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• Why is domU faster than dom0 on IO?

  - by Paco

  I have installed debian 7 on a physical machine. This is the configuration of the machine: 3 hard drives using RAID 5 Strip element size: 1M Read policy: Adaptive read ahead Write policy: Write Through /boot 200 MB ext2 / 15 GB ext3 SWAP 10GB LVM rest (~500GB) emphasized text I installed postgresql, created a big database (over 1GB). I have an SQL request that takes a lot of time to run (a SELECT statement, so it only reads data from the database). This request takes approximately 5.5 seconds to run. Then, I installed XEN, created a domU, with another debian distro. On this OS, I also installed postgresql, with the same database. The same SQL request takes only 2.5 seconds to run. I checked the kernel on both dom0 and domU. uname-a returns "Linux debian 3.2.0-4-amd64 #1 SMP Debian 3.2.41-2+deb7u2 x86_64 GNU/Linux" on both systems. I checked the kernel parameters, which are approximately the same. For those that are relevant, I changed their values to make them match on both systems using sysctl. I saw no changes (the requests still take the same amount of time). After this, I checked the file systems. I used ext3 on domU. Still no changes. I installed hdparm, and ran hdparm -Tt on both systems, on all my partitions on both systems, and I get similar results. Now, I am stuck, I don't know what is different, and what could be the cause of such a big difference. Additional Info: Debian runs on a Dell server PowerEdge 2950 postgresql: 9.1.9 (both dom0 and domU) xen-linux-system: 3.2.0 xen-hypervisor: 4.1 Thanks EDIT: As Krzysztof Ksiezyk suggested, it might be due to some file caching system. I ran the dd command to test both the read and write speed. Here is domU: root@test1:~# dd if=/dev/zero of=/root/dd count=5MB bs=1MB ^C2020+0 records in 2020+0 records out 2020000000 bytes (2.0 GB) copied, 18.8289 s, 107 MB/s root@test1:~# dd if=/root/dd of=/dev/null count=5MB bs=1MB 2020+0 records in 2020+0 records out 2020000000 bytes (2.0 GB) copied, 15.0549 s, 134 MB/s And here is dom0: root@debian:~# dd if=/dev/zero of=/root/dd count=5MB bs=1MB ^C1693+0 records in 1693+0 records out 1693000000 bytes (1.7 GB) copied, 8.87281 s, 191 MB/s root@debian:~# dd if=/root/dd of=/dev/null count=5MB bs=1MB 1693+0 records in 1693+0 records out 1693000000 bytes (1.7 GB) copied, 0.501509 s, 3.4 GB/s What can be the cause of this caching system? And how can we "fix" it? Can we apply it to dom0? EDIT 2: I switched my virtual disk type. To do so I followed this article. I did a dd if=/dev/vg0/test1-disk of=/mnt/test1-disk.img bs=16M Then in /etc/xen/test1.cfg, I changed the disk parameter to use file: instead of phy: it should have removed the file caching, but I still get the same numbers (domU being much faster for Postgres)

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• NTP daemon or ntpdate doesn't synchronize

  - by user2862333

  I'm having some problems with synchronization with an NTP server. 1) The NTP daemon doesn't sync the system clock at all, even though it's running (confirmed with /etc/init.d/ntp status). Forcing to sync with ntpd -q or ntpd -gq does not work either. 2) Stopping the NTP daemon and syncing manually with ntpdate does give me the following output: ~# ntpdate -d 0.debian.pool.ntp.org 6 Nov 16:48:53 ntpdate[4417]: ntpdate [email protected] Sat May 12 09:07:19 UTC 2012 (1) transmit(79.132.237.5) receive(79.132.237.5) transmit(85.234.197.2) receive(85.234.197.2) transmit(194.50.97.34) receive(194.50.97.34) transmit(79.132.237.1) receive(79.132.237.1) transmit(79.132.237.5) receive(79.132.237.5) transmit(85.234.197.2) receive(85.234.197.2) transmit(194.50.97.34) receive(194.50.97.34) transmit(79.132.237.1) receive(79.132.237.1) transmit(79.132.237.5) receive(79.132.237.5) transmit(85.234.197.2) receive(85.234.197.2) transmit(194.50.97.34) receive(194.50.97.34) transmit(79.132.237.1) receive(79.132.237.1) transmit(79.132.237.5) receive(79.132.237.5) transmit(85.234.197.2) receive(85.234.197.2) transmit(194.50.97.34) receive(194.50.97.34) transmit(79.132.237.1) receive(79.132.237.1) server 79.132.237.5, port 123 stratum 2, precision -20, leap 00, trust 000 refid [79.132.237.5], delay 0.05141, dispersion 0.00145 transmitted 4, in filter 4 reference time: d624e3b1.f490b90d Wed, Nov 6 2013 16:50:09.955 originate timestamp: d624e457.eaaf787c Wed, Nov 6 2013 16:52:55.916 transmit timestamp: d624e36c.4a7036fd Wed, Nov 6 2013 16:49:00.290 filter delay: 0.08537 0.05141 0.05151 0.06346 0.00000 0.00000 0.00000 0.00000 filter offset: 235.6038 235.6087 235.6095 235.6068 0.000000 0.000000 0.000000 0.000000 delay 0.05141, dispersion 0.00145 offset 235.608782 server 85.234.197.2, port 123 stratum 2, precision -20, leap 00, trust 000 refid [85.234.197.2], delay 0.05151, dispersion 0.00336 transmitted 4, in filter 4 reference time: d624e3e7.dc6cd02b Wed, Nov 6 2013 16:51:03.861 originate timestamp: d624e458.1c91031f Wed, Nov 6 2013 16:52:56.111 transmit timestamp: d624e36c.7da1d882 Wed, Nov 6 2013 16:49:00.490 filter delay: 0.05765 0.07750 0.06013 0.05151 0.00000 0.00000 0.00000 0.00000 filter offset: 235.6048 235.6014 235.6035 235.6078 0.000000 0.000000 0.000000 0.000000 delay 0.05151, dispersion 0.00336 offset 235.607826 server 194.50.97.34, port 123 stratum 3, precision -23, leap 00, trust 000 refid [194.50.97.34], delay 0.03021, dispersion 0.00090 transmitted 4, in filter 4 reference time: d624e38d.2bce952c Wed, Nov 6 2013 16:49:33.171 originate timestamp: d624e458.4dbbc114 Wed, Nov 6 2013 16:52:56.303 transmit timestamp: d624e36c.b0d38834 Wed, Nov 6 2013 16:49:00.690 filter delay: 0.03030 0.03636 0.03091 0.03021 0.00000 0.00000 0.00000 0.00000 filter offset: 235.6095 235.6085 235.6098 235.6105 0.000000 0.000000 0.000000 0.000000 delay 0.03021, dispersion 0.00090 offset 235.610589 server 79.132.237.1, port 123 stratum 3, precision -20, leap 00, trust 000 refid [79.132.237.1], delay 0.05113, dispersion 0.00305 transmitted 4, in filter 4 reference time: d624dfcb.6acea332 Wed, Nov 6 2013 16:33:31.417 originate timestamp: d624e458.838672ad Wed, Nov 6 2013 16:52:56.513 transmit timestamp: d624e36c.e405181c Wed, Nov 6 2013 16:49:00.890 filter delay: 0.06345 0.05113 0.05681 0.05656 0.00000 0.00000 0.00000 0.00000 filter offset: 235.6087 235.6038 235.6010 235.6074 0.000000 0.000000 0.000000 0.000000 delay 0.05113, dispersion 0.00305 offset 235.603888 6 Nov 16:49:00 ntpdate[4417]: step time server 79.132.237.5 offset 235.608782 sec Clearly, ntpdate can reach the NTP server(s), but after checking the clock, it hasn't changed and is still displaying the wrong time. Any ideas what would be the problem would be much appreciated.

