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  • is there a simple timed lock algorithm avoiding deadlock on multiple mutexes?

    - by Vicente Botet Escriba
    C++0x thread library or Boost.thread define a non-member variadic template function that locks all mutex at once that helps to avoid deadlock. template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...); The same can be applied to a non-member variadic template function try_lock_until, which locks all the mutex until a given time is reached that helps to avoid deadlock like lock(...). template <class Clock, class Duration, class L1, class L2, class... L3> void try_lock_until( const chrono::time_point<Clock,Duration>& abs_time, L1&, L2&, L3&...); I have an implementation that follows the same design as the Boost function boost::lock(...). But this is quite complex. As I can be missing something evident I wanted to know if: is there a simple timed lock algorithm avoiding deadlock on multiple mutexes? If no simple implementation exists, can this justify a proposal to Boost? P.S. Please avoid posting complex solutions.

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  • Java Memory Model: reordering and concurrent locks

    - by Steffen Heil
    Hi The java meomry model mandates that synchronize blocks that synchronize on the same monitor enforce a before-after-realtion on the variables modified within those blocks. Example: // in thread A synchronized( lock ) { x = true; } // in thread B synchronized( lock ) { System.out.println( x ); } In this case it is garanteed that thread B will see x==true as long as thread A already passed that synchronized-block. Now I am in the process to rewrite lots of code to use the more flexible (and said to be faster) locks in java.util.concurrent, especially the ReentrantReadWriteLock. So the example looks like this: // in thread A synchronized( lock ) { lock.writeLock().lock(); x = true; lock.writeLock().unlock(); } // in thread B synchronized( lock ) { lock.readLock().lock(); System.out.println( x ); lock.readLock().unlock(); } However, I have not seen any hints within the memory model specification that such locks also imply the nessessary ordering. Looking into the implementation it seems to rely on the access to volatile variables inside AbstractQueuedSynchronizer (for the sun implementation at least). However this is not part of any specification and moreover access to non-volatile variables is not really condsidered covered by the memory barrier given by these variables, is it? So, here are my questions: Is it safe to assume the same ordering as with the "old" synchronized blocks? Is this documented somewhere? Is accessing any volatile variable a memory barrier for any other variable? Regards, Steffen

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  • Is lock returned by ReentrantReadWriteLock equivalent to it's read and write locks?

    - by Todd
    Hello, I have been looking around for the answer to this, but no joy. In Java, is using the lock created by ReentrantReadWriteLock equivalent to getting the read and write locks as returned by readLock.lock() and writeLock.lock()? In other words, can I expect the read and write locks associated with the ReentrantReadWriteLock to be requested and held by synchronizing on the ReentrantReadWriteLock? My gut says "no" since any object can be used for synchronization. I wouldn't think that there would be special behavior for ReentrantReadWriteLock. However, special behavior is the corner case of which I may not be aware. Thanks, Todd

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  • Why are my files in /var/lock and where did they just go?!

    - by Nicky Hajal
    I am hosting a website on Debian 5.0 & Apache2. Today one of my websites was down, Apache said it couldn't find the directory. I located the files and the whole site once in /var/www/site was now /var/lock/site. All the files were present. I was confused, but figured I'd just move it back. mv /var/lock/site /var/www All looked fine... Except that only the directories moved and the files appear to be lost! I am working on restoring from backups but I would really love to know what happened and where my files went (the backups are a few days old). Thanks for your help!

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  • is it possible to lock oracle 10g database table with ADO.NET?

