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• On-Demand thumbnail creation with django and nginx

  - by sharjeel

  I want to generate thumbnails of images on the fly. My site is built with django and deployed using nginx which serves all the static content and communicates with django/apache using reverse proxy. Right now, for every image in my site, I generate all required sizes of thumbnails on-hand and deliver them when required. The problem is that whenever I change the size of a thumbnail, I have to regenerate all of them (and they are tons). However now I'd like to generate the thumbnail the first time it is accessed and later on nginx would deliver the same file over n over. If I delete that thumbnail file because of lesser accesses, it should get generated automatically the next time. Thumbnails in my case also have watermarks which require some computation logic of my application so a webserver thumbnail module might not work very well. The size of the thumbnail can be embedded in the URL. So http://www.example.com/thumbnail/abc_320x240.jpg gets the 320x240 size of the thumbnail. The approach I'm looking right now is to let nginx lookup the file and if it doesn't exist, forward the query to my django application which would create the thumbnail and send either the response or a redirect string. However I'm not sure about the concurrency issues and any other issues which might pop up later. What is the appropriate way to achieve this?

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• Is my objective possible using WCF (and is it the right way to do things?)

  - by David

  I'm writing some software that modifies a Windows Server's configuration (things like MS-DNS, IIS, parts of the filesystem). My design has a server process that builds an in-memory object graph of the server configuration state and a client which requests this object graph. The server would then serialize the graph, send it to the client (presumably using WCF), the server then makes changes to this graph and sends it back to the server. The server receives the graph and proceeds to make modifications to the server. However I've learned that object-graph serialisation in WCF isn't as simple as I first thought. My objects have a hierarchy and many have parametrised-constructors and immutable properties/fields. There are also numerous collections, arrays, and dictionaries. My understanding of WCF serialisation is that it requires use of either the XmlSerializer or DataContractSerializer, but DCS places restrictions on the design of my object-graph (immutable data seems right-out, it also requires parameter-less constructors). I understand XmlSerializer lets me use my own classes provided they implement ISerializable and have the de-serializer constructor. That is fine by me. I spoke to a friend of mine about this, and he advocates going for a Data Transport Object-only route, where I'd have to maintain a separate DataContract object-graph for the transport of data and re-implement my server objects on the client. Another friend of mine said that because my service only has two operations ("GetServerConfiguration" and "PutServerConfiguration") it might be worthwhile just skipping WCF entirely and implementing my own server that uses Sockets. So my questions are: Has anyone faced a similar problem before and if so, are there better approaches? Is it wise to send an entire object graph to the client for processing? Should I instead break it down so that the client requests a part of the object graph as it needs it and sends only bits that have changed (thus reducing concurrency-related risks?)? If sending the object-graph down is the right way, is WCF the right tool? And if WCF is right, what's the best way to get WCF to serialise my object graph?

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• Using SQL dB column as a lock for concurrent operations in Entity Framework

  - by Sid

  We have a long running user operation that is handled by a pool of worker processes. Data input and output is from Azure SQL. The master Azure SQL table structure columns are approximated to [UserId, col1, col2, ... , col N, beingProcessed, lastTimeProcessed ] beingProcessed is boolean and lastTimeProcessed is DateTime. The logic in every worker role is: public void WorkerRoleMain() { while(true) { try { dbContext db = new dbContext(); // Read foreach (UserProfile user in db.UserProfile .Where(u => DateTime.UtcNow.Subtract(u.lastTimeProcessed) > TimeSpan.FromHours(24) & u.beingProcessed == false)) { user.beingProcessed = true; // Modify db.SaveChanges(); // Write // Do some long drawn processing here ... ... ... user.lastTimeProcessed = DateTime.UtcNow; user.beingProcessed = false; db.SaveChanges(); } } catch(Exception ex) { LogException(ex); Sleep(TimeSpan.FromMinutes(5)); } } // while () } With multiple workers processing as above (each with their own Entity Framework layer), in essence beingProcessed is being used a lock for MutEx purposes Question: How can I deal with concurrency issues on the beingProcessed "lock" itself based on the above load? I think read-modify-write operation on the beingProcessed needs to be atomic but I'm open to other strategies. Open to other code refinements too.

