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  • Is it bad to be the only person supporting software you have developed?

    - by trpt4him
    My employer has a need for a web-based application to manage and share data within the department, with approximately 50-75 possible users. I feel I have the ability to write it for them. I would likely use Python/Django with a MySQL database, so it would be open source. However, I'm the only IT person in my department (our larger organization has a separate IT support staff with which I often work, but not for web development). I want to develop this application, but if I leave in 1-2 years, and someone else has to come in after me and support it, will this be seen as a bad decision? This is assuming all the obvious points -- I will write documentation, I will comment my code, and I will strive to follow good application design principles. But will that be enough? In principle, is it acceptable for one person to develop and support an entire web application? Is this a "do first, then show and ask" kind of situation, or should I be certain it will be adopted by everyone involved first?

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  • Whether to separate out methods or not

    - by Skippy
    I am new to java and want to learn best coding practices and understand why one method is better than another, in terms of efficiency and as the coding becomes more complicated. This is just an example, but I can take the principles from here to apply elsewhere. I have need an option to display stuff, and have put the method stuff separately from the method to ask if the user wants to display the stuff, as stuff has a lot of lines of code. For readability I have done this: public static void displayStuff () { String input = getInput ("Display stuff? Y/N \n"); if (input..equalsIgnoreCase ("Y")) { stuff (); } else if (input.equalsIgnoreCase ("N")) { //quit program } else { //throw error System.out.print("Error! Enter Y or N: \n"); } } private static String stuff () { //to lots of things here return stuff (); } Or public static void displayStuff () { String input = getInput ("Display stuff? Y/N \n"); if (input..equalsIgnoreCase ("Y")) { //to lots of things here stuff; } else if (input.equalsIgnoreCase ("N")) { //quit program } else { //throw error System.out.print("Error! Enter Y or N: \n"); } } Is it better to keep them together and why? Also, should the second method be private or public, if I am asking for data within the class? I am not sure if this is on topic for here. please advise.

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  • How to effectively design a piece of software

    - by ti83plus
    Im a compsci student and ive got some experience in various languages and paradigms c/java/python/ruby/html/css/scheme/sql/asp(classic). I realise that i want to have some software in my portfolio for future job hunting even tho i still have 2 years left of my education. Ive got a pretty good idea of what i want to make, its a webapp. Most shops around here are either .net or java and since i know java best and dont have access to ms developer tools im thinking i should go with java. Even tho i feel i know the principles of OOP pretty good ive got no clue how to go from my idea to a working solution. Where can i access information about designing the underlying architechture of my solution? Also i would like to know what other technologies i should train on, my current list includes javascript(and possibly a javascript library) some sort of java web framework tips are appreciated. I would like to add support for android/iphone apps in the future and this is something i have to take into account when designing the app. I have done a course on software engineering but i found this to be more centered around project management ideas then the actual design and implementation. So i would like tips on technologies i should focus on to get the most out of my time without the massive overhead of huge config processes but at the same time keep my project viable in a business sense, so that i use technologies that are relevant for business (java developer jobs). And i would also like tips on where i can learn more about the design process around a software project, i will be working mostly alone. But i find the approach ive used up until now (start coding and figure it out as you go) wont suffice.

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  • The Enterprise Side of JavaFX: Part Two

    - by Janice J. Heiss
    A new article, part of a three-part series, now up on the front page of otn/java, by Java Champion Adam Bien, titled “The Enterprise Side of JavaFX,” shows developers how to implement the LightView UI dashboard with JavaFX 2. Bien explains that “the RESTful back end of the LightView application comes with a rudimentary HTML page that is used to start/stop the monitoring service, set the snapshot interval, and activate/deactivate the GlassFish monitoring capabilities.”He explains that “the configuration view implemented in the org.lightview.view.Browser component is needed only to start or stop the monitoring process or set the monitoring interval.”Bien concludes his article with a general summary of the principles applied:“JavaFX encourages encapsulation without forcing you to build models for each visual component. With the availability of bindable properties, the boundary between the view and the model can be reduced to an expressive set of bindable properties. Wrapping JavaFX components with ordinary Java classes further reduces the complexity. Instead of dealing with low-level JavaFX mechanics all the time, you can build simple components and break down the complexity of the presentation logic into understandable pieces. CSS skinning further helps with the separation of the code that is needed for the implementation of the presentation logic and the visual appearance of the application on the screen. You can adjust significant portions of an application's look and feel directly in CSS files without touching the actual source code.”Check out the article here.

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  • How Can I Set the OS X Dock to Show Instantly Instead of Waiting Half a Second in Snow Leopard?

    - by Patrik Björklund
    Having a MacBook Pro 13" I find myself trying to maximize the screen real estate constantly which have led me to put the dock at the right hand corner of the screen. Now in an attempt to save a few more pixels I decided to give autohiding of the dock a go and ran into a problem that have plagued me all night... When you toggle the switch to automatically hide the dock you get stuck with a small delay before it appears when hovering the edge of the screen. I have learnt that this is one of the many design principles Apple have put in use. In this case it appears to be their mission to save me from seeing the dock those few times when it's not intentional to touch the screens edge. In Windows there was a way to tweak this setting to have the menubar appear instantly by altering some registry value. Is there some way I can do this in Mac OS X?

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  • ASP.NET design not SOLID

    - by w0051977
    SOLID principles are described here: http://en.wikipedia.org/wiki/SOLID_%28object-oriented_design%29 I am developing a large ASP.NET app. The previous developer created a few very large classes each with lots of different purposes. It is very difficult to maintain and extend. The classes are deployed to the web server along with the code behind files etc. I want to share a small amount of the app with another application. I am considering moving all of the classes of the ASP.NET web app to a DLL, so the small subset of functionality can be shared. I realise it would be better to only share the classes which contain code to be shared but because of the dependencies this is proving to be very difficult e.g. class A contains code that should be shared, however class A contains references to classes B, C, D, E, F, G etc, so class A cannot be shared on its own. I am planning to refactor the code in the future. As a temporary solution I am planning to convert all the classes into a single class library. Is this a bad idea and if so, is there an alternative? as I don't have time to refactor at the moment.

