simple talk - Page 220 - Developer IT

A short demo to use savepoint in MySQL available?

- by vps

MySQL tutorial format is too ugly. Can someone share a simple demo?

bluetooth connection using pybluez

I am working on bluetooth not exactly on bluetooth stack-development but to use bluetooth in one of my project. I had done all that before using some of the py-bluez commands like hciconfig, hcitool scan , then simple-agents and using serial module inside python. But that was quite random. We were able to connect only one specific device based on its bluetooth address and there was no facility of reconnection once the devices are disconnected. Now i want to try out this stuff in a sequential manner like this (i am doing that all on a RPI and for at present on ubuntu 12.04.) i) Store some names in a file along with some other information with respect to that device. ii) Run a script to find out the device in locality with those names and if any one if found, report that. For this step, i had taken a reference from BTBook , made available from MIT. Below is the script for the same, but that script only search for the single name. from bluetooth import * target_name = "XT1033" target_address = None nearby_devices = discover_devices() for address in nearby_devices: if target_name == lookup_name( address ): target_address = address break if target_address is not None: print "found target bluetooth device with address ", target_address connect_socket(target_address); else: print "could not find target bluetooth device nearby" iii) Connect the device using client sock. But i dont have any device on which i can write a simple python script. My client can be any device that will be publishing data. Now i came through a script in the same book, that actually connect to a client requesting permission to connect to server. from bluetooth import * port = 1 server_sock=BluetoothSocket( RFCOMM ) server_sock.bind(("",port)) server_sock.listen(1) client_sock, client_info = server_sock.accept() print "Accepted connection from ", client_info data = client_sock.recv(1024) print "received [%s]" % data client_sock.close() server_sock.close() here client_sock, client_info = server_sock.accept() provide the client address and port requested to be connected. Can i pass address obtained from the earlier script to this, so that it connect server to the client? iv) Then if client get disconnected, re-connect(a simple polling can be used.) All this stuff can be done using bash and py-bluez functions but i want to do that in a sequential manner.I am not a master in python but i can do some small stuff. Can any one guide me for the same or can direct me to more usefull resource through which i can continue my coding part after finding the "X", "Y" named devices.

Read the article

Controlling access to my API using SSH public key (not SSL)

- by tharrison

I have the challenge of implementing an API to be consumed by relatively non-technical clients -- pasting some sample code into their WordPress or homegrown PHP site is probably as much as we can ask. Asking them to install SSL on their servers ain't happening. So I am seeking a simple yet secure way to authenticate API clients. OAuth is the obvious solution, but I don't think it passes the "simple" test. Adding a client id and hashed secret as a parameter to the requests is closer -- it's not hard to do md5($secret . $client_id) or whatever the php would be. It seems to me that if client requests could use the same approach as SSH public keys (client gives us a key from their server(s) there should be some existing magic to make all of the subsequent transactions transparently work just as regular HTTP API requests. I am still working this out (obviously :-), so if I am being an idiot, it would be nice to know why. Thanks!

Read the article

Hyper-V with a single host NIC

- by Cylindric

I'm trying to create a simple Hyper-V test machine, and am having more problems than expected with the basic networking. All other hypervisors I've used (vmware server, vmware player, virtualbox) allow me to use the default single NIC in my machine, and then create guests with "bridged" mode so they appear like a normal machine on the network. With the Hyper-V server, when I create a Virtual Switch and set it to "external", I lose all network access from the host. The solutions I've seen online seem to rely on complex configurations requiring multiple vSwitches and RRAS on the host to get it going. Is it really that complex to get a simple, single-NIC Hyper-V server going, in such a way that I can still use the host? The host is Windows 8.

Read the article

Lightweight alternative to R for RHEL?

- by Eric Rath

I want to use R for some statistical analysis of logfile information, but found that even the "limited" R-core RPM has a lot of dependencies not already installed. I don't want to install so many packages for a peripheral need. Are there lightweight alternatives for simple statistical analysis on RHEL 6? I have an R script that accepts on stdin a large set of values -- one value per line -- and prints out the min, max, mean, median, 95th percentile, and standard deviation. For more context, I'm using grep and awk to find GET requests for a particular path in our webserver log files, get the response times, and calculate the metrics listed above in order to measure the impact on performance of changes to a web application. I don't need any graphing capabilities, just simple computation. Is there something I've overlooked?

Read the article

Running a cronjob

- by Ed01

've been puzzling over cronjobs for the last few hours. I've read documentation and examples. I understand the basics and concepts, but haven't gotten anything to work. So I would appreciate some help with this total noob dilemma. The ultimate goal is to schedule the execution of a django function every day. Before I get that far, I want to know that I can schedule any old script to run, first once, then on a regular basis. So I want to: 1) Write a simple script (perhaps a bash script) that will allow me to determine that yes, it did indeed run successfully, or that it failed. 2) schedule this script to run at the top of the hour I tried writing a bash script that simple output some text to the terminal: #!/bin/bash echo "The script ran" Then I dropped this into a .txt file MAILTO = *****.******@gmail.com 05 * * * * /home/vadmin/development/test.sh But nothing happened. I'm sure I did many things wrong. Where do I start to fix all of this?

Read the article

IPSec 2 hosts (preshared key) - network shares very slow

- by LxFlip

I'm testing a IPSec config between 2 hosts, using ipsec auth with preshared key, very simple configuration. (I want to start with a IPSec simple preshared key config, and then step up to a Certificate or kerberos...) The problem is: The connection is working but when accessing network file shares the first time it's very slow. On the same host i'm testing the shares, i have an IIS site running, and the performance seems very normal, fast. Does anybody know why does SMB shares are soo slow? Is there any ipsec policy options that should be tweaked? Thanks

Read the article

Excel Circular Reference

- by Demetre

Simple question. I'm working in Excel 2007, and I want to convert 1000 grams into 1 kilogram when I input 1000g into a particular cell. So I decided to create the if statement below. However there is a problem with this. It's stating that it is a Circular Reference, and leaves me with a 0 value. Is there a simple way to fix this issue in excel? Here is the if statement I have. =IF(C8 >= 1000,CONVERT(C8,"g","k"),C8) Tell what you guys think. Thanks in advance.

Read the article

Table Formatting in Word

- by user359217

I have a table in Word which is 5 columns wide and multiple rows. In Row 3, cells 1, 2, 3 & 5 have simple text. Cell 4 contains a large quantity of text and therefore needs to wrap over several pages. Therefore, I mark "Allow row to break across pages". Problem: on next page where row has wrapped, cells 1, 2, 3 & 5 are blank with cell 4 displaying the wrapped text. Is there any way that I can get the simple text from Row 3, cells 1, 2 and 3 to repeat on the pages which contain the wrapped text of cell 4? I do not want the data to be in the table heading, as I have multiple rows which have a similar volume of text.