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• Sql Server huge tables with no rows

  - by Mike Gates

  I have a Sql Server database that has a few tables with zero row count but take up a combined 10 GB of space. I can see this by doing right-click/properties on the tables in question (data space is huge, between 1 and 6 GB, and row count is zero on these tables). I have no clue what could be causing this as I would assume zero rows would mean nearly zero space taken. Any ideas?

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• Capturing and Transforming ASP.NET Output with Response.Filter

  - by Rick Strahl

  During one of my Handlers and Modules session at DevConnections this week one of the attendees asked a question that I didn’t have an immediate answer for. Basically he wanted to capture response output completely and then apply some filtering to the output – effectively injecting some additional content into the page AFTER the page had completely rendered. Specifically the output should be captured from anywhere – not just a page and have this code injected into the page. Some time ago I posted some code that allows you to capture ASP.NET Page output by overriding the Render() method, capturing the HtmlTextWriter() and reading its content, modifying the rendered data as text then writing it back out. I’ve actually used this approach on a few occasions and it works fine for ASP.NET pages. But this obviously won’t work outside of the Page class environment and it’s not really generic – you have to create a custom page class in order to handle the output capture. [updated 11/16/2009 – updated ResponseFilterStream implementation and a few additional notes based on comments] Enter Response.Filter However, ASP.NET includes a Response.Filter which can be used – well to filter output. Basically Response.Filter is a stream through which the OutputStream is piped back to the Web Server (indirectly). As content is written into the Response object, the filter stream receives the appropriate Stream commands like Write, Flush and Close as well as read operations although for a Response.Filter that’s uncommon to be hit. The Response.Filter can be programmatically replaced at runtime which allows you to effectively intercept all output generation that runs through ASP.NET. A common Example: Dynamic GZip Encoding A rather common use of Response.Filter hooking up code based, dynamic  GZip compression for requests which is dead simple by applying a GZipStream (or DeflateStream) to Response.Filter. The following generic routines can be used very easily to detect GZip capability of the client and compress response output with a single line of code and a couple of library helper routines: WebUtils.GZipEncodePage(); which is handled with a few lines of reusable code and a couple of static helper methods: /// <summary> ///Sets up the current page or handler to use GZip through a Response.Filter ///IMPORTANT:  ///You have to call this method before any output is generated! /// </summary> public static void GZipEncodePage() {     HttpResponse Response = HttpContext.Current.Response;     if(IsGZipSupported())     {         stringAcceptEncoding = HttpContext.Current.Request.Headers["Accept-Encoding"];         if(AcceptEncoding.Contains("deflate"))         {             Response.Filter = newSystem.IO.Compression.DeflateStream(Response.Filter,                                        System.IO.Compression.CompressionMode.Compress);             Response.AppendHeader("Content-Encoding", "deflate");         }         else        {             Response.Filter = newSystem.IO.Compression.GZipStream(Response.Filter,                                       System.IO.Compression.CompressionMode.Compress);             Response.AppendHeader("Content-Encoding", "gzip");                            }     }     // Allow proxy servers to cache encoded and unencoded versions separately    Response.AppendHeader("Vary", "Content-Encoding"); } /// <summary> /// Determines if GZip is supported /// </summary> /// <returns></returns> public static bool IsGZipSupported() { string AcceptEncoding = HttpContext.Current.Request.Headers["Accept-Encoding"]; if (!string.IsNullOrEmpty(AcceptEncoding) && (AcceptEncoding.Contains("gzip") || AcceptEncoding.Contains("deflate"))) return true; return false; } GZipStream and DeflateStream are streams that are assigned to Response.Filter and by doing so apply the appropriate compression on the active Response. Response.Filter content is chunked So to implement a Response.Filter effectively requires only that you implement a custom stream and handle the Write() method to capture Response output as it’s written. At first blush this seems very simple – you capture the output in Write, transform it and write out the transformed content in one pass. And that indeed works for small amounts of content. But you see, the problem is that output is written in small buffer chunks (a little less than 16k it appears) rather than just a single Write() statement into the stream, which makes perfect sense for ASP.NET to stream data back to IIS in smaller chunks to minimize memory usage en route. Unfortunately this also makes it a more difficult to implement any filtering routines since you don’t directly get access to all of the response content which is problematic especially if those filtering routines require you to look at the ENTIRE response in order to transform or capture the output as is needed for the solution the gentleman in my session asked for. So in order to address this a slightly different approach is required that basically captures all the Write() buffers passed into a cached stream and then making the stream available only when it’s complete and ready to be flushed. As I was thinking about the implementation I also started thinking about the few instances when I’ve used Response.Filter implementations. Each time I had to create a new Stream subclass and create my custom functionality but in the end each implementation did the same thing – capturing output and transforming it. I thought there should be an easier way to do this by creating a re-usable Stream class that can handle stream transformations that are common to Response.Filter implementations. Creating a semi-generic Response Filter Stream Class What I ended up with is a ResponseFilterStream class that provides a handful of Events that allow you to capture and/or transform Response content. The class implements a subclass of Stream and then overrides Write() and Flush() to handle capturing and transformation operations. By exposing events it’s easy to hook up capture or transformation operations via single focused methods. ResponseFilterStream exposes the following events: CaptureStream, CaptureString Captures the output only and provides either a MemoryStream or String with the final page output. Capture is hooked to the Flush() operation of the stream. TransformStream, TransformString Allows you to transform the complete response output with events that receive a MemoryStream or String respectively and can you modify the output then return it back as a return value. The transformed output is then written back out in a single chunk to the response output stream. These events capture all output internally first then write the entire buffer into the response. TransformWrite, TransformWriteString Allows you to transform the Response data as it is written in its original chunk size in the Stream’s Write() method. Unlike TransformStream/TransformString which operate on the complete output, these events only see the current chunk of data written. This is more efficient as there’s no caching involved, but can cause problems due to searched content splitting over multiple chunks. Using this implementation, creating a custom Response.Filter transformation becomes as simple as the