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  • Python Locking Implementation (with threading module)

    - by Matty
    This is probably a rudimentary question, but I'm new to threaded programming in Python and am not entirely sure what the correct practice is. Should I be creating a single lock object (either globally or being passed around) and using that everywhere that I need to do locking? Or, should I be creating multiple lock instances in each of the classes where I will be employing them. Take these 2 rudimentary code samples, which direction is best to go? The main difference being that a single lock instance is used in both class A and B in the second, while multiple instances are used in the first. Sample 1 class A(): def __init__(self, theList): self.theList = theList self.lock = threading.Lock() def poll(self): while True: # do some stuff that eventually needs to work with theList self.lock.acquire() try: self.theList.append(something) finally: self.lock.release() class B(threading.Thread): def __init__(self,theList): self.theList = theList self.lock = threading.Lock() self.start() def run(self): while True: # do some stuff that eventually needs to work with theList self.lock.acquire() try: self.theList.remove(something) finally: self.lock.release() if __name__ == "__main__": aList = [] for x in range(10): B(aList) A(aList).poll() Sample 2 class A(): def __init__(self, theList,lock): self.theList = theList self.lock = lock def poll(self): while True: # do some stuff that eventually needs to work with theList self.lock.acquire() try: self.theList.append(something) finally: self.lock.release() class B(threading.Thread): def __init__(self,theList,lock): self.theList = theList self.lock = lock self.start() def run(self): while True: # do some stuff that eventually needs to work with theList self.lock.acquire() try: self.theList.remove(something) finally: self.lock.release() if __name__ == "__main__": lock = threading.Lock() aList = [] for x in range(10): B(aList,lock) A(aList,lock).poll()

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  • Utility that helps in file locking - expert tips wanted

    - by maix
    I've written a subclass of file that a) provides methods to conveniently lock it (using fcntl, so it only supports unix, which is however OK for me atm) and b) when reading or writing asserts that the file is appropriately locked. Now I'm not an expert at such stuff (I've just read one paper [de] about it) and would appreciate some feedback: Is it secure, are there race conditions, are there other things that could be done better … Here is the code: from fcntl import flock, LOCK_EX, LOCK_SH, LOCK_UN, LOCK_NB class LockedFile(file): """ A wrapper around `file` providing locking. Requires a shared lock to read and a exclusive lock to write. Main differences: * Additional methods: lock_ex, lock_sh, unlock * Refuse to read when not locked, refuse to write when not locked exclusivly. * mode cannot be `w` since then the file would be truncated before it could be locked. You have to lock the file yourself, it won't be done for you implicitly. Only you know what lock you need. Example usage:: def get_config(): f = LockedFile(CONFIG_FILENAME, 'r') f.lock_sh() config = parse_ini(f.read()) f.close() def set_config(key, value): f = LockedFile(CONFIG_FILENAME, 'r+') f.lock_ex() config = parse_ini(f.read()) config[key] = value f.truncate() f.write(make_ini(config)) f.close() """ def __init__(self, name, mode='r', *args, **kwargs): if 'w' in mode: raise ValueError('Cannot open file in `w` mode') super(LockedFile, self).__init__(name, mode, *args, **kwargs) self.locked = None def lock_sh(self, **kwargs): """ Acquire a shared lock on the file. If the file is already locked exclusively, do nothing. :returns: Lock status from before the call (one of 'sh', 'ex', None). :param nonblocking: Don't wait for the lock to be available. """ if self.locked == 'ex': return # would implicitly remove the exclusive lock return self._lock(LOCK_SH, **kwargs) def lock_ex(self, **kwargs): """ Acquire an exclusive lock on the file. :returns: Lock status from before the call (one of 'sh', 'ex', None). :param nonblocking: Don't wait for the lock to be available. """ return self._lock(LOCK_EX, **kwargs) def unlock(self): """ Release all locks on the file. Flushes if there was an exclusive lock. :returns: Lock status from before the call (one of 'sh', 'ex', None). """ if self.locked == 'ex': self.flush() return self._lock(LOCK_UN) def _lock(self, mode, nonblocking=False): flock(self, mode | bool(nonblocking) * LOCK_NB) before = self.locked self.locked = {LOCK_SH: 'sh', LOCK_EX: 'ex', LOCK_UN: None}[mode] return before def _assert_read_lock(self): assert self.locked, "File is not locked" def _assert_write_lock(self): assert self.locked == 'ex', "File is not locked exclusively" def read(self, *args): self._assert_read_lock() return super(LockedFile, self).read(*args) def readline(self, *args): self._assert_read_lock() return super(LockedFile, self).readline(*args) def readlines(self, *args): self._assert_read_lock() return super(LockedFile, self).readlines(*args) def xreadlines(self, *args): self._assert_read_lock() return super(LockedFile, self).xreadlines(*args) def __iter__(self): self._assert_read_lock() return super(LockedFile, self).__iter__() def next(self): self._assert_read_lock() return super(LockedFile, self).next() def write(self, *args): self._assert_write_lock() return super(LockedFile, self).write(*args) def writelines(self, *args): self._assert_write_lock() return super(LockedFile, self).writelines(*args) def flush(self): self._assert_write_lock() return super(LockedFile, self).flush() def truncate(self, *args): self._assert_write_lock() return super(LockedFile, self).truncate(*args) def close(self): self.unlock() return super(LockedFile, self).close() (the example in the docstring is also my current use case for this) Thanks for having read until down here, and possibly even answering :)