Read the article

Execute a SQlite command with Entity Framework

- by Filimindji

Hi everybody, I use a SQLite database and Entity Framework (with .net framework 3.5). I'm trying to execute a simple SQL non query command to create a new table in this datase. My Entity Framework already contains the object model for this table : I just want to generate the corresponding table using a command. (By the way, there is maybe a better way to do this. Any ideas someone :) My problem is that I'm not able to execute any command, even the simple commands. Here is my code : EntityConnection entityConnection = new EntityConnection(entitiesConnectionString); Entities db = new Entities(entityConnection); DbCommand command = db.Connection.CreateCommand(); command.CommandText ="CREATE TABLE MyTable (Id int NOT NULL, OtherTable_Id nchar(40) REFERENCES OtherTable (Id) On Delete CASCADE On Update NO ACTION, SomeData nvarchar(1024) NOT NULL, Primary Key(Id) );"; command.ExecuteNonQuery(); I got this error : System.Data.EntitySqlException: The query syntax is not valid., near identifier 'TABLE', line 1, column 8. at System.Data.Common.EntitySql.CqlParser.yyerror(String s) at System.Data.Common.EntitySql.CqlParser.yyparse() at System.Data.Common.EntitySql.CqlParser.Parse(String query) at System.Data.Common.EntitySql.CqlQuery.Parse(String query, ParserOptions parserOptions) at System.Data.Common.EntitySql.CqlQuery.Compile(String query, Perspective perspective, ParserOptions parserOptions, Dictionary`2 parameters, Dictionary`2 variables, Boolean validateTree) at System.Data.EntityClient.EntityCommand.MakeCommandTree() at System.Data.EntityClient.EntityCommand.CreateCommandDefinition() at System.Data.EntityClient.EntityCommand.TryGetEntityCommandDefinitionFromQueryCache(EntityCommandDefinition& entityCommandDefinition) at System.Data.EntityClient.EntityCommand.GetCommandDefinition() at System.Data.EntityClient.EntityCommand.InnerPrepare() at System.Data.EntityClient.EntityCommand.ExecuteReader(CommandBehavior behavior) at System.Data.EntityClient.EntityCommand.ExecuteScalar[T_Result](Func`2 resultSelector) It's seem to be a syntax error, but I can't figure where is the problem and how to resolve it. The entityConnection is ok because I can use any entities generated with EF. I tried with another simple command, but it throw another exception : DbCommand command = db.Connection.CreateCommand(); command.CommandText = "SELECT COUNT(Id) From OtherTable;"; int result = (int)command.ExecuteScalar(); And I got this error, witch is not the same, but may help : System.Data.EntitySqlException: 'Groupe' could not be resolved in the current scope or context. Make sure that all referenced variables are in scope, that required schemas are loaded, and that namespaces are referenced correctly., near simple identifier, line 1, column 23. at System.Data.Common.EntitySql.CqlErrorHelper.ReportIdentifierError(Expr expr, SemanticResolver sr) at System.Data.Common.EntitySql.SemanticAnalyzer.ConvertIdentifier(Expr expr, SemanticResolver sr) at System.Data.Common.EntitySql.SemanticAnalyzer.Convert(Expr astExpr, SemanticResolver sr) at System.Data.Common.EntitySql.SemanticAnalyzer.ProcessAliasedFromClauseItem(AliasExpr aliasedExpr, SemanticResolver sr) at System.Data.Common.EntitySql.SemanticAnalyzer.ProcessFromClauseItem(FromClauseItem fromClauseItem, SemanticResolver sr) at System.Data.Common.EntitySql.SemanticAnalyzer.ProcessFromClause(FromClause fromClause, SemanticResolver sr) at System.Data.Common.EntitySql.SemanticAnalyzer.ConvertQuery(Expr expr, SemanticResolver sr) at System.Data.Common.EntitySql.SemanticAnalyzer.Convert(Expr astExpr, SemanticResolver sr) at System.Data.Common.EntitySql.SemanticAnalyzer.ConvertRootExpression(Expr astExpr, SemanticResolver sr) at System.Data.Common.EntitySql.SemanticAnalyzer.ConvertGeneralExpression(Expr astExpr, SemanticResolver sr) at System.Data.Common.EntitySql.CqlQuery.AnalyzeSemantics(Expr astExpr, Perspective perspective, ParserOptions parserOptions, Dictionary`2 parameters, Dictionary`2 variables) at System.Data.Common.EntitySql.CqlQuery.Compile(String query, Perspective perspective, ParserOptions parserOptions, Dictionary`2 parameters, Dictionary`2 variables, Boolean validateTree) at System.Data.EntityClient.EntityCommand.MakeCommandTree() at System.Data.EntityClient.EntityCommand.CreateCommandDefinition() at System.Data.EntityClient.EntityCommand.TryGetEntityCommandDefinitionFromQueryCache(EntityCommandDefinition& entityCommandDefinition) at System.Data.EntityClient.EntityCommand.GetCommandDefinition() at System.Data.EntityClient.EntityCommand.InnerPrepare() at System.Data.EntityClient.EntityCommand.ExecuteReader(CommandBehavior behavior) at System.Data.EntityClient.EntityCommand.ExecuteScalar[T_Result](Func`2 resultSelector)

Read the article

Perl CGI that sends a temporary loading page to client then later sends the actual results page

- by Kurt W. Leucht

I've wasted at least a half day of my company's time searching the Internet for an answer and I'm getting wrapped around the axle here. I can't figure out the difference between all the different technology choices (long polling, ajax streaming, comet, XMPP, etc.) and I can't get a simple hello world example working on my PC. I am running Apache 2.2 and ActivePerl 5.10.0. JavaScript is completely acceptable for this solution. All I want to do is write a simple Perl CGI script that when accessed, it immediately returns some HTML that tells the user to wait or maybe sends an animated GIF. Then without any user intervention (no mouse clicks or anything) I want the CGI script to at some time later replace the wait message or the animated GIF with the actual HTML results from their query. I know this is simple stuff and websites do it all the time, but I can't find a single working example that I can cut and paste onto my machine that will work. Here is my simple Hello World example that I've compiled from various Internet sources, but it doesn't seem to work. When I refresh this CGI URL in my web browser it prints nothing for 5 seconds, then it prints the PLEASE BE PATIENT web page, but not the results web page. What am I doing wrong? #!C:\Perl\bin\perl.exe use CGI; use CGI::Carp qw/fatalsToBrowser warningsToBrowser/; sub Create_HTML { my $html = <<EOHTML; <html> <head> <meta http-equiv="pragma" content="no-cache" /> <meta http-equiv="expires" content="-1" /> <script type="text/javascript" > var xmlhttp=false; /*@cc_on @*/ /*@if (@_jscript_version >= 5) // JScript gives us Conditional compilation, we can cope with old IE versions. // and security blocked creation of the objects. try { xmlhttp = new ActiveXObject("Msxml2.XMLHTTP"); } catch (e) { try { xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); } catch (E) { xmlhttp = false; } } @end @*/ if (!xmlhttp && typeof XMLHttpRequest!='undefined') { try { xmlhttp = new XMLHttpRequest(); } catch (e) { xmlhttp=false; } } if (!xmlhttp && window.createRequest) { try { xmlhttp = window.createRequest(); } catch (e) { xmlhttp=false; } } </script> <title>Ajax Streaming Connection Demo</title> </head> <body> Some header text. <p> <div id="response">PLEASE BE PATIENT</div> <p> Some footer text. </body> </html> EOHTML return $html; } my $cgi = new CGI; print $cgi->header; print Create_HTML(); sleep(5); print "<script type=\"text/javascript\">\n"; print "\$('response').innerHTML = 'Here are your results!';\n"; print "</script>\n";