following code. To hook up the Response.Filter using the MemoryStream version event: ResponseFilterStream filter = new ResponseFilterStream(Response.Filter); filter.TransformStream += filter_TransformStream; Response.Filter = filter; and the event handler to do the transformation: MemoryStream filter_TransformStream(MemoryStream ms) { Encoding encoding = HttpContext.Current.Response.ContentEncoding; string output = encoding.GetString(ms.ToArray()); output = FixPaths(output); ms = new MemoryStream(output.Length); byte[] buffer = encoding.GetBytes(output); ms.Write(buffer,0,buffer.Length); return ms; } private string FixPaths(string output) { string path = HttpContext.Current.Request.ApplicationPath; // override root path wonkiness if (path == "/") path = ""; output = output.Replace("\"~/", "\"" + path + "/").Replace("'~/", "'" + path + "/"); return output; } The idea of the event handler is that you can do whatever you want to the stream and return back a stream – either the same one that’s been modified or a brand new one – which is then sent back to as the final response. The above code can be simplified even more by using the string version events which handle the stream to string conversions for you: ResponseFilterStream filter = new ResponseFilterStream(Response.Filter); filter.TransformString += filter_TransformString; Response.Filter = filter; and the event handler to do the transformation calling the same FixPaths method shown above: string filter_TransformString(string output) { return FixPaths(output); } The events for capturing output and capturing and transforming chunks work in a very similar way. By using events to handle the transformations ResponseFilterStream becomes a reusable component and we don’t have to create a new stream class or subclass an existing Stream based classed. By the way, the example used here is kind of a cool trick which transforms “~/” expressions inside of the final generated HTML output – even in plain HTML controls not HTML controls – and transforms them into the appropriate application relative path in the same way that ResolveUrl would do. So you can write plain old HTML like this: <a href=”~/default.aspx”>Home</a>  and have it turned into: <a href=”/myVirtual/default.aspx”>Home</a>  without having to use an ASP.NET control like Hyperlink or Image or having to constantly use: <img src=”<%= ResolveUrl(“~/images/home.gif”) %>” /> in MVC applications (which frankly is one of the most annoying things about MVC especially given the path hell that extension-less and endpoint-less URLs impose). I can’t take credit for this idea. While discussing the Response.Filter issues on Twitter a hint from Dylan Beattie who pointed me at one of his examples which does something similar. I thought the idea was cool enough to use an example for future demos of Response.Filter functionality in ASP.NET next I time I do the Modules and Handlers talk (which was great fun BTW). How practical this is is debatable however since there’s definitely some overhead to using a Response.Filter in general and especially on one that caches the output and the re-writes it later. Make sure to test for performance anytime you use Response.Filter hookup and make sure it' doesn’t end up killing perf on you. You’ve been warned :-}. How does ResponseFilterStream work? The big win of this implementation IMHO is that it’s a reusable  component – so for implementation there’s no new class, no subclassing – you simply attach to an event to implement an event handler method with a straight forward signature to retrieve the stream or string you’re interested in. The implementation is based on a subclass of Stream as is required in order to handle the Response.Filter requirements. What’s different than other implementations I’ve seen in various places is that it supports capturing output as a whole to allow retrieving the full response output for capture or modification. The exception are the TransformWrite and TransformWrite events which operate only active chunk of data written by the Response. For captured output, the Write() method captures output into an internal MemoryStream that is cached until writing is complete. So Write() is called when ASP.NET writes to the Response stream, but the filter doesn’t pass on the Write immediately to the filter’s internal stream. The data is cached and only when the Flush() method is called to finalize the Stream’s output do we actually send the cached stream off for transformation (if the events are hooked up) and THEN finally write out the returned content in one big chunk. Here’s the implementation of ResponseFilterStream: /// <summary> /// A semi-generic Stream implementation for Response.Filter with /// an event interface for handling Content transformations via /// Stream or String. /// <remarks> /// Use with care for large output as this implementation copies /// the output into a memory stream and so increases memory usage. /// </remarks> /// </summary> public class ResponseFilterStream : Stream { /// <summary> /// The original stream /// </summary> Stream _stream; /// <summary> /// Current position in the original stream /// </summary> long _position; /// <summary> /// Stream that original content is read into /// and then passed to TransformStream function /// </summary> MemoryStream _cacheStream = new MemoryStream(5000); /// <summary> /// Internal pointer that that keeps track of the size /// of the cacheStream /// </summary> int _cachePointer = 0; /// <summary> /// /// </summary> /// <param name="responseStream"></param> public ResponseFilterStream(Stream responseStream) { _stream = responseStream; } /// <summary> /// Determines whether the stream is captured /// </summary> private bool IsCaptured { get { if (CaptureStream != null || CaptureString != null || TransformStream != null || TransformString != null) return true; return false; } } /// <summary> /// Determines whether the Write method is outputting data immediately /// or delaying output until Flush() is fired. /// </summary> private bool IsOutputDelayed { get { if (TransformStream != null || TransformString != null) return true; return false; } } /// <summary> /// Event that captures Response output and makes it available /// as a MemoryStream instance. Output is captured but won't /// affect Response output. /// </summary> public event Action<MemoryStream> CaptureStream; /// <summary> /// Event that captures Response output and makes it available /// as a string. Output is captured but won't affect Response output. /// </summary> public event Action<string> CaptureString; /// <summary> /// Event that allows you transform the stream as each chunk of /// the output is written in the Write() operation of the stream. /// This means that that it's possible/likely that the input /// buffer will not contain the full response output but only /// one of potentially many chunks. /// /// This event is called as part of the filter stream's Write() /// operation. /// </summary> public event Func<byte[], byte[]> TransformWrite; /// <summary> /// Event that allows you to transform the response stream as /// each chunk of bytep[] output is written during the stream's write /// operation. This means it's possibly/likely that the string /// passed to the handler only contains a portion of the full /// output. Typical buffer chunks are around 16k a piece. /// /// This event is called as part of the stream's Write operation. /// </summary> public event Func<string, string> TransformWriteString; /// <summary> /// This event allows capturing and transformation of the entire /// output stream by caching all write operations and delaying final /// response output until Flush() is called on the stream. /// </summary> public event Func<MemoryStream, MemoryStream> TransformStream; /// <summary> /// Event that can be hooked up to handle Response.Filter /// Transformation. Passed a string that you can modify and /// return back as a return value. The modified content /// will become the final output. /// </summary> public event Func<string, string> TransformString; protected virtual void OnCaptureStream(MemoryStream ms) { if (CaptureStream != null) CaptureStream(ms); } private void OnCaptureStringInternal(MemoryStream ms) { if (CaptureString != null) { string content = HttpContext.Current.Response.ContentEncoding.GetString(ms.ToArray()); OnCaptureString(content); } } protected virtual void OnCaptureString(string output) { if (CaptureString != null) CaptureString(output); } protected virtual byte[] OnTransformWrite(byte[] buffer) { if (TransformWrite != null) return TransformWrite(buffer); return buffer; } private byte[] OnTransformWriteStringInternal(byte[] buffer) { Encoding encoding = HttpContext.Current.Response.ContentEncoding; string output = OnTransformWriteString(encoding.GetString(buffer)); return encoding.GetBytes(output); } private string OnTransformWriteString(string value) { if (TransformWriteString != null) return TransformWriteString(value); return value; } protected virtual MemoryStream OnTransformCompleteStream(MemoryStream ms) { if (TransformStream != null) return TransformStream(ms); return ms; } /// <summary> /// Allows transforming of strings /// /// Note this handler is internal and not meant to be overridden /// as the TransformString Event has to be hooked up in order /// for this handler to even fire to avoid the overhead of string /// conversion on every pass through. /// </summary> /// <param name="responseText"></param> /// <returns></returns> private string OnTransformCompleteString(string responseText) { if (TransformString != null) TransformString(responseText); return responseText; } /// <summary> /// Wrapper method form OnTransformString that handles /// stream to string and vice versa conversions /// </summary> /// <param name="ms"></param> /// <returns></returns> internal MemoryStream OnTransformCompleteStringInternal(MemoryStream ms) { if (TransformString == null) return ms; //string content = ms.GetAsString(); string content = HttpContext.Current.Response.ContentEncoding.GetString(ms.ToArray()); content = TransformString(content); byte[] buffer = HttpContext.Current.Response.ContentEncoding.GetBytes(content); ms = new MemoryStream(); ms.Write(buffer, 0, buffer.Length); //ms.WriteString(content); return ms; } /// <summary> /// /// </summary> public override bool CanRead { get { return true; } } public override bool CanSeek { get { return true; } } /// <summary> /// /// </summary> public override bool CanWrite { get { return true; } } /// <summary> /// /// </summary> public override long Length { get { return 0; } } /// <summary> /// /// </summary> public override long Position { get { return _position; } set { _position = value; } } /// <summary> /// /// </summary> /// <param name="offset"></param> /// <param name="direction"></param> /// <returns></returns> public override long Seek(long offset, System.IO.SeekOrigin direction) { return _stream.Seek(offset, direction); } /// <summary> /// /// </summary> /// <param name="length"></param> public override void SetLength(long length) { _stream.SetLength(length); } /// <summary> /// /// </summary> public override void Close() { _stream.Close(); } /// <summary> /// Override flush by writing out the cached stream data /// </summary> public override void Flush() { if (IsCaptured && _cacheStream.Length > 0) { // Check for transform implementations _cacheStream = OnTransformCompleteStream(_cacheStream); _cacheStream = OnTransformCompleteStringInternal(_cacheStream); OnCaptureStream(_cacheStream); OnCaptureStringInternal(_cacheStream); // write the stream back out if output was delayed if (IsOutputDelayed) _stream.Write(_cacheStream.ToArray(), 0, (int)_cacheStream.Length); // Clear the cache once we've written it out _cacheStream.SetLength(0); } // default flush behavior _stream.Flush(); } /// <summary> /// /// </summary> /// <param name="buffer"></param> /// <param name="offset"></param> /// <param name="count"></param> /// <returns></returns> public override int Read(byte[] buffer, int offset, int count) { return _stream.Read(buffer, offset, count); } /// <summary> /// Overriden to capture output written by ASP.NET and captured /// into a cached stream that is written out later when Flush() /// is called. /// </summary> /// <param name="buffer"></param> /// <param name="offset"></param> /// <param name="count"></param> public override void Write(byte[] buffer, int offset, int count) { if ( IsCaptured ) { // copy to holding buffer only - we'll write out later _cacheStream.Write(buffer, 0, count); _cachePointer += count; } // just transform this buffer if (TransformWrite != null) buffer = OnTransformWrite(buffer); if (TransformWriteString != null) buffer = OnTransformWriteStringInternal(buffer); if (!IsOutputDelayed) _stream.Write(buffer, offset, buffer.Length); } } The key features are the events and corresponding OnXXX methods that handle the event hookups, and the Write() and Flush() methods of the stream implementation. All the rest of the members tend to be plain jane passthrough stream implementation code without much consequence. I do love the way Action<t> and Func<T> make it so easy to create the event signatures for the various events – sweet. A few Things to consider Performance Response.Filter is not great for performance in general as it adds another layer of indirection to the ASP.NET output pipeline, and this implementation in particular adds a memory hit as it basically duplicates the response output into the cached memory stream which is necessary since you may have to look at the entire response. If you have large pages in particular this can cause potentially serious memory pressure in your server application. So be careful of wholesale adoption of this (or other) Response.Filters. Make sure to do some performance testing to ensure it’s not killing your app’s performance. Response.Filter works everywhere A few questions came up in comments and discussion as to capturing ALL output hitting the site and – yes you can definitely do that by assigning a Response.Filter inside of a module. If you do this however you’ll want to be very careful and decide which content you actually want to capture especially in IIS 7 which passes ALL content – including static images/CSS etc. through the ASP.NET pipeline. So it is important to filter only on what you’re looking for – like the page extension or maybe more effectively the Response.ContentType. Response.Filter Chaining Originally I thought that filter chaining doesn’t work at all due to a bug in the stream implementation code. But it’s quite possible to assign multiple filters to the Response.Filter property. So the following actually works to both compress the output and apply the transformed content: WebUtils.GZipEncodePage(); ResponseFilterStream filter = new ResponseFilterStream(Response.Filter); filter.TransformString += filter_TransformString; Response.Filter = filter; However the following does not work resulting in invalid content encoding errors: ResponseFilterStream filter = new ResponseFilterStream(Response.Filter); filter.TransformString += filter_TransformString; Response.Filter = filter; WebUtils.GZipEncodePage(); In other words multiple Response filters can work together but it depends entirely on the implementation whether they can be chained or in which order they can be chained. In this case running the GZip/Deflate stream filters apparently relies on the original content length of the output and chokes when the content is modified. But if attaching the compression first it works fine as unintuitive as that may seem. Resources Download example code Capture Output from ASP.NET Pages © Rick Strahl, West Wind Technologies, 2005-2010Posted in ASP.NET  

