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  • How to write my own global lock / unlock functions for PostgreSQL

    - by rafalmag
    I have postgresql (in perlu) function getTravelTime(integer, timestamp), which tries to select data for specified ID and timestamp. If there are no data or if data is old, it downloads them from external server (downloading time ~300ms). Multiple process use this database and this function. There is an error when two process do not find data and download them and try to do an insert to travel_time table (id and timestamp pair have to be unique). I thought about locks. Locking whole table would block all processes and allow only one to proceed. I need to lock only on id and timestamp. pg_advisory_lock seems to lock only in "current session". But my processes uses their own sessions. I tried to write my own lock/unlock functions. Am I doing it right? I use active waiting, how can I omit this? Maybe there is a way to use pg_advisory_lock() as global lock? My code: CREATE TABLE travel_time_locks ( id_key integer NOT NULL, time_key timestamp without time zone NOT NULL, UNIQUE (id_key, time_key) ); ------------ -- Function: mylock(integer, timestamp) DROP FUNCTION IF EXISTS mylock(integer, timestamp) CASCADE; -- Usage: SELECT mylock(1, '2010-03-28T19:45'); -- function tries to do a global lock similar to pg_advisory_lock(key, key) CREATE OR REPLACE FUNCTION mylock(id_input integer, time_input timestamp) RETURNS void AS $BODY$ DECLARE rows int; BEGIN LOOP BEGIN -- active waiting here !!!! :( INSERT INTO travel_time_locks (id_key, time_key) VALUES (id_input, time_input); EXCEPTION WHEN unique_violation THEN CONTINUE; END; EXIT; END LOOP; END; $BODY$ LANGUAGE 'plpgsql' VOLATILE COST 1; ------------ -- Function: myunlock(integer, timestamp) DROP FUNCTION IF EXISTS myunlock(integer, timestamp) CASCADE; -- Usage: SELECT myunlock(1, '2010-03-28T19:45'); -- function tries to do a global unlock similar to pg_advisory_unlock(key, key) CREATE OR REPLACE FUNCTION myunlock(id_input integer, time_input timestamp) RETURNS integer AS $BODY$ DECLARE BEGIN DELETE FROM ONLY travel_time_locks WHERE id_key=id_input AND time_key=time_input; RETURN 1; END; $BODY$ LANGUAGE 'plpgsql' VOLATILE COST 1;

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  • Realtime MySQL search results on an advanced search page

    - by Andrew Heath
    I'm a hobbyist, and started learning PHP last September solely to build a hobby website that I had always wished and dreamed another more competent person might make. I enjoy programming, but I have little free time and enjoy a wide range of other interests and activities. I feel learning PHP alone can probably allow me to create 98% of the desired features for my site, but that last 2% is awfully appealing: The most powerful tool of the site is an advanced search page that picks through a 1000+ record game scenario database. Users can data-mine to tremendous depths - this advanced page has upwards of 50 different potential variables. It's designed to allow the hardcore user to search on almost any possible combination of data in our database and it works well. Those who aren't interested in wading through the sea of options may use the Basic Search, which is comprised of the most popular parts of the Advanced search. Because the advanced search is so comprehensive, and because the database is rather small (less than 1,200 potential hits maximum), with each variable you choose to include the likelihood of getting any qualifying results at all drops dramatically. In my fantasy land where I can wield AJAX as if it were Excalibur, my users would have a realtime Total Results counter in the corner of their screen as they used this page, which would automatically update its query structure and report how many results will be displayed with the addition of each variable. In this way it would be effortless to know just how many variables are enough, and when you've gone and added one that zeroes out the results set. A somewhat similar implementation, at least visually, would be the Subtotal sidebar when building a new custom computer on IBuyPower.com For those of you actually still reading this, my question is really rather simple: Given the time & ability constraints outlined above, would I be able to learn just enough AJAX (or whatever) needed to pull this one feature off without too much trouble? would I be able to more or less drop-in a pre-written code snippet and tweak to fit? or should I consider opening my code up to a trusted & capable individual in the future for this implementation? (assuming I can find one...) Thank you.

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  • Idiomatic scheme and generic programming, why only on numbers ?

    - by Skeptic
    Hi, In Scheme, procedures like +, -, *, / works on different types of numbers, but we don't much see any other generic procedures. For example, length works only on list so that vector-length and string-length are needed. I guess it comes from the fact that the language doesn't really offer any mechanism for defining generic procedure (except cond of course) like "type classes" in Haskell or a standardized object system. Is there an idiomatic scheme way to handle generic procedures that I'm not aware of ?

