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• C#: Can you detect whether or not the current execution context is within `lock (this)`?

  - by John Gietzen

  If I have an object that I would like to force to be accessed from within a lock, like so: var obj = new MyObject(); lock (obj) { obj.AddOne(); obj.RemoveOne(); } Is it possible, from within the AddOne and RemoveOne functions to detect whether the current execution context is within a lock? Something like: Monitor.AreWeCurrentlyEnteredInto(this)

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• Syncing Data to Remote Services, Best Practices for Caching?

  - by viatropos

  I want to be able to publish events to Eventbrite, Eventful, and Google Calendar for my Google Apps. Each service has slightly different properties for events... I will be syncing many other things too, such as users with Google Contacts and MailChimp, Documents with Google Docs and some other services, etc... So I'm wondering, what is the recommended way of retrieving the data for the end user so that it's reasonably maintainable and optimized? Here are the things I'm thinking that I'm having trouble with: My App keeps a central database of all the models (Event, Document, User, Form, etc.), and whenever Admin creates an object (e.g. create through Eventbrite or through our Admin panel), we sync them and store a copy in our local database. When User goes to the site /events, App retrieves the events from the database. Read Events from a target feed, such as the Eventbrite or Eventful feed, and scrap the local database. Basically, I'm wondering, if we're storing all of the data on a remote service, do we really need to have a local database copy of the data? When would we need to have a local database, when wouldn't we?

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• Is there a better way to throttle a high throughput job?

  - by ChaosPandion

  I created a simple class that shows what I am trying to do without any noise. Feel free to bash away at my code. That's why I posted it here. public class Throttled : IDisposable { private readonly Action work; private readonly Func<bool> stop; private readonly ManualResetEvent continueProcessing; private readonly Timer throttleTimer; private readonly int throttlePeriod; private readonly int throttleLimit; private int totalProcessed; public Throttled(Action work, Func<bool> stop, int throttlePeriod, int throttleLimit) { this.work = work; this.stop = stop; this.throttlePeriod = throttlePeriod; this.throttleLimit = throttleLimit; continueProcessing = new ManualResetEvent(true); throttleTimer = new Timer(ThrottleUpdate, null, throttlePeriod, throttlePeriod); } public void Dispose() { throttleTimer.Dispose(); ((IDisposable)continueProcessing).Dispose(); } public void Execute() { while (!stop()) { if (Interlocked.Increment(ref totalProcessed) > throttleLimit) { lock (continueProcessing) { continueProcessing.Reset(); } if (!continueProcessing.WaitOne(throttlePeriod)) { throw new TimeoutException(); } } work(); } } private void ThrottleUpdate(object state) { Interlocked.Exchange(ref totalProcessed, 0); lock (continueProcessing) { continueProcessing.Set(); } } }

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• .NET platform independant sync framework

  - by Quandary

  Question: I need to synchronize a few ActionScript files from my computer to a network share (backup). I saw a quick fix would be using Microsoft Sync Framework for this, and write a windows service. My problem is I also use Linux, and before I start with MS vendor lockin, is there any sync framework/library/whatever I could use that works accross platform? Or does the MS sync framework work on Linux, too? It is my understanding that it is a wrapper around some com objects, thus it wouldn't work. All I need is synchronizing files. So never mind the database part, although it would be nice to have it, too.

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• Develop a custom file sync provider over file transfer WCF service

  - by user336622

  Hi All I'm trying to develop a custom sync provider to sync files over the internet using WCF service. We already have a file transfer service and it's working in production but I need to implement a custom sync provider on top of it. I've checked the solution in Braynt Likes blog here but this doesn't fulfill my needs. Any advices, guidance, or sample code to help is really appreciated?

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• Do the 'up to date' guarantees provided by final field in Java's memory model extend to indirect ref

  - by mattbh

  The Java language spec defines semantics of final fields in section 17.5: The usage model for final fields is a simple one. Set the final fields for an object in that object's constructor. Do not write a reference to the object being constructed in a place where another thread can see it before the object's constructor is finished. If this is followed, then when the object is seen by another thread, that thread will always see the correctly constructed version of that object's final fields. It will also see versions of any object or array referenced by those final fields that are at least as up-to-date as the final fields are. My question is - does the 'up-to-date' guarantee extend to the contents of nested arrays, and nested objects? An example scenario: Thread A constructs a HashMap of ArrayLists, then assigns the HashMap to final field 'myFinal' in an instance of class 'MyClass' Thread B sees a (non-synchronized) reference to the MyClass instance and reads 'myFinal', and accesses and reads the contents of one of the ArrayLists In this scenario, are the members of the ArrayList as seen by Thread B guaranteed to be at least as up to date as they were when MyClass's constructor completed?

