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  • What do I need to know before working on an IM application?

    - by John
    I'm looking into building an IM-type application using Java stack (for the server at least). I'd be interested to see any information/advice on how applications like Skype/AIM/MSN work, as well as know any technologies/APIs that might be relevant. Without giving away the idea itself, it's perhaps more akin to Google Wave than Skype, but information useful for either is very welcome. Specific points I have already thought of include: Server Vs P2P... for reasons of logging my system will require all communication to go through a central server. Is this how other IM tools work... especially when audio/video comes into the equation? Cross-communication with other systems. Are there APIs for this or do all IM providers work hard to keep their protocol secret? The nature of what I'm designing means integration could probably only be limited, but it definitely seems worthwhile from a business perspective

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  • What is the best businessmodel when using NHibernate to access the database?

    - by Sem Dendoncker
    Hi, I'm starting to use NHiberante as a dataaccess tier. I'm used to work with the repository model as a business model. This means, my domain model (not really a tier), my asp.net application tier, my repository tier which finally uses my dataaccess tier. But with what I've learned by now (on NHibernate) I know that many logic is done within an ISession so I was wondering if NHibernate could be used within the repository tier. Or perhaps there are even better ways to use NHibernate. Does anyone know where I could find a propriate example of how to use NHibernate in an webapplication? Cheers, M.

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  • One application instance for two domain name

    - by dervlap
    Hello, I have two web applications in ASP.NET which are quite the same (same business logic, same DAL, same DB scheme but different instance). The only thing that I need to change is the design (logo, color,...) and the text (global and local resource) to adress two separate business sector. We cannot "subdomain" the application because we need the two app "seems to be" independant. Is it a good idea to run only one instance for the 2 web applications. For example : I will have 2 hostnames : mycompagny.com and mycompagny2.com and I will put an HTTP Module which will set a string which will be propagated in my application like 'company' and 'company2'. I will instanciate the dal only once but the connection string will change depending on the string 'company' or 'company2'. Any pros and cons ? Any other alternatives ?

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  • Abstract away a compound identity value for use in business logic?

    - by John K
    While separating business logic and data access logic into two different assemblies, I want to abstract away the concept of identity so that the business logic deals with one consistent identity without having to understand its actual representation in the data source. I've been calling this a compound identity abstraction. Data sources in this project are swappable and various and the business logic shouldn't care which data source is currently in use. The identity is the toughest part because its implementation can change per kind of data source, whereas other fields like name, address, etc are consistently scalar values. What I'm searching for is a good way to abstract the concept of identity, whether it be an existing library, a software pattern or just a solid good idea of some kind is provided. The proposed compound identity value would have to be comparable and usable in the business logic and passed back to the data source to specify records, entities and/or documents to affect, so the data source must be able to parse back out the details of its own compound ids. Data Source Examples: This serves to provide an idea of what I mean by various data sources having different identity implementations. A relational data source might express a piece of content with an integer identifier plus a language specific code. For example. content_id language Other Columns expressing details of content 1 en_us 1 fr_ca The identity of the first record in the above example is: 1 + en_us However when a NoSQL data source is substituted, it might somehow represent each piece of content with a GUID string 936DA01F-9ABD-4d9d-80C7-02AF85C822A8 plus language code of a different standardization, And a third kind of data source might use just a simple scalar value. So on and so forth, you get the idea.

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  • Singleton pattern with Web application, Not a good idea!!

    - by Tony
    Hi I found something funny, I notice it by luck while I was debugging other thing. I was applying MCP pattern and I made a singleton controller to be shared among all presentations. Suddenly I figured out that some event is called once at first postback, twice if there is two postback, 100 times if there is 100 postbacks. because Singleton is based on a static variable which hold the instance, and the static variable live across postbacks, and I wired the event assuming that it will be wired once, and rewired for each postback. I think we should think twice before applying a singleton in a web application, or I miss something?? thanks

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  • Silverlight : vers une fin programmée ? Microsoft ferme Silverlight.net, le site de référence sur la technologie

    Silverlight : vers une fin programmée ? Microsoft ferme Silverlight.net, le site de référence sur la technologie L'avènement du HTML5 a mis en mal les technologies de développement d'applications Web riches (RIA) comme Silverlight et Flash. Voici aujourd'hui pratiquement un an que Microsoft a publié Silverlight 5. Depuis, l'éditeur n'a plus fait mention d'une évolution de la technologie en dehors de la sortie discrète d'une mise à jour mineure (Silverlight 5.1). [IMG]http://idelways.developpez.com/news/images/silverlight-logo.jpg[/IMG] Par contre, Microsoft accorde une attention particulière à HTML5, qui occupe une place de...

