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• Books for Computer Networking

  - by Altimet Gaandu

  Hi, I am a student of computer engineering from Vasula University, Somalia. We have a subject called Advanced Computer Networks and the following is the list of recommended books: Text Books: 1. B. A. Forouzan, "TCP/IP Protocol Suite", Tata McGraw Hill edition, Third Edition. 2. N. Olifer, V. Olifer, "Computer Networks: Principles, Technologies and Protocols for Network design", Wiley India Edition, First edition. References: 1. W.Richard Stevens, "TCP/IP Volume1, 2, 3", Addison Wesley. 2. D.E.Comer,"TCP/IPVolumeI and II", PearsonEducation. . 3.W.R. Stevens, "Unix Network Programming", Vol. 1, Pearson Education. 4. J.Walrand, P. Var~fya, "High Performance Communication Networks", Morgan Kaufmann. . 5. A.S.Tanenbaum,"Computer Networks", Pearson Education, Fourth Edition. But we have been unable to find these either in the market or on the internet (read: torrents). Please provide download links to any of these books and oblige. Thanks.

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• Need help with threads in a client/server

  - by nunos

  For college, I am developing a local relay chat. I have to program a chat server and client that will only work on sending messages on different terminal windows on the same computer with threads and fifos. The fifos part I am having no trouble, the threads part is the one that is giving me some headaches. The server has one thread for receiving commands from a fifo (used by all clients) and another thread for each client that is connected. For each client that is connected I need to know a certain information. Firstly, I was using global variables, which worked as longs as there was only one client connected, which is much of a chat, to chat alone. So, ideally I would have some data like: -nickname -name -email -etc... per client that is connected. However, I don't know how to do that. I could create a client_data[MAX_NUMBER_OF_THREADS] where client_data was a struct with everything I needed to have access to, but this would require to, in every communication between server and client to ask for the id of the client in the array client_data and that does not seem very pratical I could also instantiate a client_data immediately after creating the thread but it would only be available in that block, and that is not very pratical either. As you can see I am in need of a little guidance here. Any comment, piece of code or link to any relevant information is greatly appreciated. Thanks.

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• Sql Server Replication: Snapshot vs Merge

  - by Zyphrax

  Background information Let's say I have two database servers, both SQL Server 2008. One is in my LAN (ServerLocal), the other one is on a remote hosting environment (ServerRemote). I have created a database on ServerLocal and have an exact copy of that database on ServerRemote. The database on ServerRemote is part of a web application and I would like to keep it's data up-to-date with the data in the database ServerLocal. ServerLocal is able to communicate with ServerRemote, this is one-way traffic. Communication from ServerRemote to ServerLocal isn't available. Current solution I thought it would be a nice solution to use replication. So I've made ServerLocal a publisher and subscriptions are pushed to the ServerRemote. This works fine, when a snapshot is transfered to ServerRemote the existing data will be purged and the ServerRemote database is once again an exact replica of the database on ServerLocal. The problem Records that exist on ServerRemote that don't exist on ServerLocal are removed. This doesn't matter for most of my tables but in some of my tables I'd like to keep the existing data (aspnet_users for instance), and update the records if necessary. What kind of replication fits my problem?

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• Proper API Design for Version Independence?

  - by Justavian

  I've inherited an enormous .NET solution of about 200 projects. There are now some developers who wish to start adding their own components into our application, which will require that we begin exposing functionality via an API. The major problem with that, of course, is that the solution we've got on our hands contains such a spider web of dependencies that we have to be careful to avoid sabotaging the API every time there's a minor change somewhere in the app. We'd also like to be able to incrementally expose new functionality without destroying any previous third party apps. I have a way to solve this problem, but i'm not sure it's the ideal way - i was looking for other ideas. My plan would be to essentially have three dlls. APIServer_1_0.dll - this would be the dll with all of the dependencies. APIClient_1_0.dll - this would be the dll our developers would actual refer to. No references to any of the mess in our solution. APISupport_1_0.dll - this would contain the interfaces which would allow the client piece to dynamically load the "server" component and perform whatever functions are required. Both of the above dlls would depend upon this. It would be the only dll that the "client" piece refers to. I initially arrived at this design, because the way in which we do inter process communication between windows services is sort of similar (except that the client talks to the server via named pipes, rather than dynamically loading dlls). While i'm fairly certain i can make this work, i'm curious to know if there are better ways to accomplish the same task.

