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  • How should I implement simple caches with concurrency on Redis?

    - by solublefish
    Background I have a 2-tier web service - just my app server and an RDBMS. I want to move to a pool of identical app servers behind a load balancer. I currently cache a bunch of objects in-process. I hope to move them to a shared Redis. I have a dozen or so caches of simple, small-sized business objects. For example, I have a set of Foos. Each Foo has a unique FooId and an OwnerId. One "owner" may own multiple Foos. In a traditional RDBMS this is just a table with an index on the PK FooId and one on OwnerId. I'm caching this in one process simply: Dictionary<int,Foo> _cacheFooById; Dictionary<int,HashSet<int>> _indexFooIdsByOwnerId; Reads come straight from here, and writes go here and to the RDBMS. I usually have this invariant: "For a given group [say by OwnerId], the whole group is in cache or none of it is." So when I cache miss on a Foo, I pull that Foo and all the owner's other Foos from the RDBMS. Updates make sure to keep the index up to date and respect the invariant. When an owner calls GetMyFoos I never have to worry that some are cached and some aren't. What I did already The first/simplest answer seems to be to use plain ol' SET and GET with a composite key and json value: SET( "ServiceCache:Foo:" + theFoo.Id, JsonSerialize(theFoo)); I later decided I liked: HSET( "ServiceCache:Foo", theFoo.FooId, JsonSerialize(theFoo)); That lets me get all the values in one cache as HVALS. It also felt right - I'm literally moving hashtables to Redis, so perhaps my top-level items should be hashes. This works to first order. If my high-level code is like: UpdateCache(myFoo); AddToIndex(myFoo); That translates into: HSET ("ServiceCache:Foo", theFoo.FooId, JsonSerialize(theFoo)); var myFoos = JsonDeserialize( HGET ("ServiceCache:FooIndex", theFoo.OwnerId) ); myFoos.Add(theFoo.OwnerId); HSET ("ServiceCache:FooIndex", theFoo.OwnerId, JsonSerialize(myFoos)); However, this is broken in two ways. Two concurrent operations can read/modify/write at the same time. The latter "wins" the final HSET and the former's index update is lost. Another operation could read the index in between the first and second lines. It would miss a Foo that it should find. So how do I index properly? I think I could use a Redis set instead of a json-encoded value for the index. That would solve part of the problem since the "add-to-index-if-not-already-present" would be atomic. I also read about using MULTI as a "transaction" but it doesn't seem like it does what I want. Am I right that I can't really MULTI; HGET; {update}; HSET; EXEC since it doesn't even do the HGET before I issue the EXEC? I also read about using WATCH and MULTI for optimistic concurrency, then retrying on failure. But WATCH only works on top-level keys. So it's back to SET/GET instead of HSET/HGET. And now I need a new index-like-thing to support getting all the values in a given cache. If I understand it right, I can combine all these things to do the job. Something like: while(!succeeded) { WATCH( "ServiceCache:Foo:" + theFoo.FooId ); WATCH( "ServiceCache:FooIndexByOwner:" + theFoo.OwnerId ); WATCH( "ServiceCache:FooIndexAll" ); MULTI(); SET ("ServiceCache:Foo:" + theFoo.FooId, JsonSerialize(theFoo)); SADD ("ServiceCache:FooIndexByOwner:" + theFoo.OwnerId, theFoo.FooId); SADD ("ServiceCache:FooIndexAll", theFoo.FooId); EXEC(); //TODO somehow set succeeded properly } Finally I'd have to translate this pseudocode into real code depending how my client library uses WATCH/MULTI/EXEC; it looks like they need some sort of context to hook them together. All in all this seems like a lot of complexity for what has to be a very common case; I can't help but think there's a better, smarter, Redis-ish way to do things that I'm just not seeing. How do I lock properly? Even if I had no indexes, there's still a (probably rare) race condition. A: HGET - cache miss B: HGET - cache miss A: SELECT B: SELECT A: HSET C: HGET - cache hit C: UPDATE C: HSET B: HSET ** this is stale data that's clobbering C's update. Note that C could just be a really-fast A. Again I think WATCH, MULTI, retry would work, but... ick. I know in some places people use special Redis keys as locks for other objects. Is that a reasonable approach here? Should those be top-level keys like ServiceCache:FooLocks:{Id} or ServiceCache:Locks:Foo:{Id}? Or make a separate hash for them - ServiceCache:Locks with subkeys Foo:{Id}, or ServiceCache:Locks:Foo with subkeys {Id} ? How would I work around abandoned locks, say if a transaction (or a whole server) crashes while "holding" the lock?