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  • Handling multiple post-defined datasource connections based on user

    - by choudeshell
    How should I handle multiple user-connection defined datasource connection in my .NET application? To illustrate this better; the user provides three (3) pieces of information, username, password and data source name. This data source name is then looked-up in a central database telling that application which main database this user should connect to. My question is, in the DataAccess layer, should we only pass down enough information to uniquely identify that user, and query the central database each time in the DAL; or, should we provide another way to pass the datasource connection down to the DAL?

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  • Should an edit of a comment be sent through POST or PUT?

    - by the_drow
    I have the following URI: Posts/{postId}/Comments/{commentId} I would like to enable users to edit a comment through my API, should the edit be done with POST or PUT? One one hand, POST updates the contents of a resource so that makes sense but on the other hand PUT replaces it with a new one. So if I understand correctly with POST I need to send only what needs to be updates and with PUT I send the whole resource. Usually in edit forms, the whole resource is loaded anyway so what's the point of using POST? If I take one approach or the other, what are the differences?

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  • .NET integration Framework

    - by buhtla
    Is there any kind of framework that provides some generic mechanism for integration between applications or something similar? By integration I presume data exchange (import and export) between two applications via some standard interface like Web Service.

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  • What is the most challenging part of a project like MyLifeBits

    - by kennyzx
    I have made a small utility to keep track of my daily expenses, and my coffee consumption (I always want to quit coffee but never succeed), and of course, this kind of utility is quite simple, just involving an xml file and a few hundred lines of code to manipulate the file. Then I find this project MyLifeBits, it is very intesting, but I think it should require a lot of effort to achieve its goal- that is, to record everything about a person that can be digitally record. So I wonder, it is possible to write an advanced version of my own utility - but a tiny version of MyLifeBits, that can capture: Every webpage I've read, no matter what browser I am using, just download its contents for offline reading, Auto archive emails/documents/notes that I edited, Auto archive Codes that I run/written. Well, these are basically what I do on my PC. And the captured records can be searched easily. My question is, What do you think is the most challenging part? Interoperating with Visual Studio/Office/Lotes Notes/Web browsers is one thing, Database is another thing given that "everything" is record, Advanced programming patterns since it is not a "toy project"? And others that I have overlooked but can be very difficult to handle?

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  • How to dynamically reference a "partial template" in MS Word?

    - by scunliffe
    I want to make use of an "external reference" in Word. (for anyone that knows AutoCAD, I want XREF abilities in Word) Essentially I have a custom "header" that I want included in a whole pile of documents... that all reference a single file... such that if my address, logo, tagline, phone, fax or email changes, I update the one file, and all of the other 101 files that use it automatically update when I next open/use them. I'm using Office 2007 if that makes any difference.

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  • How to change the style of a source reference in Word ?

    - by ldigas
    I have a number of references in Word 2007; there is several way of referencing them and the one I find the most fitting is purely numeric, e.g. (3) for reference number 3 in the list. But since I reference equations by round parenthesis, I'd like to change the literature references to square brackets. How can I do that ?

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  • How to cross-reference many character encodings with ASCII OR UTFx?