    - by matti
    I have a table that contains a maximum value that needs to be get and set by multiple programs. How can I lock the table for a while when old value is got and new is updated in C#? In other words: string sql = "lock table MaxValueTable in exclusive mode"; using (DbCommand cmd = cnctn.CreateCommand()) { cmd.CommandText = sql; // execute command somehow!! } maxValue = GetMaxValue(); SetMaxValue(maxValue + X); sql = "lock table MaxValueTable in share mode"; using (DbCommand cmd = cnctn.CreateCommand()) { cmd.CommandText = sql; // execute command somehow!! }

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  • Template type deduction with a non-copyable class

    - by Evan Teran
    Suppose I have an autolocker class which looks something like this: template <T> class autolocker { public: autolocker(T *l) : lock(l) { lock->lock(); } ~autolocker() { lock->unlock(); } private: autolocker(const autolocker&); autolocker& operator=(const autolocker&); private: T *lock; } Obviously the goal is to be able to use this autolocker with anything that has a lock/unlock method without resorting to virtual functions. Currently, it's simple enough to use like this: autolocker<some_lock_t> lock(&my_lock); // my_lock is of type "some_lock_t" but it is illegal to do: autolocker lock(&my_lock); // this would be ideal Is there anyway to get template type deduction to play nice with this (keep in my autolocker is non-copyable). Or is it just easiest to just specify the type?

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  • In ArrayBlockingQueue, why copy into ReentrantLock field into local final variable?

    - by mjlee
    In ArrayBlockingQueue, any method that requires lock will get set 'final' local variable before calling 'lock()'. public boolean offer(E e) { if (e == null) throw new NullPointerException(); final ReentrantLock lock = this.lock; lock.lock(); try { if (count == items.length) return false; else { insert(e); return true; } } finally { lock.unlock(); } } Is there any reason to set a local variable 'lock' from 'this.lock' when field 'this.lock' is final also. Additionally, it also set local variable of E[] before acting on. private E extract() { final E[] items = this.items; E x = items[takeIndex]; items[takeIndex] = null; takeIndex = inc(takeIndex); --count; notFull.signal(); return x; } Is there any reason for copying to local final variable?

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  • How to disable the Windows 8 lock screen, without disabling the password?

    - by zeel
    Windows 8 now has a slide-away lock screen like so many other OSes designed for a touch interface. However on a non-touch PC/laptop this is just an extra annoying step in logging in. There is an option to disable it, but this also turns off the password requirement. Is there a way to disable the lock screen, and have Windows wake up directly to the password entry screen? Often when pressing a key to hide it there will be a noticeable delay before password entry is accepted, and no indicator, so one can easily lose the first character or so that they attempt to type, thus failing the login attempt.

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  • In ArrayBlockingQueue, why copy final member field into local final variable?

    - by mjlee
    In ArrayBlockingQueue, any method that requires lock will get set 'final' local variable before calling 'lock()'. public boolean offer(E e) { if (e == null) throw new NullPointerException(); final ReentrantLock lock = this.lock; lock.lock(); try { if (count == items.length) return false; else { insert(e); return true; } } finally { lock.unlock(); } } Is there any reason to set a local variable 'lock' from 'this.lock' when field 'this.lock' is final also. Additionally, it also set local variable of E[] before acting on. private E extract() { final E[] items = this.items; E x = items[takeIndex]; items[takeIndex] = null; takeIndex = inc(takeIndex); --count; notFull.signal(); return x; } Is there any reason for copying to local final variable?

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  • How does lock(syncRoot) make sense on a static method?

    - by Rising Star
    The following code is excerpted from the (Windows Identity Foundation SDK) template that MS uses to create a new Security Token Service Web Site. public static CustomSecurityTokenServiceConfiguration Current { get { HttpApplicationState httpAppState = HttpContext.Current.Application; CustomSecurityTokenServiceConfiguration customConfiguration = httpAppState.Get( CustomSecurityTokenServiceConfigurationKey ) as CustomSecurityTokenServiceConfiguration; if ( customConfiguration == null ) { lock ( syncRoot ) { customConfiguration = httpAppState.Get( CustomSecurityTokenServiceConfigurationKey ) as CustomSecurityTokenServiceConfiguration; if ( customConfiguration == null ) { customConfiguration = new CustomSecurityTokenServiceConfiguration(); httpAppState.Add( CustomSecurityTokenServiceConfigurationKey, customConfiguration ); } } } return customConfiguration; } } I'm relatively new to multi-threaded programming. I assume that the reason for the lock statement is to make this code thread-safe in the event that two web requests arrive at the web site at the same time. However, I would have thought that using lock (syncRoot) would not make sense because syncRoot refers to the current instance that this method is operating on... but this is a static method? How does this make sense?

