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• Data access strategy for a site like SO - sorted SQL queries and simultaneous updates that affect th

  - by Kaleb Brasee

  I'm working on a Grails web app that would be similar in access patterns to StackOverflow or MyLifeIsAverage - users can vote on entries, and their votes are used to sort a list of entries based on the number of votes. Votes can be placed while the sorted select queries are being performed. Since the selects would lock a large portion of the table, it seems that normal transaction locking would cause updates to take forever (given enough traffic). Has anyone worked on an app with a data access pattern such as this, and if so, did you find a way to allow these updates and selects to happen more or less concurrently? Does anyone know how sites like SO approach this? My thought was to make the sorted selects dirty reads, since it is acceptable if they're not completely up to date all of the time. This is my only idea for possibly improving performance of these selects and updates, but I thought someone might know a better way.

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• How Can I Compress Texture in OpenGL on iPhone/iPad?

  - by nonamelive

  Hi, I'm making an iPad app which needs OpenGL to do a flip animation. I have a front image texture and a back image texture. Both the two textures are screenshots. // Capture an image of the screen UIGraphicsBeginImageContext(view.bounds.size); [view.layer renderInContext:UIGraphicsGetCurrentContext()]; image = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); // Allocate some memory for the texture GLubyte *textureData = (GLubyte*)calloc(maxTextureSize*4, maxTextureSize); // Create a drawing context to draw image into texture memory CGContextRef textureContext = CGBitmapContextCreate(textureData, maxTextureSize, maxTextureSize, 8, maxTextureSize*4, CGImageGetColorSpace(image.CGImage), kCGImageAlphaPremultipliedLast); CGContextDrawImage(textureContext, CGRectMake(0, maxTextureSize-size.height, size.width, size.height), image.CGImage); CGContextRelease(textureContext); // ...done creating the texture data [EAGLContext setCurrentContext:context]; glGenTextures(1, &textureToView); glBindTexture(GL_TEXTURE_2D, textureToView); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR); glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, maxTextureSize, maxTextureSize, 0, GL_RGBA, GL_UNSIGNED_BYTE, textureData); // free texture data which is by now copied into the GL context free(textureData); Each of the texture takes up about 8MB memory, which is unacceptable for an iPhone/iPad app. Could anyone tell me how can I compress the texture to reduce the memory. I'm a complete newbie to OpenGL. Any help would be appreciated!

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• Does Windows 7 have a Kiosk Mode?

  - by Jordan

  I'm writing a Kiosk on a Windows 7 computer. The Kiosk is on a touch screen computer, that will not have a keyboard. The application is a WPF application running the .NET Framework version 3.5. I'm having problems with the progress bar task bar showing up (blinking) at the bottom of the screen. This unfortunately gives the user access to the background operating system. Is there someway to lock a full screen application window into the front of the screen without using it on-top-of-everything mode. Some sort of Kiosk mode, or some tool that was designed for this.

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• How do I implement aasm in Rails 3 for what I want it to do?

  - by marcamillion

  I am a Rails n00b and have been advised that in order for me to keep track of the status of my user's accounts (i.e. paid, unpaid (and therefore disabled), free trial, etc.) I should use an 'AASM' gem. So I found one that seems to be the most popular: https://github.com/rubyist/aasm But the instructions are pretty vague. I have a Users model and a Plan model. User's model manages everything you might expect (username, password, first name, etc.). Plan model manages the subscription plan that users should be assigned to (with the restrictions). So I am trying to figure out how to use the AASM gem to do what I want to do, but no clue where to start. Do I create a new model ? Then do I setup a relationship between my User model and the model for AASM ? How do I setup a relationship? As in, a user 'has_many' states ? That doesn't seem to make much sense to me. Any guidance would be really appreciated. Thanks. Edit: If anyone else is confused by AASMs like myself, here is a nice explanation of their function in Rails by the fine folks at Envy Labs: http://blog.envylabs.com/2009/08/the-rails-state-machine/ Edit2: How does this look: include AASM aasm_column :current_state aasm_state :paid aasm_state :free_trial aasm_state :disabled #this is for accounts that have exceed free trial and have not paid #aasm_state :free_acct aasm_event :pay do transitions :to => :paid, :from => [:free_trial, :disabled] transitions :to => :disabled, :from => [:free_trial, :paid] end