    - by Garet Claborn
    I'm working with a binary structure, the goal of which is to index the significance of specific bits for any character encoding so that we may trigger events while doing specific checks against the profile. Each character encoding scheme has an associated system record. This record's leading value will be a C++ unsigned long long binary value and signifies the length, in bits, of encoded characters. Following the length are three values, each is a bit field of that length. offset_mask - defines the occurrence of non-printable characters within the min,max of print_mask range_mask - defines the occurrence of the most popular 50% of printable characters print_mask - defines the occurrence value of printable characters The structure of profiles has changed from the op of this question. Most likely I will try to factorize or compress these values in the long-term instead of starting out with ranges after reading more. I have to write some of the core functionality for these main reasons. It has to fit into a particular event architecture we are using, Better understanding of character encoding. I'm about to need it. Integrating into non-linear design is excluding many libraries without special hooks. I'm unsure if there is a standard, cross-encoding mechanism for communicating such data already. I'm just starting to look into how chardet might do profiling as suggested by @amon. The Unicode BOM would be easily enough (for my current project) if all encodings were Unicode. Of course ideally, one would like to support all encodings, but I'm not asking about implementation - only the general case. How can these profiles be efficiently populated, to produce a set of bitmasks which we can use to match strings with common characters in multiple languages? If you have any editing suggestions please feel free, I am a lightweight when it comes to localization, which is why I'm trying to reach out to the more experienced. Any caveats you may be able to help with will be appreciated.

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  • Does Zend application server provide any sort of pushed-based messaging architecture support?

    - by Alex
    Does Zend application server provide any sort of pushed-based messaging architecture support?I mean some different message-oriented middlewares that can be used for the push-based architecture or email support?There is a need in provision of the non web-based interface for certain users of the website who would get regular messages notifying them of different special offers in their field of interest(it should be push-based)

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  • Linux C++: Linker is outputting strange errors