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• Mocking non-virtual methods in C++ without editing production code?

  - by wk1989

  Hello, I am a fairly new software developer currently working adding unit tests to an existing C++ project that started years ago. Due to a non-technical reason, I'm not allowed to modify any existing code. The base class of all my modules has a bunch of methods for Setting/Getting data and communicating with other modules. Since I just want to unit testing each individual module, I want to be able to use canned values for all my inter-module communication methods. I.e. for a method Ping() which checks if another module is active, I want to have it return true or false based on what kind of test I'm doing. I've been looking into Google Test and Google Mock, and it does support mocking non-virtual methods. However the approach described (http://code.google.com/p/googlemock/wiki/CookBook#Mocking_Nonvirtual_Methods) requires me to "templatize" the original methods to take in either real or mock objects. I can't go and templatize my methods in the base class due to the requirement mentioned earlier, so I need some other way of mocking these virtual methods Basically, the methods I want to mock are in some base class, the modules I want to unit test and create mocks of are derived classes of that base class. There are intermediate modules in between my base Module class and the modules that I want to test. I would appreciate any advise! Thanks, JW EDIT: A more concrete examples My base class is lets say rootModule, the module I want to test is leafModule. There is an intermediate module which inherits from rootModule, leafModule inherits from this intermediate module. In my leafModule, I want to test the doStuff() method, which calls the non virtual GetStatus(moduleName) defined in the rootModule class. I need to somehow make GetStatus() to return a chosen canned value. Mocking is new to me, so is using mock objects even the right approach?

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• How to hide helper functions from public API in c

  - by emge

  I'm working on a project and I need to create an API. I am using sockets to communicate between the server (my application) and the clients (the other applications using my API). This project is in c not C++ I come from a linux background and this is my first project using Windows, Visual Studio 2008, and dll libraries. I have communication working between the client and server, but I have some that is duplicated on both projects. I would like to create a library (probably a dll file), that both projects can link to so I don't have to maintain extra code. I also have to create the library that has the API that I need to make available for my clients. Within the API functions that I want public are the calls to these helper functions that are "duplicated code", I don't want to expose these functions to my client, but I do want my server to be able to use those functions. How can I do this? I will try to clarify with an example. This is what I started with. Server Project: int Server_GetPacket(SOCKET sd); int ReceiveAll(SOCKET sd, char *buf, int len); int VerifyLen(char *buf); Client Project: int Client_SendCommand(int command); int Client_GetData(int command, char *buf, int len); int ReceiveAll(SOCKET sd, char *buf, int len); int VerifyLen(char *buf); This is kind of what I would like to end up with: //Server Project: int Server_GetPacket(SOCKET sd); // library with public and private types // private API (not exposed to my client) int ReceiveAll(SOCKET sd, char *buf, int len); int VerifyLen(char *buf); // public API (header file available for client) int Client_SendCommand(int command); int Client_GetData(int command, char *buf, int len); Thanks any help would be appreciated.

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• Process data BEFORE a 301 Redirect?

  - by Jesse

  So, I've been working on a PHP link shortener (I know, just what the world needs). Basically when the page loads, php determines where it needs to go and sends a 301 Header to redirect the browser, like so... Header( "HTTP/1.1 301 Moved Permanently" ); header("Location: http://newsite.com"; Now, I'm trying to add some tracking to my redirects and insert some custom analytics data into a MySQL table before the redirect happen. It works perfectly if I don't specify the a redirect type and just use: header("Location: http://newsite.com"; But, of course as soon as you add in the 301 header, nothing else gets processed. Actually, on the first request, it sends the data to MySQL, but on any subsequent requests there's no communication with the database. I assume it's a browser caching issue, once it's seen the 301 it decides they're no reason to parse anything on future requests. But, does anyone know if there's any way to get around this? I'd really like to keep it as a 301 for SEO purposes (I believe if you don't specify it sends a 404 by default?). I thought about using .htaccess to prepend a file to the page that will do the MySQL work, but with the 301, wouldn't that just get ignored as well? Anyway, I'm not sure if there's any solution other than using a different type of redirect, but I'm ready to give up just yet. So, any suggestions would be much appreciated. Thanks!

