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  • What data is actually stored in a B-tree database in CouchDB?

    - by Andrey Vlasovskikh
    I'm wondering what is actually stored in a CouchDB database B-tree? The CouchDB: The Definitive Guide tells that a database B-tree is used for append-only operations and that a database is stored in a single B-tree (besides per-view B-trees). So I guess the data items that are appended to the database file are revisions of documents, not the whole documents: +---------|### ... | | +------|###|------+ ... ---+ | | | | +------+ +------+ +------+ +------+ | doc1 | | doc2 | | doc1 | ... | doc1 | | rev1 | | rev1 | | rev2 | | rev7 | +------+ +------+ +------+ +------+ Is it true? If it is true, then how the current revision of a document is determined based on such a B-tree? Doesn't it mean, that CouchDB needs a separate "view" database for indexing current revisions of documents to preserve O(log n) access? Wouldn't it lead to race conditions while building such an index? (as far as I know, CouchDB uses no write locks).

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  • How to do concurrent modification testing for grails application

    - by werner5471
    I'd like to run tests that simulate users modifying certain data at the same time for a grails application. Are there any plug-ins / tools / mechanisms I can use to do this efficiently? They don't have to be grails specific. It should be possible to fire multiple actions in parallel. I'd prefer to run the tests on functional level (so far I'm using Selenium for other tests) to see the results from the user perspective. Of course this can be done in addition to integration testing if you'd recommend to run concurrent modification tests on integration level as well.

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  • Can the lock function be used to implement thread-safe enumeration?

    - by Daniel
    I'm working on a thread-safe collection that uses Dictionary as a backing store. In C# you can do the following: private IEnumerable<KeyValuePair<K, V>> Enumerate() { if (_synchronize) { lock (_locker) { foreach (var entry in _dict) yield return entry; } } else { foreach (var entry in _dict) yield return entry; } } The only way I've found to do this in F# is using Monitor, e.g.: let enumerate() = if synchronize then seq { System.Threading.Monitor.Enter(locker) try for entry in dict -> entry finally System.Threading.Monitor.Exit(locker) } else seq { for entry in dict -> entry } Can this be done using the lock function? Or, is there a better way to do this in general? I don't think returning a copy of the collection for iteration will work because I need absolute synchronization.

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  • cflock do not throw timeout for same url called in same browser

    - by Pritesh Patel
    I am trying lock block on page test.cfm and below is code written on page. <cfscript> writeOutput("Before lock at #now()#"); lock name="threadlock" timeout="3" type="exclusive" { writeOutput("<br/>started at #now()#"); thread action="sleep" duration="10000"; writeOutput("<br/>ended at #now()#"); } writeOutput("<br/>After lock at #now()#"); </cfscript> assuming my url for page is http://localhost.local/test.cfm and running it on browser in two different tabs. I was expecting one of the url will throw timeout error after 3 second since another url lock it atleast for 10 seconds due to thread sleep. Surprisingly I do not get any timeout error rather second page call run after 10 seconds as first call finish execution. But I am appending some url parameter (e.g. http://localhost.local/test.cfm?q=1) will throw error. Also I am calling same url in different browser then one of the call will throw timeout issue. Is lock based on session and url? Update Here is output for two different cases: Case 1: TAB1 Url: http://localhost.local/test/test.cfm Before lock at {ts '2013-10-18 09:21:35'} started at {ts '2013-10-18 09:21:35'} ended at {ts '2013-10-18 09:21:45'} After lock at {ts '2013-10-18 09:21:45'} TAB2 Url: http://localhost.local/test/test.cfm Before lock at {ts '2013-10-18 09:21:45'} started at {ts '2013-10-18 09:21:45'} ended at {ts '2013-10-18 09:21:55'} After lock at {ts '2013-10-18 09:21:55'} Case 2: TAB1 Url: http://localhost.local/test/test.cfm Before lock at {ts '2013-10-18 09:27:18'} started at {ts '2013-10-18 09:27:18'} ended at {ts '2013-10-18 09:27:28'} After lock at {ts '2013-10-18 09:27:28'} TAB2 Url: http://localhost.local/test/test.cfm? (Added ? at the end) Before lock at {ts '2013-10-18 09:27:20'} A timeout occurred while attempting to lock threadlock. The error occurred in C:/inetpub/wwwroot/test/test.cfm: line 13 11 : 12 : <cfoutput>Before lock at #now()#</cfoutput> 13 : <cflock name="threadlock" timeout="3" type="exclusive"> 14 : <cfoutput><br/>started at #now()#</cfoutput> 15 : <cfthread action="sleep" duration="10000"/> ... Result for case 2 as expected. For case 1, strange thing I just noticed is tab 2 output "Before lock at {ts '2013-10-18 09:21:45'} indicates that whole request start after 10 seconds (means after the complete execution of first tab) when I have fired it in second URL just after 2 seconds of first tabs.