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• Syntax error in aggregate argument: Expecting a single column argument with possible 'Child' qualifier.

  - by Rushabh

  DataTable distinctTable = dTable.DefaultView.ToTable(true,"ITEM_NO","ITEM_STOCK"); DataTable dtSummerized = new DataTable("SummerizedResult"); dtSummerized.Columns.Add("ITEM_NO",typeof(string)); dtSummerized.Columns.Add("ITEM_STOCK",typeof(double)); int count=0; foreach(DataRow dRow in distinctTable.Rows) { count++; //string itemNo = Convert.ToString(dRow[0]); double TotalItem = Convert.ToDouble(dRow[1]); string TotalStock = dTable.Compute("sum(" + TotalItem + ")", "ITEM_NO=" + dRow["ITEM_NO"].ToString()).ToString(); dtSummerized.Rows.Add(count,dRow["ITEM_NO"],TotalStock); } Error Message: Syntax error in aggregate argument: Expecting a single column argument with possible 'Child' qualifier. Do anyone can help me out? Thanks.

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• CruiseControl.NET: using $(CCNetLabel ) inside ccnet.config file

  - by minty

  When calling external processes like MSBuild cruise control sets environment variables. One of values is CCNetLabel. it holds the value of the current projects label. I want to use the same values in ccnet config itself but when I try ccnet config has a problem. I get the following error: [CCNet Server:ERROR] INTERNAL ERROR: Reference to unknown symbol CCNetLabel ---------- ThoughtWorks.CruiseControl.Core.Config.Preprocessor.EvaluationException: Reference to unknown symbol CCNetLabel at ThoughtWorks.CruiseControl.Core.Config.Preprocessor.ConfigPreprocessorEnvironment._GetConstantDef(String name) at ThoughtWorks.CruiseControl.Core.Config.Preprocessor.ConfigPreprocessorEnvironment.eval_text_constant(String name) ..... ---------- I actually want to append the CCNetLabel to another variable so I need to access the property in ccnet.config. is there a different way to reference these variables?

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• Getting started with Exchange Web Services 2010

  - by Adam Tuttle

  I've been tasked with writing a SOAP web-service in .Net to be middleware between EWS2010 and an application server that previously used WebDAV to connect to Exchange. (As I understand it, WebDAV is going away with EWS2010, so the application server will no longer be able to connect as it previously did, and it is exponentially harder to connect to EWS without WebDAV. The theory is that doing it in .Net should be easier than anything else... Right?!) My end goal is to be able to get and create/update email, calendar items, contacts, and to-do list items for a specified Exchange account. (Deleting is not currently necessary, but I may build it in for future consideration, if it's easy enough). I was originally given some sample code, which did in fact work, but I quickly realized that it was outdated. The types and classes used appear nowhere in the current documentation. For example, the method used to create a connection to the Exchange server was: ExchangeService svc = new ExchangeService(); svc.Credentials = new WebCredentials(AuthEmailAddress, AuthEmailPassword); svc.AutodiscoverUrl(AutoDiscoverEmailAddress); For what it's worth, this was using an assembly that came with the sample code: Microsoft.Exchange.WebServices.dll ("MEWS"). Before I realized that this wasn't the current standard way to accomplish the connection, and it worked, I tried to build on it and add a method to create calendar items, which I copied from here: static void CreateAppointment(ExchangeServiceBinding esb) { // Create the appointment. CalendarItemType appointment = new CalendarItemType(); ... } Right away, I'm confronted with the difference between ExchangeService and ExchangeServiceBinding ("ESB"); so I started Googling to try and figure out how to get an ESB definition so that the CreateAppointment method will compile. I found this blog post that explains how to generate a proxy class from a WSDL, which I did. Unfortunately, this caused some conflicts where types that were defined in the original Assembly, Microsoft.Exchange.WebServices.dll (that came with the sample code) overlapped with Types in my new EWS.dll assembly (which I compiled from the code generated from the services.wsdl provided by the Exchange server). I excluded the MEWS assembly, which only made things worse. I went from a handful of errors and warnings to 25 errors and 2,510 warnings. All kinds of types and methods were not found. Something is clearly wrong, here. So I went back on the hunt. I found instructions on adding service references and web references (i.e. the extra steps it takes in VS2008), and I think I'm back on the right track. I removed (actually, for now, just excluded) all previous assemblies I had been trying; and I added a service reference for https://my.exchange-server.com/ews/services.wsdl Now I'm down to just 1 error and 1 warning. Warning: The element 'transport' cannot contain child element 'extendedProtectionPolicy' because the parent element's content model is empty. This is in reference to a change that was made to web.config when I added the service reference; and I just found a fix for that here on SO. I've commented that section out as indicated, and it did make the warning go away, so woot for that. The error hasn't been so easy to get around, though: Error: The type or namespace name 'ExchangeService' could not be found (are you missing a using directive or an assembly reference?) This is in reference to the function I was using to create the EWS connection, called by each of the web methods: private ExchangeService getService(String AutoDiscoverEmailAddress, String AuthEmailAddress, String AuthEmailPassword) { ExchangeService svc = new ExchangeService(); svc.Credentials = new WebCredentials(AuthEmailAddress, AuthEmailPassword); svc.AutodiscoverUrl(AutoDiscoverEmailAddress); return svc; } This function worked perfectly with the MEWS assembly from the sample code, but the ExchangeService type is no longer available. (Nor is ExchangeServiceBinding, that was the first thing I checked.) At this point, since I'm not following any directions from the documentation (I couldn't find anywhere in the documentation that said to add a service reference to your Exchange server's services.wsdl -- but that does seem to be the best/farthest I've gotten so far), I feel like I'm flying blind. I know I need to figure out whatever it is that should replace ExchangeService / ExchangeServiceBinding, implement that, and then work through whatever errors crop up as a result of that switch... But I have no idea how to do that, or where to look for how to do it. Googling "ExchangeService" and "ExchangeServiceBinding" only seem to lead back to outdated blog posts and MSDN, neither of which has proven terribly helpful thus far. Help me, Obi-Wan, you're my only hope!