    - by knight666
    Alright, here is the output I get: arm-none-linux-gnueabi-ld --entry=main -dynamic-linker=/system/bin/linker -rpath-link=/home/oem/android-ndk-r3/build/platforms/android-5/arch-arm/usr/lib -L/home/oem/android-ndk-r3/build/platforms/android-5/arch-arm/usr/lib -nostdlib -lstdc++ -lm -lGLESv1_CM -rpath=/home/oem/android-ndk-r3/build/platforms/android-5/arch-arm/usr/lib -rpath=../../YoghurtGum/lib/Android -L./lib/Android intermediate/Alien.o intermediate/Bullet.o intermediate/Game.o intermediate/Player.o ../../YoghurtGum/bin/YoghurtGum.a -o bin/Galaxians.android intermediate/Game.o: In function `Galaxians::Init()': /media/YoghurtGum/Tests/Galaxians/src/Game.cpp:45: undefined reference to `__cxa_end_cleanup' /media/YoghurtGum/Tests/Galaxians/src/Game.cpp:44: undefined reference to `__cxa_end_cleanup' intermediate/Game.o:(.ARM.extab+0x18): undefined reference to `__gxx_personality_v0' intermediate/Game.o: In function `Player::Update()': /media/YoghurtGum/Tests/Galaxians/src/Player.h:41: undefined reference to `__cxa_end_cleanup' intermediate/Game.o:(.ARM.extab.text._ZN6Player6UpdateEv[_ZN6Player6UpdateEv]+0x0): undefined reference to `__gxx_personality_v0' intermediate/Game.o:(.rodata._ZTIN10YoghurtGum4GameE[_ZTIN10YoghurtGum4GameE]+0x0): undefined reference to `vtable for __cxxabiv1::__class_type_info' intermediate/Game.o:(.rodata._ZTI6Player[_ZTI6Player]+0x0): undefined reference to `vtable for __cxxabiv1::__si_class_type_info' intermediate/Game.o:(.rodata._ZTIN10YoghurtGum6EntityE[_ZTIN10YoghurtGum6EntityE]+0x0): undefined reference to `vtable for __cxxabiv1::__si_class_type_info' intermediate/Game.o:(.rodata._ZTIN10YoghurtGum6ObjectE[_ZTIN10YoghurtGum6ObjectE]+0x0): undefined reference to `vtable for __cxxabiv1::__class_type_info' intermediate/Game.o:(.rodata._ZTI6Bullet[_ZTI6Bullet]+0x0): undefined reference to `vtable for __cxxabiv1::__si_class_type_info' intermediate/Game.o:(.rodata._ZTI5Alien[_ZTI5Alien]+0x0): undefined reference to `vtable for __cxxabiv1::__si_class_type_info' intermediate/Game.o:(.rodata+0x20): undefined reference to `vtable for __cxxabiv1::__si_class_type_info' ../../YoghurtGum/bin/YoghurtGum.a(Sprite.o):(.rodata._ZTIN10YoghurtGum16SpriteDataOpenGLE[_ZTIN10YoghurtGum16SpriteDataOpenGLE]+0x0): undefined reference to `vtable for __cxxabiv1::__si_class_type_info' ../../YoghurtGum/bin/YoghurtGum.a(Sprite.o):(.rodata._ZTIN10YoghurtGum10SpriteDataE[_ZTIN10YoghurtGum10SpriteDataE]+0x0): undefined reference to `vtable for __cxxabiv1::__class_type_info' make: *** [bin/Galaxians.android] Fout 1 Here's an error I managed to decipher: intermediate/Game.o: In function `Galaxians::Init()': /media/YoghurtGum/Tests/Galaxians/src/Game.cpp:45: undefined reference to `__cxa_end_cleanup' /media/YoghurtGum/Tests/Galaxians/src/Game.cpp:44: undefined reference to `__cxa_end_cleanup' This is line 43 through 45: Assets::AddSprite(new Sprite("media\\ViperMarkII.bmp"), "ship"); Assets::AddSprite(new Sprite("media\\alien.bmp"), "alien"); Assets::AddSprite(new Sprite("media\\bat_ball.bmp"), "bullet"); So, what seems funny to me is that the first new is fine (line 43), but the second one isn't. What could cause this? intermediate/Game.o: In function `Player::Update()': /media/YoghurtGum/Tests/Galaxians/src/Player.h:41: undefined reference to `__cxa_end_cleanup' Another issue with new: Engine::game->scene_current->AddObject(new Bullet(m_X + 10, m_Y)); I have no idea where to begin with the other issues. These are my makefiles, They're a giant mess because I'm just trying to get it to work. Static library: # ====================================== # # # # YoghurtGum static library # # # # ====================================== # include ../YoghurtGum.mk PROGS = bin/YoghurtGum.a SOURCES = $(wildcard src/*.cpp) #$(YG_PATH_LIB)/libGLESv1_CM.so \ #$(YG_PATH_LIB)/libEGL.so \ YG_LINK_OPTIONS = -shared YG_LIBRARIES = \ $(YG_PATH_LIB)/libc.a \ $(YG_PATH_LIB)/libc.so \ $(YG_PATH_LIB)/libstdc++.a \ $(YG_PATH_LIB)/libstdc++.so \ $(YG_PATH_LIB)/libm.a \ $(YG_PATH_LIB)/libm.so \ $(YG_PATH_LIB)/libui.so \ $(YG_PATH_LIB)/liblog.so \ $(YG_PATH_LIB)/libGLESv2.so \ $(YG_PATH_LIB)/libcutils.so \ YG_OBJECTS = $(patsubst src/%.cpp, $(YG_INT)/%.o, $(SOURCES)) YG_NDK_PATH_LIB = /home/oem/android-ndk-r3/build/platforms/android-5/arch-arm/usr/lib all: $(PROGS) rebuild: clean $(PROGS) # remove all .o objects from intermediate and all .android objects from bin clean: rm -f $(YG_INT)/*.o $(YG_BIN)/*.a copy: acpy ../$(PROGS) $(PROGS): $(YG_OBJECTS) $(YG_ARCHIVER) -vq $(PROGS) $(YG_NDK_PATH_LIB)/crtbegin_static.o $(YG_NDK_PATH_LIB)/crtend_android.o $^ && \ $(YG_ARCHIVER) -vr $(PROGS) $(YG_LIBRARIES) $(YG_OBJECTS): $(YG_INT)/%.o : $(YG_SRC)/%.cpp $(YG_COMPILER) $(YG_FLAGS) -I $(GLES_INCLUDES) -c $< -o $@ Test game project: # ====================================== # # # # Galaxians # # # # ====================================== # include ../../YoghurtGum.mk PROGS = bin/Galaxians.android YG_COMPILER = arm-none-linux-gnueabi-g++ YG_LINKER = arm-none-linux-gnueabi-ld YG_PATH_LIB = ./lib/Android YG_LIBRARIES = ../../YoghurtGum/bin/YoghurtGum.a YG_PROGS = bin/Galaxians.android GLES_INCLUDES = ../../YoghurtGum/src ANDROID_NDK_ROOT = /home/oem/android-ndk-r3 NDK_PLATFORM_VER = 5 YG_NDK_PATH_LIB = $(ANDROID_NDK_ROOT)/build/platforms/android-$(NDK_PLATFORM_VER)/arch-arm/usr/lib YG_LIBS = -nostdlib -lstdc++ -lm -lGLESv1_CM #YG_COMPILE_OPTIONS = -g -rdynamic -Wall -Werror -O2 -w YG_COMPILE_OPTIONS = -g -Wall -Werror -O2 -w YG_LINK_OPTIONS = --entry=main -dynamic-linker=/system/bin/linker -rpath-link=$(YG_NDK_PATH_LIB) -L$(YG_NDK_PATH_LIB) $(YG_LIBS) SOURCES = $(wildcard src/*.cpp) YG_OBJECTS = $(patsubst src/%.cpp, intermediate/%.o, $(SOURCES)) all: $(PROGS) rebuild: clean $(PROGS) clean: rm -f intermediate/*.o bin/*.android $(PROGS): $(YG_OBJECTS) $(YG_LINKER) $(YG_LINK_OPTIONS) -rpath=$(YG_NDK_PATH_LIB) -rpath=../../YoghurtGum/lib/Android -L$(YG_PATH_LIB) $^ $(YG_LIBRARIES) -o $@ $(YG_OBJECTS): intermediate/%.o : src/%.cpp $(YG_COMPILER) $(YG_COMPILE_OPTIONS) -I ../../YoghurtGum/src/GLES -I ../../YoghurtGum/src -c $< -o $@ Any help would be appreciated.