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• PHP Sessions - Locking and Sharing questions

  - by Nuno Peralta

  Hi, I would like to know if it is possible to read $_SESSION attributes without locking it. Currently, session_start() locks SESSION, that means other PHP processes will wait until it is unlocked. But, some processes just want to get some $_SESSION variables, not to write on them. Is that possible to implement some function like session_get(string $id) which doesn't lock SESSION? Also, it is possible to share SESSIONs between browsers, once the user is logged in the same account, for example, using session_id('shared_vars_of_'.$userid). But, is that secure? Is this discouraged? Thanks, Nuno

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• SortList duplicated key, but it shouldn't

  - by Luca

  I have a class which implements IList interface. I requires a "sorted view" of this list, but without modifying it (I cannot sort directly the IList class). These view shall be updated when the original list is modified, keeping items sorted. So, I've introduced a SortList creation method which create a SortList which has a comparer for the specific object contained in the original list. Here is the snippet of code: public class MyList<T> : ICollection, IList<T> { ... public SortedList CreateSortView(string property) { try { Lock(); SortListView sortView; if (mSortListViews.ContainsKey(property) == false) { // Create sorted view sortView = new SortListView(property, Count); mSortListViews.Add(property, sortView); foreach (T item in Items) sortView.Add(item); } else sortView = mSortListViews[property]; sortView.ReferenceCount++; return (sortView); } finally { Unlock(); } } public void DeleteSortView(string property) { try { Lock(); // Unreference sorted view mSortListViews[property].ReferenceCount--; // Remove sorted view if (mSortListViews[property].ReferenceCount == 0) mSortListViews.Remove(property); } finally { Unlock(); } } protected class SortListView : SortedList { /// <summary> /// /// </summary> /// <param name="property"></param> /// <param name="capacity"></param> public SortListView(string property, int capacity) : base(new GenericPropertyComparer(typeof(T).GetProperty(property, BindingFlags.Instance | BindingFlags.Public)), capacity) { } /// <summary> /// Reference count. /// </summary> public int ReferenceCount = 0; /// <summary> /// /// </summary> /// <param name="item"></param> public void Add(T item) { Add(item, item); } /// <summary> /// /// </summary> /// <param name="item"></param> public void Remove(T item) { // Base implementation base.Remove(item); } /// <summary> /// Compare object on a generic property. /// </summary> class GenericPropertyComparer : IComparer { #region Constructors /// <summary> /// Construct a GenericPropertyComparer specifying the property to compare. /// </summary> /// <param name="property"> /// A <see cref="PropertyInfo"/> which specify the property to be compared. /// </param> /// <remarks> /// The <paramref name="property"/> parameter imply that the compared objects have the specified property. The property /// must be readable, and its type must implement the IComparable interface. /// </remarks> public GenericPropertyComparer(PropertyInfo property) { if (property == null) throw new ArgumentException("property doesn't specify a valid property"); if (property.CanRead == false) throw new ArgumentException("property specify a write-only property"); if (property.PropertyType.GetInterface("IComparable") == null) throw new ArgumentException("property type doesn't IComparable"); mSortingProperty = property; } #endregion #region IComparer Implementation public int Compare(object x, object y) { IComparable propX = (IComparable)mSortingProperty.GetValue(x, null); IComparable propY = (IComparable)mSortingProperty.GetValue(y, null); return (propX.CompareTo(propY)); } /// <summary> /// Sorting property. /// </summary> private PropertyInfo mSortingProperty = null; #endregion } } /// <summary> /// Sorted views of this ReactList. /// </summary> private Dictionary<string, SortListView> mSortListViews = new Dictionary<string, SortListView>(); } Practically, class users request to create a SortListView specifying the name of property which determine the sorting, and using the reflection each SortListView defined a IComparer which keep sorted the items. Whenever an item is added or removed from the original list, every created SortListView will be updated with the same operation. This seems good at first chance, but it creates me problems since it give me the following exception when adding items to the SortList: System.ArgumentException: Item has already been added. Key in dictionary: 'PowerShell_ISE [C:\Windows\sysWOW64\WindowsPowerShell\v1.0\PowerShell_ISE.exe]' Key being added: 'PowerShell_ISE [C:\Windows\system32\WindowsPowerShell\v1.0\PowerShell_ISE.exe]' As you can see from the exception message, thrown by SortedListView.Add(object), the string representation of the key (the list item object) is different (note the path of the executable). Why SortList give me that exception? To solve this I tried to implement a GetHashCode implementation for the underlying object, but without success: public override int GetHashCode() { return ( base.GetHashCode() ^ mApplicationName.GetHashCode() ^ mApplicationPath.GetHashCode() ^ mCommandLine.GetHashCode() ^ mWorkingDirectory.GetHashCode() ); }

