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  • MySQL with Java: Open connection only if possible

    - by emempe
    I'm running a database-heavy Java application on a cluster, using Connector/J 5.1.14. Therefore, I have up to 150 concurrent tasks accessing the same MySQL database. I get the following error: Exception in thread "main" com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Too many connections This happens because the server can't handle so many connections. I can't change anything on the database server. So my question is: Can I check if a connection is possible BEFORE I actually connect to the database? Something like this (pseudo code): check database for open connection slots if (slot is free) { Connection cn = DriverManager.getConnection(url, username, password); } else { wait ... } Cheers

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  • JPA in distributed Java EE configuration

    - by sof
    Hello, I'm developing a JEE application to run on Glassfish: Database (javaDB, MS SQL, MySQL or Oracle) EJB layer with JPA (Toplink essentials - from Glassfish) for database access JSF/Icefaces based web UI accessing the EJB layer The application will have a lot of concurrent web client, so I want to run it on different physical servers and use a load-balancer. My problem is now how to keep the applications synchronized. I intend to set up multiple servers, each running Glassfish with my EAR app installed. Whenever on one of the servers data is added to or removed from the database (via JPA, no direct SQL queries), this change should be reflected in the JPA layer on the other servers. I've been looking around for solutions to this, but couldn't find anything I really like (the full Toplink from Oracle claims to have a solution, but don't know). Doing a refresh before every access to a JPA entity could work, but is far from efficient. Are there any patterns, libraries, ... that could help here? Thanks a lot!

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  • how to pass session variable to model in RoR?

    - by siulamvictor
    I used a global variable in my app for passing information before. But I got a problem and thanks everyone here suggested me to store those data in session with database. I tried, but I found that I can't access the session variable in Model. I googled and knew this is the Model normal behavior, RoR won't pass the session variable to Model. So how to pass the value of the session variable into Models? or is there any other method for my use case? I need a variable storing a value, which is required in all MVCs, and should be independent between different concurrent users. Thanks everyone. :)

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  • Understanding Clojure concurrency example

    - by dusha
    Hello, I just go through various documentation on Clojure concurrency and came accross the example on the website (http://clojure.org/concurrent_programming). (import '(java.util.concurrent Executors)) (defn test-stm [nitems nthreads niters] (let [refs (map ref (replicate nitems 0)) pool (Executors/newFixedThreadPool nthreads) tasks (map (fn [t] (fn [] (dotimes [n niters] (dosync (doseq [r refs] (alter r + 1 t)))))) (range nthreads))] (doseq [future (.invokeAll pool tasks)] (.get future)) (.shutdown pool) (map deref refs))) I understand what it does and how it works, but I don't get why the second anonymous function fn[] is needed? Many thanks, dusha. P.S. Without this second fn [] I get NullPointerException.

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  • Find maximum number of logged on users in SQL

    - by lleto
    Hi, I want to keep tabs on the number of concurrent users of my application. I therefore log a time_start and a time_stop. If I now want to query the database for the maximum number of logged on users and return the start date, how would I do that. The table looks like this: id | time_start | time_stop ----+---------------------+--------------------- 1 | 2010-03-07 05:40:59 | 2010-03-07 05:41:33 2 | 2010-03-07 06:50:51 | 2010-03-07 10:50:51 3 | 2010-02-21 05:20:00 | 2010-03-07 12:23:44 4 | 2010-02-19 08:21:12 | 2010-03-07 12:37:28 5 | 2010-02-13 05:52:13 | Where time_stop is empty the user is still logged on. In this case I would expect to see 2010-03-07 returned, since all users (5) were logged on at that moment. However if I would run the query with 'where time_start BETWEEN '2010-02-17' AND '2010-02-23' I would expect to see 2010-02-21 with a maximum of 2. Is this possible direct in SQL (using postgres) or do I need to parse the results in PHP? Thanks, lleto

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  • Database suggestion

    - by superartsy
    I am working on an enterprise level product that is designed around SQL Express Database ( views,concurrent users,stored procedures,case,IF).Though we dont use any SQL Server advanced features, if the database size of 4Gb in express ends up being a limitation customers can move to advanced versions of SQL Server. However my problem is, SQL Express deployment is not easy and the installer size is huge. This is a major drawback for someone looking to try our product.You dont want end users to not buy a product becos the download is huge. Does anyone have any recommendations of a database that has a smaller footprint but all the features of Express and which can be migrated to express?