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• Will this make the object thread-safe?

  - by sharptooth

  I have a native Visual C++ COM object and I need to make it completely thread-safe to be able to legally mark it as "free-threaded" in th system registry. Specifically I need to make sure that no more than one thread ever accesses any member variable of the object simultaneously. The catch is I'm almost sure that no sane consumer of my COM object will ever try to simultaneously use the object from more than one thread. So I want the solution as simple as possible as long as it meets the requirement above. Here's what I came up with. I add a mutex or critical section as a member variable of the object. Every COM-exposed method will acquire the mutex/section at the beginning and release before returning control. I understand that this solution doesn't provide fine-grained access and this might slow execution down, but since I suppose simultaneous access will not really occur I don't care of this. Will this solution suffice? Is there a simpler solution?

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• Open source framework à la Microsoft Sync Framework suggestions?

  - by drskol

  We are implementing a warehouse management system atop an open source stack (Java, web services & friends). In this system, we want to integrate many mobile devices which should also be capable of adequate online/offline functionality, e.g. preparing database inserts while a mobile device is temporarily unconnected, and performing them on the backend database when reconnected. For a .NET stack, Microsoft Sync Framework would be a perfect solution, e.g. to do database replication and hoarding. Can anyone suggest an open source alternative to the MS Sync Framework and possibly describe his experiences with it? Thanks in advance for any answers.

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• Booking logic and architecture, database sync: Hotels, tennis courts reservation system ...

  - by coulix

  Hello Stackers, Imagine that you want to design a tennis booking system. You have 5 tennis clubs as partenrs with no online api allowing you to check on their side if a court is booked or not: You have to build this part as well. Every time a booking is done on their side you want it to be know by our system. Probably using a POST request form tennis partner to our server. Every time a booking is done on our website, we want to push the booking to their system. The difficulty is that their system need to be online and accessible from outside. Ip may change, we have to use a dns updater. In case their system is not available we still accept the booking and fallback to an async email with 'i confirm booking/reject booking' link sent to the club. I find the whole process quite complex and was wondering about the way online hotel booking system and hotel were working. Do they all have their data open and online ? The good thing is that the data will grow large and fits nicely to some no SQL ;) like couch db

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• Questions about HTML5 audio

  - by Nimbuz

  <audio src="http://upload.wikimedia.org/wikipedia/commons/8/82/Riddle_song.ogg"></audio> <ul id="lyrics"> <li>line 1</li> <li>line 2</li> <li>line 3</li> <li>and so on...</li> </ul><!-- end #lyrics --> So I want to: Highlight (change color or background) of the line that is being played. Save current time to a cookie and resume on next visit. I'm not sure if either of these are possible in HTML5, but even in Flash or other technology, I'd like to know if and how it is possible. I understand #2 is asking too much, but #1 is really important. So almost similar to this: http://randallagordon.com/jaraoke/ but all the lines are visible, just the current line is highlighted. Many thanks for your help.

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• Sync data between a windows desktop app and windows mobile client app

  - by Chris W

  I need to knock up a very quick prototype/proof of concept application to demo to someone within the next couple of days so I've minimal time to research this as fully as I normally would. The set-up is a very simple database application running on a laptop - will only ever be a single user updating a couple of tables so I was thinking of knocking up a basic Win Forms app against SQL Compact. Visual Studio's auto generated data grid edit screens will be fine with a little customisation. The second aspect is to then add a windows mobile client application that can pull data from both tables stored on the laptop, edit some data and insert some extra rows before sending the changes back to the laptop copy of the database. I've not done any WinMo development so what's the best approach for me to look at. Is it easy enough to sync data between the two databases when the WinMo device is connected to the laptop with USB? Most of the samples I've looked at so far seem to be syncing SQL Compact with SQL Standard using IIS which seems a bit overkill. The volumes of data to be synced are so small that I can easily write some manual sync code if it's easy for me to query/update the Compact DB from the laptop application when the device is connected.

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• What is best way of keeping two database schema in synch?

  - by ebeworld

  We have two SQL server database, and one is kept in US and being changed from day to day and the other one is in India. What is the best way of keeping two database schema in synch. Does any functionality like sql server replication help?

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• Emacs: Often switching between Emacs and my IDE's editor, how can I 'synch' the file?