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  • How to create "lock" (stop updating of cells) in Excel?

    - by Oozi
    My question is pretty simple, but one to which I can't really find a solution myself. I use Excel quite frequently, but rarely the fancy stuff. Anyway, to the point: How do I create a dropmenu that will lock certain cells, rows or columns? (by lock I mean unable to change via hand and stops updating itself). Example: A1 = 5 B1 = A1 * 100 Can I "lock" the B1-cell, so that changing A1 will have no effect on B1 (Will remain at value 500)? I would preferably want to be able to "unlock" the cell as well, instead of simply making function into value. Is this possible? Thanks in advance!

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  • C# async and actors

    - by Alex.Davies
    If you read my last post about async, you might be wondering what drove me to write such odd code in the first place. The short answer is that .NET Demon is written using NAct Actors. Actors are an old idea, which I believe deserve a renaissance under C# 5. The idea is to isolate each stateful object so that only one thread has access to its state at any point in time. That much should be familiar, it's equivalent to traditional lock-based synchronization. The different part is that actors pass "messages" to each other rather than calling a method and waiting for it to return. By doing that, each thread can only ever be holding one lock. This completely eliminates deadlocks, my least favourite concurrency problem. Most people who use actors take this quite literally, and there are plenty of frameworks which help you to create message classes and loops which can receive the messages, inspect what type of message they are, and process them accordingly. But I write C# for a reason. Do I really have to choose between using actors and everything I love about object orientation in C#? Type safety Interfaces Inheritance Generics As it turns out, no. You don't need to choose between messages and method calls. A method call makes a perfectly good message, as long as you don't wait for it to return. This is where asynchonous methods come in. I have used NAct for a while to wrap my objects in a proxy layer. As long as I followed the rule that methods must always return void, NAct queued up the call for later, and immediately released my thread. When I needed to get information out of other actors, I could use EventHandlers and callbacks (continuation passing style, for any CS geeks reading), and NAct would call me back in my isolated thread without blocking the actor that raised the event. Using callbacks looks horrible though. To remind you: m_BuildControl.FilterEnabledForBuilding(    projects,    enabledProjects = m_OutOfDateProjectFinder.FilterNeedsBuilding(        enabledProjects,             newDirtyProjects =             {                 ....... Which is why I'm really happy that NAct now supports async methods. Now, methods are allowed to return Task rather than just void. I can await those methods, and C# 5 will turn the rest of my method into a continuation for me. NAct will run the other method in the other actor's context, but will make sure that when my method resumes, we're back in my context. Neither actor was ever blocked waiting for the other one. Apart from when they were actually busy doing something, they were responsive to concurrent messages from other sources. To be fair, you could use async methods with lock statements to achieve exactly the same thing, but it's ugly. Here's a realistic example of an object that has a queue of data that gets passed to another object to be processed: class QueueProcessor {    private readonly ItemProcessor m_ItemProcessor = ...     private readonly object m_Sync = new object();    private Queue<object> m_DataQueue = ...    private List<object> m_Results = ...     public async Task ProcessOne() {         object data = null;         lock (m_Sync)         {             data = m_DataQueue.Dequeue();         }         var processedData = await m_ItemProcessor.ProcessData(data); lock (m_Sync)         {             m_Results.Add(processedData);         }     } } We needed to write two lock blocks, one to get the data to process, one to store the result. The worrying part is how easily we could have forgotten one of the locks. Compare that to the version using NAct: class QueueProcessorActor : IActor { private readonly ItemProcessor m_ItemProcessor = ... private Queue<object> m_DataQueue = ... private List<object> m_Results = ... public async Task ProcessOne()     {         // We are an actor, it's always thread-safe to access our private fields         var data = m_DataQueue.Dequeue();         var processedData = await m_ItemProcessor.ProcessData(data);         m_Results.Add(processedData);     } } You don't have to explicitly lock anywhere, NAct ensures that your code will only ever run on one thread, because it's an actor. Either way, async is definitely better than traditional synchronous code. Here's a diagram of what a typical synchronous implementation might do: The left side shows what is running on the thread that has the lock required to access the QueueProcessor's data. The red section is where that lock is held, but doesn't need to be. Contrast that with the async version we wrote above: Here, the lock is released in the middle. The QueueProcessor is free to do something else. Most importantly, even if the ItemProcessor sometimes calls the QueueProcessor, they can never deadlock waiting for each other. So I thoroughly recommend you use async for all code that has to wait a while for things. And if you find yourself writing lots of lock statements, think about using actors as well. Using actors and async together really takes the misery out of concurrent programming.