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• How can I use Boost::regex.hpp library in C++?

  - by MIH1406

  I tried to use Boost library but I failed, see my code: #include "listy.h" #include <boost/regex.hpp> using namespace boost; ListyCheck::ListyCheck() { } ListyCheck::~ListyCheck() { } bool ListyCheck::isValidItem(std::string &__item) { regex e("(\\d{4}[- ]){3}\\d{4}"); return regex_match(__item, e); } When I tried to compile it I get those messages: /usr/include/boost/regex/v4/regex_match.hpp:50: undefined reference to `boost::re_detail::perl_matcher<__gnu_cxx::__normal_iterator, std::allocator , std::allocator, std::allocator , boost::regex_traits ::match()' /usr/include/boost/regex/v4/basic_regex.hpp:425: undefined reference to `boost::basic_regex ::do_assign(char const*, char const*, unsigned int)' /usr/include/boost/regex/v4/perl_matcher.hpp:366: undefined reference to `boost::re_detail::perl_matcher<__gnu_cxx::__normal_iterator, std::allocator , std::allocator, std::allocator , boost::regex_traits ::construct_init(boost::basic_regex const&, boost::regex_constants::_match_flags)' etc...

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• Debugging error "The Type 'xx' is defined in an assembly that is not referenced"

  - by Abel

  The full error is as follows: The type 'System.Windows.Forms.Control' is defined in an assembly that is not referenced. You must add a reference to assembly 'System.Windows.Forms, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'. and it points at the very first statement (an Debug.Assert line) in the very first class in a library project that doesn't need System.Windows.Forms (or so I thought). I know how to solve it: add the mentioned reference. But how do I find out what library is causing this error, or better, what part of the code triggers using the WinForms library? Normally, you can add libraries that reference others, but you only need to add references to these others when they're actually used. EDIT: Alternative solution This or similar problems can also be resolved using the Binding Log Viewer Fuslogvw.exe from Microsoft's Framework Tools. It shows all attempts and successes of assemblies your application binds to.

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• jquery countdown/countUP with server side values

  - by basit.

  script: http://keith-wood.name/countdown.htm daily json response: items: { fajr: '5:23 am', sharooq: '7:23 am', dhur: '1:34 pm', asr: '4:66 pm': magrib: '6:23 pm', isha: '8:01 pm'} when site loads i make ajax request and get the above response times, these are events that happens daily for everyday, but different timing. i want to get that time and put a count down on how many minitues left or hours or seconds and show that and once the seconds are done, then show how many minitues ago that event took place, after 15 minitues later show new even count down. so following on how it will look on display dhur 2 hours left dhur 2 minutes left (if no longer hours left) dhur 55 seconds left (if no longer minutes left) dhur 5 seconds ago (if count down finished and then show how many seconds ago) dhur 9 minutes ago (if extended more then seconds, then show how many minutes ago) asr 1 hour left (after 15 minutes later time changes to new event) this is kind of very simple for pro in javascript and really complicated for me, so need your guys help, if the script im using not good and you prefer some other script for this kind of task, please share with me, it dont have to be jquery script, but helps if its jquery.

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• MDI WinForm application and duplicate child form memory leak

  - by Steve

  This is a WinForm MDI application problem (.net framework 3.0). It will be described in C#. Sorry it is a bit long because I try to make things as clear as possible. I have a MDI application. At some point I find that one MDI child form is never released. There is a menu that creates the MDI child form and show it. When the MDI child form is closed, it is supposed to be destroyed and the memory taken by it should be given back to .net. But to my surprise, this is not true. All the MDI child form instances are kept in memory. This is obviously a "memory leak". Well, it is not a real leak in .net. It is just that I think the closed form should be dead but somehow there is at least one unknown reference from outside world that still connect with the closed form. I read some articles on the Web. Some says that when the MDI child form is closing, I should unwire all the event handlers, otherwise some event handlers may keep my form alive. Some says that DataBindings should be cleaned before the form is closing otherwise the DataBindings will add references to some global Hashtable and thus keep my form alive. My form contains quite a lot things. Many event handlers and many DataBindings and many BindingSources and few suspected controls containing user control and HelpProvider. I create a big method that unwires all the event handlers from all the relevant controls, clear all the DataBindings and DataSources. The HelpProvider and user controls are disposed carefully. At the end, I find that, I don't have to clear DataBindings and DataSources. Event handlers are definitely causing the problem. And MDI form structure also contributes to something. During my experiments, I find that, if you create a MDI child form, even if you close it, there will still be one instance in the memory. The reference is from PropertyStore of the main form. This means, unless the main form is closed (application ends), there will always be one instance of MDI child form in the memory. The good news is that, no matter how many times you open and close the child form, there will be only one instance, not a big "leak". When it comes to event handlers, things become more tricky. I have to address that, all the event handlers on my form are anonymous event handlers. Here is an example code: //On MDI child form's design code... Button btnSave = new Button(); btnSave.Click += new System.EventHandler(btnSave_Click); Where btnSave_Click is also a method in MDI child form. The above is always the case for various controls and various types of event. To me, this is a bi-directional circular reference. btnSave keeps a reference of MDI child form via the event handler. MDI child form keeps a reference of btnSave instance. To me again, such bi-directional circular reference should not cause any problem for .net's garbage collector. This means that I do not have to explicitly unwire the event when the form is being disposed: btnSave.Click -= btnSave_Click; But the truth is not so. For some event handlers, they are safe. Ignoring them do not cause any duplicate instance. For some other event handlers, they will cause one instance remaining in the memory (similar effect as the MDI form structure, but this time caused by the hanging event handlers). For some other event handlers, they will cause every instance opened in the memory. I am totally confused about the differences between these three types of event handlers. The controls are created in the same way and the event is attached in the same way. What is the difference? (Don't tell me it is the event handle methods that make difference.) Anyone has experience of this wired scenario and has an answer for me? Thanks a lot. So now, for safety issue, I will have to unwire all the event handlers when the form is being disposed. That will be a long list of similar code for each control. Is there a general way of removing events from controls in recursive way using reflection? What about performance issue? That's the end of my story and I am still in the middle of my problem. For any help, I thank you.