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  • Thread locking issue with FileHelpers between calling engine.ReadNext() method and readign engine.Li

    - by Rad
    I use producer/consumer pattern with FileHelpers library to import data from one file (which can be huge) using multiple threads. Each thread is supposed to import a chunk of that file and I would like to use LineNumber property of the FileHelperAsyncEngine instance that is reading the file as primary key for imported rows. FileHelperAsyncEngine internally has an IEnumerator IEnumerable.GetEnumerator(); which is iterated over using engine.ReadNext() method. That internally sets LineNumber property (which seems is not thread safe). Consumers will have Producers assiciated with them that will supply DataTables to Consumers which will consume them via SqlBulkLoad class which will use IDataReader implementation which will iterate over a collection of DataTables which are internal to a Consumer instance. Each instance of will have one SqlBulkCopy instance associate with it. I have thread locking issue. Below is how I create multiple Producer threads. I start each thread afterwords. Produce method on a producer instance will be called determining which chunk of input file will be processed. It seems that engine.LineNumber is not thread safe and I doesn't import a proper LineNumber in the database. It seems that by the time engine.LineNumber is read some other thread called engine.ReadNext() and changed engine.LineNumber property. I don't want to lock the loop that is supposed to process a chunk of input file because I loose parallelism. How to reorganize the code to solve this threading issue? Thanks Rad for (int i = 0; i < numberOfProducerThreads; i++) DataConsumer consumer = dataConsumers[i]; //create a new producer DataProducer producer = new DataProducer(); //consumer has already being created consumer.Subscribe(producer); FileHelperAsyncEngine orderDetailEngine = new FileHelperAsyncEngine(recordType); orderDetailEngine.Options.RecordCondition.Condition = RecordCondition.ExcludeIfBegins; orderDetailEngine.Options.RecordCondition.Selector = STR_ORDR; int skipLines = i * numberOfBufferTablesToProcess * DataBuffer.MaxBufferRowCount; Thread newThread = new Thread(() => { producer.Produce(consumer, inputFilePath, lineNumberFieldName, dict, orderDetailEngine, skipLines, numberOfBufferTablesToProcess); consumer.SetEndOfData(producer); }); producerThreads.Add(newThread); thread.Start();} public void Produce(DataConsumer consumer, string inputFilePath, string lineNumberFieldName, Dictionary<string, object> dict, FileHelperAsyncEngine engine, int skipLines, int numberOfBufferTablesToProcess) { lock (this) { engine.Options.IgnoreFirstLines = skipLines; engine.BeginReadFile(inputFilePath); } int rowCount = 1; DataTable buffer = consumer.BufferDataTable; while (engine.ReadNext() != null) { lock (this) { dict[lineNumberFieldName] = engine.LineNumber; buffer.Rows.Add(ObjectFieldsDataRowMapper.MapObjectFieldsToDataRow(engine.LastRecord, dict, buffer)); if (rowCount % DataBuffer.MaxBufferRowCount == 0) { consumer.AddBufferDataTable(buffer); buffer = consumer.BufferDataTable; } if (rowCount % (numberOfBufferTablesToProcess * DataBuffer.MaxBufferRowCount) == 0) { break; } rowCount++; } } if (buffer.Rows.Count > 0) { consumer.AddBufferDataTable(buffer); } engine.Close(); }

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  • SQLite file locking and DropBox