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  • Lock-Free, Wait-Free and Wait-freedom algorithms for non-blocking multi-thread synchronization.

    - by GJ
    In multi thread programming we can find different terms for data transfer synchronization between two or more threads/tasks. When exactly we can say that some algorithem is: 1)Lock-Free 2)Wait-Free 3)Wait-Freedom I understand what means Lock-free but when we can say that some synchronization algorithm is Wait-Free or Wait-Freedom? I have made some code (ring buffer) for multi-thread synchronization and it use Lock-Free methods but: 1) Algorithm predicts maximum execution time of this routine. 2) Therad which call this routine at beginning set unique reference, what mean that is inside of this routine. 3) Other threads which are calling the same routine check this reference and if is set than count the CPU tick count (measure time) of first involved thread. If that time is to long interrupt the current work of involved thread and overrides him job. 4) Thread which not finished job because was interrupted from task scheduler (is reposed) at the end check the reference if not belongs to him repeat the job again. So this algorithm is not really Lock-free but there is no memory lock in use, and other involved threads can wait (or not) certain time before overide the job of reposed thread. Added RingBuffer.InsertLeft function: function TgjRingBuffer.InsertLeft(const link: pointer): integer; var AtStartReference: cardinal; CPUTimeStamp : int64; CurrentLeft : pointer; CurrentReference: cardinal; NewLeft : PReferencedPtr; Reference : cardinal; label TryAgain; begin Reference := GetThreadId + 1; //Reference.bit0 := 1 with rbRingBuffer^ do begin TryAgain: //Set Left.Reference with respect to all other cores :) CPUTimeStamp := GetCPUTimeStamp + LoopTicks; AtStartReference := Left.Reference OR 1; //Reference.bit0 := 1 repeat CurrentReference := Left.Reference; until (CurrentReference AND 1 = 0)or (GetCPUTimeStamp - CPUTimeStamp > 0); //No threads present in ring buffer or current thread timeout if ((CurrentReference AND 1 <> 0) and (AtStartReference <> CurrentReference)) or not CAS32(CurrentReference, Reference, Left.Reference) then goto TryAgain; //Calculate RingBuffer NewLeft address CurrentLeft := Left.Link; NewLeft := pointer(cardinal(CurrentLeft) - SizeOf(TReferencedPtr)); if cardinal(NewLeft) < cardinal(@Buffer) then NewLeft := EndBuffer; //Calcolate distance result := integer(Right.Link) - Integer(NewLeft); //Check buffer full if result = 0 then //Clear Reference if task still own reference if CAS32(Reference, 0, Left.Reference) then Exit else goto TryAgain; //Set NewLeft.Reference NewLeft^.Reference := Reference; SFence; //Try to set link and try to exchange NewLeft and clear Reference if task own reference if (Reference <> Left.Reference) or not CAS64(NewLeft^.Link, Reference, link, Reference, NewLeft^) or not CAS64(CurrentLeft, Reference, NewLeft, 0, Left) then goto TryAgain; //Calcolate result if result < 0 then result := Length - integer(cardinal(not Result) div SizeOf(TReferencedPtr)) else result := cardinal(result) div SizeOf(TReferencedPtr); end; //with end; { TgjRingBuffer.InsertLeft } RingBuffer unit you can find here: RingBuffer, CAS functions: FockFreePrimitives, and test program: RingBufferFlowTest Thanks in advance, GJ

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  • Can I lock tables in an IF statement in MySQL?