Read the article

What git branching models actually work - the final question

- by UncleCJ

In our company we have successfully deployed git and we are currently using a simple trunk/release/hotfixes branching model. However, this has it's problems, I have some key issues of confusion in the community which would be awesome to have answered here. Maybe my hopes for an Alexander stroke are too great, quite possibly I'll decompose this question into more manageable issues, but here's my first shot. Workflows / branching models - below are the three main descriptions of this I have seen, but they are partially contradicting each other or don't go far enough to sort out the subsequent issues we've run into (as described below). Thus our team so far defaults to not so great solutions. Are you doing something better? gitworkflows(7) Manual Page (nvie) A successful Git branching model (reinh) A Git Workflow for Agile Teams Merging vs rebasing (tangled vs sequential history) - the bids on this are as confusing as it gets. Should one pull --rebase or wait with merging back to the mainline until your task is finished? Personally I lean towards merging since this preserves a visual illustration of on which base a task was started and finished, and I even prefer merge --no-ff for this purpose. It has other drawbacks however. Also many haven't realized the useful property of merging - that it isn't commutative (merging a topic branch into master does not mean merging master into the topic branch). I am looking for a natural workflow - sometimes mistakes happen because our procedures don't capture a specific situation with simple rules. For example a fix needed for earlier releases should of course be based sufficiently downstream to be possible to merge upstream into all branches necessary (is the usage of these terms clear enough?). However it happens that a fix makes it into the master before the developer realizes it should have been placed further downstream, and if that is already pushed (even worse, merged or something based on it) then the option remaining is cherry-picking, with it's associated perils... What simple rules like such do you use? Also in this is included the awkwardness of one topic branch necessarily excluding other topic branches (assuming they are branched from a common baseline). Developers don't want to finish a feature to start another one feeling like the code they just wrote is not there anymore How to avoid creating merge conflicts (due to cherry-pick)? What seems like a sure way to create a merge conflict is to cherry-pick between branches, they can never be merged again? Would applying the same commit in revert (how to do this?) in either branch possibly solve this situation? This is one reason I do not dare to push for a largely merge-based workflow. How to decompose into topical branches? - We realize that it would be awesome to assemble a finished integration from topic branches, but often work by our developers is not clearly defined (sometimes as simple as "poking around") and if some code has already gone into a "misc" topic, it can not be taken out of there again, according to the question above? How do you work with defining/approving/graduating/releasing your topic branches? Proper procedures like code review and graduating would of course be lovely, but we simply cannot keep things untangled enough to manage this - any suggestions? integration branches, illustration please? Vote and comment as much as you'd like, I'll try to keep the issue page clear and informative enough. Thanks! Below is a list of related topics on stackoverflow I have checked out: What are some good strategies to allow deployed applications to be hotfixable? Workflow description for git usage for in-house development Git workflow for corporate Linux kernel development How do you maintain development code and production code? (thanks for this PDF!) git releases management Git Cherry-pick vs Merge Workflow How to cherry-pick multiple commits How do you merge selective files with git-merge? How to cherry pick a range of commits and merge into another branch ReinH Git Workflow git workflow for making modifications you’ll never push back to origin Cherry-pick a merge Proper Git workflow for combined OS and Private code? Maintaining Project with Git Why cant Git merge file changes with a modified parent/master. Git branching / rebasing good practices When will "git pull --rebase" get me in to trouble?

Read the article

WCF and Firewalls

- by Amitd

Hi guys, As a part of learning WCF, I was trying to use a simple WCF client-server code . http://weblogs.asp.net/ralfw/archive/2007/04/14/a-truely-simple-example-to-get-started-with-wcf.aspx but I'm facing strange issues.I was trying out the following. Client(My) IP address is : 192.168.2.5 (internal behind firewall) Server IP address is : 192.168.50.30 port : 9050 (internal behind firewall) Servers LIVE/External IP (on internet ) : 121.225.xx.xx (accessible from internet) When I specify the above I.P address of server(192.168.50.30), the client connects successfully and can call servers methods. Now suppose if I want to give my friend (outside network/on internet) the client with server's live I.P, i get an ENDPOINTNOTFOUND exceptions. Surprisingly if I run the above client specifying LIVE IP(121.225.xx.xx) of server i also get the same exception. I tried to debug the problem but haven't found anything. Is it a problem with the company firewall not forwarding my request? or is it a problem with the server or client . Is something needed to be added to the server/client to overcome the same problem? Or are there any settings on the firewall that need to be changed like port forwarding? (our network admin has configured the port to be accessible from the internet.) is it a authentication issue? Code is available at . http://www.ralfw.de/weblog/wcfsimple.txt http://weblogs.asp.net/ralfw/archive/2007/04/14/a-truely-simple-example-to-get-started-with-wcf.aspx i have just separated the client and server part in separate assemblies.rest is same. using System; using System.Collections.Generic; using System.Text; using System.ServiceModel; namespace WCFSimple.Contract { [ServiceContract] public interface IService { [OperationContract] string Ping(string name); } } namespace WCFSimple.Server { [ServiceBehavior(InstanceContextMode = InstanceContextMode.PerCall)] class ServiceImplementation : WCFSimple.Contract.IService { #region IService Members public string Ping(string name) { Console.WriteLine("SERVER - Processing Ping('{0}')", name); return "Hello, " + name; } #endregion } public class Program { private static System.Threading.AutoResetEvent stopFlag = new System.Threading.AutoResetEvent(false); public static void Main() { ServiceHost svh = new ServiceHost(typeof(ServiceImplementation)); svh.AddServiceEndpoint( typeof(WCFSimple.Contract.IService), new NetTcpBinding(), "net.tcp://localhost:8000"); svh.Open(); Console.WriteLine("SERVER - Running..."); stopFlag.WaitOne(); Console.WriteLine("SERVER - Shutting down..."); svh.Close(); Console.WriteLine("SERVER - Shut down!"); } public static void Stop() { stopFlag.Set(); } } } namespace WCFSimple { class Program { static void Main(string[] args) { Console.WriteLine("WCF Simple Demo"); // start server System.Threading.Thread thServer = new System.Threading.Thread(WCFSimple.Server.Program.Main); thServer.IsBackground = true; thServer.Start(); System.Threading.Thread.Sleep(1000); // wait for server to start up // run client ChannelFactory<WCFSimple.Contract.IService> scf; scf = new ChannelFactory<WCFSimple.Contract.IService>( new NetTcpBinding(), "net.tcp://localhost:8000"); WCFSimple.Contract.IService s; s = scf.CreateChannel(); while (true) { Console.Write("CLIENT - Name: "); string name = Console.ReadLine(); if (name == "") break; string response = s.Ping(name); Console.WriteLine("CLIENT - Response from service: " + response); } (s as ICommunicationObject).Close(); // shutdown server WCFSimple.Server.Program.Stop(); thServer.Join(); } } } Any help?