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• WM_CONCAT use CASE

  - by Ruslan

  i have a select: select substr(acc,1,4),currency, amount, module, count(*), wm_concat(trn_ref_no) trn from all_entries where date = to_date ('01012010','DDMMYYYY') group by substr(acc,1,4),currency, amount, module in this case i have error: *ORA-06502: PL/SQL: : character string buffer too small ... "WMSYS.WM_CONCAT_IMPL"* if i change to: select substr(acc,1,4),currency, amount, module, count(), (case when count() < 10 then wm_concat(trn_ref_no) else null end) trn from fcc.acvw_all_ac_entries where trn_dt = to_date ('05052010','DDMMYYYY') group by substr(acc,1,4),currency, amount, module to avoid buffer limit error. But even in this case i have the same error. How can i avoid this error?

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• MySql - Get row number on select

  - by George

  Can I run a select statement and get the row number if the items are sorted? I have a table like this: mysql> describe orders; +-------------+---------------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +-------------+---------------------+------+-----+---------+----------------+ | orderID | bigint(20) unsigned | NO | PRI | NULL | auto_increment | | itemID | bigint(20) unsigned | NO | | NULL | | +-------------+---------------------+------+-----+---------+----------------+ I can then run this query to get the number of orders by ID: SELECT itemID, COUNT(*) as ordercount FROM orders GROUP BY itemID ORDER BY ordercount DESC; This gives me a count of each itemID in the table like this: +--------+------------+ | itemID | ordercount | +--------+------------+ | 388 | 3 | | 234 | 2 | | 3432 | 1 | | 693 | 1 | | 3459 | 1 | +--------+------------+ I want to get the row number as well, so I could tell that itemID 388 is the first row, 234 is second, etc (essentially the ranking of the orders, not just a raw count). I know I can do this in java when I get the result set back, but I was wondering if there was a way to handle it purely in SQL.

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• Is there a way to get programmatic access to the columns of a ActiveReports detail section?

  - by Seth Spearman

  Hello, I have a report in Data Dynamics ActiveReports for .NET. In this report I am programmatically setting the ColumnCount property of the detail section to X. The detail section has one databound textbox. The ColumnDirection property of the detail section is set to AcrossDown and the and then the data binding mechanism automatically fills across with data after setting the DataSource and DataMember. Here is the code... Public Sub RunReport Dim count As Integer = 0 ' ... get count Detail1.ColumnCount = count Me.DataSource = ds Me.DataMember = ds.Tables(0).TableName End Sub That code works fine and the data is automatically filled across the report. Now I need to alter the report and circle or highlight one of the items that is auto-filled across columns in the report. I cannot find any way to programmatically access the auto-generated columns so I can turn on a border or draw a circle or something. Any ideas how I would do that? Seth

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• WPF Resource Dictionary in a separate assembly

  - by Gustavo Cavalcanti

  I have resource dictionary files (MenuTemplate.xaml, ButtonTemplate.xaml, etc) that I want to use in multiple separate applications. I could add them to the applications' assemblies, but it's better if I compile these resources in one single assembly and have my applications reference it, right? After the resource assembly is built, how can I reference it in the App.xaml of my applications? Currently I use ResourceDictionary.MergedDictionaries to merge the individual dictionary files. If I have them in an assembly, how can I reference them in xaml? Thanks for your help! Gustavo

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• passing json from servlet to dojo

  - by Moev4

  I am currently trying to pass a generated JSON string to dojo for parsing and am having some issues. The servlet simply writes it as a string as so: response.getWriter().append("{ \"data\": {"); response.getWriter().append("\"type\": \"facing\","); response.getWriter().append("\"score\": " + "\"" + score + "\","); response.getWriter().append("\"count\":" + "\"" + count + "\"" ); response.getWriter().append("}}"); which prints as: {"data":{"type":"facing","score":"10","count":"24"}} And the parsing on the dojo end looks as so: dojo.xhrPost({ url: url, handleAs: "json", load: function(data) { alert(data); /* Parse Not working */ alert(data.data[0].type); }, error: function(error) { alert("No dice") } }); The main issue is the data.data[0].type is returning nothing but when i print this out as text the json seems to be correctly formatted. Any help with this would be appreciated.

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• Charater string buffer too small

  - by Ruslan

  I have select: select v.accs, v.currency,v.amount,v.drcr_ind, count(*) qua,wm_concat(ids) npx_IDS, wm_concat(px_dtct) npx_DTCT from table v group by accs, currency, amount, drcr_ind but i get error ORA-06502: PL/SQL: : character string buffer too small if i'll remove one string, because sometimes (when v.accs= 3570) count(*) = 215 but when i try to skip using wm_concat for v.accs= 3570 for example this way: select v.accs, v.currency,v.amount,v.drcr_ind, count(*) qua,wm_concat(ids) npx_IDS, (case when v.accs = 3570 then wm_concat(px_dtct) else 'too many' end) npx_DTCT from table v group by accs, currency, amount, drcr_ind i still have the same error message. But why? How can i fix it? Thanx

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• Equality between two enumerables

  - by Berryl

  I have two enumerables with the exact same reference elements, and wondering why Equals wouldn't be true. As a side question, the code below to compare each element works, but there must be a more elegant way Cheers, Berryl var other = (ActivityService) obj; if (!AllAccounts.Count().Equals(other.AllAccounts.Count())) return false; for (int i = 0; i < AllAccounts.Count(); i++) { if (!AllAccounts.ElementAt(i).Equals(other.AllAccounts.ElementAt(i))) { return false; } } return true;

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• UIImageWriteToSavedPhotosAlbum not working in iOS 4?

  - by nickthedude

  Ive been using this function: (http://developer.apple.com/iphone/library/documentation/uikit/reference/UIKitFunctionReference/Reference/reference.html#//apple_ref/c/func/UIImageWriteToSavedPhotosAlbum) ...to save a uiimage to the users photo album and it works great on everything except a 4.0 simulator, dont have a device to test on so it may be boned there as well. ive also tried it in both the latest xcode (3.2.3 gm seed) and a previous build of 3.2.3. can anyone confirm that this is still function is still "functional" in iOS 4. the documentation gives no indication that it would be removed and I havent seen anything in searches related to it. Thanks, Nick

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• How does Garbage Collection in Java work?

  - by Bright010957

  I was wondering how the garbage collector in Java deals with the following situation. Object A has a reference to Object B and Object B has a reference to Object C. The main program has a reference to Object A. So you can use Object B trough Object A, and Object C trough Object B trough Object A. What happens to Object B and Object C, if the link between Object A and Object B is set to null? Should Object B and Object C now been collected by the Garbage Collector? I mean there is still a connection between Object B and Object C.