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  • Java - JPA - @Version annotation

    - by Yatendra Goel
    I am new to JPA. I am cofused about the @Version annotation. How it works? I have googled it and found various answers whose extract is as follows: JPA uses a version field in your entities to detect concurrent modifications to the same datastore record. When the JPA runtime detects an attempt to concurrently modify the same record, it throws an exception to the transaction attempting to commit last. But still I am not sure how it works? ================================================================================== Also as from the following lines: You should consider version fields immutable. Changing the field value has undefined results. Does it mean that we should declare our version field as final

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  • How do I choose a database?

    - by liamzebedee
    I need a comparison table of some sort for database varieties (MySQL, SQLite etc.). I can't find one. My use case is, I am implementing storage of objects in a distributed hash table. I need a database solution that is: Fast for sorting Simplistic (no users, preferably no additional structures like multiple tables etc.) Concurrent (if possible) Multi-platform File based (not stored in memory primarily) Centralized I will be programming in Go. As I understand, I believe I need what is called a Document Orientated Database, because I am storing objects, identified by keys. EDIT: While I am implementing a DHT, I will also be storing metadata about the objects, such as access counts etc. It would also be preferable to have TLL (time to live)

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  • Why are C, C++, and LISP so prevalent in embedded devices and robots?

    - by David
    It seems that the software language skills most sought for embedded devices and robots are C, C++, and LISP. Why haven't more recent languages made inroads into these applications? For example, Erlang would seem particularly well-suited to robotic applications, since it makes concurrent programming easier and allows hot swapping of code. Python would seem to be useful, if for no other reason than its support of multiple programming paradigms. I'm even surprised that Java hasn't made a foray into general robotic programming. I'm sure one argument would be, "Some newer languages are interpreted, not compiled" - implying that compiled languages are quicker and use fewer computational resources. Is this still the case, in a time when we can put a Java Virtual Machine on a cell phone or a SunSpot? (and isn't LISP interpreted anyway?)

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  • PHP no wait sem_acquire?

    - by SerEnder
    Not a specific code question, but more of a general coding question. I'm trying to use a semaphore in a work project to limit the number of users that can access certain processes at a concurrent time. From my understanding the following: $iKey = ftock($sSomeFileLocation,'sOneCharacterString'); //Generate the key if($sem_id = sem_get($iKey){ //1 user allowed if(sem_acquire($sem_id)){ //Do the limited process here sem_release($sem_id); } } The problem that I see here is that if there is already one user who has the semaphore key, then the next user just waits until the first user is done rather than just faulting out. Anyone know of a way that if the max_acquire number has been reached, sem_acquire (or similar) will just return false? Thanks

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  • Java 7 New Features

    - by John W.
    I have done some good reading on the new java.util.concurrent features being introduced with the java 7 release. For instance, Phaser, TransferQueue and the more exciting Fork Join Framework. I recently saw a power point made by Josh Bloch about even more features that are going to be introduced however that link has been lost. For example I remember one change is being able to build a Map the same way you can build an array for: Map myMap = {"1,Dog","2,Cat"}; and so forth (this may not be 100% correct but the idea is there) Does anyone know of a list or just can name some new things to look forward to? Note: I did see a question asked http://stackoverflow.com/questions/213958/new-features-in-java-7 however it was asked ~2 years ago and I am sure the list of updates are more concrete. Thanks!

