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• Operation is not valid due to the current state of the object

  - by DBa

  Sometimes, when running a Mono .NET application (it seems to not depend on the input data, as it does not change), I run into following situation: when trying to Dequeue an element from a (non-empty, I check its Count beforehand) Queue, an exception is thrown: Unhandled Exception: System.InvalidOperationException: Operation is not valid due to the current state of the object at System.Collections.Generic.Queue1[DBWorkItem].Peek ()[0x00000] at System.Collections.Generic.Queue1[DBWorkItem].Dequeue () [0x00000] at DBProcessor.process (System.Object q) [0x0006b] in <... Though I can certainly catch this exception, the element is dequeued and lost. Has anyone else encountered this behavior, too?

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• Using a JMS Session from different threads

  - by Evan

  From the javadoc for Session it states: A Session object is a single-threaded context for producing and consuming messages. So I understand that you shouldn't use a Session object from two different threads at the same time. What I'm unclear on is if you could use the Session object (or children such as a Queue) from a different thread than the one it created. In the case I'm working on, I'm considering putting my Session objects into a pool of available sessions that any thread could borrow from, use, and return to the pool when it is finished with it. Is this kosher? (Using ActiveMQ BTW, if that impacts the answer at all.)

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• Porting Java app to Go - any advice?

  - by Devrim

  We want to rewrite kodingen.com backend with Go which currently is Java, running as daemon using jsvc. I have never touched any C in my life, am only experienced in Java so I don't know if this is something that I should even start. However, task is pretty simple read shell commands from mysql database queue and execute them in parallel save each shell output to the database that's it. So these simple requirements gives me hope that I can start using this wonderful language. What would you advise?

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• Anyone up to creating a tomcat based alternative for GAE?

  - by bach

  Hi, If we had the possibility to run GAE app without any code change on our servlet engine that would be great because: in case that google changes their billing policy we can just jump to our own server or in case their current policy doesn't fit our app needs we can do stuff which is not allowed in the GAE, compromising a 1 JVM, 1 DB We don't actually need a distributed system but more of a realtime system with synchronize, true locking mechanisms, other servers/software installed on the server machine, socket interface etc... Such a package should include at least: TomCat (or equivalent) DataNucleus Access Platform (Task Queue service) Any idea if it's easy to get such a thing or if it's already exist somewhere? Thanks

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• Python PyQT4 - Adding an unknown number of QComboBox widgets to QGridLayout

  - by ZZ

  Hi all, I want to retrieve a list of people's names from a queue and, for each person, place a checkbox with their name to a QGridLayout using the addWidget() function. I can successfully place the items in a QListView, but they just write over the top of each other rather than creating a new row. Does anyone have any thoughts on how I could fix this? self.chk_People = QtGui.QListView() items = self.jobQueue.getPeopleOffQueue() for item in items: QtGui.QCheckBox('%s' % item, self.chk_People) self.jobQueue.getPeopleOffQueue() would return something like ['Bob', 'Sally', 'Jimmy'] if that helps.

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• Artificial Intelligence in online game using Google App Engine

  - by Hortinstein

  I am currently in the planning stages of a game for google app engine, but cannot wrap my head around how I am going to handle AI. I intend to have persistant NPCs that will move about the map, but short of writing a program that generates the same XML requests I use to control player actions, than run it on another server I am stuck on how to do it. I have looked at the Task Queue feature, but due to long running processes not being an option on the App engine, I am a little stuck. I intend to run multiple server instances with 200+ persistant NPC entities that I will need to update. Most action is slowly roaming around based on player movements/concentrations, and attacking close range players...(you can probably guess the type of game im developing)

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• What's the purpose of GC.SuppressFinalize(this) in Dispose() method?

  - by mr.b

  I have code that looks like this: /// <summary> /// Dispose of the instance /// </summary> public void Dispose() { if (_instance != null) { _instance = null; // Call GC.SupressFinalize to take this object off the finalization // queue and prevent finalization code for this object from // executing a second time. GC.SuppressFinalize(this); } } Although there is a comment that explains purpose of that GC-related call, I still don't understand why it's there. Isn't object destined for garbage collection once all instances cease from existence (like, when used in using() block)? What's the use case scenario where this would play important role? Thanks!