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  • SQL Server race condition issue with range lock

    - by Freek
    I'm implementing a queue in SQL Server (please no discussions about this) and am running into a race condition issue. The T-SQL of interest is the following: set transaction isolation level serializable begin tran declare @RecordId int declare @CurrentTS datetime2 set @CurrentTS=CURRENT_TIMESTAMP select top 1 @RecordId=Id from QueuedImportJobs with (updlock) where Status=@Status and (LeaseTimeout is null or @CurrentTS>LeaseTimeout) order by Id asc if @@ROWCOUNT> 0 begin update QueuedImportJobs set LeaseTimeout = DATEADD(mi,5,@CurrentTS), LeaseTicket=newid() where Id=@RecordId select * from QueuedImportJobs where Id = @RecordId end commit tran RecordId is the PK and there is also an index on Status,LeaseTimeout. What I'm basically doing is select a record of which the lease happens to be expired, while simultaneously updating the lease time with 5 minutes and setting a new lease ticket. So the problem is that I'm getting deadlocks when I run this code in parallel using a couple of threads. I've debugged it up to the point where I found out that the update statement sometimes gets executing twice for the same record. Now, I was under the impression that the with (updlock) should prevent this (it also happens with xlock btw, not with tablockx). So it actually look like there is a RangeS-U and a RangeX-X lock on the same range of records, which ought to be impossible. So what am I missing? I'm thinking it might have something to do with the top 1 clause or that SQL Server does not know that where Id=@RecordId is actually in the locked range? Deadlock graph: Table schema (simplified):

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  • How Indices Cope with MVCC ?

    - by geeko
    Greetings Overflowers, To my understanding (and I hope I'm not right) changes to indices cannot be MVCCed. I'm wondering if this is also true with big records as copies can be costly. Since records are accessed via indices (usually), how MVCC can be effective ? Do, for e.g., indices keep track of different versions of MVCCed records ? Any recent good reading on this subject ? Really appreciated ! Regards

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  • GAE update different fields of the same entity

    - by bach
    Hi, UserA and UserB are changing objectA.filedA objectA.filedB respectively and at the same time. Because they are not changing the same field one might think that there are no overlaps. Is that true? or the implementation of pm.makePersistnace() actually override the whole object... good to know...

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  • How do I can linux flock command to prevent another root process deleting a file?

    - by Danmaxis
    Hello there, I would like to prevent one of my root process from deleting a certaing file. So I came across the flock command, it seems to fit my need, but I didnt get its sintax. If I only indicate a shared lock, it doesnt work: flock -s "./file.xml" If I add a timeout parameter, it still doesnt work flock -s -w5 "./file.xml" It seems that way, it fits in flock [-sxun][-w #] fd# way. (What is this fd# parameter?) So, I tried the flock [-sxon][-w #] file [-c] command Using flock -s -w5 "./file.xml" -c "tail -3 ./file.xml" and it worked, tail command at ./file.xml was executed. But I would like to know, does the lock end after the command or does it last 5 seconds after the end of the command execution? My main question is, how can I prevent another root process deleting a file in linux?

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  • Mysql- "FLUSH TABLES WITH READ LOCK" started automatically

    - by ming yeow
    I would like to understand how this happened. I was running a query that would take a long time, but should not lock up any table. However, my dbs were practically down - it seems like it was being locked up by "FLUSH TABLES WITH READ LOCK" 03:21:31 select type_id, count(*) from guid_target_infos group by type_id 02:38:11 select type_id, count(*) from guid_infos group by type_id 02:24:29 FLUSH TABLES WITH READ LOCK But i did not start this command. can someone tell me why it was started automatically?

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  • How do I avoid a race condition in my Rails app?