    - by Alex Jenter
    I'm developing an app in Visual C++ that uses an SQLite3 DB for storing data. Usually it sits in the tray most of the time. I also would like to enable putting my app in a DropBox folder to share it across several PCs. It worked really well up until DropBox has recently updated itself. And now it says that it "can't sync the file in use". The SQLite file is open in my app, but the lock is shared. There are some prepared statements, but all are reset immediately after using step. Is there any way to enable synchronizing of an open SQLite database file? Thanks! Here is the simple wrapper that I use just for testing (no error handling), in case this helps: class Statement { private: Statement(sqlite3* db, const std::wstring& sql) : db(db) { sqlite3_prepare16_v2(db, sql.c_str(), sql.length() * sizeof(wchar_t), &stmt, NULL); } public: ~Statement() { sqlite3_finalize(stmt); } public: void reset() { sqlite3_reset(stmt); } int step() { return sqlite3_step(stmt); } int getInt(int i) const { return sqlite3_column_int(stmt, i); } tstring getText(int i) const { const wchar_t* v = (const wchar_t*)sqlite3_column_text16(stmt, i); int sz = sqlite3_column_bytes16(stmt, i) / sizeof(wchar_t); return std::wstring(v, v + sz); } private: friend class Database; sqlite3* db; sqlite3_stmt* stmt; }; class Database { public: Database(const std::wstring& filename = L"")) : db(NULL) { sqlite3_open16(filename.c_str(), &db); } ~Database() { sqlite3_close(db); } void exec(const std::wstring& sql) { auto_ptr<Statement> st(prepare(sql)); st->step(); } auto_ptr<Statement> prepare(const tstring& sql) const { return auto_ptr<Statement>(new Statement(db, sql)); } private: sqlite3* db; };

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  • Column locking in innodb?

    - by ming yeow
    I know this sounds weird, but apparently one of my columns is locked. select * from table where type_id = 1 and updated_at < '2010-03-14' limit 1; select * from table where type_id = 3 and updated_at < '2010-03-14' limit 10; the first one would not finish running even in a few hours, while the second one completes smoothly. the only difference is the type_id between the 2 queries. a bit of background, the first statement screwed up before which i had to kill manually. Thanks in advance for your help - i have an urgent data job to finish, and this problem is driving me crazy

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  • Locking on an object...

    - by Mystere Man
    I often see code like that which is shown here, ie where an object is allocated and then used as a "lock object". It seems to me that you could use any object for this, including the event itself as the lock object. Why allocate a new object that does nothing? My understanding is that calling lock() on an object doesn't actually alter the object itself, nor does it actually lock it from being used, it's simply used as a placeholder for multiple lock statements to anchor on. So my question is, is this really a good thing to do?

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  • Locking behaviour is different via network shares

    - by MattH
    I have been trying to lock a file so that other cloned services cannot access the file. I then read the file, and then move the file when finished. The Move is allowed by using FileShare.Delete. However in later testing, we found that this approach does not work if we are looking at a network share. I appreciate my approach may not have been the best, but my specific question is: Why does the below demo work against the local file, but not against the network file? The more specific you can be the better, as I've found very little information in my searches that indicates network shares behave differently to local disks. string sourceFile = @"C:\TestFile.txt"; string localPath = @"C:\MyLocalFolder\TestFile.txt"; string networkPath = @"\\MyMachine\MyNetworkFolder\TestFile.txt"; File.WriteAllText(sourceFile, "Test data"); if (!File.Exists(localPath)) File.Copy(sourceFile, localPath); foreach (string path in new string[] { localPath, networkPath }) { using (FileStream fsLock = File.Open(path, FileMode.Open, FileAccess.ReadWrite, (FileShare.Read | FileShare.Delete))) { string target = path + ".out"; File.Move(path, target); //This is the point of failure, when working with networkPath if (File.Exists(target)) File.Delete(target); } if (!File.Exists(path)) File.Copy(sourceFile, path); }

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  • SQLserver multithreaded locking with TABLOCKX

    - by WilfriedVS
    I have a table "tbluser" with 2 fields: userid = integer (autoincrement) user = nvarchar(100) I have a multithreaded/multi server application that uses this table. I want to accomplish the following: Guarantee that field user is unique in my table Guarantee that combination userid/user is unique in each server's memory I have the following stored procedure: CREATE PROCEDURE uniqueuser @user nvarchar(100) AS BEGIN BEGIN TRAN DECLARE @userID int SET nocount ON SET @userID = (SELECT @userID FROM tbluser WITH (TABLOCKX) WHERE [user] = @user) IF @userID <> '' BEGIN SELECT userID = @userID END ELSE BEGIN INSERT INTO tbluser([user]) VALUES (@user) SELECT userID = SCOPE_IDENTITY() END COMMIT TRAN END Basically the application calls the stored procedure and provides a username as parameter. The stored procedure either gets the userid or insert the user if it is a new user. Am I correct to assume that the table is locked (only one server can insert/query)?