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  • C# similar technologies and books in Java

    - by MigNix
    Hi, I know this can look like dublicate of other discussions and I have already reviewed them, but they are different. I want to learn Java and I want to know are there any books similar to those that I have seen for C#. Here is the list: C# 4.0 in a Nutshell: The Definitive Reference Probably Java in a Nutshell :) Professional C# 4.0 and .NET 4 Pro C# 2010 and the .NET 4 Platform Accelerated C# 2010 Programming WCF Services Essential Windows Communication Foundation (WCF): For .NET Framework 3.5 C# in Depth: What you need to master C# 2 and 3 CLR via C# Professional ADO.NET 3.5 with LINQ and the Entity Framework Professional Enterprise .NET Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Libraries Mainly I am interested in Java 6 and want to know about JVM , Cuncurency, Data access and distributed application technologies. I have seen there are some books on JVM like Inside the Java 2 Virtual Machine also some free books on official web site. Also I found concurrent programming in Java. But may be you can suggest any others. Thank you

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  • Chat Server with sockets in C

    - by Andrew
    I'm trying to write a chat server in C that allows communication between two clients using POSIX sockets. I'm not sure I have a good grasp on this concept or how I should set up the communication protocol between the clients and the server. I know I need one socket to bind() the server port to so I can accept incoming connections from clients, but in order to have two clients connected at the same time do I need to create a socket for each of these clients when I accept() or should I accept() a client and then fork() so I can have another client accept? I'm not worried about concurrent chatting yet, it's more of a ping-pong approach where the clients need to wait for a recv() after they send() before they can type a new message.

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  • How should I capture clickstream data?

    - by editor
    I'd like to start using clickstream analysis to improve a dynamic site's user experience. I'd like to rule out two options: parameterizing URLs (index.php?src=http://www.example.com) and immediate database logging. The former makes pretty ugly URLs and isn't great for SEO and the latter might slow down page render when there are lots of concurrent users. Assuming these aren't viable options, I think I'm left with doing an asynchronous POST to a server side script that runs a database query and returns a 204 (no data) response. Is this the best option for capturing clickstream data?

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  • Suspend TimerTask until the next execution

    - by user1052518
    I am using a TimerTask to run some periodic tasks, the task being processing a set of files. I have a requirement where if the number of files to be processed exceeds a pre-determined limit, the thread suspends execution and waits till the next cycle to start processing the files again. Is there a way to suspend the TimerTask until the next execution period or do I have to extend the TimerTask class to achieve this functionality? I saw there is a TimerTask.cancel method, but this will cancel all further executions of this thread. I don't want this to happen. I just want the thread to be suspended until the next execution period. I don't have the luxury of moving to any of the other concurrent classes in Java as our framework uses TimerTask, and I have to stick with it. Any suggestions, pointers or tips are greatly appreciated. thanks, Asha

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  • UIPageViewController blanking page