Read the article

How to send web browser a loading page, then some time later a results page

- by Kurt W. Leucht

I've wasted at least a half day of my company's time searching the Internet for an answer and I'm getting wrapped around the axle here. I can't figure out the difference between all the different technology choices (long polling, ajax streaming, comet, XMPP, etc.) and I can't get a simple hello world example working on my PC. I am running Apache 2.2 and ActivePerl 5.10.0. JavaScript is completely acceptable for this solution. All I want to do is write a simple Perl CGI script that when accessed, it immediately returns some HTML that tells the user to wait or maybe sends an animated GIF. Then without any user intervention (no mouse clicks or anything) I want the CGI script to at some time later replace the wait message or the animated GIF with the actual results from their query. I know this is simple stuff and websites do it all the time using JavaScript, but I can't find a single working example that I can cut and paste onto my machine that will work in Perl. Here is my simple Hello World example that I've compiled from various Internet sources, but it doesn't seem to work. When I refresh this Perl CGI script in my web browser it prints nothing for 5 seconds, then it prints the PLEASE BE PATIENT web page, but not the results web page. So the Ajax XMLHttpRequest stuff obviously isn't working right. What am I doing wrong? #!C:\Perl\bin\perl.exe use CGI; use CGI::Carp qw/fatalsToBrowser warningsToBrowser/; sub Create_HTML { my $html = <<EOHTML; <html> <head> <meta http-equiv="pragma" content="no-cache" /> <meta http-equiv="expires" content="-1" /> <script type="text/javascript" > var xmlhttp=false; /*@cc_on @*/ /*@if (@_jscript_version >= 5) // JScript gives us Conditional compilation, we can cope with old IE versions. // and security blocked creation of the objects. try { xmlhttp = new ActiveXObject("Msxml2.XMLHTTP"); } catch (e) { try { xmlhttp = new ActiveXObject("Microsoft.XMLHTTP"); } catch (E) { xmlhttp = false; } } @end @*/ if (!xmlhttp && typeof XMLHttpRequest!='undefined') { try { xmlhttp = new XMLHttpRequest(); } catch (e) { xmlhttp=false; } } if (!xmlhttp && window.createRequest) { try { xmlhttp = window.createRequest(); } catch (e) { xmlhttp=false; } } </script> <title>Ajax Streaming Connection Demo</title> </head> <body> Some header text. <p> <div id="response">PLEASE BE PATIENT</div> <p> Some footer text. </body> </html> EOHTML return $html; } my $cgi = new CGI; print $cgi->header; print Create_HTML(); sleep(5); print "<script type=\"text/javascript\">\n"; print "\$('response').innerHTML = 'Here are your results!';\n"; print "</script>\n";

Read the article

How do I convert Data::Dumper output back into a Perl data structure?

- by newbee_me

Hi all! I was wondering if you could shed some lights regarding the code I've been doing for a couple of days. I've been trying to convert a Perl-parsed hash back to XML using the XMLout() and XMLin() method and it has been quite successful with this format. #!/usr/bin/perl -w use strict; # use module use IO::File; use XML::Simple; use XML::Dumper; use Data::Dumper; my $dump = new XML::Dumper; my ( $data, $VAR1 ); Topology:$VAR1 = { 'device' => { 'FOC1047Z2SZ' => { 'ChassisID' => '2009-09', 'Error' => undef, 'Group' => { 'ID' => 'A1', 'Type' => 'Base' }, 'Model' => 'CATALYST', 'Name' => 'CISCO-SW1', 'Neighbor' => {}, 'ProbedIP' => 'TEST', 'isDerived' => 0 } }, 'issues' => [ 'TEST' ] }; # create object my $xml = new XML::Simple (NoAttr=>1, RootName=>'data', SuppressEmpty => 'true'); # convert Perl array ref into XML document $data = $xml->XMLout($VAR1); #reads an XML file my $X_out = $xml->XMLin($data); # access XML data print Dumper($data); print "STATUS: $X_out->{issues}\n"; print "CHASSIS ID: $X_out->{device}{ChassisID}\n"; print "GROUP ID: $X_out->{device}{Group}{ID}\n"; print "DEVICE NAME: $X_out->{device}{Name}\n"; print "DEVICE NAME: $X_out->{device}{name}\n"; print "ERROR: $X_out->{device}{error}\n"; I can access all the element in the XML with no problem. But when I try to create a file that will house the parsed hash, problem arises because I can't seem to access all the XML elements. I guess, I wasn't able to unparse the file with the following code. #!/usr/bin/perl -w use strict; #!/usr/bin/perl # use module use IO::File; use XML::Simple; use XML::Dumper; use Data::Dumper; my $dump = new XML::Dumper; my ( $data, $VAR1, $line_Holder ); #this is the file that contains the parsed hash my $saveOut = "C:/parsed_hash.txt"; my $result_Holder = IO::File->new($saveOut, 'r'); while ($line_Holder = $result_Holder->getline){ print $line_Holder; } # create object my $xml = new XML::Simple (NoAttr=>1, RootName=>'data', SuppressEmpty => 'true'); # convert Perl array ref into XML document $data = $xml->XMLout($line_Holder); #reads an XML file my $X_out = $xml->XMLin($data); # access XML data print Dumper($data); print "STATUS: $X_out->{issues}\n"; print "CHASSIS ID: $X_out->{device}{ChassisID}\n"; print "GROUP ID: $X_out->{device}{Group}{ID}\n"; print "DEVICE NAME: $X_out->{device}{Name}\n"; print "DEVICE NAME: $X_out->{device}{name}\n"; print "ERROR: $X_out->{device}{error}\n"; Do you have any idea how I could access the $VAR1 inside the text file? Regards, newbee_me

Read the article

PHP: How do I loop through every XML file in a directory?