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  • Assign TableView column binding to a specific core-data object in to-many related entity based on se

    - by snown27
    How would I assign a column of a TableView to be a specific entry in a Core Data entity that is linked to the NSArrayController via a to-many relationship? For example Entity: MovieEntity Attributes: title(NSString), releaseDate(NSDate) Relationship: posters<-->> PosterEntity Entity:PosterEntity Attributes: imageLocation(NSURL), default(BOOL) Relationships: movie<<--> MovieEntity So I have a three column table that I want to display the Poster, Title, and Release Date attributes in, but as one movie could potentially have several different style's of posters how do I get the poster column to only show the one that's marked default? Since the rest of the table is defined in Interface Builder I would prefer to keep it that way for the sake of keeping the code as clean as possible, but if this can only be done programmatically then I'm all ears. I just wanted to give preference in case there's more than one way to skin a cat, so to speak.

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  • When does logic belong in the Business Object/Entity, and when does it belong in a Service?

    - by Casey
    In trying to understand Domain Driven Design I keep returning to a question that I can't seem to definitively answer. How do you determine what logic belongs to a Domain entity, and what logic belongs to a Domain Service? Example: We have an Order class for an online store. This class is an entity and an aggregate root (it contains OrderItems). Public Class Order:IOrder { Private List<IOrderItem> OrderItems Public Order(List<IOrderItem>) { OrderItems = List<IOrderItem> } Public Decimal CalculateTotalItemWeight() //This logic seems to belong in the entity. { Decimal TotalWeight = 0 foreach(IOrderItem OrderItem in OrderItems) { TotalWeight += OrderItem.Weight } return TotalWeight } } I think most people would agree that CalculateTotalItemWeight belongs on the entity. However, at some point we have to ship this order to the customer. To accomplish this we need to do two things: 1) Determine the postage rate necessary to ship this order. 2) Print a shipping label after determining the postage rate. Both of these actions will require dependencies that are outside the Order entity, such as an external webservice to retrieve postage rates. How should we accomplish these two things? I see a few options: 1) Code the logic directly in the domain entity, like CalculateTotalItemWeight. We then call: Order.GetPostageRate Order.PrintLabel 2) Put the logic in a service that accepts IOrder. We then call: PostageService.GetPostageRate(Order) PrintService.PrintLabel(Order) 3) Create a class for each action that operates on an Order, and pass an instance of that class to the Order through Constructor Injection (this is a variation of option 1 but allows reuse of the RateRetriever and LabelPrinter classes): Public Class Order:IOrder { Private List<IOrderItem> OrderItems Private RateRetriever _Retriever Private LabelPrinter _Printer Public Order(List<IOrderItem>, RateRetriever Retriever, LabelPrinter Printer) { OrderItems = List<IOrderItem> _Retriever = Retriever _Printer = Printer } Public Decimal GetPostageRate { _Retriever.GetPostageRate(this) } Public void PrintLabel { _Printer.PrintLabel(this) } } Which one of these methods do you choose for this logic, if any? What is the reasoning behind your choice? Most importantly, is there a set of guidelines that led you to your choice?

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  • Programmatically determine whether to describe an object with "a" or "an"?

    - by MarathonStudios
    I have a database of nouns (ex "house", "exclamation point", "apple") that I need to output and describe in my application. It's hard to put together a natural-sounding sentence to describe an item without using "a" or "an" - "a house is BIG", "an exclamation point is SMALL", etc. Is there any function, library, or hack i can use in PHP to determine whether it is more appropriate to describe any given noun with A or AN?

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  • How to get maximum/largest value from json object with Android?