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  • For kernel/OS is C still it

    - by Recursion
    I like operating systems and would eventually like to become a OS developer mostly working on kernels. In the future will C still be the language of choice and what else should I be trying to learn.

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  • iPhone - extending a class delegate

    - by Digital Robot
    OK, I know how to create a class extension, using something like that: on .h @interface UIButton (myExtensionName) // my extended methods @end and then on .m @implementation UIButton (myExtensionName) // my implementations @end But how do I declare the extended delegates I may create? If this was a normal class I would do @protocol myExtensionName <NSObject> // my delegate declarations @end but how do I do that on a class extension? thanks

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  • Are there any LIDIL documentation?

    - by Eskat0n
    As a part of my univercity work I have to write a program in LIDIL and describe its operators and functions but I can't find any documentation on this language. Naturally I've tried to google but nothing found on the first 40 pages of search results so I gave up. SO is my last hope. Any trace of info about LIDIL is fine for me.

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  • GAE update different fields of the same entity

    - by bach
    Hi, UserA and UserB are changing objectA.filedA objectA.filedB respectively and at the same time. Because they are not changing the same field one might think that there are no overlaps. Is that true? or the implementation of pm.makePersistnace() actually override the whole object... good to know...

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  • How to read expected child nodes of a given node from schema in PHP?

    - by MartyIX
    I was wondering if there's an implementation of a XML schema reader that for an arbitrary node in XML schema provides list of nodes which are supposed to be present as child nodes of given node, restrictions on nodes and so on. I'm planning to program it for my purposes but I would like to know if it isn't solved somewhere. I really need only a small subset what I described above. Thanks for tips!

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  • Use Shakespeare-text and external file

    - by Adam
    How can I convert the below example to use an external file instead of the embedded lazy text quasi quotes? {-# LANGUAGE QuasiQuotes, OverloadedStrings #-} import Text.Shakespeare.Text import qualified Data.Text.Lazy.IO as TLIO import Data.Text (Text) import Control.Monad (forM_) data Item = Item { itemName :: Text , itemQty :: Int } items :: [Item] items = [ Item "apples" 5 , Item "bananas" 10 ] main :: IO () main = forM_ items $ \item -> TLIO.putStrLn [lt|You have #{show $ itemQty item} #{itemName item}.|] This is from the yesod online book.

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  • Lambda functions

    - by Oden
    Hey, I'm really interested in the way of using lambda functions. Does it make sense to use them in a high-level programming language? If yes, why? Is this really just a function embedded in a function (like this) or is there more behind?

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  • writing boot sector code

    - by JGC
    hi I want to write a code which put something in bootsector but when I run the assembly 8086 code which does this purpose, nothing happens. does any one know what can I do or does any one has code (in any language) which answer my need?

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  • How to infer the type of a derived class in base class?

    - by enzi
    I want to create a method that allows me to change arbitrary properties of classes that derive from my base class, the result should look like this: SetPropertyValue("size.height", 50); – where size is a property of my derived class and height is a property of size. I'm almost done with my implementation but there's one final obstacle that I want to solve before moving on, to describe this I will first have to explain my implementation a bit: Properties that can be modified are decorated with an attribute There's a method in my base class that searches for all derived classes and their decorated properties For each property I generate a "property modifier", a class that contains 2 delegates: one to set and one to get the value of the property. Property Modifiers are stored in a dictionary, with the name of the property as key In my base class, there is another dictionary that contains all property-modifier-dictionaries, with the Type of the respective class as key. What the SetPropertyValue method does is this: Get the correct property-modifier-dictionary, using the concrete type of the derived class (<- yet to solve) Get the property modifier of the property to change (e.g. of the property size) Use the get or set delegate to modify the property's value Some example code to clarify further: private static Dictionary<RuntimeTypeHandle, object> EditableTypes; //property-modifier-dictionary protected void SetPropertyValue<T>(EditablePropertyMap<T> map, string property, object value) { var property = map[property]; // get the property modifier property.Set((T)this, value); // use the set delegate (encapsulated in a method) } In the above code, T is the Type of the actual (derived) class. I need this type for the get/set delegates. The problem is how to get the EditablePropertyMap<T> when I don't know what T is. My current (ugly) solution is to pass the map in an overriden virtual method in the derived class: public override void SetPropertyValue(string property, object value) { base.SetPropertyValue((EditablePropertyMap<ExampleType>)EditableTypes[typeof(ExampleType)], property, value); } What this does is: get the correct dictionary containing the property modifiers of this class using the class's type, cast it to the appropiate type and pass it to the SetPropertyValue method. I want to get rid of the SetPropertyValue method in my derived class (since there are a lot of derived classes), but don't know yet how to accomplish that. I cannot just make a virtual GetEditablePropertyMap<T> method because I cannot infer a concrete type for T then. I also cannot acces my dictionary directly with a type and retrieve an EditablePropertyMap<T> from it because I cannot cast to it from object in the base class, since again I do not know T. I found some neat tricks to infere types (e.g. by adding a dummy T parameter), but cannot apply them to my specific problem. I'd highly appreciate any suggestions you may have for me.