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• java GC periodically enters into several full GC cycles

  - by Peter

  Environment: sun JDK 1.6.0_16 vm settings: -XX:+DisableExplicitGC -XX:+UseConcMarkSweepGC -Xms1024 -Xmx1024M -XX:MaxNewSize=448m -XX:NewSize=448m -XX:SurvivorRatio=4(6 also checked) -XX:MaxPermSize=128M OS: windows server 2003 processor: 4 cores of INTEL XEON 5130, 2000 Hz my application description: high intensity of concurrent(java 5 concurrency used) operations completed each time by commit to oracle. it's about 20-30 threads run non stop, doing tasks. application runs in JBOSS web container. My GC starts work normally, I see a lot of small GCs and all that time CPU shows good load, like all 4 cores loaded to 40-50%, CPU graph is stable. Then , after 1 min of good work, CPU starts drop to 0% on 2 cores from 4, it's graph becomes unstable, goes up and down("teeth"). I see, that my threads work slower(I have monitoring), I see that GC starts produce a lot of FULL GC during that time and next 4-5 minutes this situation remains as is, then for short period of time, like 1 minute, it gets back to normal situation, but shortly after that all bad thing repeats. Question: Why I have so frequent full GC??? How to prevent that? I played with SurvivorRatio - does not help. I noticed, that application behaves normally until first FULL GC occurs, while I have enough memory. Then it runs badly. my GC LOG: starts good then long period of FULL GCs(many of them) 1027.861: [GC 942200K-623526K(991232K), 0.0887588 secs] 1029.333: [GC 803279K(991232K), 0.0927470 secs] 1030.551: [GC 967485K-625549K(991232K), 0.0823024 secs] 1030.634: [GC 625957K(991232K), 0.0763656 secs] 1033.126: [GC 969613K-632963K(991232K), 0.0850611 secs] 1033.281: [GC 649899K(991232K), 0.0378358 secs] 1035.910: [GC 813948K(991232K), 0.3540375 secs] 1037.994: [GC 967729K-637198K(991232K), 0.0826042 secs] 1038.435: [GC 710309K(991232K), 0.1370703 secs] 1039.665: [GC 980494K-972462K(991232K), 0.6398589 secs] 1040.306: [Full GC 972462K-619643K(991232K), 3.7780597 secs] 1044.093: [GC 620103K(991232K), 0.0695221 secs] 1047.870: [Full GC 991231K-626514K(991232K), 3.8732457 secs] 1053.739: [GC 942140K(991232K), 0.5410483 secs] 1056.343: [Full GC 991232K-634157K(991232K), 3.9071443 secs] 1061.257: [GC 786274K(991232K), 0.3106603 secs] 1065.229: [Full GC 991232K-641617K(991232K), 3.9565638 secs] 1071.192: [GC 945999K(991232K), 0.5401515 secs] 1073.793: [Full GC 991231K-648045K(991232K), 3.9627814 secs] 1079.754: [GC 936641K(991232K), 0.5321197 secs]

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• Returning objects from another thread?

  - by Mark

  Trying to follow the hints laid out here, but she doesn't mention how to handle it when your collection needs to return a value, like so: private delegate TValue DequeueHandler(); public virtual TValue Dequeue() { if (dispatcher.CheckAccess()) { --count; var pair = dict.First(); var queue = pair.Value; var val = queue.Dequeue(); if (queue.Count == 0) dict.Remove(pair.Key); OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, val)); return val; } else { dispatcher.BeginInvoke(new DequeueHandler(Dequeue)); } } This obviously won't work, because dispatcher.BeginInvoke doesn't return anything. What am I supposed to do? Or maybe I could replace dequeue with two functions, Peek and Pop, where Peek doesn't really need to be on the UI thread because it doesn't modify anything, right? As a side question, these methods don't need to be "locked" either, do they? If they're all forced to run in the UI thread, then there shouldn't be any concurrency issues, right?

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• Design patterns for Agent / Actor based concurrent design.

  - by nso1

  Recently i have been getting into alternative languages that support an actor/agent/shared nothing architecture - ie. scala, clojure etc (clojure also supports shared state). So far most of the documentation that I have read focus around the intro level. What I am looking for is more advanced documentation along the gang of four but instead shared nothing based. Why ? It helps to grok the change in design thinking. Simple examples are easy, but in a real world java application (single threaded) you can have object graphs with 1000's of members with complex relationships. But with agent based concurrency development it introduces a whole new set of ideas to comprehend when designing large systems. ie. Agent granularity - how much state should one agent manage - implications on performance etc or are their good patterns for mapping shared state object graphs to agent based system. tips on mapping domain models to design. Discussions not on the technology but more on how to BEST use the technology in design (real world "complex" examples would be great).