    - by blankon91
    I've json string like this: {"GetReportResult":[ {"bulan":"4","total":"2448","type":"CHEESE1K","uang":"8847823"},{"bulan":"4","total":"572476","type":"ESL","uang":"5863408410"},{"bulan":"4","total":"46008","type":"ESL500ML","uang":"234498301"},{"bulan":"4","total":"228985","type":"UHT","uang":"1367172990"},{"bulan":"4","total":"40995","type":"WHP","uang":"235750452"},{"bulan":"5","total":"5703","type":"CHEESE1K","uang":"134929306"},{"bulan":"5","total":"648663","type":"ESL","uang":"6645764498"},{"bulan":"5","total":"49305","type":"ESL500ML","uang":"266817346"},{"bulan":"5","total":"287867","type":"UHT","uang":"1446897805"},{"bulan":"5","total":"51958","type":"WHP","uang":"631994613"},{"bulan":"6","total":"4390","type":"CHEESE1K","uang":"104527773"},{"bulan":"6","total":"443335","type":"ESL","uang":"4540123082"},{"bulan":"6","total":"28462","type":"ESL500ML","uang":"148290912"},{"bulan":"6","total":"213250","type":"UHT","uang":"1197646870"},{"bulan":"6","total":"27049","type":"WHP","uang":"189802525"} ]} I want to get maximum/largest value of bulan, which is 6. How to do that on android?

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  • C++ Filling an 1D array to represent a n-dimensional object based on a straight line segment

    - by Ben
    I'm struggling to find a good way to put this question but here goes. I'm making a system that uses a 1D array implemented as double * parts_ = new double[some_variable];. I want to use this to hold co-ordinates for a particle system that can run in various dimensions. What I want to be able to do is write a generic fill algorithm for filling this in n-dimensions with a common increment in all direction to a variable size. Examples will serve best I think. Consider the case where the number of particles stored by the array is 4 In 1D this produces 4 elements in the array because each particle only has one co-ordinate. 1D: {0, 25, 50, 75}; In 2D this produces 8 elements in the array because each particle has two co-ordinates.. 2D: {0, 0, 0, 25, 25, 0, 25, 25} In 3D this produces 12 elements in the array because each particle now has three co-ordinates {0, 0, 0, 0, 0, 25, 0, 0, 50, ... } These examples are still not quite accurate, but they hopefully will suffice. The way I would do this normally for two dimensions: int i = 0; for(int x = 0; x < parts_size_ / dims_ / dims_ * 25; x += 25) { for(int y = 0; y < parts_size_ / dims_ / dims_ * 25; y += 25) { parts_[i] = x; parts_[i+1] = y; i+=2; // Indentation hates me today .< How can I implement this for n-dimensions where 25 can be any number? The straight line part is because it seems to me logical that a line is a somewhat regular shape in 1D, as is a square in 2D, and a cube in 3D. It seems to me that it would follow that there would be similar shapes in this family that could be implemented for 4D and higher dimensions via a similar fill pattern. This is the shape I wish to set my array to represent.

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  • How to find the class object of Java generic type?

    - by Samuel Yung
    Assume I have a generic type P which is an Enum, that is <P extends Enum<P>>, and I want to get the Enum value from a string, for example: String foo = "foo"; P fooEnum = Enum.valueOf(P.class, foo); This will get a compile error because P.class is invalid. So what can I do in order to make the above code work?

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  • Extract information from a Func<bool, T> or alike lambda

    - by Syska
    I''m trying to build a generic cache layer. ICacheRepository Say I have the following: public class Person { public int PersonId { get; set; } public string Firstname { get; set; } public string Lastname { get; set; } public DateTime Added { get; set; } } And I have something like this: list.Where(x => x.Firstname == "Syska"); Here I want to extract the above information, to see if the query supplied the "PersonId" which it did not, so I dont want to cache it. But lets say I run a query like this: list.Where(x => x.PersonId == 10); Since PersonId is my key ... I want to cache it. with the key like "Person_10" and I later can fetch it from the cache. I know its possible to extract the information with Expression<Func<>> but there seems to be a big overhead of doing this (when running compile and extract the Constant values etc. and a bunch of cache to be sure to parse right) Are there a framework for this? Or some smart/golden way of doing this ?

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  • How do I change the class of an object to a subclass of its current class in C++?

    - by Jared P
    I have an array of pointers to a base class, so that I can make those pointers point to (different) subclasses of the base class, but still interact with them. (really only a couple of methods which I made virtual and overloaded) I'm wondering if I can avoid using the pointers, and instead just make an array of the base class, but have some way to set the class to the subclass of my choosing. I know there must be something there specifying the class, as it needs to use that to look up the function pointer for virtual methods. By the way, the subclasses all have the same ivars and layout. Note: the design is actually based on using a template argument instead of a variable, due to performance increases, so really the abstract base class is just the interface for the subclasses, which are all the same except for their compiled code. Thanks

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  • Is it possible to create ostream object, which outputs to multiple destinations?