    - by Cathal
    Hi, I have a really simple Rails application that allows users to register their attendance on a set of courses. The ActiveRecord models are as follows: class Course < ActiveRecord::Base has_many :scheduled_runs ... end class ScheduledRun < ActiveRecord::Base belongs_to :course has_many :attendances has_many :attendees, :through => :attendances ... end class Attendance < ActiveRecord::Base belongs_to :user belongs_to :scheduled_run, :counter_cache => true ... end class User < ActiveRecord::Base has_many :attendances has_many :registered_courses, :through => :attendances, :source => :scheduled_run end A ScheduledRun instance has a finite number of places available, and once the limit is reached, no more attendances can be accepted. def full? attendances_count == capacity end attendances_count is a counter cache column holding the number of attendance associations created for a particular ScheduledRun record. My problem is that I don't fully know the correct way to ensure that a race condition doesn't occur when 1 or more people attempt to register for the last available place on a course at the same time. My Attendance controller looks like this: class AttendancesController < ApplicationController before_filter :load_scheduled_run before_filter :load_user, :only => :create def new @user = User.new end def create unless @user.valid? render :action => 'new' end @attendance = @user.attendances.build(:scheduled_run_id => params[:scheduled_run_id]) if @attendance.save flash[:notice] = "Successfully created attendance." redirect_to root_url else render :action => 'new' end end protected def load_scheduled_run @run = ScheduledRun.find(params[:scheduled_run_id]) end def load_user @user = User.create_new_or_load_existing(params[:user]) end end As you can see, it doesn't take into account where the ScheduledRun instance has already reached capacity. Any help on this would be greatly appreciated.

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  • Do I need to syncronize thread access to an int

    - by Martin Harris
    I've just written a method that is called by multiple threads simultaneously and I need to keep track of when all the threads have completed, the code uses this pattern: private void RunReport() { _reportsRunning++; try { //code to run the report } finally { _reportsRunning--; } } This is the only place within the code that _reportsRunning's value is changed, and the method takes about a second to run. Occasionally when I have more than six or so threads running reports together the final result for _reportsRunning can get down to -1, if I wrap the calls to _runningReports++ and _runningReports-- in a lock then the behaviour appears to be correct and consistant. So, to the question: When I was learning multithreading in C++ I was taught that you didn't need to synchronize calls to increment and decrement operations because they were always one assembly instruction and therefore it was impossible for the thread to be switched out mid-call. Was I taught correctly, and if so how come that doesn't hold true for C#?

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  • Do really need a count lock on Multi threads with one CPU core?

    - by MrROY
    If i have some code looks like this(Please ignore the syntax, i want to understand it without a specified language): count = 0 def countDown(): count += 1 if __name__ == '__main__': thread1(countDown) thread2(countDown) thread3(countDown) Here i have a CPU with only one core, do i really need a lock to the variable count in case of it could be over-written by other threads. I don't know, but if the language cares a lot, please explain it under Java?C and Python, So many thanks.

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  • Using locks inside a loop

    - by Xaqron
    // Member Variable private readonly object _syncLock = new object(); // Now inside a static method foreach (var lazyObject in plugins) { if ((string)lazyObject.Metadata["key"] = "something") { lock (_syncLock) { if (!lazyObject.IsValueCreated) lazyObject.value.DoSomething(); } return lazyObject.value; } } Here I need synchronized access per loop. There are many threads iterating this loop and based on the key they are looking for, a lazy instance is created and returned. lazyObject should not be created more that one time. Although Lazy class is for doing so and despite of the used lock, under high threading I have more than one instance created (I track this with a Interlocked.Increment on a volatile shared int and log it somewhere). The problem is I don't have access to definition of Lazy and MEF defines how the Lazy class create objects. My questions: 1) Why the lock doesn't work ? 2) Should I use an array of locks instead of one lock for performance improvement ?

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  • how can we achieve second application read that file when first application not modifying it

    - by soField
    i have two application first application is bash second is java which one of them is periodically deleting and recreating a specific file (first) the other one is also periodically reading this file and process it in it's own logic (second) how can we achieve second application read that file when first application not modifying it my aim is to force second app read the file only when content of file fully written inside it how can achieve this goal ?

