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  • Java: Making concurrent MySQL queries from multiple clients synchronised

    - by Misha Gale
    I work at a gaming cybercafe, and we've got a system here (smartlaunch) which keeps track of game licenses. I've written a program which interfaces with this system (actually, with it's backend MySQL database). The program is meant to be run on a client PC and (1) query the database to select an unused license from the pool available, then (2) mark this license as in use by the client PC. The problem is, I've got a concurrency bug. The program is meant to be launched simultaneously on multiple machines, and when this happens, some machines often try and acquire the same license. I think that this is because steps (1) and (2) are not synchronised, i.e. one program determines that license #5 is available and selects it, but before it can mark #5 as in use another copy of the program on another PC tries to grab that same license. I've tried to solve this problem by using transactions and table locking, but it doesn't seem to make any difference - Am I doing this right? Here follows the code in question: public LicenseKey Acquire() throws SmartLaunchException, SQLException { Connection conn = SmartLaunchDB.getConnection(); int PCID = SmartLaunchDB.getCurrentPCID(); conn.createStatement().execute("LOCK TABLE `licensekeys` WRITE"); String sql = "SELECT * FROM `licensekeys` WHERE `InUseByPC` = 0 AND LicenseSetupID = ? ORDER BY `ID` DESC LIMIT 1"; PreparedStatement statement = conn.prepareStatement(sql); statement.setInt(1, this.id); ResultSet results = statement.executeQuery(); if (results.next()) { int licenseID = results.getInt("ID"); sql = "UPDATE `licensekeys` SET `InUseByPC` = ? WHERE `ID` = ?"; statement = conn.prepareStatement(sql); statement.setInt(1, PCID); statement.setInt(2, licenseID); statement.executeUpdate(); statement.close(); conn.commit(); conn.createStatement().execute("UNLOCK TABLES"); return new LicenseKey(results.getInt("ID"), this, results.getString("LicenseKey"), results.getInt("LicenseKeyType")); } else { throw new SmartLaunchException("All licenses of type " + this.name + "are in use"); } }

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  • How much do you think this job should pay hourly?

    - by Silent
    Well i got this job offer and they expect alot i say. i know most of this but, i would like to know what type of pay i should expect. I dont well to sell short you know. with Web Designer: Dreamweaver, HTML, JavaScript, Graphic Design/Manipulation, Templates, Layouts, Navigation, Flash/Multimedia Objects. Programmer: PHP, Web Application Development, MVC, Joomla, AJAX, JQuery, My SQL (SQL, Database).

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  • Search by Letters using LINQ

    - by Biswo
    hi i m using Linq & C# and i want to filter my data as this syntax in Sql Syntax in sql is I hav one table name Customer in which name is field 'Select name from customer where name like 'C%'' can u help to solve this code in Linq

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  • Stored Queries?

    - by phpeffedup
    Is it considered crazy to store common SQL queries for my web app in a database for use in execution? Or is that common practice? Or is it impossible? My thinking is, this way, I avoid hard-coding SQL into my application files, and add another level of abstraction. Is this crazy? Is this what a stored procedure is? Or is that something else?

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  • using haskell for "business applications"

    - by snorlaks
    Hello, I would like to know if there is any posiibility that I can use Haskell with small database like sql server compact so that client wont have to install any server on his desktop. Is there any api providing sql statements and so on ... What is the best solution to achieve small database application using haskell. thanks for help

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  • Is there any memory leak in the normal routine of sqlite3_*()?

    - by reer
    A normal routine of sqlite3_prepare_v2() + sqlite3_step() + sqlite3_finalize() could contain leak. It sound ridiculous. But the test code seems to say it. Or I used the sqlite3_*() wrongly. Appreciate for any reply. __code________________________ include include // for usleep() include int multi_write (int j); sqlite3 *db = NULL; int main (void) { int ret = -1; ret = sqlite3_open("test.db", &db); ret = sqlite3_exec(db,"CREATE TABLE data_his (id INTEGER PRIMARY KEY, d1 CHAR(16))", NULL,NULL,NULL); usleep (100000); int j=0; while (1) { multi_write (j++); usleep (2000000); printf (" ----------- %d\n", j); } ret = sqlite3_close (db); return 0; } int multi_write (int j) { int ret = -1; char *sql_f = "INSERT OR REPLACE INTO data_his VALUES (%d, %Q)"; char *sql = NULL; sqlite3_stmt *p_stmt = NULL; ret = sqlite3_prepare_v2 (db, "BEGIN TRANSACTION", -1, &p_stmt, NULL); ret = sqlite3_step ( p_stmt ); ret = sqlite3_finalize ( p_stmt ); int i=0; for (i=0; i<100; i++) { sql = sqlite3_mprintf ( sql_f, j*100000 + i, "00000000000068FD"); ret = sqlite3_prepare_v2 (db, sql, -1, &p_stmt, NULL ); sqlite3_free ( sql ); //printf ("sqlite3_prepare_v2(): %d, %s\n", ret, sqlite3_errmsg (db)); ret = sqlite3_step ( p_stmt ); //printf ("sqlite3_step(): %d, %s\n", ret, sqlite3_errmsg (db)); ret = sqlite3_finalize ( p_stmt ); //printf ("sqlite3_finalize(): %d, %s\n\n", ret, sqlite3_errmsg (db)); } ret = sqlite3_prepare_v2 (db, "COMMIT TRANSACTION", -1, &p_stmt, NULL ); ret = sqlite3_step ( p_stmt ); ret = sqlite3_finalize ( p_stmt ); return 0; } __result________________________ And I watch the the process's run by top. At first, the memory statistics is: PID PPID USER STAT VSZ %MEM %CPU COMMAND 17731 15488 root S 1104 5% 7% ./sqlite3multiwrite When the printf() in while(1){} of main() prints the 150, the memory statistics is: PID PPID USER STAT VSZ %MEM %CPU COMMAND 17731 15488 root S 1552 5% 7% ./sqlite3multiwrite It sounds that after 150 for-cycles, the memory used by sqlite3multiwrite increase from 1104KB to 1552KB. What does it mean? memory leak or other thing?