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  • Basic is Best

    - by Eric A. Stephens
    Fellow foodies will recognize the recent movement towards "farm-to-table" restaurants. These venues attempt to simplify their menus and source ingredients as close to the source as possible. I had the opportunity to dine at such a restaurant the other evening. I was gushing about the appetizer to my server when she described the preparation for the item and then punctuated her comments with "basic is best". I reminded my fellow enterprise architect diners there was an architecture lesson in that statement. They rolled their eyes and chuckled. But they also knew I was right. I'm reminded of Frederick Brooks' book The Mythical Man Month and his latest The Design of Design. The former must read book talks about complexity. But he refrains from damning all complexity. The world we live in and enterprises we strive to transform with enterprise architecture are complicated organisms, much like the human body. But sometimes a simple solution is the best approach. Fewer applications (think: portfolio rationalization). Fewer components. Fewer lines of code. Whatever level of abstraction you are working at, less is more. I'm reminded of the enterprise architecture principle "Control Technical Diversity". At one firm I created pithy catch phrases for each principles. I named this one "Less is More". But perhaps another variation is what my server said the other night, "Basic is Best".

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  • How to plan/manage multi-platform (mobile) products?

    - by PhD
    Say I've to develop an app that runs on iOS, Android and Windows 8 Mobile. Now all three platforms are technically in different program languages. The only 'reuse' that I can see is that of the boxes-and-lines drawings (UML :) charts and nothing else. So how do companies/programmers manage the variation of the same product across different platforms especially since the implementation languages differ? It's 'easier' in the desktop world IMO given the plethora of languages and cross-platform libraries to make your life easier. Not so in the mobile world. More so, product line management principles don't seem to be all that applicable - what is same and variant doesn't really matter - the application is the same (conceptually) and the implementation is variant. Some difficulties that come to mind: Bug Fixing: Applications maybe designed in a similar manner but the bug identification and fixing would be radically different. A bug on iOS may/may-not be existent for that on Android. Or a bug fix approach on one platform may not be the same on another (unless it's a semantic bug like a!=b instead of a==b which would require the same 'approach' to fixing in essence Enhancements: Making a change on one platform would be radically different than on another Code-Design Divergence: They way the code is written/organized, the class structures etc., could be very different given the different implementation environments - leading to further reuse of the (above) UML models. There are of course many others - just keeping the development in sync and making sure all applications are up to the same version with the same set of features etc. Seems the effort is 3x that of a single application. So how exactly does one manage this nightmarish situation? Some thoughts: Split application to client/server to minimize the effect to client side only (not always doable) Use frameworks like Unity-3D that could take care of the cross-platform problem (mostly applicable to games and probably not to other applications etc.) Any other ways of managing a platform line? What are some proven approaches to managing/taming the effects?

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  • Relationship between SOA and OOA

    - by TheSilverBullet
    Thomas Erl defines SOA as follows in his site: Service-oriented computing represents a new generation distributed computing platform. As such, it encompasses many things, including its own design paradigm and design principles, design pattern catalogs, pattern languages, a distinct architectural model, and related concepts, technologies, and frameworks. This definitely sounds like a whole new category which is parallel to object orientation. Almost one in which you would expect an entirely new language to exist for. Like procedural C and object oriented C#. Here is my understanding: In real life, we don't have entirely new language for SOA. And most application which have SOA architecture have an object oriented design underneath it. SOA is a "strategy" to make the entire application/service distributed and reliable. SOA needs OOPS working underneath it. Is this correct? Where does SOA (if at all it does) fit in with object oriented programming practices? Edit: I have learnt through answers that OOA and SOA work with each other and cannot be compared (in a "which is better" way). I have changed the title to "Relationship between SOA and OOA" rather than "comparison".

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  • Windows 7: Use VirtualStore technology as user for own benefit?

    - by mfn
    I've researched a bit and came to the understanding that VirtualStore is part of the new UAC feature in Vista/W7 which is the file system part of the transparent data redirection and redirects write access to folders like program files to C:\User\<username>\AppData\Local\VirtualStore\ in lack of applications respecting the LUA principles. Now I'm interested if that kind of transparent redirection can also be used as a power to the user. Here's an example which comes to my mind: I install any kind of software to e.g. D:\Whatever\ThisAndThisApp\ and I set up things that, after initial installation, any write access to this folder is transparently redirected to e.g. D:\MyOwnVirtualStore\Whatever\ThisAndThisApp\file_only_writable_here.txt. Is this thinking too far or can I actually use that power of VirtualStore as a user on Windows 7? I'm using the Professional version of that matters.

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  • PeopleSoft CRM 9.2 Release Value Proposition

    - by Race Bannon
    Oracle's PeopleSoft Customer Relationship Management (CRM) delivers solutions that have been tailored to fit your industry business processes, your customer strategies, and your success criteria. With PeopleSoft CRM 9.2, organizations will be able to deploy a solution that delivers built-in best practices specific to your industry with a highly configurable, tightly integrated platform, ensuring that solutions will be fast to implement. The result is less configuration, less customization, and less integration. PeopleSoft Customer Relationship Management (CRM) is a world-class solution for organizations of every size and Oracle’s planned product roadmap for PeopleSoft applications is to deliver valuable, needed features for all of an organization’s constituents along three design principles — Simplicity, Productivity, and Lowered Total Cost of Ownership — as well as new application functionality as prioritized by our customers. The upcoming 9.2 release of PeopleSoft Customer Relationship Management focuses on these themes of Simplicity, Productivity, and Lower Total Cost of Ownership while also delivering robust new functionality to help your organization succeed. The recently published PeopleSoft CRM 9.2 Release Value Proposition provides overviews of the new features and enhancements planned for these applications for Release 9.2. This document offers customers a road map intended to help them assess the business benefits of upgrading to the 9.2 release while also helping them plan their IT projects and investments. (Link is to a My Oracle Support page, available to customers and partners.) Oracle continues to deliver enterprise-wide features that enhance our customer ownership experience and helps them run their businesses more efficiently and profitably. With the CRM 9.2 release, we continue to abide by this firm commitment we’ve made to our customers.