    - by MalcomTucker
    This is throwing a syntax error - --from body of a stored proc IF (name = in_name) SET out_id = temp; ELSE LOCK TABLE People WRITE; INSERT INTO People (Name) VALUES (in_name); UNLOCK TABLE; SELECT LAST_INSERT_ID() INTO out_id END IF do I have to lock any tables I need at the start of the SP?

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  • dpkg stuck downloading font files

    - by Bob Bowles
    I have been reinstalling Ubuntu 12.04. The install from USB works fine, and I could update everything OK, but when I got to re-installing my application software I hit a snag. One of the packages I tried to re-install was ttf-mscorefonts-installer. dpkg stalled during this setup, downloading a font file (it had tried to download it all night). I stopped dpkg, and attempted to re-start downloading something else, but it would not let me. The commands I typed are as follows: bob@bobStudio:~$ sudo rm /var/lib/dpkg/lock This unlocks dpkg, but if I try to do something I get the following message (eg): bob@bobStudio:~$ sudo apt-get install synaptic E: dpgk was interrupted, you must manually run 'sudo dpkg --configure -a' to correct the problem So, I did just that: bob@bobStudio:~$ sudo dpkg --configure -a whereupon it started the previously failed download all over again. I went round the loop here a few times and each time after the configure command it re-started the failing download, but then I got this: bob@bobStudio:~$ sudo dpkg --configure -a Setting up update-notifier-common (0.119ubuntu8.4) ... ttf-mscorefonts-installer: downloading http://downloads.sourceforge.net/corefonts/andale32.exe Traceback (most recent call last): File "/usr/lib/update-notifier/package-data-downloader", line 234, in process_download_requests dest_file = urllib.urlretrieve(files[i])[0] File "/usr/lib/python2.7/urllib.py", line 93, in urlretrieve return _urlopener.retrieve(url, filename, reporthook, data) File "/usr/lib/python2.7/urllib.py", line 239, in retrieve fp = self.open(url, data) File "/usr/lib/python2.7/urllib.py", line 207, in open return getattr(self, name)(url) File "/usr/lib/python2.7/urllib.py", line 344, in open_http h.endheaders(data) File "/usr/lib/python2.7/httplib.py", line 954, in endheaders self._send_output(message_body) File "/usr/lib/python2.7/httplib.py", line 814, in _send_output self.send(msg) File "/usr/lib/python2.7/httplib.py", line 776, in send self.connect() File "/usr/lib/python2.7/httplib.py", line 757, in connect self.timeout, self.source_address) File "/usr/lib/python2.7/socket.py", line 553, in create_connection for res in getaddrinfo(host, port, 0, SOCK_STREAM): IOError: [Errno socket error] [Errno -2] Name or service not known Setting up ttf-mscorefonts-installer (3.4ubuntu3) ... bob@bobStudio:~$ sudo apt-get update E: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable) E: Unable to lock directory /var/lib/apt/lists/ bob@bobStudio:~$ sudo rm /var/lib/dpkg/lock bob@bobStudio:~$ sudo apt-get update E: Could not get lock /var/lib/apt/lists/lock - open (11: Resource temporarily unavailable) E: Unable to lock directory /var/lib/apt/lists/ The good news is that, once I sorted out the file locks, this seems to have permanently aborted the setup of the font package, so at least I can do something else with dpkg. That leaves two questions: 1) How could I have broken the loop without actually crashing out of dpkg? 2) How can I set up the ttf-mscorefonts-installer package in the future? Is this download really broken, or is it 'just' a bad Internet connection?