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  • crystal reports sticky problem

    - by mahesh
    hi I have a project with following sql table companymaster expbill invcarat companyId companyid companyId name invno invno address buyer srno telno consignee qty rate amount I have tried many times by the way of ADO.Dataset with multiple sql query but reports show like sticky problems. the details section repeating from diferent companyID and invno am tired don't know how to overcome from this kindly provide me better solution with good example.

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  • How can I make the Entity data model designer use my database column descriptions?

    - by Jack
    I am using EF4 with Visual Studio 2010. I have a SQL database already created and all my columns are documented with the Description property in SQL management studio. When I create the edmx in Visual Studio, how can I make it read those descriptions and use them to add comments to my generated classes? I know I can use the Summary and LongDescription in the edmx properties but I'm looking for a way to do this without having to copy and paste every description. Thanks!

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  • Databases: Migrate data between MS Access DB and MYSQL

    - by Dean
    Hello, I have 2 databases, one is MS Access DB from an old website, and the other one is MYSQL from the new Joomla+VirtueMart based website. I need to migrate existing products from MS Access to MYSQL. I thought of putting both on server and writing SQL queries in MYSQL workbench, untill I have a good script for that, but I'm very new to SQL, so I'd rather avoid that. I there a better way and more efficient for that?

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  • CodeIgniter static class question

    - by Josh K
    If I would like to have several static methods in my models so I can say User::get_registered_users() and have it do something like public static function get_registered_users() { $sql = "SELECT * FROM `users` WHERE `is_registered` = 0"; $this->db->query($sql); // etc... } Is it possible to access the $this->db object or create a new one for a static method?

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  • How to evaluate the string in ruby if the string contains ruby command?

    - by Arun
    In my case, I was storing the sql query in my database as text. I am showing you one record which is present in my database Query.all :id => 1, :sql => "select * from user where id = #{params[:id]}" str = Query.first Now 'str' has value "select * from user where id = #{params[:id]}" Here, I want to parsed the string like If my params[:id] is 1 then "select * from user where id = 1" I used eval(str). Is this correct?

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  • Best Performing ORM for .NET

    - by steve_c
    I'm curious if anyone has done any performance comparisons with any or all of the main players in the .NET ORM space. Specifically I'm interested in comparisons between the following: Linq to SQL NHibernate LLBL Gen Entity Framework Though it seems people don't really consider Linq to SQL a true ORM, I am still including it in this list. Some performance metrics would be nice to see.

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  • insert ... select with divide operator in select errors?

    - by Mark
    Hi, the following query CREATE TABLE IF NOT EXISTS XY ( x INT NOT NULL , y FLOAT NULL , PRIMARY KEY(x) ) INSERT INTO XY (x,y) (select 1 as x ,(1/7) as y); errors with Error code 1064, SQL state 42000: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'INSERT INTO XY (x,y) (select 1 as x ,(1/7) as y)' at line 7 Line 1, column 1 any ideas?

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  • What's the difference between 'service' and 'server'?

    - by bonefisher
    I can imagine that the 'server' can be a machine/host but can be also a program like ftp server, smtp server, etc.. The 'service' on the other hand refers mainly to applications/programms.. Why can then for example the Sql Server cannot be called as Sql Service? It has the same semanthics. Or the other way round: MS Azure service: why it isn't called Azure Server? :)

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  • Regarding data storage in ecommerce applications

    - by geeth
    Why is MainFrame better choice to stores data related to customer, orders etc? I work with a online retail store, they use .net technologies for applications and MainFrames for storing product, customer, order etc related data. Is SQL DB server not a good option for these type of applications? How is MainFrames better choice than SQL DB? Please provide your insight.

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  • restoring with mysqldump, but where is the data?

    - by Ole Media
    Ok, so I'm in need to restore a table and I do: mysqldump --opt database table_name < table_name.sql I hit enter and Done! Well, not really, when I go to see if there is anything on the table it show 0 records. I have look into the table_name.sql and I see two records. What am I doing wrong?

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  • What is a good free ODBC viewer?

    - by wshato
    At my job I have to work with three different databases. One is a SQL Server 2008, for which I use SQL Server Management Studio, and another is MySQL, for which I use the MySQL tools. The third however is a Cache database that I can only access through a ODBC connection. Currently I use the SLIK software ODBC View tool to access it, but it is severely lacking when compared with the other two tools. Can anyone recommend a good tool for this type of work?

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