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• Firefox, prevent rendering between javascript statements.

  - by Erik

  I'm trying to create some kind of zoom around the mouse cursor feature on my website which ultimately runs these two lines (+ the same for height/scrollTop). canvas.style.width = someValue; canvas.parentNode.scrollLeft = someOtherValue; The problem is that in firefox(3.6) the page is re-rendered directly after the first row has been executed and since the view is depending on both values this means that every time i recalculate the view firefox will will render an invalid view before the correct one, in other words creating flicker. I've tried swapping the two rows but get the same problem. In chrome, opera and IE this doesn't happen. Both lines are executed before any rendering is done. Is there any way to lock the rendering manually, maybe something like this? document.disableRendering(); //fantasy function canvas.style.width = someValue; canvas.parentNode.scrollLeft = someOtherValue; document.enableRendering(); //fantasy function

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• directx audio video error message in debugmode

  - by clamp

  I have a c#/winforms application that uses directx to play some video and audio. whenever i start my application in debugmode i get this annoying message. i can click "continue" and everything seems to work fine. but i still want to get rid of this message. it does not show up in releasemode. Managed Debugging Assistant 'LoaderLock' has detected a problem in 'C:\pathtoexe.exe'. Additional Information: DLL 'C:\WINDOWS\assembly\GAC\Microsoft.DirectX.AudioVideoPlayback\1.0.2902.0__31bf3856ad364e35\Microsoft.DirectX.AudioVideoPlayback.dll' is attempting managed execution inside OS Loader lock. Do not attempt to run managed code inside a DllMain or image initialization function since doing so can cause the application to hang.

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• Singleton constructor question

  - by gillyb

  Hi, I created a Singleton class in c#, with a public property that I want to initialize when the Singleton is first called. This is the code I wrote : public class BL { private ISessionFactory _sessionFactory; public ISessionFactory SessionFactory { get { return _sessionFactory; } set { _sessionFactory = value; } } private BL() { SessionFactory = Dal.SessionFactory.CreateSessionFactory(); } private object thisLock = new object(); private BL _instance = null; public BL Instance { get { lock (thisLock) { if (_instance == null) { _instance = new BL(); } return _instance; } } } } As far as I know, when I address the Instance BL object in the BL class for the first time, it should load the constructor and that should initialize the SessionFactory object. But when I try : BL.Instance.SessionFactory.OpenSession(); I get a Null Reference Exception, and I see that SessionFactory is null... why?

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• How to force programs out of swap file when a resources-intensive batch finishes?

  - by sharptooth

  We use employees' desktops for CPU-intensive simulation during the night. Desktops run Windows - usually Windows XP. Employees don't log off, they just lock the desktops, switch off their monitors and go. Every employee has a configuration file which he can edit to specify when he is most likely out of office. When that time comes a background program grabs data for simulation from the server, spawns worker processes, watches them, gets results and sends them to the server. When the time specified by the employee elapses simulation stops so that normal desktop usage is not interfered. The problem is that simuation consumes a lot of memory, so when the worker processes run they force other programs into the swap file. So when the employee comes all the programs he left are luggish and slow until he opens them one by one so that they are unswapped. Is there a way the program can force other programs out of swap file when it stops simulation so that they again run smoothly?

