records like 20 30 70 30 40 10 20 45 65 20 are removed because of they are subset of other records.

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  • Handling multiple async HTTP requests in Silverlight serially

    - by Jeb
    Due to the async nature of http access via WebClient or HttpWebRequest in Silverlight 4, when I want to do multiple http get/posts serially, I find myself writing code that looks like this: doFirstGet(someParams, () => { doSecondGet(someParams, () => { doThirdGet(... } }); Or something similar. I'll end up nesting subsequent calls within callbacks usually implemented using lambdas of some sort. Even if I break things out into Actions or separate methods, it still ends up being hard to read. Does anyone have a clean solution to executing multiple http requests in SL 4 serially? I don't need things to be synchronous, I just want serial execution.

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  • ASP.Net Treeview - Client side event handling (jQuery??)

    - by The Great Gonzo
    I have a Treeview (bog standard ASP.Net Treeview) that is bound to an Xml source which allows the user to navigate to various parts of our system. Nothing special.... However, I wanted to generate a breadcrumb for use in the main content area on the client side when the user clicks a node in the Treeview control. I know there are mechanisms available for breadcrumb generation (such as site maps) but for various reasons I need to do it when a node is clicked. So far hooking up a click event using the document ready function available as part of jQuery is easy. However, having spent sometime looking I can not see how I navigate back up the tree nodes to get to each parent nodes text value to build the breadcrumb. I have been playing with .parent(), .parents() and .closest() available via jQuery but don't seem to be getting anywhere. Has anyone done anything like this or can anyone provide a better method? Thanks in advance....

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  • Selecting items from events, as evenly spaced as possible

    - by Matt Warren
    If I have an event that happens 15 times a second (numbered 1 - 15), but I only want to process it 3 times I can choose [1], [6] and [11],. It's important that the events I process are as evenly spaced as possible and take into account wrap-around, i.e. the events are continuous 13, 14, 15, 1, 2, 3 etc. If I want 4 items the best I can do is [1], [5], [9] & [13]. Is there a general algorithm that will calculate which events I need to process given the total number of events (total) and the number to process (processAmount).

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  • download file exception handling

    - by klaus-vlad
    Hi, In my application I download several critical files from a server, and I want to write some code that handles the case where the a file download didn't complete for a reason or other ,to retry downloading it at next startup. The function that downloads a file at a time however throws only MalformedURLException and IOException , but if these exceptions are thrown that means that the download didn't even begin. How should I arrange things so I can treat the case where a download failed , even if it began ? download(String file) throws MalformedURLException ,IOException { }

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  • Does SQL Server 2005 error message numbers back to the asp.net application?

    - by Duke
    I'd like to get the message number and severity level information from SQL Server upon execution of an erroneous query. For example, when a user attempts to delete a row being referenced by another record, and the cascade relationship is "no action", I'd like the application to be able to check for error message 547 ("The DELETE statement conflicted with the REFERENCE constraint...") and return a user friendly and localized message to the user. When running such a query directly on SQL Server, the following message is printed: Msg 547, Level 16, State 0, Line 1 <Error message...> In an Asp.Net app is this information available in an event handler parameter or elsewhere? Also, I don't suppose anyone knows where I can find a definitive reference of SQL Server message numbers?

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  • ASP.NET MVC - How to Preserve ModelState Errors Across RedirectToAction?

    - by RPM1984
    Hi Guys, I have the following two action methods (simplified for question): [HttpGet] public ActionResult Create(string uniqueUri) { // get some stuff based on uniqueuri, set in ViewData. return View(); } [HttpPost] public ActionResult Create(Review review) { // validate review if (validatedOk) { return RedirectToAction("Details", new { postId = review.PostId}); } else { ModelState.AddModelError("ReviewErrors", "some error occured"); return RedirectToAction("Create", new { uniqueUri = Request.RequestContext.RouteData.Values["uniqueUri"]}); } } So, if the validation passes, i redirect to another page (confirmation). If an error occurs, i need to display the same page with the error. If i do return View(), the error is displayed, but if i do return RedirectToAction (as above), it loses the Model errors. I'm not surprised by the issue, just wondering how you guys handle this? I could of course just return the same View instead of the redirect, but i have logic in the "Create" method which populates the view data, which i'd have to duplicate. Any suggestions?