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  • MySQL locking problem

    - by teehoo
    I have a simple setup of a set of writers and a set of readers working with a MySQL ISAM table. The writers are only inserting rows while the readers are only checking for new rows. OK, so I know that I don't need a lock in this situation, since I'm not modifying existing rows. However my Writers are accessing one more table that does need a lock. I piece of information seems irrelevant except for the following limitation stated in the MySQL documentation: A session that requires locks must acquire all the locks that it needs in a single LOCK TABLES statement. While the locks thus obtained are held, the session can access only the locked tables. For example, in the following sequence of statements, an error occurs for the attempt to access t2 because it was not locked in the LOCK TABLES statement: So to access the table I want to insert rows into, I NEED to lock it, which is causing me performance problems. Any suggestions of how to get around this?

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  • locking database record for editing

    - by sd_dracula
    I have a SQL 2008 DB and an asp.net frontend. I would like to implement a lock when a user is currently editing a record but unsure of which is the best approach. My idea is to have a isLocked column for the records and it gets set to true when a user pulls that record, meaning all other users have read only access until the first user finishes the editing. However, what if the session times out and he/she never saves/updates the record, the record will remain with isLocked = true, meaning others cannot edit it, right? How can I implement some sort of session time out and have isLocked be automatically set to false when the session times out (or after a predefined period) Should this be implemented on the asp.net side or the SQL side?

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  • Explain the code: c# locking feature and threads

    - by Mendy
    I used this pattern in a few projects, (this snipped of code is from CodeCampServer), I understand what it does, but I'm really interesting in an explanation about this pattern. Specifically: Why is the double check of _dependenciesRegistered. Why to use lock (Lock){}. Thanks. public class DependencyRegistrarModule : IHttpModule { private static bool _dependenciesRegistered; private static readonly object Lock = new object(); public void Init(HttpApplication context) { context.BeginRequest += context_BeginRequest; } public void Dispose() { } private static void context_BeginRequest(object sender, EventArgs e) { EnsureDependenciesRegistered(); } private static void EnsureDependenciesRegistered() { if (!_dependenciesRegistered) { lock (Lock) { if (!_dependenciesRegistered) { new DependencyRegistrar().ConfigureOnStartup(); _dependenciesRegistered = true; } } } } }

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  • Strange Locking Behaviour in SQL Server 2005

    - by SQL Learner
    Can anyone please tell me why does the following statement inside a given stored procedure returns repeated results even with locks on the rows used by the first SELECT statement? BEGIN TRANSACTION DECLARE @Temp TABLE ( ID INT ) INSERT INTO @Temp SELECT ID FROM SomeTable WITH (ROWLOCK, UPDLOCK, READPAST) WHERE SomeValue <= 10 INSERT INTO @Temp SELECT ID FROM SomeTable WITH (ROWLOCK, UPDLOCK, READPAST) WHERE SomeValue >= 5 SELECT * FROM @Temp COMMIT TRANSACTION Any values in SomeTable for which SomeValue is between 5 and 10 will be returned twice, even though they were locked in the first SELECT. I thought that locks were in place for the whole transaction, and so I wasn't expecting the query to return repeated results. Why is this happening?

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  • Basics of SQL Server 2008 Locking

    Relational databases are designed for multiple simultaneous users, and Microsoft SQL Server is no different. However, supporting multiple users requires some form of concurrency control, which in SQL Server's case means transaction isolation and locking. Read on to learn how SQL Server 2008 implements locking.

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  • Basics of SQL Server 2008 Locking

    Relational databases are designed for multiple simultaneous users, and Microsoft SQL Server is no different. However, supporting multiple users requires some form of concurrency control, which in SQL Server's case means transaction isolation and locking. Read on to learn how SQL Server 2008 implements locking.

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  • Why avoid pessimistic locking in a version control system?

    - by raven
    Based on a few posts I've read concerning version control, it seems people think pessimistic locking in a version control system is a bad thing. Why? I understand that it prevents one developer from submitting a change while another has the file checked out, but so what? If your code files are so big that you constantly have more than one person working on them at the same time, I submit that you should reorganize your code. Break it up into smaller functional units. Integration of concurrent code changes is a tedious and error-prone process even with the tools a good version control system provides to make it easier. I think it should be avoided if at all possible. So, why is pessimistic locking discouraged?