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  • What is a Relational Database Management System (RDBMS)?

    A Relational Database Management System (RDBMS)  can also be called a traditional database that uses a Structured Query Language (SQL) to provide access to stored data while insuring the integrity of the data. The data is stored in a collection of tables that is defined by relationships between data items. In addition, data permitted to be joined in new relationships. Traditional databases primarily process data through transactions called transaction processing. Transaction processing is the methodology of grouping related business operations based predefined business events. An example of this can be seen when a person attempts to purchase an item from an online e-tailor. The business must execute specific operations for a related  business event. In this case, a business must store the following information: Customer Info, Order Info, Order Item Info, Customer Payment Data, Payment Results, and Current Order Status. Example: Pseudo SQL Operations needed for processing an online e-tailor sale. Insert Customer into Customers Insert New Order into Orders Insert Each New Order Item into OrderItems Insert Customer Payment Info into PaymentInfo Insert Payment Processing Result into PaymentDetails Update Customer for Current Order Status Common Relational Database Management System Microsoft SQL Server Microsoft Access Oracle MySQL DB2 It is important to note that no current RDBMS has fully implemented all of the Relational Principles. Common RDBMS Traits Volatile Data Supports Transaction Processing Optimized for Updates and Simple Queries 

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  • A Better Way to Plan, Execute and Manage Enterprise Architecture

    - by JuergenKress
    IT Strategies from Oracle is an authorized library of guidelines and reference architectures that will help you better plan, execute, and manage your enterprise architecture and IT initiatives. The IT Strategies from Oracle library offers two types of best practice documents: practitioner guides containing pragmatic advice and approaches, and reference architectures containing the proven technology patterns to jumpstart your initiative. The IT Strategies from Oracle library can help you establish a reliable set of principles and standards to guide your use of Oracle technology. We will expand this library over time across all of Oracle's technologies. Today, you can access: Overview documents providing an introduction to all the resources available in the library and best practices maturity models Oracle Reference Architectures covering the application infrastructure foundation, management and monitoring, security, software engineering, service-oriented integration, service orientation, user interaction, engineered systems, and a master glossary. Enterprise Technology Strategies for Service-Oriented Architecture offering practitioner guides on creating a SOA roadmap, frameworks for governance, determining ROI, identifying services, software engineering, and white papers. Enterprise Technology Strategies for Event-Driven Architecture offering practitioner guides on creating an EDA roadmap and reference architectures on an EDA foundation and EDA infrastructure. Enterprise Technology Strategies for Business Process Management including practitioner guides on creating a BPM roadmap, business process engineering, governance, and reference architectures on a BPM foundation and BPM infrastructure. Enterprise Technology Strategies for Cloud Computing including reference architectures on a Cloud foundation and Cloud infrastructure. Enterprise Technology Strategies for Business Analytics includes a practitioner guide for creating a BA roadmap, and reference architectures for a BA foundation and BA infrastructure. Get the Oracle Enterprise Architecture content here. SOA & BPM Partner Community For regular information on Oracle SOA Suite become a member in the SOA & BPM Partner Community for registration please visit www.oracle.com/goto/emea/soa (OPN account required) If you need support with your account please contact the Oracle Partner Business Center. Blog Twitter LinkedIn Facebook Wiki Mix Forum Technorati Tags: Architecture,SOA Community,Oracle SOA,Oracle BPM,Community,OPN,Jürgen Kress

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  • Windows 7: Use VirtualStore as user for own benefit?

    - by mfn
    I've researched a bit and came to the understanding that VirtualStore is part of the new UAC feature in Vista/W7 which is the file system part of the transparent data redirection and redirects write access to folders like program files to C:\User\<username>\AppData\Local\VirtualStore\ in lack of applications respecting the LUA principles. Now I'm interested if that kind of transparent redirection can also be used as a power to the user. Here's an example which comes to my mind: I install any kind of software to e.g. D:\Whatever\ThisAndThisApp\ and I set up things that, after initial installation, any write access to this folder is transparently redirected to e.g. D:\MyOwnVirtualStore\Whatever\ThisAndThisApp\file_only_writable_here.txt. Is this thinking too far or can I actually use that power of VirtualStore as a user on Windows 7? I'm using the Professional version of that matters.

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  • What is Paging in memory management?

    - by Fasih Khatib
    I was just reading Operating System Principles by Silberschatz et al when I came across paging in memory management.I'm slightly confused about it. It states that Physical Memory(I assume it's RAM) is divided into frames, and logical memory is divided into pages. CPU generates logical addresses containing page number and an offset. This page number is used to retrieve the frame number from a page table which gives the base address so the physical address is calculated as base+offset. My question is: is the page table maintained for every process? I logically think that the answer would be yes as every process will need to map its own pages to frames. I may be wrong. Please clarify. Also: paging and segmentation(where 'holes' are created in memory) are two totally different techniques that are not used in combination. Correct?