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  • program won't find math.h anymore

    - by 130490868091234
    After a long time, I downloaded a program I co-developed and tried to recompile it on my Ubuntu Linux 12.04, but it seems it does not find math.h anymore. This may be because something has changed recently in gcc, but I can't figure out if it's something wrong in src/Makefile.am or a missing dependency: Download from http://www.ub.edu/softevol/variscan/: tar xzf variscan-2.0.2.tar.gz cd variscan-2.0.2/ make distclean sh ./autogen.sh make I get: [...] gcc -DNDEBUG -O3 -W -Wall -ansi -pedantic -lm -o variscan variscan.o statistics.o common.o linefile.o memalloc.o dlist.o errabort.o dystring.o intExp.o kxTok.o pop.o window.o free.o output.o readphylip.o readaxt.o readmga.o readmaf.o readhapmap.o readxmfa.o readmav.o ran1.o swcolumn.o swnet.o swpoly.o swref.o statistics.o: In function `calculate_Fu_and_Li_D': statistics.c:(.text+0x497): undefined reference to `sqrt' statistics.o: In function `calculate_Fu_and_Li_F': statistics.c:(.text+0x569): undefined reference to `sqrt' statistics.o: In function `calculate_Fu_and_Li_D_star': statistics.c:(.text+0x63b): undefined reference to `sqrt' statistics.o: In function `calculate_Fu_and_Li_F_star': statistics.c:(.text+0x75c): undefined reference to `sqrt' statistics.o: In function `calculate_Tajima_D': statistics.c:(.text+0x85d): undefined reference to `sqrt' statistics.o:statistics.c:(.text+0xcb1): more undefined references to `sqrt' follow statistics.o: In function `calcRunMode21Stats': statistics.c:(.text+0xe02): undefined reference to `log' statistics.o: In function `correctedDivergence': statistics.c:(.text+0xe5a): undefined reference to `log' statistics.o: In function `calcRunMode22Stats': statistics.c:(.text+0x104a): undefined reference to `sqrt' statistics.o: In function `calculate_Fu_fs': statistics.c:(.text+0x11a8): undefined reference to `fabsl' statistics.c:(.text+0x11ca): undefined reference to `powl' statistics.c:(.text+0x11f2): undefined reference to `logl' statistics.o: In function `calculateStatistics': statistics.c:(.text+0x13f2): undefined reference to `log' collect2: ld returned 1 exit status make[1]: *** [variscan] Error 1 make[1]: Leaving directory `/home/avilella/variscan/latest/variscan-2.0.2/src' make: *** [all-recursive] Error 1 The libraries are there because this simple example works perfectly well: $ gcc test.c -o test -lm $ cat test.c #include <stdio.h> #include <math.h> int main(void) { double x = 0.5; double result = sqrt(x); printf("The hyperbolic cosine of %lf is %lf\n", x, result); return 0; } Any ideas?