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• Is there a FSRef in iPhone SDK or is there something that can be FSRef's alternative?

  - by unknownthreat

  The question may sound stupid, but the thing is this: I am learning how to use a Audio Queue, and the example I've taken (aqtest) has been a nice guide for me until I recently found out that aqtest is not for iPhone. (stupid me) I served around the Internet and found out that there is no FSRef for iPhone. If possible, I want to find a way to work around FSRef thinggie. So here comes the question: can I use something else instead of FSRef that exists on the iPhone SDK? Or am I missing something?

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• What is the best way for communication between cluster nodes

  - by Tom

  I have an application written in a combination of ASP/VB6/VBScript and ASP.NET/C# that consists of a website part, SOAP-like webservice part and a queue processing part processing incoming files in a hotfolder. We are used to running under load balancers (Microsoft or other make). Often we need to communicate between the different load balanced servers. Currently we do this through the SQL Server database that is common for all nodes, however, this comes with a performance penalty as each message requires a transaction and continual polling from the other nodes. What would be better ways to achieve this? Tom, Appelby

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• List of generic algorithms and data structures

  - by Jake Petroules

  As part of a library project, I want to include a plethora of generic algorithms and data structures. This includes algorithms for searching and sorting, data structures like linked lists and binary trees, path-finding algorithms like A*... the works. Basically, any generic algorithm or data structure you can think of that you think might be useful in such a library, please post or add it to the list. Thanks! (NOTE: Because there is no single right answer I've of course placed this in community wiki... and also, please don't suggest algorithms which are too specialized to be provided by a generic library) The List: Data structures AVL tree B-tree B*-tree B+-tree Binary tree Binary heap Binary search tree Linked lists Singly linked list Doubly linked list Stack Queue Sorting algorithms Binary tree sort Bubble sort Heapsort Insertion sort Merge sort Quicksort Selection sort Searching algorithms

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• Python threads all executing on a single core

  - by Rob Lourens

  I have a Python program that spawns many threads, runs 4 at a time, and each performs an expensive operation. Pseudocode: for object in list: t = Thread(target=process, args=(object)) # if fewer than 4 threads are currently running, t.start(). Otherwise, add t to queue But when the program is run, Activity Monitor in OS X shows that 1 of the 4 logical cores is at 100% and the others are at nearly 0. Obviously I can't force the OS to do anything but I've never had to pay attention to performance in multi-threaded code like this before so I was wondering if I'm just missing or misunderstanding something. Thanks.

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• Plupload - how to stop uploads individualy

  - by Manny Calavera

  Hello. I'm using plupload script with PHP and I am uploading using HTML5. I would like to stop uploads separately, from the list, not the whole process. Is this possible with plupload ? I think it would be useful to stop individual files. Thanks. EDIT: Let's just say that a user tries to upload 2 files and they are added to queue. After that, he clicks the upload button and the upload starts. At some point during the upload he realize that he doesn't really want to upload the second file, he needs to stop the 2nd upload and the 1st upload should continue. I can send an uploader.start(); on the plupload but it will stop all upload processes...and I need to stop just one. Thanks.

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• LINQ-to-SQL and SQL Compact - database file sharing problem

  - by Eye of Hell

  Hello. I'm learing LINQ-to-SQL right now and i have wrote a simple application that define SQL data: [Table( Name = "items" )] public class Item { [ Column( IsPrimaryKey = true, IsDbGenerated = true ) ] public int Id; [ Column ] public string Name; } I have launched 2 copy of application connected to the same .sdf file and tested if all database modifications in one application affects another application. But strange thing arise. If i use InsertOnSubmit() and DeleteOnSubmit() in one application, added/removed items are instantly visible in other application via 'select' LINQ queue. But if i try to modify 'Name' field in one application, it is NOT visible in other applicaton until it reconnects the database :(. The test code i use: var Items = from c in db.Items where Id == c.Id select c; foreach( var Item in Items ) { Item.Name = "new name"; break; } db.SubmitChanges(); Can anyone suggest what i'm doing wrong and why InsertOnSubmit()/DeleteOnSubmit works and SubmitChanges() don't?