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  • Manipulate data in the DB query or in the code

    - by DrDro
    How do you decide on which side you perform your data manipulation when you can either do it in the code or in the query ? When you need to display a date in a specific format for example. Do you retrieve the desired format directly in the sql query or you retrieve the date then format it through the code ? What helps you to decide : performance, best practice, preference in SQL vs the code language, complexity of the task... ?

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  • Define a default .properties file in JAVA

    - by bnabilos
    Hello, I have 2 .properties files in my java project and I want to define one of them as default file to be used when the language of the operating system is different from the 2 languages already defined. Can you tell me please what should I add to my code to make that possible? Thank you

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  • What libraries provide cross-platform 3D and P2P support?

    - by uckelman
    I'm trying to find a constellation of libraries which, taken together, meet the following requirements: Smooth scaling, rotation, panning (in two dimensions). I'll have a large bitmap (or SVG, in some cases), maybe up to 10000x10000 pixels, which serves as map, with some middling number of small bitmaps (or, again, possibly SVG) that can be dragged around over it. I need to be able to zoom, rotate, and pan this scene; however, the view will always be normal to (i.e., looking head-on at) the large bitmap, so I'm not really using the depth dimension. Peer-to-peer. I'd like for multiple users to be able to connect in order to share one of the scenes mentioned above, preferably peer-to-peer, without much configuration by the user. I'm intending to have a server running for cases where users are unable to connect P2P; I'd like to have the failover happen automatically, or possibly have some way of promoting clients who are capable to be servers themselves. Synchronization. Once a user has started dragging one of the small bitmaps (a piece), no other user should be able to drag that piece until the drag stops. I haven't thought of exactly how to do this---there might be a simple solution, or this kind of synchronization might be something that a library provides. Cross(ish)-platform. I need to be able to run on Linux, Windows, and Mac OS. It would be nice to also be able to run on tablets. Having mostly the same code for all platforms is a plus, but not absolutely necessary. (L)GPL compatible. I'm planning to release under the LGPL or GPL, preferably the latter, so I need libraries which have compatible licenses. I'm not set on any particular language, I'd like to use the library or libraries which make the work easiest, though my preference is to work in at most two languages for the project. (The Model could potentially be in one language and the View in another, so they could talk to each other via some protocol I define, if that would get me a better selection of libraries to use.) Can anyone offer suggestions for what to use?

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  • What is the use of mysql_rollback() function in PHP?

    - by OM The Eternity
    What is the use of mysql_rollback() function in PHP? How is it used? please explain me with some example, PS: Please Do not give me link to the php.net or, mysql site, I dont need AUTHOR language to understand it, I need the Developer way to understand... I hope u understand... Thanks in advance. UPDATE if i have Updated something in a tanle can i Reset the previous value using this function?

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  • Safe Cross Thread Signals/Slot C++

    - by JP
    It seem that the only implementation that provide Safe Cross-Thread Signals for both the Signal class and what's being called in the slot is QT. (Maybe I'm wrong?). But I cannot use QT in the project I'm doing. So how could I provide safe Slots call from a different thread (Using Boost::signals2 for example)? Are mutex inside the slot the only way? I think signals2 protect themself but not what's being done inside the slot. Thanks

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  • Standard library function in R for finding the mode?

    - by Nick
    In statistical language R, mean() and median() are standard functions which do what you'd expect. mode() tells you the internal storage mode of the R object, not the value that occurs the most in its argument. But surely there is a standard library function that implements mode for a vector (or list).

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  • What can you do in C without "std" includes? Are they part of "C," or just libraries?

    - by Chris Cooper
    I apologize if this is a subjective or repeated question. It's sort of awkward to search for, so I wasn't sure what terms to include. What I'd like to know is what the basic foundation tools/functions are in C when you don't include standard libraries like stdio and stdlib. What could I do if there's no printf(), fopen(), etc? Also, are those libraries technically part of the "C" language, or are they just very useful and effectively essential libraries?

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