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  • Input prediction and server re-simultaion

    - by Lope
    I have read plenty of articles about multiplayer principles and I have basic client-server system set up. There is however one thing I am not clear on. When player enters input, it is sent to the server and steps back in time to check if what should have happened at the time of that input and it resimulates the world again. So far everything's clear. All articles took shooting as an example, because it is easy to explain and it is pretty straightforward, but I believe movement is more complicated. Imagine following situation: 2 players move towards each other. A------<------B Player A stops halfway towards the collision point, but there is lag spike so the command does not arrive on the server for a second or so. Current state of the world on the server (and on the other clients as well) at the time when input arrives is this: [1]: -------AB------- The command arrives and we go back in time and re-simulate the world, the result is this: [2]: ---AB----------- Player A sees situation [2] which is correct, but the player is suddenly teleported from the position in [1] (center) to the position in [2]. Is this how this is supposed to work? Point of the client prediction is to give lagged player feeling that everything is smooth, not to ruin experience for other players. Alternative is to discard timestamp on the player's input and handle it when it arrives on the server without going back in time. This, however, creates even more severe problems for lagged player (even if he is lagging just a bit)

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  • Java desktop programmer starting to learn Android development: how different is it?

    - by Prog
    I'm a Java programmer. All of my experience is on desktop applications, using Swing for the GUI. I spend a lot of time studying OOP, I have decent understanding of OO concepts and I design and program by the OO approach. I'm thinking of starting to learn Android development soon, and I'm wondering how different it is from desktop development. Obviously the GUI libraries will be different (not Swing), but other than that, I want to know if there are significant differences. I will divide this question to two parts: Apart from the GUI libraries, am I still going to use the standard Java libarary I'm used to? Aka same data structues, same utility classes, etc.? If not, what are the main differences between the libraries I'm used to and the libraries I'll be using? How different is Android development in regard to OO design? Are all of the familiar principles, design patterns, techniques and best pratices just as valid and used? Or is OOP and OOD in Android development significantly different than OO in desktop development? To summarize: apart from GUI design, how different is Java Android development than Java desktop development?

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  • SPARC SuperCluster Papers

    - by user12616590
    Oracle has been publishing white papers that describe uses and characteristics of the SPARC SuperCluster product. Here are just a few: A Technical Overview of the Oracle SPARC SuperCluster T4-4SPARC SuperCluster T4-4 is a high performance, multi-purpose engineered system that has been designed, tested and integrated to run a wide array of enterprise applications. It is well suited for multi-tier enterprise applications with Web, database and application components. This 20-page paper discusses the components and technical characteristics of this product. SPARC SuperCluster T4-4 Platform Security Principles and CapabilitiesThe security capabilities designed into the SPARC SuperCluster, and architectural, deployment, and operational best practices for taking advantage of them. Consolidating Oracle E-Business Suite on Oracle’s SPARC SuperClusterThis Oracle Optimized Solution describes the implementation and use of SPARC SuperCluster as a consolidation platform for E-Business Suite in 30 pages. Oracle Optimized Solution for Oracle PeopleSoft Human Capital Management on SPARC SuperClusterThe Oracle Optimized Solution for PeopleSoft Human Capital Management on SPARC SuperCluster is the industry's only proven, tested, applications-to-disk solution that maintains excellence managing absences, optimizing collaborative activities, streamlining knowledge and honing processes; 31 pages. I hope you find some of those papers useful.

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  • Should I be concerned that I can't program very fast without Google? [closed]

    - by seth
    Possible Duplicate: Google is good or bad for programmer? I'm currently in college to be a software engineer, and one of the main principles taught to us is how to learn for ourselves, and how to search the web when we have a doubt. This leads to a proactive attitude - when I need something, I go get it. Recently, I started wondering how much development would I be able to do without internet access and the answer bugged me quite a bit. I know the concept of the languages and how to use them, but I was amazed by how "slow" things were without having the Google to help in the development. Most of the problems I have are related to specific syntax. For example, reading and writing to a file in Java. I have done this about a dozen times in my life, yet every time I need to do it, I end up googling "read file java" and refreshing my memory. I completely understand the code and fully understand what it does, but I am sure that without Google it would take me a few tries to get the code correct. Is this normal? Should I be worried and try to change something in my programming behaviour?

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  • How to make sure clients update their browser cache when my website is updated?

    - by user64204
    I am using the HTTP 1.1 Cache-Control header to implement client-side caching. Since I update my website only once a month I would like the CSS and JS files to be cached for 30 days with Cache-Control: max-age=2592000. The problem is that the 30-day period defined by Cache-Control doesn't coincide with the website update cycle, it starts from the moment the users visit the site and ends 30 days later, which means an update could occur in the meantime and users would be running with outdated content for a while, which could break the rendering of the website if for instance the HTML and CSS no longer match. How can I perform client-side caching of content for periods of several days but somehow get users to refresh their CSS/JS files after the website has been updated? One solution I could think of is that if website updates can be schedule, the max-age returned by the server could be decreased every day accordingly so that no matter when people visit the website, the end of caching period would coincide with the update of the website, but changing the server configuration every day goes against one of my sysadmin principles (once it's running, don't touch it).