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• Producer consumer with qualifications

  - by tgguy

  I am new to clojure and am trying to understand how to properly use its concurrency features, so any critique/suggestions is appreciated. So I am trying to write a small test program in clojure that works as follows: there 5 producers and 2 consumers a producer waits for a random time and then pushes a number onto a shared queue. a consumer should pull a number off the queue as soon as the queue is nonempty and then sleep for a short time to simulate doing work the consumers should block when the queue is empty producers should block when the queue has more than 4 items in it to prevent it from growing huge Here is my plan for each step above: the producers and consumers will be agents that don't really care for their state (just nil values or something); i just use the agents to send-off a "consumer" or "producer" function to do at some time. Then the shared queue will be (def queue (ref [])). Perhaps this should be an atom though? in the "producer" agent function, simply (Thread/sleep (rand-int 1000)) and then (dosync (alter queue conj (rand-int 100))) to push onto the queue. I am thinking to make the consumer agents watch the queue for changes with add-watcher. Not sure about this though..it will wake up the consumers on any change, even if the change came from a consumer pulling something off (possibly making it empty) . Perhaps checking for this in the watcher function is sufficient. Another problem I see is that if all consumers are busy, then what happens when a producer adds something new to the queue? Does the watched event get queued up on some consumer agent or does it disappear? see above I really don't know how to do this. I heard that clojure's seque may be useful, but I couldn't find enough doc on how to use it and my initial testing didn't seem to work (sorry don't have the code on me anymore)

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• Which Queue implementation to use in Java?

  - by devoured elysium

  I need to use a FIFO structure in my application. It needs to have at most 5 elements. I'd like to have something easy to use (I don't care for concurrency) that implements the Collection interface. I've tried the LinkedList, that seems to come from Queue, but it doesn't seem to allow me to set it's maximum capacity. It feels as if I just want at max 5 elements but try to add 20, it will just keep increasing in size to fit it. I'd like something that'd work the following way: XQueue<Integer> queue = new XQueue<Integer>(5); //where 5 is the maximum number of elements I want in my queue. for (int i = 0; i < 10; ++i) { queue.offer(i); } for (int i = 0; i < 5; ++i) { System.out.println(queue.poll()); } That'd print: 5 6 7 8 9 Thanks

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• sqlite3 'database is locked' won't go away with retries

  - by Azarias

  I have a sqlite3 database that is accessed by a few threads (3-4). I am aware of the general limitations of sqlite3 with regards to concurrency as stated http://www.sqlite.org/faq.html#q6 , but I am convinced that is not the problem. All of the threads both read and write from this database. Whenever I do a write, I have the following construct: try: Cursor.execute(q, params) Connection.commit() except sqlite3.IntegrityError: Notify except sqlite3.OperationalError: print sys.exc_info() print("DATABASE LOCKED; sleeping for 3 seconds and trying again") time.sleep(3) Retry On some runs, I won't even hit this block, but when I do, it never comes out of it (keeps retrying, but I keep getting the 'database is locked' error from exc_info. If I understand the reader/writer lock usage correctly, some amount of waiting should help with the contention. What this sounds like is deadlock, but I do not use any transactions in my code, and every SELECT or INSERT is simply a one off. Some threads, however, keep the same connection when they do their operation (which includes a mix of SELECTS and INSERTS and other modifiers). I would appericiate it if you could shade a light on this, and also ways around fixing it (besides using a different database engine.)

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• How would you code an efficient Circular Buffer in Java or C#

  - by Cheeso

  I want a simple class that implements a fixed-size circular buffer. It should be efficient, easy on the eyes, generically typed. EDIT: It need not be MT-capable, for now. I can always add a lock later, it won't be high-concurrency in any case. Methods should be: .Add and I guess .List, where I retrieve all the entries. On second thought, Retrieval I think should be done via an indexer. At any moment I will want to be able to retrieve any element in the buffer by index. But keep in mind that from one moment to the next Element[n] may be different, as the Circular buffer fills up and rolls over. This isn't a stack, it's a circular buffer. Regarding "overflow": I would expect internally there would be an array holding the items, and over time the head and tail of the buffer will rotate around that fixed array. But that should be invisible from the user. There should be no externally-detectable "overflow" event or behavior. This is not a school assignment - it is most commonly going to be used for a MRU cache or a fixed-size transaction or event log.

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• Would an immutable keyword in Java be a good idea?

  - by berry120

  Generally speaking, the more I use immutable objects in Java the more I'm thinking they're a great idea. They've got lots of advantages from automatically being thread-safe to not needing to worry about cloning or copy constructors. This has got me thinking, would an "immutable" keyword go amiss? Obviously there's the disadvantages with adding another reserved word to the language, and I doubt it will actually happen primarily for the above reason - but ignoring that I can't really see many disadvantages. At present great care has to be taken to make sure objects are immutable, and even then a dodgy javadoc comment claiming a component object is immutable when it's in fact not can wreck the whole thing. There's also the argument that even basic objects like string aren't truly immutable because they're easily vunerable to reflection attacks. If we had an immutable keyword the compiler could surely recursively check and give an iron clad guarantee that all instances of a class were immutable, something that can't presently be done. Especially with concurrency becoming more and more used, I personally think it'd be good to add a keyword to this effect. But are there any disadvantages or implementation details I'm missing that makes this a bad idea?