- by celebritarian

Hi! I'm building a simple application. It's a user interface to an online order system. Basically, the system is going to work like this: Other companies upload their purchase orders to our FTP server. These orders are simple XML files (containing things like customer data, address information, ordered products and the quantities…) I've built a simple user interface in HTML5, jQuery and CSS — all powered by PHP. PHP reads the content of an order (using the built-in features of SimpleXML) and displays it on the web page. So, it's a web app, supposed to always be running in a browser at the office. The PHP app will display the content of all orders. Every fifteen minutes or so, the app will check for new orders. How do I loop through all XML files in a directory? Right now, my app is able to read the content of a single XML file, and display it in a nice way on the page. My current code looks like this: // pick a random order that I know exists in the Order directory: $xml_file = file_get_contents("Order/6366246.xml",FILE_TEXT); $xml = new SimpleXMLElement($xml_file); // start echo basic order information, like order number: echo $xml->OrderHead->ShopPO; // more information about the order and the customer goes here… echo "<ul>"; // loop through each order line, and echo all quantities and products: foreach ($xml->OrderLines->OrderLine as $orderline) { echo "<tr>\n". "<li>".$orderline->Quantity." st.</li>\n". "<li>".$orderline->SKU."</li>\n"; } echo "</ul>"; // more information about delivery options, address information etc. goes here… So, that's my code. Pretty simple. It only needs to do one thing — print out the content of all order files on the screen — so me and my colleagues can see the order, confirm it and deliver it. That's it. But right now — as you can see — I'm selecting one single order at a time, located in the Order directory. But how do I loop through the entire Order directory, and read aand display the content of each order (like above)? I'm stuck. I don't really know how you get all (xml) files in a directory and then do something with the files (like reading them and echo out the data, like I want to). -- I'd really appreciate some help. I'm not very experienced with PHP/server-side programming, so if you could help me out here I'd be very grateful. Thanks a lot in advance! // Björn (celebritarian at me dot com)

Read the article

Mysql 100% CPU + Slow query

- by felipeclopes

Read the article

Unable to show Desktop Notifications using Google Chrome

- by Praveen Kumar

I followed the instructions as given in Using The Notifications API. Also I faced many problems like the below, because I added the document.querySelector() inside the <head> part: Uncaught TypeError: Cannot call method 'addEventListener' of null Now I have the below source, where I am able to Check Notification Support, and Check Notification Permissions links. Guide me how to bring in notifications in a simpler way. Also, I tried this: $("#html").click(function() { if (window.webkitNotifications.checkPermission() == 0) { createNotificationInstance({ notificationType: 'html' }); } else { window.webkitNotifications.requestPermission(); } }); Now I am stuck with this source. I need to generate HTML & Simple Notifications. Am I missing something? Please guide me. Source: <!DOCTYPE HTML> <html lang="en-US"> <head> <meta charset="UTF-8"> <title>Desktop Notifications</title> <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script> <script type="text/javascript"> function checkNotifications() { if (window.webkitNotifications) alert("Notifications are supported!"); else alert("Notifications are not supported for this Browser/OS version yet."); } function createNotificationInstance(options) { if (window.webkitNotifications.checkPermission() == 0) { // 0 is PERMISSION_ALLOWED if (options.notificationType == 'simple') { return window.webkitNotifications.createNotification('icon.png', 'Notification Title', 'Notification content...'); } else if (options.notificationType == 'html') { return window.webkitNotifications.createHTMLNotification('http://localhost/'); } } else { window.webkitNotifications.requestPermission(); } } </script> <style type="text/css"> * {font-family: Verdana, sans-serif;} body {font-size: 10pt; margin: 0; padding: 0;} p {margin: 5px;} a {color: #09f; text-decoration: none;} a:hover {color: #f00;} </style> </head> <body> <p><strong>Desktop Notifications</strong></p> <p>Lets see how the notifications work in this browser.</p> <p> <a href="#" onclick="checkNotifications(); return false;">Check Notification Support</a>. Next <a href="#" onclick="alert('Notifications are ' + ((window.webkitNotifications.checkPermission() == 0) ? '' : 'not ') + 'allowed!'); return false;">Check Notification Permissions</a> and if permissions are not there, <a href="#" onclick="window.webkitNotifications.requestPermission(); return false;">Request Permissions</a>. Create a <a href="#" id="text">Simple Notification</a> or <a href="#" id="html">HTML Notification</a>. </p> </body> <script type="text/javascript"> document.querySelector("#html").addEventListener('click', function() { if (window.webkitNotifications.checkPermission() == 0) { createNotificationInstance({ notificationType: 'html' }); } else { window.webkitNotifications.requestPermission(); } }, false); document.querySelector("#text").addEventListener('click', function() { if (window.webkitNotifications.checkPermission() == 0) { createNotificationInstance({ notificationType: 'simple' }); } else { window.webkitNotifications.requestPermission(); } }, false); </script> </html>

Read the article

Android refresh StateListDrawable problem

- by Max

Hi all, I have a strange problem with StateListDrawable or maybe (probably) I'm missing something. I created a test application for it and the same problem occurs. So, this is my StateListDrawable resourse in file test_selection.xml <?xml version="1.0" encoding="utf-8"?> <selector xmlns:android="http://schemas.android.com/apk/res/android"> <item android:state_selected="true"> <shape android:shape="rectangle" android:background="#ff0000"> <corners android:radius="10dp" /> <gradient android:startColor="#ff5555" android:endColor="#ff5555" android:angle="0" /> </shape> </item> <item android:state_selected="false"> <shape android:shape="rectangle" android:background="#eeeeee"> <corners android:radius="10dp" /> <gradient android:startColor="#eeeeee" android:endColor="#eeeeee" android:angle="0" /> </shape> </item> </selector> It's a very simple selector that draw a red color for selected state and a white rect for the unselected one. My main.xml template is very simple. I simply use a TextView that uses the selection as background. <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent"> <TextView android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="@string/hello" android:textSize="30dp" android:id="@+id/test_view_example" android:background="@drawable/test_selection"/> <Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:id="@+id/refresh" android:onClick="updateView" android:text="refresh"></Button> </LinearLayout> My Activity code is also very simple. public class TestDrawableStateActivity extends Activity { private final static int[] SELECTED_STATE = { android.R.attr.state_selected }; private final static int[] UNSELECTED_STATE = {}; private TextView textView; /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); textView = (TextView) findViewById(R.id.test_view_example); } @Override protected void onResume() { super.onResume(); // Carichiamo la Drawable if(textView.getBackground().setState(SELECTED_STATE)){ textView.invalidate(); } } public void updateView(View view) { if(textView.getBackground().setState(SELECTED_STATE)){ textView.invalidate(); }; } } When Activity starts I try to set the state of my Drawable (the StateListDrawable) with the value SELECTED. It seems all very simple.... but the problem is that the state is not shown. If, later, I click a button and execute the method updateView() the state changes. Where is my problem? Where am I wrong? Thankx a lot Max

Read the article

extensible database design: automatic ALTER TABLE or serialize() field BLOB ?