  - by WizardOfOdds

  I very often need to do some Emacs magic on some files and I need to go back and forth my IDE (IntelliJ IDEA) and Emacs. When a change is made under Emacs (and after I've saved the file) and I go back to IntelliJ the change appears immediately (if I recall correctly I configured IntelliJ to "always reload file when a modification is detected on disk" or something like that). I don't even need to reload: as soon as IntelliJ IDEA gains focus, it instantly reloads the file (and I hence have immediately access to the modifications I made from Emacs). So far, so very good. However "the other way round", it doesn't work yet. Can I configure Emacs so that everytime a file is changed on disk it reloads it? Or make Emacs, everytime it "gains focus", verify if any file currently opened has been modified on disk? I know I can start modifying the buffer under Emacs and it shall instantly warn that it has been modified, but I'd rather have it do it immediately (for example if I used my IDE to do some big change, when I come back to Emacs what I see may not be at all anymore what the file contains and it's a bit weird).

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• MS Sync framework - Identity crisis resolution by partitioning the primary key.

  - by user326136

  Hello, We implementing offline feature to an existing application. We have implemented the syn with SQL Server internal change tracking and over WCF using MS Sync Framework (http://msdn.microsoft.com/en-us/sync/default.aspx) All of our tables have primary key as integer, we cannot move to GUID. So as you are thinking we will have identity crises between applications. So we decided to go with the way Merge replication does(http://msdn.microsoft.com/en-us/library/aa179416(SQL.80).aspx) partition the primary key range. Below is the example scenario - Server Table A - ID Range - 0 to 100 Client 1 Table A - ID Range - 101 to 200 Client 2 Table A - ID Range - 201 to 300 how to implement this ? i know we can use BCC CHECKIDENT (yourtable, reseed, value) CHECK (([ID]<=(100))) but this does not solve the issue.... Merge replication provides an option of "Not for replication"(http://msdn.microsoft.com/en-us/library/aa237102(SQL.80).aspx) to achieve insert form clients and still maintain the set range.. can i use that somehow here? please help...

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• How to synchronize two (or n) replication processes for MS SQL databases?

  - by Yauheni Sivukha

  There are two master databases and two read-only copies updated by standard transactional replication. It is needed to map some entity from both read-only databases, lets say that A databases contains orders and B databases contains lines. The problem is that replication to one database can lag behind replication of second database, and at the moment of mapping R-databases will have inconsistent data. For example. We stored 2 orders with lines at 19:00 and 19:03. Mapping process started at 19:05, but to the moment of mapping A database replication processed all changes up to 19:03, but B database replication processed only changes up to 19:00. After mapping we will have order entity with order as of 19:03 and lines as of 19:00. The troubles are guaranteed:) In my particular case both databases have temporal model, so it is possible to fetch data for every time slice, but the problem is to identify time of latest replication. Question: How to synchronize replication processes for several databases to avoid situation described above?

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• Does the Java Memory Model (JSR-133) imply that entering a monitor flushes the CPU data cache(s)?

  - by Durandal

  There is something that bugs me with the Java memory model (if i even understand everything correctly). If there are two threads A and B, there are no guarantees that B will ever see a value written by A, unless both A and B synchronize on the same monitor. For any system architecture that guarantees cache coherency between threads, there is no problem. But if the architecture does not support cache coherency in hardware, this essentially means that whenever a thread enters a monitor, all memory changes made before must be commited to main memory, and the cache must be invalidated. And it needs to be the entire data cache, not just a few lines, since the monitor has no information which variables in memory it guards. But that would surely impact performance of any application that needs to synchronize frequently (especially things like job queues with short running jobs). So can Java work reasonably well on architectures without hardware cache-coherency? If not, why doesn't the memory model make stronger guarantees about visibility? Wouldn't it be more efficient if the language would require information what is guarded by a monitor? As i see it the memory model gives us the worst of both worlds, the absolute need to synchronize, even if cache coherency is guaranteed in hardware, and on the other hand bad performance on incoherent architectures (full cache flushes). So shouldn't it be more strict (require information what is guarded by a monitor) or more lose and restrict potential platforms to cache-coherent architectures? As it is now, it doesn't make too much sense to me. Can somebody clear up why this specific memory model was choosen? EDIT: My use of strict and lose was a bad choice in retrospect. I used "strict" for the case where less guarantees are made and "lose" for the opposite. To avoid confusion, its probably better to speak in terms of stronger or weaker guarantees.

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• How to synchronize two (or n) replication processes for SQL Server databases?