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• Configure nHibernate for multiple-project solution

  - by NoOne

  Hello, Im doing a project with C# winforms. This project is composed by: Client project: Windows Forms where user will do CRUD operations on the models; Server project; Common Project: This project will hold the models (in the image only have the model Item); ListSingleton: Remote Object that will do the operations on the models; I already have all the communication working, but now I need to work on the persistence of the data in a mysql database. I was trying to use nHibernate but I’m having some troubles. My main problem is how to organize my hibernate configuration. In which project do I keep the mapping? Common project? In which project do I keep the hibernate configuration file (App.config)? ListSingleton project? In which project do I do this: Configuration cfg = new Configuration(); cfg.AddXmlFile("Item.hbm.xml"); ISessionFactory factory = cfg.BuildSessionFactory(); ISession session = factory.OpenSession(); ITransaction transaction = session.BeginTransaction(); Item newItem = new Item("BLAA"); // Tell NHibernate that this object should be saved session.Save(newItem); // commit all of the changes to the DB and close the ISession transaction.Commit(); session.Close(); In the ListSingleton project? Altho I had reference to the Common Project in the ListSingleton I keep getting error in the addXml line… My mapping is correct because I tried with a one-project solution and it worked :X

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• Passing Extras and screen rotation

  - by Luis A. Florit

  This kind of questions appear periodically. Sorry if this has been covered before, but I'm a newbie and couldn't find the appropriate answer. It deals with the correct implementation of communication between classes and activities. I made a gallery app. It has 3 main activities: the Main one, to search for filenames using a pattern; a Thumb one, that shows all the images that matched the pattern as thumbnails in a gridview, and a Photo activity, that opens a full sized image when you click a thumb in Thumbs. I pass to the Photo activity via an Intent the filenames (an array), and the position (an int) of the clicked thumb in the gridview. This third Photo activity has only one view on it: a TouchImageView, that I adapted for previous/next switching and zooming according to where you shortclick on the image (left, right or middle). Moreover, I added a longclick listener to Photo to show EXIF info. The thing is working, but I am not happy with the implementation... Some things are not right. One of the problems I am experiencing is that, if I click on the right of the image to see the next in the Photo activity, it switches fine (position++), but when rotating the device the original one at position appears. What is happening is that Photo is destroyed when rotating the image, and for some reason it restarts again, without obeying super.onCreate(savedInstanceState), loading again the Extras (the position only changed in Photo, not on the parent activities). I tried with startActivityForResult instead of startActivity, but failed... Of course I can do something contrived to save the position data, but there should be something "conceptual" that I am not understanding about how activities work, and I want to do this right. Can someone please explain me what I am doing wrong, which is the best method to implement what I want, and why? Thanks a lot!!!

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• How do you unit test new code that uses a bunch of classes that cannot be instantiated in a test har

  - by trendl

  I'm writing a messaging layer that should handle communication with a third party API. The API has a bunch of classes that cannot be easily (if at all) instantiated in a test harness. I decided to wrap each class that I need in my unit tests with an adapter/wrapper and expose the members I need through this adapter class. Often I need to expose the wrapped type as well which I do by exposing it as an object. I have also provided an interface for for each or the adapter classes to be able to use them with a mocking framework. This way I can substitute the classes in test for whatever I need. The downside is that I have a bunch of adapter classes that so far server no other reason but testing. For me this is a good reason by itself but others may find this not enough. Possibly, when I write an implementation for another third party vendor's API, I may be able to reuse much of my code and only provide the adapters specific to the vendor's API. However, this is a bit of a long shot and I'm not actually sure it will work. What do you think? Is this approach viable or am I writing unnecessary code that serves no real purpose? Let me say that I do want to write unit tests for my messaging layer and I do now know how to do it otherwise.

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• Loading GWT Messages from a Database

  - by Lars Tackmann

  In GWT one typically loads i18n strings using a interface like this: public interface StatusMessage extends Messages { String error(String username); : } which then loads the actual strings from a StatusMessage.property file: error=User: {0} does not have access to resource This is a great solution, however my client is unbendable in his demand for putting the i18n strings in a database so they can be changed at runtime (though its not a requirement that they be changed realtime). One solution is to create a async service which takes a message ID and user locale and returns a string. I have implemented this and find it terribly ugly (and it introduces a huge amount of extra communication with the server, plus it makes property placeholder replacement rather complicated). So my question is this, can I in some nice way implement a custom message provider that loads the messages from the backend in one big swoop (for the current user session). If it can also hook into the default GWT message mechanism, then I would be completely happy (i.e. so I can create a interface like above and keep using the the nice {0}, {1}... property replacement format). Other suggestions for clean database driven messages in GWT are also welcome.