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  • SQL Server Table Polling by Multiple Subscribers

    - by Daniel Hester
    Background Designing Stored Procedures that are safe for multiple subscribers (to call simultaneously) can be challenging.  For example let’s say that you want multiple worker processes to poll a shared work queue that’s encapsulated as a SQL Table. This is a common scenario and through experience you’ll find that you want to use Table Hints to prevent unwanted locking when performing simultaneous queries on the same table. There are three table hints to consider: NOLOCK, READPAST and UPDLOCK. Both NOLOCK and READPAST table hints allow you to SELECT from a table without placing a LOCK on that table. However, SELECTs with the READPAST hint will ignore any records that are locked due to being updated/inserted (or otherwise “dirty”), whereas a SELECT with NOLOCK ignores all locks including dirty reads. For the initial update of the flag (that marks the record as available for subscription) I don’t use the NOLOCK Table Hint because I want to be sensitive to the “active” records in the table and I want to exclude them.  I use an Update Lock (UPDLOCK) in conjunction with a WHERE clause that uses a sub-select with a READPAST Table Hint in order to explicitly lock the records I’m updating (UPDLOCK) but not place a lock on the table when selecting the records that I’m going to update (READPAST). UPDATES should be allowed to lock the rows affected because we’re probably changing a flag on a record so that it is not included in a SELECT from another subscriber. On the UPDATE statement we should explicitly use the UPDLOCK to guard against lock escalation. A SELECT to check for the next record(s) to process can result in a shared read lock being held by more than one subscriber polling the shared work queue (SQL table). It is expected that more than one worker process (or server) might try to process the same new record(s) at the same time. When each process then tries to obtain the update lock, none of them can because another process has a shared read lock in place. Thus without the UPDLOCK hint the result would be a lock escalation deadlock; however with the UPDLOCK hint this condition is mitigated against. Note that using the READPAST table hint requires that you also set the ISOLATION LEVEL of the transaction to be READ COMMITTED (rather than the default of SERIALIZABLE). Guidance In the Stored Procedure that returns records to the multiple subscribers: Perform the UPDATE first. Change the flag that makes the record available to subscribers.  Additionally, you may want to update a LastUpdated datetime field in order to be able to check for records that “got stuck” in an intermediate state or for other auditing purposes. In the UPDATE statement use the (UPDLOCK) Table Hint on the UPDATE statement to prevent lock escalation. In the UPDATE statement also use a WHERE Clause that uses a sub-select with a (READPAST) Table Hint to select the records that you’re going to update. In the UPDATE statement use the OUTPUT clause in conjunction with a Temporary Table to isolate the record(s) that you’ve just updated and intend to return to the subscriber. This is the fastest way to update the record(s) and to get the records’ identifiers within the same operation. Finally do a set-based SELECT on the main Table (using the Temporary Table to identify the records in the set) with either a READPAST or NOLOCK table hint.  Use NOLOCK if there are other processes (besides the multiple subscribers) that might be changing the data that you want to return to the multiple subscribers; or use READPAST if you're sure there are no other processes (besides the multiple subscribers) that might be updating column data in the table for other purposes (e.g. changes to a person’s last name).  NOLOCK is generally the better fit in this part of the scenario. See the following as an example: CREATE PROCEDURE [dbo].[usp_NewCustomersSelect] AS BEGIN -- OVERRIDE THE DEFAULT ISOLATION LEVEL SET TRANSACTION ISOLATION LEVEL READ COMMITTED -- SET NOCOUNT ON SET NOCOUNT ON -- DECLARE TEMP TABLE -- Note that this example uses CustomerId as an identifier; -- you could just use the Identity column Id if that’s all you need. DECLARE @CustomersTempTable TABLE ( CustomerId NVARCHAR(255) ) -- PERFORM UPDATE FIRST -- [Customers] is the name of the table -- [Id] is the Identity Column on the table -- [CustomerId] is the business document key used to identify the -- record globally, i.e. in other systems or across SQL tables -- [Status] is INT or BIT field (if the status is a binary state) -- [LastUpdated] is a datetime field used to record the time of the -- last update UPDATE [Customers] WITH (UPDLOCK) SET [Status] = 1, [LastUpdated] = GETDATE() OUTPUT [INSERTED].[CustomerId] INTO @CustomersTempTable WHERE ([Id] = (SELECT TOP 100 [Id] FROM [Customers] WITH (READPAST) WHERE ([Status] = 0) ORDER BY [Id] ASC)) -- PERFORM SELECT FROM ENTITY TABLE SELECT [C].[CustomerId], [C].[FirstName], [C].[LastName], [C].[Address1], [C].[Address2], [C].[City], [C].[State], [C].[Zip], [C].[ShippingMethod], [C].[Id] FROM [Customers] AS [C] WITH (NOLOCK), @CustomersTempTable AS [TEMP] WHERE ([C].[CustomerId] = [TEMP].[CustomerId]) END In a system that has been designed to have multiple status values for records that need to be processed in the Work Queue it is necessary to have a “Watch Dog” process by which “stale” records in intermediate states (such as “In Progress”) are detected, i.e. a [Status] of 0 = New or Unprocessed; a [Status] of 1 = In Progress; a [Status] of 2 = Processed; etc.. Thus, if you have a business rule that states that the application should only process new records if all of the old records have been processed successfully (or marked as an error), then it will be necessary to build a monitoring process to detect stalled or stale records in the Work Queue, hence the use of the LastUpdated column in the example above. The Status field along with the LastUpdated field can be used as the criteria to detect stalled / stale records. It is possible to put this watchdog logic into the stored procedure above, but I would recommend making it a separate monitoring function. In writing the stored procedure that checks for stale records I would recommend using the same kind of lock semantics as suggested above. The example below looks for records that have been in the “In Progress” state ([Status] = 1) for greater than 60 seconds: CREATE PROCEDURE [dbo].[usp_NewCustomersWatchDog] AS BEGIN -- TO OVERRIDE THE DEFAULT ISOLATION LEVEL SET TRANSACTION ISOLATION LEVEL READ COMMITTED -- SET NOCOUNT ON SET NOCOUNT ON DECLARE @MaxWait int; SET @MaxWait = 60 IF EXISTS (SELECT 1 FROM [dbo].[Customers] WITH (READPAST) WHERE ([Status] = 1) AND (DATEDIFF(s, [LastUpdated], GETDATE()) > @MaxWait)) BEGIN SELECT 1 AS [IsWatchDogError] END ELSE BEGIN SELECT 0 AS [IsWatchDogError] END END Downloads The zip file below contains two SQL scripts: one to create a sample database with the above stored procedures and one to populate the sample database with 10,000 sample records.  I am very grateful to Red-Gate software for their excellent SQL Data Generator tool which enabled me to create these sample records in no time at all. References http://msdn.microsoft.com/en-us/library/ms187373.aspx http://www.techrepublic.com/article/using-nolock-and-readpast-table-hints-in-sql-server/6185492 http://geekswithblogs.net/gwiele/archive/2004/11/25/15974.aspx http://grounding.co.za/blogs/romiko/archive/2009/03/09/biztalk-sql-receive-location-deadlocks-dirty-reads-and-isolation-levels.aspx