  - by Yauheni Sivukha

  There are two master databases and two read-only copies updated by standard transactional replication. It is needed to map some entity from both read-only databases, lets say that A databases contains orders and B databases contains lines. The problem is that replication to one database can lag behind replication of second database, and at the moment of mapping R-databases will have inconsistent data. For example. We stored 2 orders with lines at 19:00 and 19:03. Mapping process started at 19:05, but to the moment of mapping A database replication processed all changes up to 19:03, but B database replication processed only changes up to 19:00. After mapping we will have order entity with order as of 19:03 and lines as of 19:00. The troubles are guaranteed:) In my particular case both databases have temporal model, so it is possible to fetch data for every time slice, but the problem is to identify time of latest replication. Question: How to synchronize replication processes for several databases to avoid situation described above? Or, in other words, how to compare last time of replication in each database? UPD: The only way I see to synchronize is to continuously write timestamps into service tables in each database and to check these timestamps on replicated servers. Is that acceptable solution?

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• Sync My Documents folder, using Active Directory?

  - by GuHum

  Hey I'm trying to set up a backup feature for the users in a Active Directory Domain. The wish is for the clients "my docuemnts" folder to sync to a folder on the file server. Is this doable using Active Directory only, or do one need any third party applications? If its doable, what steps is needed to set this up, on the server, and clients? Thank you in advance

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• What's the most auditable way to organize database maintenance scripting/scheduling?

  - by RenderIn

  I'm using PHP, Oracle and crontab. Crontab is invoking a php script, which is going to synchronize some data between a local and remote database. First I thought about putting this all in one large, anonymous inline PL/SQL block and executing it in PHP. The problem is that without creating a table to audit the procedure it's really opaque to my PHP code until it returns. And then when it does return, the best I can do is receive a string in an out parameter which I've concatenated together as an audit log. Then I considered breaking it up into several SQL statements and have PHP do some intermediate auditing/logging and manipulation of the data between each of them. I'm not really satisfied with either of these. How do you organize maintenance code that is called in a cronjob?

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• Why is CompareAndSwap instruction considered expensive?

  - by eyalw

  Why is CompareAndSwap instruction considered expensive? I read in a book: "Memory barriers are expensive, about as expensive as an atomic compareAndSet() instruction." Thanks!

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• How to sync two folders in PHP?

  - by OM The Eternity

  I am using windows, Mysql DB, PHP I am building the Joomla Component whose one of the functionality is to synchronize the folders I want to sync two folders of different name.. How can I do this? It has to be dne in the same machine, no two different servers are involved in it.. How to sync two folders in PHP?

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• How can I sync my www files b/w servers?

  - by Bqm

  I want to sync my data b/w all my web servers. How can this be done?

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• How to address thread-safety of service data used for maintaining static local variables in C++?

  - by sharptooth

  Consider the following scenario. We have a C++ function with a static local variable: void function() { static int variable = obtain(); //blahblablah } the function needs to be called from multiple threads concurrently, so we add a critical section to avoid concurrent access to the static local: void functionThreadSafe() { CriticalSectionLockClass lock( criticalSection ); static int variable = obtain(); //blahblablah } but will this be enough? I mean there's some magic that makes the variable being initialized no more than once. So there's some service data maintained by the runtime that indicates whether each static local has already been initialized. Will the critical section in the above code protect that service data as well? Is any extra protection required for this scenario?

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• singleton pattern in java- lazy Intialization

  - by flash

  public static MySingleton getInstance() { if (_instance==null) { synchronized (MySingleton.class) { _instance = new MySingleton(); } } return _instance; } 1.is there a flaw with the above implementation of the getInstance method? 2.What is the difference between the two implementations.? public static synchronized MySingleton getInstance() { if (_instance==null) { _instance = new MySingleton(); } return _instance; } I have seen a lot of answers on the singleton pattern in stackoverflow but the question I have posted is to know mainly difference of 'synchronize' at method and block level in this particular case.

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• How to synchronize access to many objects

  - by vividos

  I have a thread pool with some threads (e.g. as many as number of cores) that work on many objects, say thousands of objects. Normally I would give each object a mutex to protect access to its internals, lock it when I'm doing work, then release it. When two threads would try to access the same object, one of the threads has to wait. Now I want to save some resources and be scalable, as there may be thousands of objects, and still only a hand full of threads. I'm thinking about a class design where the thread has some sort of mutex or lock object, and assigns the lock to the object when the object should be accessed. This would save resources, as I only have as much lock objects as I have threads. Now comes the programming part, where I want to transfer this design into code, but don't know quite where to start. I'm programming in C++ and want to use Boost classes where possible, but self written classes that handle these special requirements are ok. How would I implement this? My first idea was to have a boost::mutex object per thread, and each object has a boost::shared_ptr that initially is unset (or NULL). Now when I want to access the object, I lock it by creating a scoped_lock object and assign it to the shared_ptr. When the shared_ptr is already set, I wait on the present lock. This idea sounds like a heap full of race conditions, so I sort of abandoned it. Is there another way to accomplish this design? A completely different way?

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