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• NHibernate / multiple sessions and nested objects

  - by bernhardrusch

  We are using NHibernate in a rich client application. It is a pretty open application (the user searches for a dataset or creates a new one, changes the data and saves the data set. We leave the session open, because sometimes we have to lazy load some properties of the object (nested object structure). This means one big problem if we leave the session open, the db (MySQL) closes the connection and we are not able to find this out and it throws an exception (socket communication error) when accessing the database (we are thinking about testing the db connection before accessing the object - but this is not really optimal neither, the other option would be to set back the timeout of the db connection , but this just doesn't seem to well). So - is it possible to reconnect the session to a new database connection ? Another problem is it possible to get an object from one session and then re-attach it to another session ? (I often hear that session.lock should work for this - but this doesn't work so well in our application - so I ended up getting a "fresh" object from the session and copy the data over manually - which is a little bit cumbersome) Any ideas for this ?

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• Android Service setForeground While Display Sleeps

  - by c12

  I have a background Android Service that's purpose is to communicate with another device (non-phone) using a Bluetooth socket. Everything works fine except that the service gets stopped and restarted by the OS when the phone display is sleeping. This restart sometimes leaves a 15-20 minute gaps where there is no communication between the Service and the Bluetooth device and I need to be able to query the device every minute. Is startForeground the proper approach? Attempted to use: startForeground(int, Notification) //still see gaps when phone sleeps Service: public class ForegroundBluetoothService extends Service{ private boolean isStarted = false; @Override public void onDestroy() { super.onDestroy(); stop(); } /** * @see android.app.Service#onBind(android.content.Intent) */ @Override public IBinder onBind(Intent intent) { return null; } /** * @see android.app.Service#onStartCommand(android.content.Intent, int, int) */ @Override public int onStartCommand(Intent intent, int flags, int startId) { loadServiceInForeground(); return(START_STICKY); } private void loadServiceInForeground() { if (!isStarted) { isStarted=true; Notification notification = new Notification(R.drawable.icon, "Service is Running...", System.currentTimeMillis()); Intent notificationIntent = new Intent(this, MainScreen.class); PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, notificationIntent, 0); notification.setLatestEventInfo(this, "Notification Title", "Service is Running...", pendingIntent); startForeground(12345, notification); try{ queryTheBluetoothDevice(); } catch(Exception ex){ ex.printStackTrace(); } } } private void stop() { if (isStarted) { isStarted=false; stopForeground(true); } } }

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• calling same function on different buttons not loaded yet

  - by Jordan Faust

  I can not get this to work for every button and I cannot find anything explaining why. I guessing it is something small that I am missing $(document).ready(function() { // delete the selected row from the database $(document).on('click', '#business-area-delete-button', { model: "BusinessArea" }, deleteRow); $(document).on('click', '#business-type-delete-button', { model: "BusinessType" }, deleteRow); $(document).on('click', '#client-delete-button', { model: "Client" }, deleteRow); $(document).on('click', '#client-type-delete-button', { model: "ClientType" }, deleteRow); $(document).on('click', '#communication-channel-type', { model: "CommunicationChannelType" }, deleteRow); $(document).on('click', '#parameter-type-delete-button', { model: "ParameterType" }, deleteRow); $(document).on('click', '#validation-method-delete-button', { model: "ValidationMethod" }, deleteRow); } the event function deleteRow(event){ $.ajax( { type:'POST', data: { id: $(".delete-row").attr("id") }, url:"/mysite/admin/delete" + event.data.model, success:function(data,textStatus){ $('#main-content').html(data); }, error:function(XMLHttpRequest,textStatus,errorThrown){ jQuery('#alerts').html(XMLHttpRequest.responseText); }, complete:function(XMLHttpRequest,textStatus){ placeAlerts() } } ); return false }; This works only for a the button with id validation-method-delete-button. I use document and not the button its self because the button is contained in a template that is loaded later via ajax. I have this working for a similar function that is selecting a row in a table however I am not attempting to pass data in that scenario.