- by mario

I want an adaptable database scheme. But still use a simple table data gateway in my application, where I just pass an $data[] array for storing. The basic columns are settled in the initial table scheme. There will however arise a couple of meta fields later (ca 10-20). I want some flexibility there and not adapt the database manually each time, or -worse- change the application logic just because of new fields. So now there are two options which seem workable yet not overkill. But I'm not sure about the scalability or database drawbacks. (1) Automatic ALTER TABLE. Whenever the $data array is to be saved, the keys are compared against the current database columns. New columns get defined before the $data is INSERTed into the table. Actually seems simple enough in test code: function save($data, $table="forum") { // columns if ($new_fields = array_diff(array_keys($data), known_fields($table))) { extend_schema($table, $new_fields, $data); } // save $columns = implode("`, `", array_keys($data)); $qm = str_repeat(",?", count(array_keys($data)) - 1); echo ("INSERT INTO `$table` (`$columns`) VALUES (?$qm);"); function known_fields($table) { return unserialize(@file_get_contents("db:$table")) ?: array("id"); function extend_schema($table, $new_fields, $data) { foreach ($new_fields as $field) { echo("ALTER TABLE `$table` ADD COLUMN `$field` VARCHAR;"); Since it is mostly meta information fields, adding them just as VARCHAR seems sufficient. Nobody will query by them anyway. So the database is really just used as storage here. However, while I might want to add a lot of new $data fields on the go, they will not always be populated. (2) serialize() fields into BLOB. Any new/extraneous meta fields could be opaque to the database. Simply sorting out the virtual fields from the real database columns is simple. And the meta fields can just be serialize()d into a blob/text field then: function ext_save($data, $table="forum") { $db_fields = array("id", "content", "flags", "ext"); // disjoin foreach (array_diff(array_keys($data),$db_fields) as $key) { $data["ext"][$key] = $data[$key]; unset($data[$key]); } $data["ext"] = serialize($data["ext"]); Unserializing and unpacking this 'ext' column on read queries is a minor overhead. The advantage is that there won't be any sparsely filled columns in the database, so I guess it's compacter and faster than the AUTO ALTER TABLE approach. Of course, this method prevents ever using one of the new fields in a WHERE or GROUP BY clause. But I think none of the possible meta fields (user_agent, author_ip, author_img, votes, hits, last_modified, ..) would/should ever be used there anyway. So I currently prefer the 'ext' blob approach, even if it's a one-way ticket. How are such columns called usually? (looking for examples/doc) Would you use XML serialization for (very theoretical) in-database queries? The adapting table scheme seems a "cleaner" interface, even if most columns might remain empty then. How does that impact speed? How many such sparse VARCHAR fields can MySQL/innodb stomach? But most importantly: Is there any standard implementation for this? A pseudo ORM with automatic ALTER TABLE tricks? Storing a simple column list seems workable, but something like pdo::getColumnMeta would be more robust.

Read the article

Migrate Spring JPA DAO unit testing to google app engine

- by twingocerise

I'm trying to put together a simple environment where I can get Spring, Maven, JPA, Google App Engine and DAO unit testing working happily all together. The goal is to be able to run a simple DAO unit test creating an entity and then load it again with a simple find to check it's been created properly - all of this from my maven build. My dao is making use of the JPA entity manager (query(), persist(), etc.) I've got it working no problem with hsqldb and a datasource, etc. but I'm struggling to get it working with appengine. My questions are: 1) I'm using an entity manager, injecting my persistence unit as followed. Is it OK? Is there any need for a datasource or something special? I thought not but correct me if I'm wrong. applicationContext.xml <bean id='entityManagerFactory' class='org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean'> <property name="persistenceUnitName" value="transactions-optional" /> </bean> Persistence.xml <persistence-unit name="transactions-optional"> <provider>org.datanucleus.store.appengine.jpa.DatastorePersistenceProvider</provider> <properties> <property name="datanucleus.NontransactionalRead" value="true"/> <property name="datanucleus.NontransactionalWrite" value="true"/> <property name="datanucleus.ConnectionURL" value="appengine"/> </properties> </persistence-unit> 2) what are the dependencies I need to add to my pom file to be able to run the unit test making use of the entityManager? What about versions ? I found loads of things about appengine-api-labs/stubs/testing but none them got it working i.e. I'm getting jdo dependency missing while I'm using JPA... I also get loads of conflicts when I try to add some jars (datanucleus and stuff). So far I'm trying appengine-api-1.0-sdk v1.7.0 - ASM-all v3.3 - datanucleus core/api-jpa/enhancer v3.1.0 - datanucleus-appengine v2.0.1.1 and all the gae testing jars v1.7.0 3) Is there anything I need to add to my surefire plugin (test runner) to make sure it picks up all the dependencies? I'm getting an exhausting ClassNotFound on DatastorePersistenceProvider while it is in my classpath (I checked the jars and the mvn dependency:tree) I had a look at this but it doesn't seem to be working at all: http://www.vertigrated.com/blog/2011/02/working-maven-3-google-app-engine-plugin-with-gwt-support/ 4) Do I need to use any sot of localhelper to test my DAOs? Ideally I'd want to test my dao layer "as is" with the entity manager... what's your opinion ? Has anyone managed to run a unit test using JPA on google app engine ? 5) Do I need to set up any sort of gae.home somewhere in my pom file? Would anyone make use of it (a plugin or something) ? 6) Is the gwt-maven plugin any helpful if I don't use gwt - I'm writing a simple webservice making use of appengine, not a GWT app... Any help would be much appreciated as I've been struggling for 2 days now... Cheers, V.

Read the article

What is the best free or low-cost Java reporting library (e.g. BIRT, JasperReports, etc.) for making

- by Max3000

I want to print, email and write to PDF very simple reports. The reports are basically a list of items, divided in various sections/columns. The sections are not necessarily identical. Think newspaper. I just wasted a solid 2 days of work trying to make this kind of reports using JasperReports. I find that Jasper is great for outputing "normalized" data. The kind that would come out of a database for instance, each row neatly describing an item and each item printed on a line. I'm simplifying a bit but that's the idea. However, given what I want to do I always ended up completely lost. Data not being displayed for no apparent reason, columns of texts never the correct size, column positioning always ending up incorrect, pagination not sanely possible (I was never able to figure it out; the FAQ gives an obscure workaround), etc. I came to the conclusion that Jasper is really not built to make the kind of reports I want. Am I missing something? I'm ready to pay for a tool, as long as the price is reasonable. By reasonable I mean a few $100s. Thanks. EDIT: To answer cetus, here is more information about the report I made in Jasper. What I want is something like this: text text text text ------------------- text | text text |---------- text | text text | text --------| text text |---------- text | text What I made in jasper is this: (detail band) subreport | subreport ------------------------------------ subreport | subreport ------------------------------------ subreport | subreport The subreports are all the same actual report. This report has one field (called "field") and basically just prints this field in a detail band. Hence, running a single subreport simply lists all items from the datasource. The datasource itself is a simple custom JRDatasource containing a collection of strings in the field "field". The datasource iterates over the collection until there are no more strings. Each subreport has its own datasource. I tried many different variations of the above, with all sorts of different properties for the report, subreports, etc. IMO, this is fairly simple stuff. However, the problems I encounter are as follows: Subreports starting from the 3rd don't show up when their position type is 'float'. They do show up when they have 'fix relative to top'. However, I don't want to do this because the first two subreports can be of any length. I can't make each subreport to stretch according to its own length. Instead, they either don't stretch at all (which is not desirable because they have different lenghts) or they stretch according to the longest subreport. This makes a weird layout for sure. Pagination doesn't happen. If some subreports fall outside the page, they simple don't show. One alternative is to increase the 'page height' considerably and the 'detail band height' accordingly. However, in this case it is not really possibly to know the total height in advance. So I'm stuck with calculating/guessing it myself, before the report is even generated. More importantly, long reports end up on one page and this is not acceptable (the printout text is too small, it's ugly/non-professional to have different reports with different PDF page lengths, etc.). BTW, I used iReport so it's possibly limitations of iReport I'm listing here and not of Jasper itself. That's one of the things I'm trying to find out asking this question here. One alternative would be to generate the jrxml myself with just static text but I'm afraid I'll encounter the very same limitations. Anyway, I just generally wasted so much time getting anything done with Jasper that I can't help thinking its not the right tool for the job. (Not to say that Jasper doesn't excel in what it's good at).