    - by fiktor
    In 0-th approximation I have a class class MyClass{ public: ... std::ostream & getOStream(){return f;} private: ofstream f; ... }; Which is used sometimes in the following way: MyClass myclass; myclass.getOStream()<<some<<information<<printed<<here; But now I want to change the class MyClass, so that information will be printed both to f and to std::out, i.e. I want the above line to be equivalent to myclass.f<<some<<information<<printed<<here; std::cout<<some<<information<<printed<<here; I don't know any good way to do that. Do you? Is there any standard solution (for example in stl or in boost)? P.S. I tried to search on this, but it seems that I don't know good keywords. Words multiple, output, ostream, C++, boost seem to be too general.

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  • Why changing the images name on server results in calling the old ones?

    - by moderns
    I am running a slideshow on Ubuntu 12.04.1 that loads the images (slide1.jpg, slide2.jpg, slide3.jpg.., slide5.jpg) using the Javascript and styles as below: document.getElementById('slide_area').className='slide'+step; .slide1{background-image: url(../upload/slide1.jpg)} .slide2{background-image: url(../upload/slide2.jpg)} .slide3{background-image: url(../upload/slide3.jpg)} .slide4{background-image: url(../upload/slide4.jpg)} .slide5{background-image: url(../upload/slide5.jpg)} When I change the images names (show1.jpg, show2.jpg, show3.jpg.., show5.jpg) and also change the style as below: .slide1{background-image: url(../upload/show1.jpg)} .slide2{background-image: url(../upload/show2.jpg)} .slide3{background-image: url(../upload/show3.jpg)} .slide4{background-image: url(../upload/show4.jpg)} .slide5{background-image: url(../upload/show5.jpg)} And open the network section on Chrome, I see the server is calling the new name and old name for images! I added the header in the index.php: header("Cache-Control: no-store, no-cache, must-revalidate, max-age=0"); header("Expires: Sat, 26 Jul 1997 05:00:00 GMT"); // Date in the past header("Cache-Control: post-check=0, pre-check=0", false); header("Pragma: no-cache"); Nothing worked out with me and the slideshow doesn't work properly when I change the name of images even when clearing the browser cache as I load images sequentially (one by one) depending on imageObject.complete property! But without changing the name everything is going perfect and the images are loaded smoothly! Thank you for your help!

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  • test php disabled caching

    - by user1691389
    My site had a problem in that certain browsers (especially opera and gecko) were "over-caching" (caching far too much for my taste). I've just added the following PHP snippet to hopefully disable caching in all browsers: <?php header("Expires: Tue, 01 Jan 2000 00:00:00 GMT"); header("Last-Modified: " . gmdate("D, d M Y H:i:s") . " GMT"); header("Cache-Control: no-store, no-cache, must-revalidate, max-age=0"); header("Cache-Control: post-check=0, pre-check=0", false); header("Pragma: no-cache"); ?> Question: How would you test this out, to make sure it actually works?

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  • How to access a method of a closure's parent object?

    - by Bytecode Ninja
    I have defined a class named MyClass and I have defined two methods myMethod1 and myMethod2 for it: function MyClass() {} MyClass.prototype.myMethod1 = function() {...}; MyClass.prototype.myMethod2 = function() {...}; Inside myMethod1, I use jQuery and there's a callback closure defined there: MyClass.prototype.myMethod2 = function() { $.jQuery({success: function(data) { this.myMethod2(); }, ...}); } Now the problem is that this no longer is referring to MyClass. The question is how can I refer to it? At the moment I have assigned it to a variable named thisObj and access it this way: MyClass.prototype.myMethod2 = function() { var thisObj = this; $.jQuery({success: function(data) { thisObj.myMethod2(); }, ...}); } Is there a better way to access MyClass.this from the closure nested in myMethod2? Thanks in advance.

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  • Search BitmapData object for matching pixel values from another Bitmap.

    - by Cos
    Using Actionscript 3 is there a way to search one bitmap for the coordinates matching pixels of another bitmap? http://dl.dropbox.com/u/1914/wired.png Somehow you would have to loop through the bigger bitmap to find and the the pixel range that matches and return those coordinates. For example the Bitmap with the "E" is 250 pixels over and 14 pixels down in the bigger bitmap. I haven't been able to come up with the solution on my own. Thanks.

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