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• Response time increasing (worsening) over time with consistent load

  - by NJ

  Ok. I know I don't have a lot of information. That is, essentially, the reason for my question. I am building a game using Flash/Flex and Rails on the back-end. Communication between the two is via WebORB. Here is what is happening. When I start the client an operation calls the server every 60 seconds (not much, right?) which results in two database SELECTS and an UPDATE and a resulting response to the client. This repeats every 60 seconds. I deployed a test version on heroku and NewRelic's RPM told me that response time degraded over time. One client with one task every 60 seconds. Over several hours the response time drifted from 150ms to over 900ms in response time. I have been able to reproduce this in my development environment (my Macbook Pro) so it isn't a problem on Heroku's side. I am not doing anything sophisticated (by design) in the server app. An action gets called, gets some data from the database, performs an AR update and then returns a response. No caching, etc. Any thoughts? Anyone? I'd really appreciate it.

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• How to model and handle presentation DTO's to abstract from complicated domain model?

  - by arrages

  Hi I am developing an application that needs to work with a complex domain model using Hibernate. This application uses Spring MVC and using the domain objects in the presentation layer is very messy so I think I should use DTO's that go to and from my service layer so that these match what I need in my views. Now lets assume I have a CarLease entity whose properties are not simple java primitives but it's composed with other entities like Make, Model, etc public class CarLease { private Make make; Private Model model; . . . } most properties are in this fashion and they are selectable using drop down selects on the jsp view, each will post back an ID to the controller. Now considering some standard use cases: create, edit, display How would you go about modeling the presentation DTO's to be used as form backing objects and communication between presentation and service layers?? Would you create a different DTO for each case (create, edit, display), would you make DTO's for the complex attributes? if so where would you translate the ID to entity? how and where would you handle validation, DTO/Domain assembly, what would you return from service layer methods? (create, edit, get) As you can see, I now I will benefit by separating my view from the domain objects (very complex with lots of stuff I don't need.) but I am having a hard time finding any real world examples and best practices for this. I need some architecture guidance from top to bottom, please keep in mind I will use Spring MVC in case that may leverage on your anwser. thanks in advance.

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• Is it possible to use a serial port like session in c#?

  - by Pandiya Chendur

  I am using serial port communication in my asp.net webform application... private bool sendSMS(int portNo, string mobNo, string details) { try { SerialPort SerialPort1 = new SerialPort(); SerialPort1.PortName = "COM" + portNo.ToString(); SerialPort1.BaudRate = 9600; SerialPort1.Parity = Parity.None; SerialPort1.DataBits = 8; SerialPort1.StopBits = StopBits.One; SerialPort1.RtsEnable = true; SerialPort1.DtrEnable = true; SerialPort1.Encoding.GetEncoder(); SerialPort1.ReceivedBytesThreshold = 1; SerialPort1.NewLine = Environment.NewLine; SerialPort1.Open(); SerialPort1.Write("AT" + SerialPort1.NewLine); Sleep(500); SerialPort1.Write("AT+CMGF=1" + SerialPort1.NewLine); Sleep(500); SerialPort1.Write("AT+CMGS=" + (char)34 + mobNo + (char)34 + SerialPort1.NewLine); Sleep(1000); SerialPort1.Write(details + (char)26); Sleep(2000); SerialPort1.Close(); } catch { } return true; } This method works when i send i single message... But when want to send sms in bulk opening and closing port everytime is not a good idea... So my question is it possible to use a serial port like session in c#?... When i open a port i want it to be open for 1 hour and then if my time expires i want to close the port and open it the next time... Any suggestion...

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• Compiling my Boost/NTL program with c++ on Linux.

  - by Martin Lauridsen

  Hi SO, I wrote a client program and a server program, that uses the NTL library and Boost::Asio, to do client/server communication for an integer factorization application, in C++. Both sides consist of several headers and cpp files. Both project compile fine individually on Windows in Visual Studio. All I did, was add the include path of NTL and Boost to both projects: Additional include paths: "D:\Downloads\WinNTL-5_5_2\include";D:\boost_1_42_0 Furthermore, for both projects, I added the two library paths to both projects in VS: Additional library directories: D:\boost_1_42_0\stage\lib;"D:\Documents\Visual Studio 2008\Projects\ntl\Debug" And added under Additional dependencies: ntl.lib As said, it compiles fine on Windows. But when I put the code on a Linux machine provided by university, I try to compile with the following statement c++ -I/appl/htopopt/Linux_x86_64/NTL-5.4.2/include -I/appl/htopopt/Linux_x86_64/boost_1_43_0/include client_protocol.cpp mpqs_client.cpp mpqs_sieve.cpp mpqs_helper.cpp -o mpqs_helper -L/appl/htopopt/Linux_x86_64/NTL-5.4.2/lib -lntl -L/appl/htopopt/Linux_x86_64/gmp-4.2.1/lib -lgmp -lm -L/appl/htopopt/Linux_x86_64/boost_1_43_0/lib -lboost_system -static Upon doing this, I get a huuuge error, which I posted here. Any idea how to fix this, please??