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  • Show friendly message on ASP.NET Ajax error

    - by balexandre
    You all know how annoying is this: I do have a log system and the correct error is well explicit there, but I want to give a better message to the user. I keep trying several ways but I'm using Telerik components and well jQuery and I ended up using both ASP.NET Ajax methods and jQuery, so I use function pageLoad() { try { var manager = Sys.WebForms.PageRequestManager.getInstance(); manager.add_endRequest(endRequest); manager.add_beginRequest(OnBeginRequest); manager } catch (err) { alert(err); } } as well $(document).ready(function() { ... } that alert(err) is never fired even upon OnClick events what's the best approach to avoid this message errors and provide a cleaner way? all this happens in <asp:UpdatePanel> as I use that when I didn't know better (3 years ago!) and I really don't want to mess up and build all again from scratch :( Any help is greatly appreciated Updated with more error windows after volpav solution

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  • Magento: Why do controller action predispatch events not fire if the controller is rewritten?

    - by mattalexx
    Why do controller action predispatch events not fire if the controller is rewritten? Here is a snippet of store/app/code/core/Mage/Core/Controller/Varien/Action.php: abstract class Mage_Core_Controller_Varien_Action { // [...] public function preDispatch() { // [...] if ($this->_rewrite()) { return; // [What is the purpose if this?] } // [...] // [This is where my event needs to be firing, but this code never gets // executed because the controller is rewritten] Mage::dispatchEvent( 'controller_action_predispatch_'.$this->getFullActionName(), array('controller_action'=>$this) ); } // [...] } I don't know where to start fixing this problem. Anyone out there ever dealt with this before?

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  • ArgumentException or ArgumentNullException for string parameters?

    - by Anna Lear
    Far as best practices are concerned, which is better: public void SomeMethod(string str) { if(string.IsNullOrEmpty(str)) { throw new ArgumentException("str cannot be null or empty."); } // do other stuff } or public void SomeMethod(string str) { if(str == null) { throw new ArgumentNullException("str"); } if(str == string.Empty) { throw new ArgumentException("str cannot be empty."); } // do other stuff } The second version seems more precise, but also more cumbersome than the first. I usually go with #1, but figured I'd check if there's an argument to be made for #2.

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  • handling activity destruction in multithreaded android app

    - by Jayesh
    Hi, I have a multithreded app where background threads are used to load data over network or from disk/db. Every once in a while user will perform some action e.g. fetch news over network, which will spawn a background AsyncTask, but for some reason user will quit the app (press back button so that activity gets destroyed). In most such scenarios, I make appropriate checks in the background thread after it returns from n/w i/o, so that it won't crash by accessing members of the activity that is destroyed by now. However some corner cases are left where crashes happen, because the background thread would access some member of activity that is now null. Do other Android developers have some generic/recommended framework to handle such scenarios? These are the times when I wish android would have guaranteed termination of all threads when activity destroys (in the same way that regular linux process cleans up when it's quit)... but I guess Android devs had good reasons for not exposing process lifetimes through the api.

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  • Handling close-to-impossible collisions on should-be-unique values

    - by balpha
    There are many systems that depend on the uniqueness of some particular value. Anything that uses GUIDs comes to mind (eg. the Windows registry or other databases), but also things that create a hash from an object to identify it and thus need this hash to be unique. A hash table usually doesn't mind if two objects have the same hash because the hashing is just used to break down the objects into categories, so that on lookup, not all objects in the table, but only those objects in the same category (bucket) have to be compared for identity to the searched object. Other implementations however (seem to) depend on the uniqueness. My example (that's what lead me to asking this) is Mercurial's revision IDs. An entry on the Mercurial mailing list correctly states The odds of the changeset hash colliding by accident in your first billion commits is basically zero. But we will notice if it happens. And you'll get to be famous as the guy who broke SHA1 by accident. But even the tiniest probability doesn't mean impossible. Now, I don't want an explanation of why it's totally okay to rely on the uniqueness (this has been discussed here for example). This is very clear to me. Rather, I'd like to know (maybe by means of examples from your own work): Are there any best practices as to covering these improbable cases anyway? Should they be ignored, because it's more likely that particularly strong solar winds lead to faulty hard disk reads? Should they at least be tested for, if only to fail with a "I give up, you have done the impossible" message to the user? Or should even these cases get handled gracefully? For me, especially the following are interesting, although they are somewhat touchy-feely: If you don't handle these cases, what do you do against gut feelings that don't listen to probabilities? If you do handle them, how do you justify this work (to yourself and others), considering there are more probable cases you don't handle, like a supernonva?