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• c++11 atomic ordering: extended total order memory_order_seq_cst for locks

  - by itaj

  There's this note in c++11 29.3-p3: [ Note: Although it is not explicitly required that S include locks, it can always be extended to an order that does include lock and unlock operations, since the ordering between those is already included in the "happens before" ordering. - end note ] What does it mean by "always"? I can understand that any certain implementation can be designed to support such an extended S. But in some general implementation that wasn't designed for it, I don't see that S can be extended so. I had sent this question to comp.std.c++ but got no answers there. http://groups.google.com/group/comp.std.c++/browse_frm/thread/5242fa70d0594d1b#

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• How can I utilize or mimic Application OnStart in an HttpModule?

  - by Sailing Judo

  We are trying to remove the global.asax from our many web applications in favor of HttpModules that are in a common code base. This works really well for many application events such as BeginRequest and PostAuthentication, but there is no Application Start event exposed in the HttpModule. I can think of a couple of smelly ways to overcome this deficit. For example, I can probably do this: protected virtual void BeginRequest(object sender, EventArgs e) { Log.Debug("Entered BeginRequest..."); var app = HttpContext.Current.Application; var hasBeenSet app["HasBeenExecuted"] == null ? false : true; if(!hasBeenSet) { app.Lock(); // ... do app level code app.Add("HasBeenSet", true); app.Unlock(); } // do regular begin request stuff ... } But this just doesn't smell well to me. What is the best way to invoke some application begin logic without having a global.asax?

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• Is this ISession handled by the SessionScope - Castle ActiveRecord

  - by oillio

  Can I do this? I have the following in my code: public class ARClass : ActiveRecordBase<ARClass> { ---SNIP--- public void DoStuff() { using (new SessionScope()) { holder.CreateSession(typeof(ARClass)).Lock(this, LockMode.None); ...Do some work... } } } So, as I'm sure you can guess, I am doing this so that I can access lazy loaded references in the object. It works great, however I am worried about creating an ISession like this and just dropping it. Does it get properly registered with the SessionScope and will the scope properly tare my ISession down when it is disposed of? Or do I need to do more to manage it myself?

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• Is Work Stealing always the most appropriate user-level thread scheduling algorithm?

  - by Il-Bhima

  I've been investigating different scheduling algorithms for a thread pool I am implementing. Due to the nature of the problem I am solving I can assume that the tasks being run in parallel are independent and do not spawn any new tasks. The tasks can be of varying sizes. I went immediately for the most popular scheduling algorithm "work stealing" using lock-free deques for the local job queues, and I am relatively happy with this approach. However I'm wondering whether there are any common cases where work-stealing is not the best approach. For this particular problem I have a good estimate of the size of each individual task. Work-stealing does not make use of this information and I'm wondering if there is any scheduler which will give better load-balancing than work-stealing with this information (obviously with the same efficiency). NB. This question ties up with a previous question.

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• I have a problem with mysql and php

  - by neo skosana

  Hi I have a problem, this is my code: $db = new mysqli("localhost", "root", "", "blah"); $result1 = $db-query("select * from c_register where email = '$eml' and password = '$pass'"); if($result1-fetch_array()) { $auth->createSession(); $_SESSION['user'] = 'client'; promptUser("You have successfully logged in!!!","index.php"); } $db = new mysqli("localhost", "root", "", "blah"); $result2 = $db-query("select * from b_register where email = '$eml' and password = '$pass'"); if($result2-fetch_array()) { $auth->createSession(); $_SESSION['user'] = 'business'; promptUser("You have successfully logged in!!!","index.php"); } $db = new mysqli("localhost", "root", "", "blah"); $result3 = $db-query("select * from g_register where email = '$eml' and password = '$pass'"); if($result3-fetch_array()) { $auth->createSession(); $_SESSION['user'] = 'employee'; promptUser("You have successfully logged in!!!","index.php"); } $db = new mysqli("localhost", "root", "", "blah"); $result4 = $db-query("select * from k_register where email = '$eml' and password = '$pass'"); if($result4-fetch_array()) { $auth->createSession(); $_SESSION['user'] = 'super'; promptUser("You have successfully logged in!!!","index.php"); } else { promptUser("Username/Password do not match. Please try again!!!",""); } Funny enough this code works, but I no that I went about it the wrong way. I am new with php and mysql, so please help. I also tried e.gresult4->free(); for all the variable that save the data, and I got this error: Fatal error: Call to a member function free() on a non-object in...