Read the article

Employee Info Starter Kit - Visual Studio 2010 and .NET 4.0 Version (4.0.0) Available

- by joycsharp

Employee Info Starter Kit is a ASP.NET based web application, which includes very simple user requirements, where we can create, read, update and delete (crud) the employee info of a company. Based on just a database table, it explores and solves all major problems in web development architectural space. This open source starter kit extensively uses major features available in latest Visual Studio, ASP.NET and Sql Server to make robust, scalable, secured and maintanable web applications quickly and easily. Since it's first release, this starter kit achieved a huge popularity in web developer community and includes 1,40,000+ download from project web site. Visual Studio 2010 and .NET 4.0 came up with lots of exciting features to make software developers life easier. A new version (v4.0.0) of Employee Info Starter Kit is now available in both MSDN Code Gallery and CodePlex. Chckout the latest version of this starter kit to enjoy cool features available in Visual Studio 2010 and .NET 4.0. [ Release Notes ] Architectural Overview Simple 2 layer architecture (user interface and data access layer) with 1 optional cache layer ASP.NET Web Form based user interface Custom Entity Data Container implemented (with primitive C# types for data fields) Active Record Design Pattern based Data Access Layer, implemented in C# and Entity Framework 4.0 Sql Server Stored Procedure to perform actual CRUD operation Standard infrastructure (architecture, helper utility) for automated integration (bottom up manner) and unit testing Technology UtilizedProgramming Languages/Scripts Browser side: JavaScript Web server side: C# 4.0 Database server side: T-SQL .NET Framework Components .NET 4.0 Entity Framework .NET 4.0 Optional/Named Parameters .NET 4.0 Tuple .NET 3.0+ Extension Method .NET 3.0+ Lambda Expressions .NET 3.0+ Aanonymous Type .NET 3.0+ Query Expressions .NET 3.0+ Automatically Implemented Properties .NET 3.0+ LINQ .NET 2.0 + Partial Classes .NET 2.0 + Generic Type .NET 2.0 + Nullable Type ASP.NET 3.5+ List View (TBD) ASP.NET 3.5+ Data Pager (TBD) ASP.NET 2.0+ Grid View ASP.NET 2.0+ Form View ASP.NET 2.0+ Skin ASP.NET 2.0+ Theme ASP.NET 2.0+ Master Page ASP.NET 2.0+ Object Data Source ASP.NET 1.0+ Role Based Security Visual Studio Features Visual Studio 2010 CodedUI Test Visual Studio 2010 Layer Diagram Visual Studio 2010 Sequence Diagram Visual Studio 2010 Directed Graph Visual Studio 2005+ Database Unit Test Visual Studio 2005+ Unit Test Visual Studio 2005+ Web Test Visual Studio 2005+ Load Test Sql Server Features Sql Server 2005 Stored Procedure Sql Server 2005 Xml type Sql Server 2005 Paging support

Read the article

Employee Info Starter Kit - Visual Studio 2010 and .NET 4.0 Version (4.0.0) Available

- by Mohammad Ashraful Alam

Employee Info Starter Kit is a ASP.NET based web application, which includes very simple user requirements, where we can create, read, update and delete (crud) the employee info of a company. Based on just a database table, it explores and solves most of the major problems in web development architectural space. This open source starter kit extensively uses major features available in latest Visual Studio, ASP.NET and Sql Server to make robust, scalable, secured and maintanable web applications quickly and easily. Since it's first release, this starter kit achieved a huge popularity in web developer community and includes 1,40,000+ download from project web site. Visual Studio 2010 and .NET 4.0 came up with lots of exciting features to make software developers life easier. A new version (v4.0.0) of Employee Info Starter Kit is now available in both MSDN Code Gallery and CodePlex. Chckout the latest version of this starter kit to enjoy cool features available in Visual Studio 2010 and .NET 4.0. [ Release Notes ] Architectural Overview Simple 2 layer architecture (user interface and data access layer) with 1 optional cache layer ASP.NET Web Form based user interface Custom Entity Data Container implemented (with primitive C# types for data fields) Active Record Design Pattern based Data Access Layer, implemented in C# and Entity Framework 4.0 Sql Server Stored Procedure to perform actual CRUD operation Standard infrastructure (architecture, helper utility) for automated integration (bottom up manner) and unit testing Technology UtilizedProgramming Languages/Scripts Browser side: JavaScript Web server side: C# 4.0 Database server side: T-SQL .NET Framework Components .NET 4.0 Entity Framework .NET 4.0 Optional/Named Parameters .NET 4.0 Tuple .NET 3.0+ Extension Method .NET 3.0+ Lambda Expressions .NET 3.0+ Aanonymous Type .NET 3.0+ Query Expressions .NET 3.0+ Automatically Implemented Properties .NET 3.0+ LINQ .NET 2.0 + Partial Classes .NET 2.0 + Generic Type .NET 2.0 + Nullable Type ASP.NET 3.5+ List View (TBD) ASP.NET 3.5+ Data Pager (TBD) ASP.NET 2.0+ Grid View ASP.NET 2.0+ Form View ASP.NET 2.0+ Skin ASP.NET 2.0+ Theme ASP.NET 2.0+ Master Page ASP.NET 2.0+ Object Data Source ASP.NET 1.0+ Role Based Security Visual Studio Features Visual Studio 2010 CodedUI Test Visual Studio 2010 Layer Diagram Visual Studio 2010 Sequence Diagram Visual Studio 2010 Directed Graph Visual Studio 2005+ Database Unit Test Visual Studio 2005+ Unit Test Visual Studio 2005+ Web Test Visual Studio 2005+ Load Test Sql Server Features Sql Server 2005 Stored Procedure Sql Server 2005 Xml type Sql Server 2005 Paging support