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  • Are there cloud network drives that let users lock files or mark them as "in use"?

    - by Brandon Craig Rhodes
    Having spent several hours reading about the features and limitations of services like DropBox and Jungle Disk and the hundreds of competitors they seem to have (as though everyone with an AWS account these days goes ahead and writes a file sharing application just for fun), I have yet to find one that would let a team of people at a small business collaborate without stepping all over each other's toes. At a small business there are often many small documents per project — estimates, contracts, project plans, budgets — and team members frequently have to open and edit them, with all sorts of problems happening if two people edit a file at once. Even if a sharing service is smart enough to keep both versions of the file created, most small-business software (like word processors, spreadsheets, estimating software, or billing systems) has no way to compare — much less to merge! — the changes in two rival versions of a file that two people edited at the same time without each other's knowledge. So, my question: are their cloud-based file sharing solutions that not only provide a virtual network drive that people can access, but that also let users lock files — even if it's not a real lock but just a flag or indicator — that could possibly prevent remote workers from both editing the same file at once? Having one person wait for another person to finish editing is a very, very small inconvenience compared to the hour or more than it can take to compare two estimates by hand until you find and resolve the rival changes. Given this fact, I am surprised that almost none of the popular file sharing solutions seem to recognize this problem and provide some solution! Does anyone know of a service that does?