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• leak in fgets when assigning to buffer

  - by monkeyking

  I'm having problems understanding why following code leaks in one case, and not in the other case. The difference is while(NULL!=fgets(buffer,length,file))//doesnt leak while(NULL!=(buffer=fgets(buffer,length,file))//leaks I thought it would be the same. Full code below. #include <stdio.h> #include <stdlib.h> #define LENS 10000 void no_leak(const char* argv){ char *buffer = (char *) malloc(LENS); FILE *fp=fopen(argv,"r"); while(NULL!=fgets(buffer,LENS,fp)){ fprintf(stderr,"%s",buffer); } fclose(fp); fprintf(stderr,"%s\n",buffer); free(buffer); } void with_leak(const char* argv){ char *buffer = (char *) malloc(LENS); FILE *fp=fopen(argv,"r"); while(NULL!=(buffer=fgets(buffer,LENS,fp))){ fprintf(stderr,"%s",buffer); } fclose(fp); fprintf(stderr,"%s\n",buffer); free(buffer); }

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• My iPhone app ran fine in simulator but crashed on device (iPod touch 3.1.2) test, I got the followi

  - by Mickey Shine

  I was running myapp on an iPod touch and I noticed it missed some libraries. Is that the reason? [Session started at 2010-03-19 15:57:04 +0800.] GNU gdb 6.3.50-20050815 (Apple version gdb-1128) (Fri Dec 18 10:08:53 UTC 2009) Copyright 2004 Free Software Foundation, Inc. GDB is free software, covered by the GNU General Public License, and you are welcome to change it and/or distribute copies of it under certain conditions. Type "show copying" to see the conditions. There is absolutely no warranty for GDB. Type "show warranty" for details. This GDB was configured as "--host=i386-apple-darwin --target=arm-apple-darwin".tty /dev/ttys007 Loading program into debugger… Program loaded. target remote-mobile /tmp/.XcodeGDBRemote-237-78 Switching to remote-macosx protocol mem 0x1000 0x3fffffff cache mem 0x40000000 0xffffffff none mem 0x00000000 0x0fff none run Running… [Switching to thread 11779] [Switching to thread 11779] sharedlibrary apply-load-rules all (gdb) continue warning: Unable to read symbols for "/Library/MobileSubstrate/MobileSubstrate.dylib" (file not found). 2010-03-19 15:57:18.892 myapp[2338:207] MS:Notice: Installing: com.yourcompany.myapp [myapp] (478.52) 2010-03-19 15:57:19.145 myapp[2338:207] MS:Notice: Loading: /Library/MobileSubstrate/DynamicLibraries/Backgrounder.dylib warning: Unable to read symbols for "/Library/MobileSubstrate/DynamicLibraries/Backgrounder.dylib" (file not found). warning: Unable to read symbols for "/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS3.1.2.sdk/usr/lib/libsubstrate.dylib" (file not found). MS:Warning: message not found [myappAppDelegate applicationWillResignActive:] MS:Warning: message not found [myappAppDelegate applicationDidBecomeActive:] 2010-03-19 15:57:19.550 myapp[2338:207] in FirstViewController 2010-03-19 15:57:20.344 myapp[2338:207] in load table view 2010-03-19 15:57:20.478 myapp[2338:207] in loading splash view 2010-03-19 15:57:22.793 myapp[2338:207] in set interface Program received signal: “0”. warning: check_safe_call: could not restore current frame

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• Why doesn't access database update?