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• Functioning Socket read no longer works when called in AsyncTask

  - by bibismcbryde

  I'm making an app that sends a string to a server over a socket and then reads the output after the server has processed that data. It worked perfectly when it was my foreground task, but I have since used AsyncTask to show a process dialog while the socket communication runs in the background, and things start breaking after I read the output from the server and then try to close the socket. private class Progressor extends AsyncTask<String, Void, Void> { ProgressDialog dialog; protected void onPreExecute() { dialog = ProgressDialog.show(ClearTalkInputActivity.this, "Loading..", "Analyzing Text", true, false); } protected Void doInBackground(String... strings) { String language = strings[0].toLowerCase(); String the_text = strings[1]; Socket socket = null; DataOutputStream dos = null; DataInputStream dis = null; try { socket = new Socket(my_ip, port); dos = new DataOutputStream(socket.getOutputStream()); dis = new DataInputStream(socket.getInputStream()); dos.writeUTF(language+"****"+the_text); String in = ""; while (in.indexOf("</content>") < 0) { in += dis.readUTF(); } socket.close(); save_str(OUTPUT_KEY, in); } catch (UnknownHostException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } finally { if (socket != null) { try { socket.close(); } catch (IOException e) { e.printStackTrace(); } } } if (dos != null) { try { dos.close(); } catch (IOException e) { e.printStackTrace(); } } if (dis != null) { try { dis.close(); } catch (IOException e) { e.printStackTrace(); } } return null; } protected void onPostExecute() { if (dialog.isShowing()) dialog.dismiss(); startActivity(new Intent (output_intent)); } }

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• Flash browser game - HTTP + PHP vs Socket + Something else

  - by Maurycy Zarzycki

  I am developing a non-real time browser RPG game (think Kingdom of Loathing) which would be played from within a Flash app. At first I just wanted to make the communication with server using simply URLLoader to tell PHP what I am doing, and using $_SESSION to store data needed in-between request. I wonder if it wouldn't be better to base it on a socket connection, an app residing on a server written in Java or Python. The problem is I have never ever written such an app so I have no idea how much I'd have to "shift" my thoughts from simple responding do request (like PHP) to continuously working application. I won't hide I am also concerned about the memory and CPU usage of such Server app, when for example there would be hundreds of users connected. I've done some research. I have tried to do some research, but thanks to my nil knowledge on the sockets subject I haven't found anything helpful. So, considering the fact I don't need real time data exchange, will it be wise to develop the server side part as socket server, not in plain ol' PHP?

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• Hashing 11 byte unique ID to 32 bits or less

  - by MoJo

  I am looking for a way to reduce a 11 byte unique ID to 32 bits or fewer. I am using an Atmel AVR microcontroller that has the ID number burned in at the factory, but because it has to be transmitted very often in a very low power system I want to reduce the length down to 4 bytes or fewer. The ID is guaranteed unique for every microcontroller. It is made up of data from the manufacturing process, basically the coordinates of the silicone on the wafer and the production line that was used. They look like this: 304A34393334-16-11001000 314832383431-0F-09000C00 Obviously the main danger is that by reducing these IDs they become non-unique. Unfortunately I don't have a large enough sample size to test how unique these numbers are. Having said that because there will only be tens of thousands of devices in use and there is secondary information that can be used to help identify them (such as their approximate location, known at the time of communication) collisions might not be too much of an issue if they are few and far between. Is something like MD5 suitable for this? My concern is that the data being hashed is very short, just 11 bytes. Do hash functions work reliably on such short data?

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• Please suggest other ways of communicating between server & client.