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  • Partial upgrade on 12.04, how to stop nagging after locking to a working NVIDIA & xorg

    - by alsk
    How to stop the upgrade manager from offering updates and upgrades that potentially would harm my working 2D and 3D graphics? Finally, I got 12.04 working as it should: with nvidia-173 drivers by downgrading xorg and locking the version: On my 32-bit system on Athlon64, with (Albatron) NVIDIA GeForce FX5700XT, locked (/pinned) to xorg 1:7.6-7ubuntu7, xserver-xorg-core 2:11.1-0obuntu10.07, nvidia-173 173.14.35-0ubuntu0.2? An annoying thing left is that every time the updates are checked, I get warning of partial updates, and ambiguous options of "partial update" and "close". Ambiguous in that sense that if I click close, I will get option to update a few packages, which has been OK, while "partial update" would like to update my kernel to 3.2, alter xorg, remove nvidia-173 etc., and update mesa etc. This is not what I call appropriate, after locking XORG and NVIDIA drivers to working ones. One may say according to package management logic it may be correct, but to me as an user it makes little sense. Last Ubuntu that worked without big mess for me was 10.10, hence I will not put 12.10 to my "production" system, until I can be sure it will not trash the system again. P.S. Is there a recommended way to keep NVIDIA GeForce FX working with 3D on Ubuntu... in future?

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  • Using the Coherence ConcurrentMap Interface (Locking API)

    - by jpurdy
    For many developers using Coherence, the first place they look for concurrency control is the com.tangosol.util.ConcurrentMap interface (part of the NamedCache interface). The ConcurrentMap interface includes methods for explicitly locking data. Despite the obvious appeal of a lock-based API, these methods should generally be avoided for a variety of reasons: They are very "chatty" in that they can't be bundled with other operations (such as get and put) and there are no collection-based versions of them. Locks do directly not impact mutating calls (including puts and entry processors), so all code must make explicit lock requests before modifying (or in some cases reading) cache entries. They require coordination of all code that may mutate the objects, including the need to lock at the same level of granularity (there is no built-in lock hierarchy and thus no concept of lock escalation). Even if all code is properly coordinated (or there's only one piece of code), failure during updates that may leave a collection of changes to a set of objects in a partially committed state. There is no concept of a read-only lock. In general, use of locking is highly discouraged for most applications. Instead, the use of entry processors provides a far more efficient approach, at the cost of some additional complexity.

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  • How can I get SQL Server transactions to use record-level locks?

    - by Joe White
    We have an application that was originally written as a desktop app, lo these many years ago. It starts a transaction whenever you open an edit screen, and commits if you click OK, or rolls back if you click Cancel. This worked okay for a desktop app, but now we're trying to move to ADO.NET and SQL Server, and the long-running transactions are problematic. I found that we'll have a problem when multiple users are all trying to edit (different subsets of) the same table at the same time. In our old database, each user's transaction would acquire record-level locks to every record they modified during their transaction; since different users were editing different records, everyone gets their own locks and everything works. But in SQL Server, as soon as one user edits a record inside a transaction, SQL Server appears to get a lock on the entire table. When a second user tries to edit a different record in the same table, the second user's app simply locks up, because the SqlConnection blocks until the first user either commits or rolls back. I'm aware that long-running transactions are bad, and I know that the best solution would be to change these screens so that they no longer keep transactions open for a long time. But since that would mean some invasive and risky changes, I also want to research whether there's a way to get this code up and running as-is, just so I know what my options are. How can I get two different users' transactions in SQL Server to lock individual records instead of the entire table? Here's a quick-and-dirty console app that illustrates the issue. I've created a database called "test1", with one table called "Values" that just has ID (int) and Value (nvarchar) columns. If you run the app, it asks for an ID to modify, starts a transaction, modifies that record, and then leaves the transaction open until you press ENTER. I want to be able to start the program and tell it to update ID 1; let it get its transaction and modify the record; start a second copy of the program and tell it to update ID 2; have it able to update (and commit) while the first app's transaction is still open. Currently it freezes at step 4, until I go back to the first copy of the app and close it or press ENTER so it commits. The call to command.ExecuteNonQuery blocks until the first connection is closed. public static void Main() { Console.Write("ID to update: "); var id = int.Parse(Console.ReadLine()); Console.WriteLine("Starting transaction"); using (var scope = new TransactionScope()) using (var connection = new SqlConnection(@"Data Source=localhost\sqlexpress;Initial Catalog=test1;Integrated Security=True")) { connection.Open(); var command = connection.CreateCommand(); command.CommandText = "UPDATE [Values] SET Value = 'Value' WHERE ID = " + id; Console.WriteLine("Updating record"); command.ExecuteNonQuery(); Console.Write("Press ENTER to end transaction: "); Console.ReadLine(); scope.Complete(); } } Here are some things I've already tried, with no change in behavior: Changing the transaction isolation level to "read uncommitted" Specifying a "WITH (ROWLOCK)" on the UPDATE statement