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  • Guest Blog: Secure your applications based on your business model, not your application architecture, by Yaldah Hakim

    - by Darin Pendergraft
    Today’s businesses are looking for new ways to engage their customers, embrace mobile applications, while staying in compliance, improving security and driving down costs.  For many, the solution to that problem is to host their applications with a Cloud Services provider, but concerns that a hosted application will be less secure continue to cause doubt. Oracle is recognized by Gartner as a leader in the User Provisioning and Identity and Access Governance magic quadrants, and has helped thousands of companies worldwide to secure their enterprise applications and identities.  Now those same world class IDM capabilities are available as a managed service, both for enterprise applications, as well has Oracle hosted applications. --- Listen to our IDM in the cloud podcast to hear Yvonne Wilson, Director of the IDM Practice in Cloud Service, explain how Oracle Managed Services provides IDM as a service ---Selecting OracleManaged Cloud Services to deploy and manage Oracle Identity Management Services is a smart business decision for a variety of reasons. Oracle hosted Identity Management infrastructure is deployed securely, resilient to failures, and supported by Oracle experts. In addition, Oracle  Managed Cloud Services monitors customer solutions from several perspectives to ensure they continue to work smoothly over time. Customers gain the benefit of Oracle Identity Management expertise to achieve predictable and effective results for their organization.Customers can select Oracle to host and manage any number of Oracle IDM products as a service as well as other Oracle’s security products, providing a flexible, cost effective alternative to onsite hardware and software costs.Security is a major concern for all organizations- making it increasingly important to partner with a company like Oracle to ensure consistency and a layered approach to security and compliance when selecting a cloud provider.  Oracle Cloud Service makes this possible for our customers by taking away the headache and complexity of managing Identity management infrastructure and other security solutions. For more information:http://www.oracle.com/us/solutions/cloud/managed-cloud-services/overview/index.htmlTwitter-https://twitter.com/OracleCloudZoneFacebook - http://www.facebook.com/OracleCloudComputing

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  • Architecture for Social Graph data that has a Time Frame Associated?

    - by Jay Stevens
    I am adding some "social" type features to an existing application. There are a limited # of node & edge types. Overall the data itself is relatively small (50,000 - 70,000 for each type of node) there will be a number of edges (relationships) between them (almost all directional). This, I know, is relatively easy to represent with an SDF store (such as BrightstarDB) or something like Microsoft's Trinity (or really many of the noSQL options). The thing that, I think, makes this a unique use case is that each relationship will have a timeframe associated with it (start and end dates). Right now, I'm thinking of just storing this in a relational structure and dealing with the headaches of "traversing the graph", but I'm looking for suggestions on a better approach (both in terms of data structure and server): Column ================ From_Node_ID Relationship To_Node_ID StartDate EndDate Any suggestions or thoughts are welcomed.

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  • How do I install Cacti 8.7i with PIA 3.1(Plug In Architecture) for 12.04 Server LTS?

    - by Pininy
    I am planning to upgrade my current 9.04 server version to 12.04LTS server. I've successfully backup and restore my Cacti version 8.7d to 8.7i that comes with the distribution. However, the plugin patch installation for PIA 3.1 it is not working for only ubuntu. Can you assist? or it's there a way to include Cacti 8.8a which is a stable version which comes with PIA 3.1 preinstalled. regards, thomas

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  • Architecture Standards &ndash; BPMN vs. BPEL for Business Process Management

    - by pat.shepherd
    I get asked often which business process standard an organization should use; BPMN or BPEL?  As I explain to folks, they both have strengths.  Here is a great article that helps understand the benefits of both and where to use them.  The good news is that, with Oracle SOA Suite and BPM suite, you have the option and flexibility to use both in the same SCA model and runtime container.  Good stuff. Here is the great article that Mark Nelson wrote: The right tool for the right job BPEL and BPMN are both ‘languages’ or ‘notations’ for describing and executing business processes. Both are open standards. Most business process engines will support one or the other of these languages. Oracle however has chosen to support both and treat them as equals. This means that you have the freedom to choose which language to use on a process by process basis. And you can freely mix and match, even within a single composite. (A composite is the deployment unit in an SCA environment.) So why support both? Well it turns out that BPEL is really well suited to modeling some kinds of processes and BPMN is really well suited to modeling other kinds of processes. Of course there is a pretty significant overlap where either will do a great job What BPM adds to SOA Suite | RedStack