  - by Ryan

  Recently I met a strange problem, see code snips as below: var sqlCommand: string; connection: TADOConnection; qry: TADOQuery; begin connection := TADOConnection.Create(nil); try connection.ConnectionString := 'Provider=Microsoft.Jet.OLEDB.4.0;Data Source=Test.MDB;Persist Security Info=False'; connection.Open(); qry := TADOQuery.Create(nil); try qry.Connection := connection; qry.SQL.Text := 'Select * from aaa'; qry.Open; qry.Append; qry.FieldByName('TestField1').AsString := 'test'; qry.Post; beep; finally qry.Free; end; finally connection.Free; end; end; First, Create a new access database named test.mdb and put it under the directory of this test project, we can create a new table named aaa in it which has only one text type field named TestField1. We set a breakpoint at line of "beep", then lunch the test application under ide debug mode, when ide stops at the breakpoint line (qry.post has been executed), at this time we use microsoft access to open test.mdb and open table aaa you will find there are no any changes in table aaa, if you let the ide continue running after pressing f9 you can find a new record is inserted in to table aaa, but if you press ctrl+f2 to terminate the application at the breakpoint, you will find the table aaa has no record been inserted, but in normal circumstance, a new record should be inserted in to the table aaa after qry.post executed. who can explain this problem , it troubles me so long time. thanks !!! BTW, the ide is delphi 2010, and the access mdb file is created by microsoft access 2007 under windows 7

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• How to get coordinates when iPhone is locked/sleeping?

  - by lionfly

  For normal situations, the didUpdateToLocation (of CLLocationManager) does not work any more when the lock/sleep button is pushed, and your app cannot get further newLocation coordinates during the locked period. However, some apps CAN track during the locked period, and they will show the correct route when you unlocked and see the Mapview. Obviously there are ways to get the coordinates during the locked period. How to get the coordinates when the iPhone is locked/sleeping? Who has hints and further info, please post. Thanks.

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• PHP/MySQL won't update decimal field

  - by Serhiy

  I have this query: UPDATE table_name SET field_1 = field_1 +100, field_2 = field_2 +100, field_3 = field_3 +100 WHERE id = 1 LIMIT 1; Where Field_1 is regular integer, Field_2 is decimal(15,6) and Field_3 is double(15,6). When I run this query from php script they update just field_1 and nothing happen with field_2 and field_3 they just stay as before. But when I run in phpMyAdmin it's work without any problems. I'm tried to lock tables, make round() before run update, nothing help. Please help... why I can't update decimal and float fields from php? PHP version: 5.2 Mysql version 5

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• Google App Engine - Dealing with concurrency issues of storing an object

  - by Spines

  My User object that I want to create and store in the datastore has an email, and a username. How do I make sure when creating my User object that another User object doesn't also have either the same email or the same username? If I just do a query to see if any other users have already used the username or the email, then there could be a race condition. UPDATE: The solution I'm currently considering is to use the MemCache to implement a locking mechanism. I would acquire 2 locks before trying to store the User object in the datastore. First a lock that locks based on email, then another that locks based on username. Since creating new User objects only happens at user registration time, and it's even rarer that two people try to use either the same username or the same email, I think it's okay to take the performance hit of locking. I'm thinking of using the MemCache locking code that is here: http://appengine-cookbook.appspot.com/recipe/mutex-using-memcache-api/ What do you guys think?

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• Mysql - Operand should contain 1 column(s) : What's wrong with the query...?

  - by SpikETidE

  Hi everybody.... I am trying to query a database to find the following If a customer searches for a hotel in a city between dates A and B, find and return the hotels in which rooms are free between the two dates. There will be more than one room in each room type(i.e. 5 Rooms in type A, 10 rooms in Type B etc) and we have to query the db to find only those hotels in which there is atleast one room free in atleast one type. This is my table structure.... **Structure for table 'reservations'** reservation_id hotel_id room_id customer_id payment_id no_of_rooms check_in_date check_out_date reservation_date **Structure for table 'hotels'** hotel_id hotel_name hotel_description hotel_address hotel_location hotel_country hotel_city hotel_type hotel_stars hotel_image hotel_deleted **Structure for table 'rooms'** room_id hotel_id room_name max_persons total_rooms room_price room_image agent_commision room_facilities service_tax vat city_tax room_description room_deleted And this is my query $city_search = '15'; $check_in_date = '29-03-2010'; $check_out_date = '31-03-2010'; $dateFormat_check_in = "DATE_FORMAT('$reservations.check_in_date','%d-%m-%Y')"; $dateFormat_check_out = "DATE_FORMAT('$reservations.check_out_date','%d-%m-%Y')"; $dateCheck = "$dateFormat_check_in >= '$check_in_date' AND $dateFormat_check_out <= '$check_out_date'"; $query = "SELECT $rooms.room_id, $rooms.room_name, $rooms.max_persons, $rooms.room_price, $hotels.hotel_id, $hotels.hotel_name, $hotels.hotel_stars, $hotels.hotel_type FROM $hotels,$rooms,$reservations WHERE $hotels.hotel_city = '$city_search' AND $hotels.hotel_id = $rooms.hotel_id AND $hotels.hotel_deleted = '0' AND $rooms.room_deleted = '0' AND $rooms.total_rooms - (SELECT SUM($reservations.no_of_rooms) as tot, $rooms.room_id as id FROM $reservations,$rooms WHERE $dateCheck GROUP BY $reservations.room_id) > '0'"; The number of rooms already reserved in each room type in each hotel will be stored in the reservations table... The query returns the error : Operand should contain 1 column(s) I tried running the sub-query alone but i don't get any result... And i have lost quite some amount of hair trying to de-bug this query from yesterday... What's wrong with this...? Or is there a better way to do what i mentioned above...? Thanks for your time...