Read the article

Parallelism in .NET – Part 4, Imperative Data Parallelism: Aggregation

- by Reed

In the article on simple data parallelism, I described how to perform an operation on an entire collection of elements in parallel. Often, this is not adequate, as the parallel operation is going to be performing some form of aggregation. Simple examples of this might include taking the sum of the results of processing a function on each element in the collection, or finding the minimum of the collection given some criteria. This can be done using the techniques described in simple data parallelism, however, special care needs to be taken into account to synchronize the shared data appropriately. The Task Parallel Library has tools to assist in this synchronization. The main issue with aggregation when parallelizing a routine is that you need to handle synchronization of data. Since multiple threads will need to write to a shared portion of data. Suppose, for example, that we wanted to parallelize a simple loop that looked for the minimum value within a dataset: double min = double.MaxValue; foreach(var item in collection) { double value = item.PerformComputation(); min = System.Math.Min(min, value); } .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } This seems like a good candidate for parallelization, but there is a problem here. If we just wrap this into a call to Parallel.ForEach, we’ll introduce a critical race condition, and get the wrong answer. Let’s look at what happens here: // Buggy code! Do not use! double min = double.MaxValue; Parallel.ForEach(collection, item => { double value = item.PerformComputation(); min = System.Math.Min(min, value); }); This code has a fatal flaw: min will be checked, then set, by multiple threads simultaneously. Two threads may perform the check at the same time, and set the wrong value for min. Say we get a value of 1 in thread 1, and a value of 2 in thread 2, and these two elements are the first two to run. If both hit the min check line at the same time, both will determine that min should change, to 1 and 2 respectively. If element 1 happens to set the variable first, then element 2 sets the min variable, we’ll detect a min value of 2 instead of 1. This can lead to wrong answers. Unfortunately, fixing this, with the Parallel.ForEach call we’re using, would require adding locking. We would need to rewrite this like: // Safe, but slow double min = double.MaxValue; // Make a "lock" object object syncObject = new object(); Parallel.ForEach(collection, item => { double value = item.PerformComputation(); lock(syncObject) min = System.Math.Min(min, value); }); This will potentially add a huge amount of overhead to our calculation. Since we can potentially block while waiting on the lock for every single iteration, we will most likely slow this down to where it is actually quite a bit slower than our serial implementation. The problem is the lock statement – any time you use lock(object), you’re almost assuring reduced performance in a parallel situation. This leads to two observations I’ll make: When parallelizing a routine, try to avoid locks. That being said: Always add any and all required synchronization to avoid race conditions. These two observations tend to be opposing forces – we often need to synchronize our algorithms, but we also want to avoid the synchronization when possible. Looking at our routine, there is no way to directly avoid this lock, since each element is potentially being run on a separate thread, and this lock is necessary in order for our routine to function correctly every time. However, this isn’t the only way to design this routine to implement this algorithm. Realize that, although our collection may have thousands or even millions of elements, we have a limited number of Processing Elements (PE). Processing Element is the standard term for a hardware element which can process and execute instructions. This typically is a core in your processor, but many modern systems have multiple hardware execution threads per core. The Task Parallel Library will not execute the work for each item in the collection as a separate work item. Instead, when Parallel.ForEach executes, it will partition the collection into larger “chunks” which get processed on different threads via the ThreadPool. This helps reduce the threading overhead, and help the overall speed. In general, the Parallel class will only use one thread per PE in the system. Given the fact that there are typically fewer threads than work items, we can rethink our algorithm design. We can parallelize our algorithm more effectively by approaching it differently. Because the basic aggregation we are doing here (Min) is communitive, we do not need to perform this in a given order. We knew this to be true already – otherwise, we wouldn’t have been able to parallelize this routine in the first place. With this in mind, we can treat each thread’s work independently, allowing each thread to serially process many elements with no locking, then, after all the threads are complete, “merge” together the results. This can be accomplished via a different set of overloads in the Parallel class: Parallel.ForEach<TSource,TLocal>. The idea behind these overloads is to allow each thread to begin by initializing some local state (TLocal). The thread will then process an entire set of items in the source collection, providing that state to the delegate which processes an individual item. Finally, at the end, a separate delegate is run which allows you to handle merging that local state into your final results. To rewriting our routine using Parallel.ForEach<TSource,TLocal>, we need to provide three delegates instead of one. The most basic version of this function is declared as: public static ParallelLoopResult ForEach<TSource, TLocal>( IEnumerable<TSource> source, Func<TLocal> localInit, Func<TSource, ParallelLoopState, TLocal, TLocal> body, Action<TLocal> localFinally ) The first delegate (the localInit argument) is defined as Func<TLocal>. This delegate initializes our local state. It should return some object we can use to track the results of a single thread’s operations. The second delegate (the body argument) is where our main processing occurs, although now, instead of being an Action<T>, we actually provide a Func<TSource, ParallelLoopState, TLocal, TLocal> delegate. This delegate will receive three arguments: our original element from the collection (TSource), a ParallelLoopState which we can use for early termination, and the instance of our local state we created (TLocal). It should do whatever processing you wish to occur per element, then return the value of the local state after processing is completed. The third delegate (the localFinally argument) is defined as Action<TLocal>. This delegate is passed our local state after it’s been processed by all of the elements this thread will handle. This is where you can merge your final results together. This may require synchronization, but now, instead of synchronizing once per element (potentially millions of times), you’ll only have to synchronize once per thread, which is an ideal situation. Now that I’ve explained how this works, lets look at the code: // Safe, and fast! double min = double.MaxValue; // Make a "lock" object object syncObject = new object(); Parallel.ForEach( collection, // First, we provide a local state initialization delegate. () => double.MaxValue, // Next, we supply the body, which takes the original item, loop state, // and local state, and returns a new local state (item, loopState, localState) => { double value = item.PerformComputation(); return System.Math.Min(localState, value); }, // Finally, we provide an Action<TLocal>, to "merge" results together localState => { // This requires locking, but it's only once per used thread lock(syncObj) min = System.Math.Min(min, localState); } ); Although this is a bit more complicated than the previous version, it is now both thread-safe, and has minimal locking. This same approach can be used by Parallel.For, although now, it’s Parallel.For<TLocal>. When working with Parallel.For<TLocal>, you use the same triplet of delegates, with the same purpose and results. Also, many times, you can completely avoid locking by using a method of the Interlocked class to perform the final aggregation in an atomic operation. The MSDN example demonstrating this same technique using Parallel.For uses the Interlocked class instead of a lock, since they are doing a sum operation on a long variable, which is possible via Interlocked.Add. By taking advantage of local state, we can use the Parallel class methods to parallelize algorithms such as aggregation, which, at first, may seem like poor candidates for parallelization. Doing so requires careful consideration, and often requires a slight redesign of the algorithm, but the performance gains can be significant if handled in a way to avoid excessive synchronization.

Search Results

Search found 31491 results on 1260 pages for 'simple talk'.

Page 220/1260 | < Previous Page | 216 217 218 219 220 221 222 223 224 225 226 227 | Next Page >

- by vps

- by srj0408

- by tharrison

- by Cylindric

- by Eric Rath

- by Ed01

- by LxFlip

- by Demetre

- by user359217

- by Filimindji

- by Kurt W. Leucht

- by UncleCJ

- by Amitd

- by Kurt W. Leucht

- by newbee_me

- by celebritarian

- by felipeclopes

- by Praveen Kumar

- by Max

- by mario

- by twingocerise

- by Max3000

- by joycsharp

- by Mohammad Ashraful Alam

- by Reed

< Previous Page | 216 217 218 219 220 221 222 223 224 225 226 227 | Next Page >