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  • Are there cloud network drives that let users lock files or mark them as "in use"?

    - by Brandon Craig Rhodes
    Having spent several hours reading about the features and limitations of services like DropBox and Jungle Disk and the hundreds of competitors they seem to have (as though everyone with an AWS account these days goes ahead and writes a file sharing application just for fun), I have yet to find one that would let a team of people at a small business collaborate without stepping all over each other's toes. At a small business there are often many small documents per project — estimates, contracts, project plans, budgets — and team members frequently have to open and edit them, with all sorts of problems happening if two people edit a file at once. Even if a sharing service is smart enough to keep both versions of the file created, most small-business software (like word processors, spreadsheets, estimating software, or billing systems) has no way to compare — much less to merge! — the changes in two rival versions of a file that two people edited at the same time without each other's knowledge. So, my question: are their cloud-based file sharing solutions that not only provide a virtual network drive that people can access, but that also let users lock files — even if it's not a real lock but just a flag or indicator — that could possibly prevent remote workers from both editing the same file at once? Having one person wait for another person to finish editing is a very, very small inconvenience compared to the hour or more than it can take to compare two estimates by hand until you find and resolve the rival changes. Given this fact, I am surprised that almost none of the popular file sharing solutions seem to recognize this problem and provide some solution! Does anyone know of a service that does?

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  • Easy way to lock a file on a remote machine (windows)?

    - by roufamatic
    I've tracked down an error in my logs, and am trying to reproduce it. My theory is that a file sometimes gets locked in a specific folder, and when the application (ASP.NET) tries to delete that folder it hangs. I don't have the application running on my own machine so I'm debugging this on a remote server. But for the life of me, I can't seem to figure out a way to lock a file that prevents it from being deleted by the process. My first thought was to map the network path to a local drive and just leave a command prompt open to that folder. Locally that always fouls up my folder deletes, but apparently SMB is a bit more robust and doesn't grant me a lock. After that I created an infinte loop vbscript in the folder and executed it remotely. The file was deleted out from underneath the executing code. Man! I then tried creating a file on the server in that folder and removing all permissions. That didn't do the trick. I don't have access to the IIS settings so perhaps it's running under a privileged system account. So: what's a program that you know is free and I can quickly use to create an exclusive lock on a file so I can test my delete theory? Like a really, really bad Notepad clone or something. :-)

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  • how to disable RemoteApp sessions lock if idle for 10 minutes and require no user needs to input password to unlock?

    - by Carlos Sanchez
    RemoteApp sessions lock if idle for 10 minutes, user needs to input password to unlock. My users are running an application from Win2008 Terminal server using RemoteApp. If the application remains idle for 10 minutes it gets "locked" and the user is required to enter username and password to continue using it. This is VERY VERY annoying as the app usually sits idle for bout 20-30 minutes, used for 1 min... repeat.

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  • how to disable RemoteApp sessions lock if idle for 10 minutes and require no user needs to input password to unlock?

    - by Carlos Sanchez
    RemoteApp sessions lock if idle for 10 minutes, user needs to input password to unlock. My users are running an application from Win2008 Terminal server using RemoteApp. If the application remains idle for 10 minutes it gets "locked" and the user is required to enter username and password to continue using it. This is VERY VERY annoying as the app usually sits idle for bout 20-30 minutes, used for 1 min... repeat.

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  • Can I use ext3 as a shared fs if I add a lock manager ?

    - by edomaur
    I need a cluster filesystem for an iSCSI device. The problem is that the servers to which it is connected generate datafiles which must be read by every other servers. Except for the writing and deleting of such files, I do not need a full locking scheme like in OCFS2 or GFS2. So, can I use a distributed lock manager (DLM) on top of an ext3 filesystem or must I use only specialized filesystem ?

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