  - by Geo

  I'm writing a TCP chat server ( programming language does not mather ). It's a school project for my nephew, so it won't be released, and all questions I'm asking are just for my knowledge :). . Some of the things it will support: chatting between users ( doh ), it will be multithreaded sending each other files I know I could easily get away with all the stuff above if I go with serialization, and just send objects from client to server and back. But, if I do that, it will be limited to a specific programming language ( meaning clients written in other programming languages may not be able to deserialize the objects ). What would be the way to go so that other clients written in other languages could be supported? One way to go, off the top of my head, would be to go in this direction: the server & the client communicate by sending messages & chunks ( in lieu of other names ). Here's what I mean by this: every time the client/server wants to send something ( text message or file ) it will first send a simple text message ( newline terminated ) with the number of the chunks it will send. Example: command 4,20,30,40,50 Where command would be something like instant-message or file,4 would be the number of chunks to be sent, 20 would be the size in bytes of the first chunk, 30 of the 2nd, and so forth. after the message was sent, the client/server will start sending chunks ( of sizes mentioned in the sent message ). What do you think about implementing the client/server communication this way? What better options are there?

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• How often should network traffic/collisions cause SNMP Sets to fail?

  - by A. Levy

  My team has a situation where an SNMP SET will fail once every two weeks or so. Since this set happens automatically, we don't necessarily notice it immediately when it fails, and this can result in an inconsistent configuration and associated wailing and gnashing of teeth. The plan is to fix this by having our software automatically retry the SET when it fails. The problem is, we aren't sure why the failure is happening. My (extremely limited) knowledge of SNMP isn't particularly helpful in diagnosing this problem, so I thought I'd ask StackOverflow for some advice. We think that every so often a spike in network traffic will cause the SET to fail. Since SNMP uses UDP for communication, I would think it would be relatively easy for a command to be drowned out if traffic was high for a short period of time. However, I have no idea how common this is. We have a small network with a single cisco router and there are less than a dozen SNMP controlled devices on that network. In addition to the SNMP traffic, there are some status web pages being loaded from the various devices. In case it makes a difference, I believe we are using the AdventNet SNMP API version 4.0.4 for Java. Does it sound reasonable that there will be some SET commands dropped occasionally, or should we be looking for other causes?

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• What happens if I just add a second IP to a domain?

  - by tntu

  We have two servers that are in constant sync. We have two applications that connect to them. Each app to different server. We devised a new version of those apps that will read a dns entry and get a list of IP addresses and try them in order. Now problem is old apps. We have noticed that some ppl still use the old ones even if we have released the new. If we were to add two IP's to each domain would they receive the IP's in the order we set them or random? Either way it will still work for us but I'm just curious. If first server goes offline will the client application try the other? To be noted for old version: Interruption does not affect in any way the continuation once connection is reestablished. Each communication is independent of previous ones. Applications connect at set intervals of time anywhere between 5 seconds to 1 hour. Connection is done simply using an http post to the URL in question.

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• Is a web-server (e.g servlets) a good solution for an IM server?

  - by John

  I'm looking at a new app, broadly speaking an IM application with a strong client-server model - all communications go through a server so they can be logged centrally. The server will be Java in some form, clients could at this point be anything from a .NET Desktop app to Flex/Silverlight, to a simple web-interface using JS/AJAX. I had anticipated doing the server using standard J2EE so I get a thread-safe, multi-user server for 'free'... to make things simple let's say using Servlets (but in practice SpringMVC would be likely). This all seemed very neat but I'm concerned if the stateless nature of Servlets is the best approach. If my memory of servlets (been a year or two) is right, each time a client sent a HTTP request, typically a new message entered by the user, the servlet could not assume it had the user/chat in memory and might have to get it from the DB... regardless it has to look it up. Then it either has to use some PUSH system to inform other members of the chat, or cache that there are new messages, for other clients who poll the server using AJAX or similar - and when they poll it again has to lookup the chat, including new messages, and send the new data. I'm wondering if a better system would be the server is running core Java, and implements a socket-based communication with clients. This allows much more immediate data transfer and is more flexible if say the IM client included some game you could play. But then you're writing a custom server and sockets don't sound very friendly to a browser-based client on current browsers. Am I missing some big piece of the puzzle here, it kind of feels like I am? Perhaps a better way to ask the question would simply be "if the client was browser-based using HTML/JS and had to run on IE7+,FF2+ (i.e no HTML5), how would you implement the server?" edit: if you are going to suggest using XMPP, I have been trying to get my head around this in another question, so please consider if that's a more appropriate place to discuss this specifically.

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