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  • Multi-threading concept and lock in c#

    - by Neeraj
    I read about lock, though not understood nothing at all. My question is why do we use a un-used object and lock that and how this makes something thread-safe or how this helps in multi-threading ? Isn't there other way to make thread-safe code. public class test { private object Lock { get; set; } ... lock (this.Lock) { ... } ... } Sorry is my question is very stupid, but i don't understand, although i've used it many times.

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  • Can I lock rows in a cursor if the cursor only returns a single count(*) row?

    - by RenderIn
    I would like to restrict users from inserting more than 3 records with color = 'Red' in my FOO table. My intentions are to A) retrieve the current count so that I can determine whether another record is allowed and B) prevent any other processes from inserting any Red records while this one is in process, hence the for update of. I'd like to do something like: cursor cur_cnt is select count(*) cnt from foo where foo.color = 'Red' for update of foo.id; Will this satisfy both my requirements or will it not lock only the rows in the count(*) who had foo.color = 'Red'?

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  • Lock innoDB table temporarily

    - by Industrial
    Hi everyone, I make bigger inserts consisting of a couple of thousand rows in my current web app and I would like to make sure that no one can do anything but read the table, until the inserts have been done. What is the best way to do this while keeping the read availability open for normal, non-admin users? Thanks!

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  • Thread-Safe lazy instantiating using MEF

    - by Xaqron
    // Member Variable private static readonly object _syncLock = new object(); // Now inside a static method foreach (var lazyObject in plugins) { if ((string)lazyObject.Metadata["key"] = "something") { lock (_syncLock) { // It seems the `IsValueCreated` is not up-to-date if (!lazyObject.IsValueCreated) lazyObject.value.DoSomething(); } return lazyObject.value; } } Here I need synchronized access per loop. There are many threads iterating this loop and based on the key they are looking for, a lazy instance is created and returned. lazyObject should not be created more that one time. Although Lazy class is for doing so and despite of the used lock, under high threading I have more than one instance created (I track this with a Interlocked.Increment on a volatile static int and log it somewhere). The problem is I don't have access to definition of Lazy and MEF defines how the Lazy class create objects. I should notice the CompositionContainer has a thread-safe option in constructor which is already used. My questions: 1) Why the lock doesn't work ? 2) Should I use an array of locks instead of one lock for performance improvement ?

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  • detection of 'flush tables with read lock' in php

    - by theduke0
    I would like to know from my application if a myisam table can accept writes (i.e. not locked). If an exception is thrown, everything is fine as I can catch this and log the failed statement to a file. However, if a 'flush tables with read lock' command has been issued (possibly for backup), the query I send will pretty much hang out forever. If one table is locked at a time, insert delayed works well. But when this global lock is applied, my query just waits. The query I run is an insert statement. If this statement fails or hangs, user experience is degraded. I need a way to send the query to the server and forget about it (pretty much). Does anyone have any suggestions on how to deal with this? -set a query timeout? -run asyncronous request and allow for the lock to expire while application continues? -fork my php process? Please let me know if I can provide and clarification or details.

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  • VB.NET Program Locks Up with Internet Explorer Opened

    - by aaronsj
    I'm using Visual Studio 2008 and developing a VB.NET application. I'm having strange lockup problems with my program, but only when Internet explorer 8 is opened. When I cover my form with another window and then uncover it, I find that it has locked up. My program has no references to IE and the only thing it even has to do with IE is using Process.Start with a web address. My program works fine and exactly as it should, but only when IE is not opened. Does anyone know why a program would lock up only while IE is running? Edit: I've done some digging and I've found the offending thread in my program. I don't know what starts this thread or what it does, but when I kill it, my program no longer freezes. The thread is one of the CreateApplicationContext threads, here is the last few items in the stack trace of that thread. 6 ntkrnlpa.exe+0x897bc 7 ntdll.dll!KiFastSystemCallRet 8 mscorwrks.dll!LogHelp_TerminateOnAssert+0x61 9 mscorwrks.dll!DllUnregisterServerInternal+0x10523 10 mscorwrks.dll!DllUnregisterServerInternal+0x10542 11 mscorwrks.dll!StrongNameErrorInfo+0x34387 12 mscorwrks.dll!StrongNameErrorInfo+0x34815 13 mscorwrks.dll!CreateApplicationContext+0xbc35 14 KERNEL32.dll!GetModuleHandleA+0xdf Process explorer says my program is using no CPU nor throwing any exceptions while it is hung.

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