    - by CrazyEoin
    I've been trying to use the UIPageViewController to display 3 different nibs for a few days on and off now and have almost got it working. I still have one weird bug that I cant figure out. Basically the app starts, I can scroll between the 3 pages one after another with out any problems, eg: Page1-Page2-Page3 and then back to the start: Page3-Page2-Page1. No Problems. The issue is that if I scroll, for example from Page3-Page2, then BACK to Page3, Page3 Dissappears when it snaps into place. If I scroll to where a forth page would be, then I get Page3. Here is the code relevant to the UIPageViewController, the nibs and the delegate methods for the UIPageViewController: - (void)viewDidLoad{ [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. self.pageViewController = [[UIPageViewController alloc] initWithTransitionStyle:UIPageViewControllerTransitionStyleScroll navigationOrientation:UIPageViewControllerNavigationOrientationHorizontal options:nil]; self.pageViewController.delegate = self; [[self.pageViewController view] setFrame:[[self view] bounds]]; indexTest = 0; Page1 *p1 = [[Page1 alloc]initWithNibName:@"Page1" bundle:nil]; p1.view.tag = 1; Page2 *p2 = [[Page2 alloc]initWithNibName:@"Page2" bundle:nil]; p2.view.tag = 2; Page3 *p3 = [[Page3 alloc]initWithNibName:@"Page3" bundle:nil]; p3.view.tag = 3; NSArray *arr = [[NSArray alloc] initWithObjects:p1,nil]; viewControllers = [[NSMutableArray alloc] initWithObjects:p1,p2,p3, nil]; [self.pageViewController setViewControllers:arr direction:UIPageViewControllerNavigationDirectionForward animated:NO completion:nil]; self.pageViewController.dataSource = self; [self addChildViewController:self.pageViewController]; [[self view] addSubview:[self.pageViewController view]]; [self.pageViewController didMoveToParentViewController:self]; self.view.gestureRecognizers = self.pageViewController.gestureRecognizers; } #pragma mark - page view controller stuff - (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerBeforeViewController:(UIViewController *)viewController { if (indexTest > 0) { switch (indexTest) { case 1:{ NSLog(@"NO page is BEFORE current page"); break; } case 2:{ NSLog(@"Page BEFORE is Page: %@", [NSString stringWithFormat:@"%@",[viewControllers objectAtIndex:0] ] ); indexTest--; return [viewControllers objectAtIndex:0]; break; } default:{ NSLog(@"PROBLEM in viewBEFORE, indexTest = %d!!!!", indexTest); break; } } } return nil; } - (UIViewController *)pageViewController:(UIPageViewController *)pageViewController viewControllerAfterViewController:(UIViewController *)viewController { if (indexTest < NUM_OF_PAGES) { switch (indexTest) { case 0:{ NSLog(@"Page AFTER is Page: %@", [NSString stringWithFormat:@"%@",[viewControllers objectAtIndex:1] ] ); indexTest++; return [viewControllers objectAtIndex:1]; break; } case 1:{ NSLog(@"Page AFTER is Page: %@", [NSString stringWithFormat:@"%@",[viewControllers objectAtIndex:2] ] ); indexTest++; return [viewControllers objectAtIndex:2]; break; } case 2:{ NSLog(@"No pages AFTER this current page %d", indexTest); break; } default:{ NSLog(@"PROBLEM in viewAFTER, indexTest = %d!!!!", indexTest); break; } } } return nil; } Finally the page index dots code #pragma mark - dot controller - (NSInteger)presentationCountForPageViewController:(UIPageViewController *)pageViewController { // The number of items reflected in the page indicator. return NUM_OF_PAGES; } - (NSInteger)presentationIndexForPageViewController:(UIPageViewController *)pageViewController { // The selected item reflected in the page indicator. return 0; } Any and all help is much appreciated, I think I'm just doing something silly that I cant see as I'm so close to it fully working. If anythings not clear or I haven't give enough information please let me know and I'll answer it as best as I can. Thanks

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  • How do I guarantee row uniqueness in MySQL without the use of a UNIQUE constraint?

    - by MalcomTucker
    Hi I have some fairly simple requirements but I'm not sure how I implement them: I have multiple concurrent threads running the same query The query supplies a 'string' value - if it exists in the table, the query should return the id of the matching row, if not the query should insert the 'string' value and return the last inserted id The 'string' column is (and must be) a text column (it's bigger than varchar 255) so I cannot set it as unique - uniqueness must be enforced through the access mechanism The query needs to be in stored procedure format (which doesnt support table locks in MySQL) How can I guarantee that 'string' is unique? How can I prevent other threads writing to the table after another thread has read it and found no matching 'string' item? Thanks for any advice..

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  • Keeping track of threads when creating them recursively

    - by 66replica
    I'm currently working on some code for my Programming Languages course. I can't post the code but I'm permitted to talk about some high level concepts that I'm struggling with and receive input on them. Basically the code is a recursive DFS on a undirected graph that I'm supposed to convert to a concurrent program. My professor already specified that I should create my threads in the recursive DFS method and then join them in another method. Basically, I'm having trouble thinking of how I should keep track of the threads I'm creating so I can join all of them in the other method. I'm thinking an array of Threads but I'm unsure how to add each new thread to the array or even if that's the right direction.

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  • Grails background call vie camle or background-thread

    - by user304217
    I need to improve the response time for a Grails application, so I need to use concurrent processing to separate work that can be done after the users web page is refreshed It seems like the Camel and background-thread plugins can do this for me. I tried the Camel way, but get 'Session does not exists' errors, which looks like hibernate can not operate in an Camel acynchonouse call. In the background-thread description they mention that they solved this problem. Can any one tell me which which will be the better choice and which ones plays nicely with Hibernate? All the best Ulrich

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  • C++11: thread_local or array of OpenCL 1.2 cl_kernel objects?