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  • Crash when checking BOF property of pessimistic locked ADO recordset

    - by Patrick
    Bit of an odd one for you: I've got two connections to a database, on one I've opened a _RecordsetPtr with a pessimistic lock. I can no longer send an UPDATE command on the other connection. I can send a SELECT command on the second connection and data is returned. If I use a read only lock then there are no problems however when I use a pessimistic lock on the second connection as well I can check the State == adStateOpen but the program hangs when I test the BOF property! If I don't test the BOF property and try to call moveNext on the second connection the software hangs If I do neither of these I am able to access the data via the second connection but trying to access the data from the first connection causes the software to hang. Any one seen anything similar as I'm a bit stuck? EDIT : it wasn't hanging, someone had put a 30 minute timeout on the connection and I wasn't waiting that long while testing...

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  • This Wed, Reading - Service Broker, Indexing, Normalisation, Sets, RI and Locking, Surrogate Keys

    - by tonyrogerson
    Registration is a must so we know numbers and for security, register here: http://sqlserverfaq.com/events/213/Service-Broker-Intro-Guidance-Indexing-Selection-Usage-Fragmentation-etc-Normalisation-Surrogate-Keys-Locking-considerations.aspx Network, learn, ask a question, meet other folk, get fed - these are all things that happen at user group events. These events are a really great opportunity to socialise in an informal learning experience - if you want your own exposure then come and do a 1 -...(read more)

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  • Read Committed isolation level, indexed views and locking behavior

    - by Michael Zilberstein
    From BOL, " Key-Range Locking " article: Key-range locks protect a range of rows implicitly included in a record set being read by a Transact-SQL statement while using the serializable transaction isolation level . The serializable isolation level requires that any query executed during a transaction must obtain the same set of rows every time it is executed during the transaction. A key range lock protects this requirement by preventing other transactions from inserting new rows whose...(read more)

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  • Do we need Record Level Locking when we already have Transaction for online ordering? (of concert ti

    - by Jian Lin
    For online ordering of concert seat or airline ticket, do we need Record Level Locking or is Transaction good enough? For concert ticket (say, seat Number 20B), or airline ticket (even with overbooking, the limit is 210, for example), I think the website cannot lock any record or begin transaction when showing the ticket purchase screen. But after the user clicks "Confirm Purchase", then the server should Begin a Transaction, Purchase Seat Number 20B, and try to Commit. If another user already bought Seat 20B in a previous transaction, then it is the "Commit" part that the current transaction will fail? So... we don't need Record Level Locking? Do Transactions always go serialized (one after another), so that's why we can know for sure there is no "race condition"? In what situation is Record Level Locking needed then?

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  • optimistic and pessimistic locks

    - by billmce
    Working on my first php/Codeigniter project and I’ve scoured the ‘net for information on locking access to editing data and haven’t found very much information. I expect it to be a fairly regular occurrence for 2 users to attempt to edit the same form simultaneously. My experience (in the stateful world of BBx, filePro, and other RAD apps) is that the data being edited is locked using a pessimistic lock—one user has access to the edit form at the time. The second user basically has to wait for the first to finish. I understand this can be done using Ajax sending XMLHttpRequests to maintain a ‘lock’ database. The php world, lacking state, seems to prefer optimistic locking. If I understand it correctly it works like this: both users get to access the data and they each record a ‘before changes’ version of the data. Before saving their changes, the data is once again retrieved and compared the ‘before changes’ version. If the two versions are identical then the users changes are written. If they are different; the user is shown what has changed since he/she started editing and some mechanism is added to resolve the differences—or the user is shown a ‘Sorry, try again’ message. I’m interested in any experience people here have had with implementing both pessimistic and optimistic locking. If there are any libraries, tools, or ‘how-to’s available I’m appreciate a link. Thanks

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