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• ActiveMQ & Camel - How to create dependency in routing paths

  - by CodeMedic

  I have a message routing to implement, which has routes that vary according to the message content. Some strands of the route are dependent on other. If for example I have Data_A which has Task_A and Task_B to be performed on it. Whereas Data_B has only Task_B to be performed on it. Here each Task has a queue served by consumers. If Task_A should be performed only after Task_B if Task_B is requested on the data, how do I set-up such dependencies?

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• Drupal - editing tab options for a content type not showing up

  - by easement

  I'm working on a D5 site and a lot of the content types have the menus at the top of the edit page. Examples: Overview | Grants | Dev load | Dev render | Edit | Revisions | Track | Workflow Overview | Dev load | Dev render | Edit | Revisions | Track | Workflow | Node queue | RelatedContent I'm not sure where these tabs come from, but I need to enable the menu for a content type that doesn't have them visible. I checked the workflow setting and the content type in question was specified to use workflow. So there should be at least another tab in addition to edit. The end goal is to have revisions show up in the menu, but I need to get the menu showing first. Any ideas where to start looking?

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• How to implement buffering with timeout in RX

  - by Gaspar Nagy

  I need to implement an event processing, that is done delayed when there are no new events arriving for a certain period. (I have to queue up a parsing task when the text buffer changed, but I don't want to start the parsing when the user is still typing.) I'm new in RX, but as far as I see, I would need a combination of BufferWithTime and the Timeout methods. I imagine this to be working like this: it buffers the events until they are received regularly within a specified time period between the subsequent events. If there is a gap in the event flow (longer than the timespan) it should return propagate the events buffered so far. Having a look at how Buffer and Timeout is implemented, I could probably implement my BufferWithTimeout method (if everyone have one, please share with me), but I wonder if this can be achieved just by combining the existing methods. Any ideas?

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• CollectionChanged notification across threads?

  - by Mark

  I'm writing a download manager using C#/WPF, and I just encountered this error: This type of CollectionView does not support changes to its SourceCollection from a thread different from the Dispatcher thread. The basic flow of my program is that a few web pages/downloads are enqueued at the start, and then they're downloaded asynchronously. When an HTML page has completed downloading, I parse it and look for more stuff to download, then enqueue it directly from within the worker thread. I get that error when trying to send out the CollectionChanged event on my customized queue class. However, I need to fire that event so that the GUI can get updated. What are my options?

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• Can you dynamically resize a java.util.concurrent.ThreadPoolExecutor while it still has tasks waitin

  - by Edward Shtern

  I'm working with a java.util.concurrent.ThreadPoolExecutor to process a number of items in parallel. Although the threading itself works fine, at times we've run into other resource constraints due to actions happening in the threads, which made us want to dial down the number of Threads in the pool. I'd like to know if there's a way to dial down the number of the threads while the threads area actually working. I know that you can call setMaximumPoolSize() and/or setCorePoolSize(), but these only resize the pool once threads become idle, but they don't become idle until there are no tasks waiting in the queue.

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• What should the Java main method be for a standalone application (for Spring JMS) ?

  - by Brandon

  I am interested in creating a Spring standalone application that will run and wait to receive messages from an ActiveMQ queue using Spring JMS. I have searched a lot of places and cannot find a consistent way of implementing the main method for such a standalone application. There appears to be few examples of Spring standalone applications. I have looked at Tomcat, JBoss, ActiveMQ and other examples from the around the web but I have not come to a conclusion so ... What is the best practice for implementing a main method for a Java application (specifically Spring with JMS) ?

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• Buffer management for socket application best practice

  - by Poni

  Having a Windows IOCP app............ I understand that for async i/o operation (on network) the buffer must remain valid for the duration of the send/read operation. So for each connection I have one buffer for the reading. For sending I use buffers to which I copy the data to be sent. When the sending operation completes I release the buffer so it can be reused. So far it's nice and not of a big issue. What remains unclear is how do you guys do this? Another thing is that even when having things this way, I mean multi-buffers, the receiver side might be flooded (talking from experience) with data. Even setting SO_RCVBUF to 25MB didn't help in my testings. So what should I do? Have a to-be-sent queue?