    - by user926918
    I need to run several C++11 threads (GCC 4.7.1) parallely in host. Each of them needs to use a device, say a GPU. As per OpenCL 1.2 spec (p. 357): All OpenCL API calls are thread-safe75 except clSetKernelArg. clSetKernelArg is safe to call from any host thread, and is safe to call re-entrantly so long as concurrent calls operate on different cl_kernel objects. However, the behavior of the cl_kernel object is undefined if clSetKernelArg is called from multiple host threads on the same cl_kernel object at the same time. An elegant way would be to use thread_local cl_kernel objects and the other way I can think of is to use an array of these objects such that i'th thread uses i'th object. As I have not implemented these earlier I was wondering if any of the two are good or are there better ways of getting things done. TIA, S

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  • Multible jquery .load() methods not running concurrently in MVC 2

    - by Boob
    In one of my aspx pages i have something like this in the head: <script type="text/javascript"> $(function() { // Jquery stuff loadStuff(); }); function loadStuff() { $('#result1').load({ source: url }); $('#result2').load({ source: url }); $('#result3').load({ source: url }); }; </script> The idea being that as the page is being displayed these loads will populate the specified divs with results from different webservices as and when the info becomes available. My problem is that this requests are being queued and being sent one at a time. However I want these to be sent at the same time and whichever returns first gets displayed first! Also if I try to kick off another request while these divs are loading the request seems to get queued after the others. How do I make these requests concurrent instead of queued? Thanks

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  • Which language should I learn to create a sudoku game?

    - by Brandan
    I'd like to learn a new programming language, something besides all the scripting languages I've used for the past many years (Ruby, JavaScript, Perl, PHP, bash). I figured it might be interesting to make a sudoku game since there are plenty of documented algorithms and it only requires fairly simple data structures. It might start out as either a generator or a solver of puzzles, not necessarily both and not necessarily with a GUI. My goal is primarily to learn some new programming concepts beyond MVC and UI design, secondarily for this thing to be pretty fast. Is there a language that particularly shines for these sorts of constraint satisfaction problems? Is it suited to a functional language like Haskell or a highly concurrent language like Erlang (say for solving puzzles much larger than 9 x 9)? Or is this question mostly meaningless?

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  • Suggestions on including free database products to include in an application - SQL Server Express or

    - by superartsy
    I am working on an enterprise level product that is designed around SQL Server Express and specifically its features (views, concurrent users, stored procedures, CASE and IF statements). Though we don't use any advanced SQL Server features, the database size limit of 4GB in the Express edition may up being a limitation. A work-around is that customers can move to more full-featured versions of SQL Server. The problem is that SQL Server Express deployment is not easy, and the installer size is huge. This is a major drawback for someone looking to try our product. You don't want end-users to not buy a product because the download is huge. Does anyone have any recommendations of a database that has a smaller footprint but all the features of Express and which can be migrated to express?

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  • Choosing an Open Source Application Server for J2EE

    - by Rafael
    Hello, I know this may be a recurring topic, but I have read a lot of articles and I still have doubts. Also, I would like to hear more recent opinions about this. The main requirements of my application server are: flexible configuration, support for a extremely high number of concurrent users. It will be a system for the mobile communications industry, so it must have high availability as well. I am going to develop a J2EE application and Open Source Applications Servers are my only option. I have use GlassFish for a very small project and I really liked it. Thank you very much for your advise.

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  • executorservice to read data from database in chuncks and run process on them

    - by TazMan
    I'm trying to write a process that would read data from a database and upload it onto a cloud datastore. How can I decide the partition strategy of the data? I want to query the table in chunks and process each chunk in 10 threads. Each thread basically will send the data to an individual node on a 10 node cluster on the cloud.. Where in the below multi threading code will the dataquery to extract and send 10 concurrent requests for uploading data to cloud would be? public class Caller { public static void main(String[] args) { ExecutorService executor = Executors.newFixedThreadPool(10); for (int i = 0; i < 10; i++) { Runnable worker = new DomainCDCProcessor(i); executor.execute(worker); } executor.shutdown(); while (!executor.isTerminated()) { } System.out.println("Finished all threads"); } }

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