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  • Repository query conditions, dependencies and DRY

    - by vFragosop
    To keep it simple, let's suppose an application which has Accounts and Users. Each account may have any number of users. There's also 3 consumers of UserRepository: An admin interface which may list all users Public front-end which may list all users An account authenticated API which should only list it's own users Assuming UserRepository is something like this: class UsersRepository extends DatabaseAbstraction { private function query() { return $this->database()->select('users.*'); } public function getAll() { return $this->query()->exec(); } // IMPORTANT: // Tons of other methods for searching, filtering, // joining of other tables, ordering and such... } Keeping in mind the comment above, and the necessity to abstract user querying conditions, How should I handle querying of users filtering by account_id? I can picture three possible roads: 1. Should I create an AccountUsersRepository? class AccountUsersRepository extends UserRepository { public function __construct(Account $account) { $this->account = $account; } private function query() { return parent::query() ->where('account_id', '=', $this->account->id); } } This has the advantage of reducing the duplication of UsersRepository methods, but doesn't quite fit into anything I've read about DDD so far (I'm rookie by the way) 2. Should I put it as a method on AccountsRepository? class AccountsRepository extends DatabaseAbstraction { public function getAccountUsers(Account $account) { return $this->database() ->select('users.*') ->where('account_id', '=', $account->id) ->exec(); } } This requires the duplication of all UserRepository methods and may need another UserQuery layer, that implements those querying logic on chainable way. 3. Should I query UserRepository from within my account entity? class Account extends Entity { public function getUsers() { return UserRepository::findByAccountId($this->id); } } This feels more like an aggregate root for me, but introduces dependency of UserRepository on Account entity, which may violate a few principles. 4. Or am I missing the point completely? Maybe there's an even better solution? Footnotes: Besides permissions being a Service concern, in my understanding, they shouldn't implement SQL query but leave that to repositories since those may not even be SQL driven.

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  • How do I develop database-utilizing application in an agile/test-driven-development way?

    - by user39019
    I want to add databases (traditional client/server RDBMS's like Mysql/Postgresql as opposed to NoSQL, or embedded databases) to my toolbox as a developer. I've been using SQLite for simpler projects with only 1 client, but now I want to do more complicated things (ie, db-backed web development). I usually like following agile and/or test-driven-development principles. I generally code in Perl or Python. Questions: How do I test my code such that each run of the test suite starts with a 'pristine' state? Do I run a separate instance of the database server every test? Do I use a temporary database? How do I design my tables/schema so that it is flexible with respect to changing requirements? Do I start with an ORM for my language? Or do I stick to manually coding SQL? One thing I don't find appealing is having to change more than one thing (say, the CREATE TABLE statement and associated crud statements) for one change, b/c that's error prone. On the other hand, I expect ORM's to be a low slower and harder to debug than raw SQL. What is the general strategy for migrating data between one version of the program and a newer one? Do I carefully write ALTER TABLE statements between each version, or do I dump the data and import fresh in the new version?

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  • How do you read from a file into an array of struct?

    - by Thomas.Winsnes
    I'm currently working on an assignment and this have had me stuck for hours. Can someone please help me point out why this isn't working for me? struct book { char title[25]; char author[50]; char subject[20]; int callNumber; char publisher[250]; char publishDate[11]; char location[20]; char status[11]; char type[12]; int circulationPeriod; int costOfBook; }; void PrintBookList(struct book **bookList) { int i; for(i = 0; i < sizeof(bookList); i++) { struct book newBook = *bookList[i]; printf("%s;%s;%s;%d;%s;%s;%s;%s;%s;%d;%d\n",newBook.title, newBook.author, newBook.subject, newBook.callNumber,newBook.publisher, newBook.publishDate, newBook.location, newBook.status, newBook.type,newBook.circulationPeriod, newBook.costOfBook); } } void GetBookList(struct book** bookList) { FILE* file = fopen("book.txt", "r"); struct book newBook[1024]; int i = 0; while(fscanf(file, "%s;%s;%s;%d;%s;%s;%s;%s;%s;%d;%d", &newBook[i].title, &newBook[i].author, &newBook[i].subject, &newBook[i].callNumber,&newBook[i].publisher, &newBook[i].publishDate, &newBook[i].location, &newBook[i].status, &newBook[i].type,&newBook[i].circulationPeriod, &newBook[i].costOfBook) != EOF) { bookList[i] = &newBook[i]; i++; } /*while(fscanf(file, "%s;%s;%s;%d;%s;%s;%s;%s;%s;%d;%d", &bookList[i].title, &bookList[i].author, &bookList[i].subject, &bookList[i].callNumber, &bookList[i].publisher, &bookList[i].publishDate, &bookList[i].location, &bookList[i].status, &bookList[i].type, &bookList[i].circulationPeriod, &bookList[i].costOfBook) != EOF) { i++; }*/ PrintBookList(bookList); fclose(file); } int main() { struct book *bookList[1024]; GetBookList(bookList); } I get no errors or warnings on compile it should print the content of the file, just like it is in the file. Like this: OperatingSystems Internals and Design principles;William.S;IT;741012759;Upper Saddle River;2009;QA7676063;Available;circulation;3;11200 Communication skills handbook;Summers.J;Accounting;771239216;Milton;2010;BF637C451;Available;circulation;3;7900 Business marketing management:B2B;Hutt.D;Management;741912319;Mason;2010;HF5415131;Available;circulation;3;1053 Patient education rehabilitation;Dreeben.O;Education;745121511;Sudbury;2010;CF5671A98;Available;reference;0;6895 Tomorrow's technology and you;Beekman.G;Science;764102174;Upper Saddle River;2009;QA76B41;Out;reserved;1;7825 Property & security: selected essay;Cathy.S;Law;750131231;Rozelle;2010;D4A3C56;Available;reference;0;20075 Introducing communication theory;Richard.W;IT;714789013;McGraw-Hill;2010;Q360W47;Available;circulation;3;12150 Maths for computing and information technology;Giannasi.F;Mathematics;729890537;Longman;Scientific;1995;QA769M35G;Available;reference;0;13500 Labor economics;George.J;Economics;715784761;McGraw-Hill;2010;HD4901B67;Available;circulation;3;7585 Human physiology:from cells to systems;Sherwood.L;Physiology;707558936;Cengage Learning;2010;QP345S32;Out;circulation;3;11135 bobs;thomas;IT;701000000;UC;1006;QA7548;Available;Circulation;7;5050 but when I run it, it outputs this: OperatingSystems;;;0;;;;;;0;0 Internals;;;0;;;;;;0;0 and;;;0;;;;;;0;0 Design;;;0;;;;;;0;0 principles;William.S;IT;741012759;Upper;41012759;Upper;;0;;;;;;0;0 Saddle;;;0;;;;;;0;0 River;2009;QA7676063;Available;circulation;3;11200;lable;circulation;3;11200;;0;;;;;;0;0 Communication;;;0;;;;;;0;0 Thanks in advance, you're a life saver