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• Stack & heap understanding question

  - by Petr

  Hi, I would really appreciate if someone could tell me whether I understand it well: class X { A a1=new A() //reference on the stack, object value on the heap a1.VarA=5; //on the stack - value type A a2=new A() //reference on the stack, object value on the heap a2.VarA=10; //on the stack - value type a1=a2; //on the stack, the target of a1 reference is updated to a2 value on the heap //also both a1 and a2 references are on the stack, while their "object" values on the heap. But what about VarA variable, its still pure value type? } class A { int VarA; }

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• Boost::Interprocess Container Container Resizing No Default Constructor

  - by CuppM

  Hi, After combing through the Boost::Interprocess documentation and Google searches, I think I've found the reason/workaround to my issue. Everything I've found, as I understand it, seems to be hinting at this, but doesn't come out and say "do this because...". But if anyone can verify this I would appreciate it. I'm writing a series of classes that represent a large lookup of information that is stored in memory for fast performance in a parallelized application. Because of the size of data and multiple processes that run at a time on one machine, we're using Boost::Interprocess for shared memory to have a single copy of the structures. I looked at the Boost::Interprocess documentation and examples, and they typedef classes for shared memory strings, string vectors, int vector vectors, etc. And when they "use" them in their examples, they just construct them passing the allocator and maybe insert one item that they've constructed elsewhere. Like on this page: http://www.boost.org/doc/libs/1_42_0/doc/html/interprocess/allocators_containers.html So following their examples, I created a header file with typedefs for shared memory classes: namespace shm { namespace bip = boost::interprocess; // General/Utility Types typedef bip::managed_shared_memory::segment_manager segment_manager_t; typedef bip::allocator<void, segment_manager_t> void_allocator; // Integer Types typedef bip::allocator<int, segment_manager_t> int_allocator; typedef bip::vector<int, int_allocator> int_vector; // String Types typedef bip::allocator<char, segment_manager_t> char_allocator; typedef bip::basic_string<char, std::char_traits<char>, char_allocator> string; typedef bip::allocator<string, segment_manager_t> string_allocator; typedef bip::vector<string, string_allocator> string_vector; typedef bip::allocator<string_vector, segment_manager_t> string_vector_allocator; typedef bip::vector<string_vector, string_vector_allocator> string_vector_vector; } Then for one of my lookup table classes, it's defined something like this: class Details { public: Details(const shm::void_allocator & alloc) : m_Ids(alloc), m_Labels(alloc), m_Values(alloc) { } ~Details() {} int Read(BinaryReader & br); private: shm::int_vector m_Ids; shm::string_vector m_Labels; shm::string_vector_vector m_Values; }; int Details::Read(BinaryReader & br) { int num = br.ReadInt(); m_Ids.resize(num); m_Labels.resize(num); m_Values.resize(num); for (int i = 0; i < num; i++) { m_Ids[i] = br.ReadInt(); m_Labels[i] = br.ReadString().c_str(); int count = br.ReadInt(); m_Value[i].resize(count); for (int j = 0; j < count; j++) { m_Value[i][j] = br.ReadString().c_str(); } } } But when I compile it, I get the error: 'boost::interprocess::allocator<T,SegmentManager>::allocator' : no appropriate default constructor available And it's due to the resize() calls on the vector objects. Because the allocator types do not have a empty constructor (they take a const segment_manager_t &) and it's trying to create a default object for each location. So in order for it to work, I have to get an allocator object and pass a default value object on resize. Like this: int Details::Read(BinaryReader & br) { shm::void_allocator alloc(m_Ids.get_allocator()); int num = br.ReadInt(); m_Ids.resize(num); m_Labels.resize(num, shm::string(alloc)); m_Values.resize(num, shm::string_vector(alloc)); for (int i = 0; i < num; i++) { m_Ids[i] = br.ReadInt(); m_Labels[i] = br.ReadString().c_str(); int count = br.ReadInt(); m_Value[i].resize(count, shm::string(alloc)); for (int j = 0; j < count; j++) { m_Value[i][j] = br.ReadString().c_str(); } } } Is this the best/correct way of doing it? Or am I missing something. Thanks!

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• Accessing ruby counter cache

  - by Julian

  Hi all, I'm playing around with a fork of acts_as_taggable_on_steroids as a learning exercise. The version I'm looking at does some stuff I don't understand to calculate Tag counts. So I thought I'd do a version using PORC (Plain Old Rails Counters): class Tagging < ActiveRecord::Base #:nodoc: belongs_to :tag, :counter_cache => "tagging_counter_cache" ... I thought tagging_counter_cache was transparently accessed when I access tag.taggings.count but apparently not? Do I really have to access tag.tagging_counter_cache explicitly? >> tag.taggings.count SQL (0.7ms) SELECT count(*) AS count_all FROM `taggings` WHERE (`taggings`.tag_id = 16) Same for size. It's cool if that's the case but just wanted to check.

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• Why I am not able to display image using swing worker?

  - by Vimal Basdeo

  I was trying some codes to implement a scheduled task and came up with these codes . import java.util.*; class Task extends TimerTask { int count = 1; // run is a abstract method that defines task performed at scheduled time. public void run() { System.out.println(count+" : Mahendra Singh"); count++; } } class TaskScheduling { public static void main(String[] args) { Timer timer = new Timer(); // Schedule to run after every 3 second(3000 millisecond) timer.schedule( new Task(), 3000); } } My output : 1 : Mahendra Singh I expected the compiler to print a series of Mahendra Singh at periodic interval of 3 s but despite waiting for around 15 minutes, I get only one output...How do I solve this out?

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• Help to translate SQL query to Relational Algebra

  - by Mestika

  Hi everyone, I'm having some difficulties with translating some queries to Relational Algebra. I've a great book about Database Design and here is a chapter about Relational Algebra but I still seem to have some trouble creating the right one: Thoes queries I've most difficuelt with is these: SELECT COUNT( cs.student_id ) AS counter FROM course c, course_student cs WHERE c.id = cs.course_id AND c.course_name = 'Introduction to Database Design' SELECT COUNT( cs.student_id ) FROM Course c INNER JOIN course_student cs ON c.id = cs.course_id WHERE c.course_name = 'Introduction to Database Design' and SELECT COUNT( * ) FROM student JOIN grade ON student.f_name = "Andreas" AND student.l_name = "Pedersen" AND student.id = grade.student_id I know the notation can be a bit hard to paste into HTML forum, but maybe just use some common name or the Greek name. Thanks in advance Mestika

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