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• Delay function from running for n seconds then run it once. (2minute question)

  - by Ozaki

  TLDR I have a function that runs on the end of a pan in an openlayers map. Don't want it to fire continously. I have a function that runs on the end of panning a map. I want it so that it will not fire the function until say 3 secconds after the pan has finished. Although I don't want to queue up the function to fire 10 or so times like setTimeout is currently doing. How can I delay a function from running for n seconds then run it only once no matter how many times it has been called?

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• Inheritance classes in Scheme

  - by DreamWalker

  Now I research OOP-part of Scheme. I can define class in Scheme like this: (define (create-queue) (let ((mpty #t) (the-list '())) (define (enque value) (set! the-list (append the-list (list value))) (set! mpty #f) the-list) (define (deque) (set! the-list (cdr the-list)) (if (= (length the-list) 0) (set! mpty #t)) the-list) (define (isEmpty) mpty) (define (ptl) the-list) (define (dispatch method) (cond ((eq? method 'enque) enque) ((eq? method 'deque) deque) ((eq? method 'isEmpty) isEmpty) ((eq? method 'print) ptl))) dispatch)) (Example from css.freetonik.com) Can I implement class inheritance in Scheme?

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• How to make a thread try to reconnect to the Database x times using JDBCTemplate

  - by gillJ

  Hi, I have a single thread trying to connect to a database using JDBCTemplate as follows: JDBCTemplate jdbcTemplate = new JdbcTemplate(dataSource); try{ jdbcTemplate.execute(new CallableStatementCreator() { @Override public CallableStatement createCallableStatement(Connection con) throws SQLException { return con.prepareCall(query); } }, new CallableStatementCallback() { @Override public Object doInCallableStatement(CallableStatement cs) throws SQLException { cs.setString(1, subscriberID); cs.execute(); return null; } }); } catch (DataAccessException dae) { throw new CougarFrameworkException( "Problem removing subscriber from events queue: " + subscriberID, dae); } I want to make sure that if the above code throws DataAccessException or SQLException, the thread waits a few seconds and tries to re-connect, say 5 more times and then gives up. How can I achieve this? Also, if during execution the database goes down and comes up again, how can i ensure that my program recovers from this and continues running instead of throwing an exception and exiting?

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• ArrayBlockingQueue - How to "interrupt" a thread that is wating on .take() method

  - by bernhard

  I use an ArrayBlockingQueue in my code. Clients will wait untill an element becomes available: myBlockingQueue.take(); How can I "shutdown" my service in case no elements are present in the queue and the take() ist wating indefenitely for an element to become available? This method throws an InterruptedException. My question is, how can I "evoke" an Interrupted Exception so that take() will quit? (I also tought about notify(), but it seems I doesnt help here..) I know I could insert an special "EOF/QUIT" marker Element but is this really the only solution? UPDATE (regarding the comment, that points to another question with two solutions: one mentioned above using a "Poisoning Pill Object" and the second one is Thread.interrupt(): The myBlockingQueue.take() is used NOT in a Thread (extending Thread) but rather implements Runnable. It seems a Runnable does not provide the .interrupt() method? How could I interrupt the Runnable? Million Thanks Bernhard

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• How to limit the number of the same Activity on the stack for an Android application

  - by johnrock

  Is this possible in an Android app? I want to make it so that no matter how many times a user starts activityA, when they hit the back button they will never get more than one occurence of activityA. What I am finding in my current code is that I have only two options: 1. I can call finish() in activityA which will prevent it from being accessible via the back button completely, or 2. I do not call finish(), and then if the user starts activityA (n) times during their usage, there will be (n) instances when hitting the back button. Again, I want to have activityA accessible by hitting the back button, but there is no reason to keep multiple instances of the same activity on the stack. Is there a way to limit the number of instances of an activity in the queue to only 1?

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