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  • Hibernate N+1 from select across multiple tables

    - by Marty Pitt
    Given the following hibernate query: String sql = "select distinct changeset " + "from Changeset changeset " + "join fetch changeset.changeEntries as changeEntry " + "join fetch changeEntry.repositoryEntity as repositoryEntity " + "join fetch repositoryEntity.project as project " + "join fetch changeset.author as changesetAuthor " + "where project.id = :projectID "; Why is this resulting in an N+1 problem? I expect this to generate the following single SQL statement (or something similar) select * from Changeset inner join changeEntry on changeset.id = changeEntry.changeset_id inner join repositoryEntity on changeEntry.repositoryentity_id = repositoryentity.id inner join project on repositoryentity.project_id = project.id where project.id = ? Instead, I see many many select statements firing. The data model here looks like this: I would like the full object graph returned from the Select statement in a single trip to the database, which is why I'm explicitly using "fetch" in the hibernate query. The Hibernate log statements are as follows: Hibernate: select distinct changeset0_.id as id2_0_, changeentr1_.id as id1_1_, repository2_.id as id9_2_, project3_.id as id6_3_, user4_.id as id7_4_, changeset0_.author_id as author5_2_0_, changeset0_.createDate as createDate2_0_, changeset0_.message as message2_0_, changeset0_.revision as revision2_0_, changeentr1_.changeType as changeType1_1_, changeentr1_.changeset_id as changeset4_1_1_, changeentr1_.diff as diff1_1_, changeentr1_.repositoryEntity_id as reposito5_1_1_, changeentr1_.repositoryEntityVersion_id as reposito6_1_1_, changeentr1_.sourceChangeEntry_id as sourceCh7_1_1_, changeentr1_.changeset_id as changeset4_0__, changeentr1_.id as id0__, repository2_.project_id as connecti6_9_2_, repository2_.name as name9_2_, repository2_.parent_id as parent7_9_2_, repository2_.path as path9_2_, repository2_.state as state9_2_, repository2_.type as type9_2_, project3_.projectName as connecti2_6_3_, project3_.driverName as driverName6_3_, project3_.isAnonymous as isAnonym4_6_3_, project3_.lastUpdatedRevision as lastUpda5_6_3_, project3_.password as password6_3_, project3_.url as url6_3_, project3_.username as username6_3_, user4_.username as username7_4_, user4_.email as email7_4_, user4_.name as name7_4_, user4_.password as password7_4_, user4_.principles as principles7_4_, user4_.userType as userType7_4_ from Changeset changeset0_ inner join ChangeEntry changeentr1_ on changeset0_.id=changeentr1_.changeset_id inner join RepositoryEntity repository2_ on changeentr1_.repositoryEntity_id=repository2_.id inner join project project3_ on repository2_.project_id=project3_.id inner join users user4_ on changeset0_.author_id=user4_.id where project3_.id=? order by changeset0_.revision desc Hibernate: select repository0_.id as id10_9_, repository0_.changeEntry_id as changeEn2_10_9_, repository0_.repositoryEntity_id as reposito3_10_9_, changeentr1_.id as id1_0_, changeentr1_.changeType as changeType1_0_, changeentr1_.changeset_id as changeset4_1_0_, changeentr1_.diff as diff1_0_, changeentr1_.repositoryEntity_id as reposito5_1_0_, changeentr1_.repositoryEntityVersion_id as reposito6_1_0_, changeentr1_.sourceChangeEntry_id as sourceCh7_1_0_, changeset2_.id as id2_1_, changeset2_.author_id as author5_2_1_, changeset2_.createDate as createDate2_1_, changeset2_.message as message2_1_, changeset2_.revision as revision2_1_, user3_.id as id7_2_, user3_.username as username7_2_, user3_.email as email7_2_, user3_.name as name7_2_, user3_.password as password7_2_, user3_.principles as principles7_2_, user3_.userType as userType7_2_, repository4_.id as id9_3_, repository4_.project_id as connecti6_9_3_, repository4_.name as name9_3_, repository4_.parent_id as parent7_9_3_, repository4_.path as path9_3_, repository4_.state as state9_3_, repository4_.type as type9_3_, project5_.id as id6_4_, project5_.projectName as connecti2_6_4_, project5_.driverName as driverName6_4_, project5_.isAnonymous as isAnonym4_6_4_, project5_.lastUpdatedRevision as lastUpda5_6_4_, project5_.password as password6_4_, project5_.url as url6_4_, project5_.username as username6_4_, repository6_.id as id9_5_, repository6_.project_id as connecti6_9_5_, repository6_.name as name9_5_, repository6_.parent_id as parent7_9_5_, repository6_.path as path9_5_, repository6_.state as state9_5_, repository6_.type as type9_5_, repository7_.id as id10_6_, repository7_.changeEntry_id as changeEn2_10_6_, repository7_.repositoryEntity_id as reposito3_10_6_, repository8_.id as id9_7_, repository8_.project_id as connecti6_9_7_, repository8_.name as name9_7_, repository8_.parent_id as parent7_9_7_, repository8_.path as path9_7_, repository8_.state as state9_7_, repository8_.type as type9_7_, changeentr9_.id as id1_8_, changeentr9_.changeType as changeType1_8_, changeentr9_.changeset_id as changeset4_1_8_, changeentr9_.diff as diff1_8_, changeentr9_.repositoryEntity_id as reposito5_1_8_, changeentr9_.repositoryEntityVersion_id as reposito6_1_8_, changeentr9_.sourceChangeEntry_id as sourceCh7_1_8_ from RepositoryEntityVersion repository0_ left outer join ChangeEntry changeentr1_ on repository0_.changeEntry_id=changeentr1_.id left outer join Changeset changeset2_ on changeentr1_.changeset_id=changeset2_.id left outer join users user3_ on changeset2_.author_id=user3_.id left outer join RepositoryEntity repository4_ on changeentr1_.repositoryEntity_id=repository4_.id left outer join project project5_ on repository4_.project_id=project5_.id left outer join RepositoryEntity repository6_ on repository4_.parent_id=repository6_.id left outer join RepositoryEntityVersion repository7_ on changeentr1_.repositoryEntityVersion_id=repository7_.id left outer join RepositoryEntity repository8_ on repository7_.repositoryEntity_id=repository8_.id left outer join ChangeEntry changeentr9_ on changeentr1_.sourceChangeEntry_id=changeentr9_.id where repository0_.id=? The 2nd one is repeated many times - for a result set of 17 objects, the 2nd statement executed 521 times. I suspect this is as a result of the parent/child relationship in the RepositoryEntity object. For the purposes of this select, I actually only require the parent object fetched. Any suggestions?