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  • Organization &amp; Architecture UNISA Studies &ndash; Chap 6

    - by MarkPearl
    Learning Outcomes Discuss the physical characteristics of magnetic disks Describe how data is organized and accessed on a magnetic disk Discuss the parameters that play a role in the performance of magnetic disks Describe different optical memory devices Magnetic Disk The way data is stored on and retried from magnetic disks Data is recorded on and later retrieved form the disk via a conducting coil named the head (in many systems there are two heads) The writ mechanism exploits the fact that electricity flowing through a coil produces a magnetic field. Electric pulses are sent to the write head, and the resulting magnetic patterns are recorded on the surface below with different patterns for positive and negative currents The physical characteristics of a magnetic disk   Summarize from book   The factors that play a role in the performance of a disk Seek time – the time it takes to position the head at the track Rotational delay / latency – the time it takes for the beginning of the sector to reach the head Access time – the sum of the seek time and rotational delay Transfer time – the time it takes to transfer data RAID The rate of improvement in secondary storage performance has been considerably less than the rate for processors and main memory. Thus secondary storage has become a bit of a bottleneck. RAID works on the concept that if one disk can be pushed so far, additional gains in performance are to be had by using multiple parallel components. Points to note about RAID… RAID is a set of physical disk drives viewed by the operating system as a single logical drive Data is distributed across the physical drives of an array in a scheme known as striping Redundant disk capacity is used to store parity information, which guarantees data recoverability in case of a disk failure (not supported by RAID 0 or RAID 1) Interesting to note that the increase in the number of drives, increases the probability of failure. To compensate for this decreased reliability RAID makes use of stored parity information that enables the recovery of data lost due to a disk failure.   The RAID scheme consists of 7 levels…   Category Level Description Disks Required Data Availability Large I/O Data Transfer Capacity Small I/O Request Rate Striping 0 Non Redundant N Lower than single disk Very high Very high for both read and write Mirroring 1 Mirrored 2N Higher than RAID 2 – 5 but lower than RAID 6 Higher than single disk Up to twice that of a signle disk for read Parallel Access 2 Redundant via Hamming Code N + m Much higher than single disk Highest of all listed alternatives Approximately twice that of a single disk Parallel Access 3 Bit interleaved parity N + 1 Much higher than single disk Highest of all listed alternatives Approximately twice that of a single disk Independent Access 4 Block interleaved parity N + 1 Much higher than single disk Similar to RAID 0 for read, significantly lower than single disk for write Similar to RAID 0 for read, significantly lower than single disk for write Independent Access 5 Block interleaved parity N + 1 Much higher than single disk Similar to RAID 0 for read, lower than single disk for write Similar to RAID 0 for read, generally  lower than single disk for write Independent Access 6 Block interleaved parity N + 2 Highest of all listed alternatives Similar to RAID 0 for read; lower than RAID 5 for write Similar to RAID 0 for read, significantly lower than RAID 5  for write   Read page 215 – 221 for detailed explanation on RAID levels Optical Memory There are a variety of optical-disk systems available. Read through the table on page 222 – 223 Some of the devices include… CD CD-ROM CD-R CD-RW DVD DVD-R DVD-RW Blue-Ray DVD Magnetic Tape Most modern systems use serial recording – data is lade out as a sequence of bits along each track. The typical recording used in serial is referred to as serpentine recording. In this technique when data is being recorded, the first set of bits is recorded along the whole length of the tape. When the end of the tape is reached the heads are repostioned to record a new track, and the tape is again recorded on its whole length, this time in the opposite direction. That process continued back and forth until the tape is full. To increase speed, the read-write head is capable of reading and writing a number of adjacent tracks simultaneously. Data is still recorded serially along individual tracks, but blocks in sequence are stored on adjacent tracks as suggested. A tape drive is a sequential access device. Magnetic tape was the first kind of secondary memory. It is still widely used as the lowest-cost, slowest speed member of the memory hierarchy.

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