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• Is ReaderWriterLockSlim.EnterUpgradeableReadLock() essentially the same as Monitor.Enter()?

  - by Neil Barnwell

  So I have a situation where I may have many, many reads and only the occasional write to a resource shared between multiple threads. A long time ago I read about ReaderWriterLock, and have read about ReaderWriterGate which attempts to mitigate the issue where many writes coming in trump reads and hurt performance. However, now I've become aware of ReaderWriterLockSlim... From the docs, I believe that there can only be one thread in "upgradeable mode" at any one time. In a situation where the only access I'm using is EnterUpgradeableReadLock() (which is appropriate for my scenario) then is there much difference to just sticking with lock(){}? Here's the excerpt: A thread that tries to enter upgradeable mode blocks if there is already a thread in upgradeable mode, if there are threads waiting to enter write mode, or if there is a single thread in write mode. Or, does the recursion policy make any difference to this?

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• unique constraint (w/o Trigger) on "one-to-many" relation

  - by elgcom

  To illustrate the problem, I make an example: A tag_bundle consists of one or more than one tags. A unique tag combination can map to a unique tag_bundle, vice versa. tag_bundle tag tag_bundle_relation +---------------+ +--------+ +---------------+--------+ | tag_bundle_id | | tag_id | | tag_bundle_id | tag_id | +---------------+ +--------+ +---------------+--------+ | 1 | | 100 | | 1 | 100 | +---------------+ +--------+ +---------------+--------+ | 101 | | 1 | 101 | +--------+ +---------------+--------+ There can't be another tag_bundle having the combination from tag 100 and tag 101. How can I ensure such unique constraint when executing SQL "concurrently"!! that is, to prevent concurrently adding two bundles with the same tag combination Adding a simple unique constraint on any table does not work, Is there any solution other than Trigger or explicit lock. I come to only this simple way: make tag combination into string, and let it be unique. tag_bundle (unique on tags) tag tag_bundle_relation +---------------+--------+ +--------+ +---------------+--------+ | tag_bundle_id | tags | | tag_id | | tag_bundle_id | tag_id | +---------------+--------+ +--------+ +---------------+--------+ | 1 | 100,101| | 100 | | 1 | 100 | +---------------+--------+ +--------+ +---------------+--------+ | 101 | | 1 | 101 | +--------+ +---------------+--------+ but it seems not a good way :(

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• Debugging in visual studio 2008 freezes entire system

  - by mat690

  Any time i try to debug in visual studio 2008 my entire system will freeze whenever a breakpoint is hit. I can move the mouse around and that remains responsive but nothing i click on does anything, I can bring up the task manager but can't do anything with it and i am able to lock/unlock the machine. I tried a fix that i found via google that suggested i disable advanced text services but it didn't work. I am using visual studio 2008 with SP1 running on Windows XP Pro, asking here is my last hope before formating/reinstalling so i hope someone can help me out. I debug by running the application in debug mode, the freeze happens no matter what the project size and it seems as if it is just the entire GUI that freezes, the computer carries on working just fine i just can't do anything with it because the GUI is totaly unresponsive.

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