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  • Inside the Concurrent Collections: ConcurrentDictionary

    - by Simon Cooper
    Using locks to implement a thread-safe collection is rather like using a sledgehammer - unsubtle, easy to understand, and tends to make any other tool redundant. Unlike the previous two collections I looked at, ConcurrentStack and ConcurrentQueue, ConcurrentDictionary uses locks quite heavily. However, it is careful to wield locks only where necessary to ensure that concurrency is maximised. This will, by necessity, be a higher-level look than my other posts in this series, as there is quite a lot of code and logic in ConcurrentDictionary. Therefore, I do recommend that you have ConcurrentDictionary open in a decompiler to have a look at all the details that I skip over. The problem with locks There's several things to bear in mind when using locks, as encapsulated by the lock keyword in C# and the System.Threading.Monitor class in .NET (if you're unsure as to what lock does in C#, I briefly covered it in my first post in the series): Locks block threads The most obvious problem is that threads waiting on a lock can't do any work at all. No preparatory work, no 'optimistic' work like in ConcurrentQueue and ConcurrentStack, nothing. It sits there, waiting to be unblocked. This is bad if you're trying to maximise concurrency. Locks are slow Whereas most of the methods on the Interlocked class can be compiled down to a single CPU instruction, ensuring atomicity at the hardware level, taking out a lock requires some heavy lifting by the CLR and the operating system. There's quite a bit of work required to take out a lock, block other threads, and wake them up again. If locks are used heavily, this impacts performance. Deadlocks When using locks there's always the possibility of a deadlock - two threads, each holding a lock, each trying to aquire the other's lock. Fortunately, this can be avoided with careful programming and structured lock-taking, as we'll see. So, it's important to minimise where locks are used to maximise the concurrency and performance of the collection. Implementation As you might expect, ConcurrentDictionary is similar in basic implementation to the non-concurrent Dictionary, which I studied in a previous post. I'll be using some concepts introduced there, so I recommend you have a quick read of it. So, if you were implementing a thread-safe dictionary, what would you do? The naive implementation is to simply have a single lock around all methods accessing the dictionary. This would work, but doesn't allow much concurrency. Fortunately, the bucketing used by Dictionary allows a simple but effective improvement to this - one lock per bucket. This allows different threads modifying different buckets to do so in parallel. Any thread making changes to the contents of a bucket takes the lock for that bucket, ensuring those changes are thread-safe. The method that maps each bucket to a lock is the GetBucketAndLockNo method: private void GetBucketAndLockNo( int hashcode, out int bucketNo, out int lockNo, int bucketCount) { // the bucket number is the hashcode (without the initial sign bit) // modulo the number of buckets bucketNo = (hashcode & 0x7fffffff) % bucketCount; // and the lock number is the bucket number modulo the number of locks lockNo = bucketNo % m_locks.Length; } However, this does require some changes to how the buckets are implemented. The 'implicit' linked list within a single backing array used by the non-concurrent Dictionary adds a dependency between separate buckets, as every bucket uses the same backing array. Instead, ConcurrentDictionary uses a strict linked list on each bucket: This ensures that each bucket is entirely separate from all other buckets; adding or removing an item from a bucket is independent to any changes to other buckets. Modifying the dictionary All the operations on the dictionary follow the same basic pattern: void AlterBucket(TKey key, ...) { int bucketNo, lockNo; 1: GetBucketAndLockNo( key.GetHashCode(), out bucketNo, out lockNo, m_buckets.Length); 2: lock (m_locks[lockNo]) { 3: Node headNode = m_buckets[bucketNo]; 4: Mutate the node linked list as appropriate } } For example, when adding another entry to the dictionary, you would iterate through the linked list to check whether the key exists already, and add the new entry as the head node. When removing items, you would find the entry to remove (if it exists), and remove the node from the linked list. Adding, updating, and removing items all follow this pattern. Performance issues There is a problem we have to address at this point. If the number of buckets in the dictionary is fixed in the constructor, then the performance will degrade from O(1) to O(n) when a large number of items are added to the dictionary. As more and more items get added to the linked lists in each bucket, the lookup operations will spend most of their time traversing a linear linked list. To fix this, the buckets array has to be resized once the number of items in each bucket has gone over a certain limit. (In ConcurrentDictionary this limit is when the size of the largest bucket is greater than the number of buckets for each lock. This check is done at the end of the TryAddInternal method.) Resizing the bucket array and re-hashing everything affects every bucket in the collection. Therefore, this operation needs to take out every lock in the collection. Taking out mutiple locks at once inevitably summons the spectre of the deadlock; two threads each hold a lock, and each trying to acquire the other lock. How can we eliminate this? Simple - ensure that threads never try to 'swap' locks in this fashion. When taking out multiple locks, always take them out in the same order, and always take out all the locks you need before starting to release them. In ConcurrentDictionary, this is controlled by the AcquireLocks, AcquireAllLocks and ReleaseLocks methods. Locks are always taken out and released in the order they are in the m_locks array, and locks are all released right at the end of the method in a finally block. At this point, it's worth pointing out that the locks array is never re-assigned, even when the buckets array is increased in size. The number of locks is fixed in the constructor by the concurrencyLevel parameter. This simplifies