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  • PHP: Handling Multiple Submit Buttons

    - by sebb
    Would like to get a consensus as to what the best practice is in this scenario: Muliple submit buttons, is it better to handle this by having separate FORMS for each one of the submits, OR is it okay to have one form and check which button was pressed? thank you for your input :D

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  • IO Exception: directory name is invalid C#

    - by Bi
    My C# application throws a System.IO.IOExcepton (The directory name is invalid) for the following code for implementing a filewatcher: public void OnChanged(object source, FileSystemEventArgs e) { DirectoryInfo dList = new DirectoryInfo(e.FullPath); FileInfo[] TxtFiles = dList.GetFiles("*.TXT"); } e.FullPath is "C:/Documents and Settings/Bi/Application Data/TestApp/Reports\\0MA01P62240_000005798_TRI_4947712701738551.TXT". If you notice it seems to append a "\\" to the path when it tracks the file. Any idea what the problem may be? Thanks

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  • Can i access outer class objects in inner class

    - by Shantanu Gupta
    I have three classes like this. class A { public class innerB { //Do something } public class innerC { //trying to access objB here directly or indirectly over here. //I dont have to create an object of innerB, but to access the object created by A //i.e. innerB objInnerB = objB; //not like this innerB objInnerB= new innerB(); } public innerB objB{get;set;} } I want to access the object of class B in Class C that is created by class A. Is it possible somehow to make changes on object of Class A in Class C. Can i get Class A's object by creating event or anyhow.

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  • Handling Input with Zend Framework outside MVC

    - by Pekka
    In a classic Zend Framework MVC setup, there seems to be access to a generic _request object from within the model/view/controller instance as outlined here: $this->_request->getPost('variablename'); is this request object somehow available in a non-MVC setup as well? If yes: how would I initialize and access it?

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  • What exactly does it mean when $_FILES is empty?

    - by Mike
    I am working on an upload script and when testing my error checks, I attempted to upload a 17MB TIFF file. When I do this the $_FILES array is empty. The script works fine for what I need it to do, which is to upload JPEG files. My solution is to test if $_FILES is empty or not before continuing with the upload script. Can anybody explain why $_FILES is empty when a TIFF is attempted to be uploaded? Is my solution, to check if $_FILES is empty or not, an okay one?

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  • problem in handling menu - submenu based on spring security

    - by Nirmal
    Hi All... I have configured spring security core plugin using requestmap table inside the database.. Now inside requestmap table I have all the possible urls and it's equivalent roles who can access that url... Now I want to generate menus and submenus based on the urls stored in requestmap table... So my requirement is to check the urls of menu & submenus against the logged in users privileges... And if logged in user has any one privilege then I need to display that main menu and the available submenus.... For e.g. I have a menu in my project called user which has a following submenus : **Users (main menu)** Manage Users (sub menu) Import Users (sub menu) Now inside my header.gsp I have successfully achieved the above requirement using if else condition, like : if ( privs.contains("/users/manageUsers") || privs.contains("/users/importUsers")) here privs are the list of urls from requestmap table for logged in user. But I want to achieve these using spring security tag lib, so for comparing urls I have find following tag from spring security core documentation : <sec:access url="/users/manageUsers"> But i am bit confuse that how I can replace or condition using tag library.. Is there any tag available which checks from multiple urls and evaluate it to true or false ? Of course I can do using sec:access tag with some flag logic, but is there any tags available which can fulfill my requirement directly ? Thanks in advance...

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