programming the locks; you don't have to check if the locks array has changed or been re-assigned before taking out a lock object. And you can be sure that when a thread takes out a lock, another thread isn't going to re-assign the lock array. This would create a new series of lock objects, thus allowing another thread to ignore the existing locks (and any threads controlling them), breaking thread-safety. Consequences of growing the array Just because we're using locks doesn't mean that race conditions aren't a problem. We can see this by looking at the GrowTable method. The operation of this method can be boiled down to: private void GrowTable(Node[] buckets) { try { 1: Acquire first lock in the locks array // this causes any other thread trying to take out // all the locks to block because the first lock in the array // is always the one taken out first // check if another thread has already resized the buckets array // while we were waiting to acquire the first lock 2: if (buckets != m_buckets) return; 3: Calculate the new size of the backing array 4: Node[] array = new array[size]; 5: Acquire all the remaining locks 6: Re-hash the contents of the existing buckets into array 7: m_buckets = array; } finally { 8: Release all locks } } As you can see, there's already a check for a race condition at step 2, for the case when the GrowTable method is called twice in quick succession on two separate threads. One will successfully resize the buckets array (blocking the second in the meantime), when the second thread is unblocked it'll see that the array has already been resized & exit without doing anything. There is another case we need to consider; looking back at the AlterBucket method above, consider the following situation: Thread 1 calls AlterBucket; step 1 is executed to get the bucket and lock numbers. Thread 2 calls GrowTable and executes steps 1-5; thread 1 is blocked when it tries to take out the lock in step 2. Thread 2 re-hashes everything, re-assigns the buckets array, and releases all the locks (steps 6-8). Thread 1 is unblocked and continues executing, but the calculated bucket and lock numbers are no longer valid. Between calculating the correct bucket and lock number and taking out the lock, another thread has changed where everything is. Not exactly thread-safe. Well, a similar problem was solved in ConcurrentStack and ConcurrentQueue by storing a local copy of the state, doing the necessary calculations, then checking if that state is still valid. We can use a similar idea here: void AlterBucket(TKey key, ...) { while (true) { Node[] buckets = m_buckets; int bucketNo, lockNo; GetBucketAndLockNo( key.GetHashCode(), out bucketNo, out lockNo, buckets.Length); lock (m_locks[lockNo]) { // if the state has changed, go back to the start if (buckets != m_buckets) continue; Node headNode = m_buckets[bucketNo]; Mutate the node linked list as appropriate } break; } } TryGetValue and GetEnumerator And so, finally, we get onto TryGetValue and GetEnumerator. I've left these to the end because, well, they don't actually use any locks. How can this be? Whenever you change a bucket, you need to take out the corresponding lock, yes? Indeed you do. However, it is important to note that TryGetValue and GetEnumerator don't actually change anything. Just as immutable objects are, by definition, thread-safe, read-only operations don't need to take out a lock because they don't change anything. All lockless methods can happily iterate through the buckets and linked lists without worrying about locking anything. However, this does put restrictions on how the other methods operate. Because there could be another thread in the middle of reading the dictionary at any time (even if a lock is taken out), the dictionary has to be in a valid state at all times. Every change to state has to be made visible to other threads in a single atomic operation (all relevant variables are marked volatile to help with this). This restriction ensures that whatever the reading threads are doing, they never read the dictionary in an invalid state (eg items that should be in the collection temporarily removed from the linked list, or reading a node that has had it's key & value removed before the node itself has been removed from the linked list). Fortunately, all the operations needed to change the dictionary can be done in that way. Bucket resizes are made visible when the new array is assigned back to the m_buckets variable. Any additions or modifications to a node are done by creating a new node, then splicing it into the existing list using a single variable assignment. Node removals are simply done by re-assigning the node's m_next pointer. Because the dictionary can be changed by another thread during execution of the lockless methods, the GetEnumerator method is liable to return dirty reads - changes made to the dictionary after GetEnumerator was called, but before the enumeration got to that point in the dictionary. It's worth listing at this point which methods are lockless, and which take out all the locks in the dictionary to ensure they get a consistent view of the dictionary: Lockless: TryGetValue GetEnumerator The indexer getter ContainsKey Takes out every lock (lockfull?): Count IsEmpty Keys Values CopyTo ToArray Concurrent principles That covers the overall implementation of ConcurrentDictionary. I haven't even begun to scratch the surface of this sophisticated collection. That I leave to you. However, we've looked at enough to be able to extract some useful principles for concurrent programming: Partitioning When using locks, the work is partitioned into independant chunks, each with its own lock. Each partition can then be modified concurrently to other partitions. Ordered lock-taking When a method does need to control the entire collection, locks are taken and released in a fixed order to prevent deadlocks. Lockless reads Read operations that don't care about dirty reads don't take out any lock; the rest of the collection is implemented so that any reading thread always has a consistent view of the collection. That leads us to the final collection in this little series - ConcurrentBag. Lacking a non-concurrent analogy, it is quite different to any other collection in the class libraries. Prepare your thinking hats!

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