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• java: retrieving the "canonical value" from a Set<T> where T has a custom equals()

  - by Jason S

  I have a class Foo which overrides equals() and hashCode() properly. I would like to also would like to use a HashSet<Foo> to keep track of "canonical values" e.g. I have a class that I would like to write like this, so that if I have two separate objects that are equivalent I can coalesce them into references to the same object: class Canonicalizer<T> { final private Set<T> values = new HashSet<T>(); public T findCanonicalValue(T value) { T canonical = this.values.get(value); if (canonical == null) { // not in the set, so put it there for the future this.values.add(value); return value; } else { return canonical; } } } except that Set doesn't have a "get" method that would return the actual value stored in the set, just the "contains" method that returns true or false. (I guess that it assumes that if you have an object that is equal to a separate object in the set, you don't need to retrieve the one in the set) Is there a convenient way to do this? The only other thing I can think of is to use a map and a list: class Canonicalizer<T> { // warning: neglects concurrency issues final private Map<T, Integer> valueIndex = new HashMap<T, Integer>(); final private List<T> values = new ArrayList<T>(); public T findCanonicalValue(T value) { Integer i = this.valueIndex.get(value); if (i == null) { // not in the set, so put it there for the future i = this.values.size(); this.values.add(value); this.valueIndex.put(value, i); return value; } else { // in the set return this.values.get(i); } } }

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• Protecting sensitive entity data

  - by Andreas

  Hi, I'm looking for some advice on architecture for a client/server solution with some peculiarities. The client is a fairly thick one, leaving the server mostly to peristence, concurrency and infrastructure concerns. The server contains a number of entities which contain both sensitive and public information. Think for example that the entities are persons, assume that social security number and name are sensitive and age is publicly viewable. When starting the client, the user is presented with a number of entities, not disclosing any sensitive information. At any time the user can choose to log in and authenticate against the server, given the authentication is successful the user is granted access to the sensitive information. The client is hosting a domain model and I was thinking of implementing this as some kind of "lazy loading", making the first request instantiating the entities and later refreshing them with sensitive data. The entity getters would throw exceptions on sensitive information when they've not been disclosed, f.e.: class PersonImpl : PersonEntity { private bool undisclosed; public override string SocialSecurityNumber { get { if (undisclosed) throw new UndisclosedDataException(); return base.SocialSecurityNumber; } } } Another more friendly approach could be to have a value object indicating that the value is undisclosed. get { if (undisclosed) return undisclosedValue; return base.SocialSecurityNumber; } Some concerns: What if the user logs in and then out, the sensitive data has been loaded but must be disclosed once again. One could argue that this type of functionality belongs within the domain and not some infrastructural implementation(i.e. repository implementations). As always when dealing with a larger number of properties there's a risk that this type of functionality clutters the code Any insights or discussion is appreciated!

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• Java "Pool" of longs or Oracle sequence with reusable values

  - by Anthony Accioly

  Several months ago I implemented a solution to choose unique values from a range between 1 and 65535 (16 bits). This range is used to generate unique Route Targets suffixes, which for this customer massive network (it's a huge ISP) are a very disputed resource, so any free index needs to become immediately available to the end user. To tackle this requirement I used a BitSet. Allocate on the RT index with set and deallocate a suffix with clear. The method nextClearBit() can find the next available index. I handle synchronization / concurrency issues manually. This works pretty well for a small range... The entire index is small (around 10k), it is blazing fast and can be easy serialized into a Blob field. The problem is, some new devices can handle RTs of 32 bits (range 1 / 4294967296). Which can't be managed with a BitSet (it would, by itself, consume around 600Mb, plus be limited to int range). Even with this massive range available, the client still wants to free available Route Targets for the end user, mainly because the lowest ones (up to 65535) - which are compatible with old routers - are being heavily disputed. Before I tell the customer that this is impossible and he will have to conform with my reusable index for lower RTs (up to 65550) and use a database sequence for the other ones (which means that when the user frees a Route Target, it will not become available again). Would anyone shed some light? Maybe some kind soul already implemented a high performance number pool for Java (6 if it matters), or I am missing a killer feature of Oracle database (11R2 if it matters)... Wishful thinking. Thank you very much in advance.

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• When should EntityManagerFactory instance be created/opened ?

  - by masato-san

  Ok, I read bunch of articles/examples how to write Entity Manager Factory in singleton. One of them easiest for me to understand a bit: http://javanotepad.blogspot.com/2007/05/jpa-entitymanagerfactory-in-web.html I learned that EntityManagerFactory (EMF) should only be created once preferably in application scope. And also make sure to close the EMF once it's used (?) So I wrote EMF helper class for business methods to use: public class EmProvider { private static final String DB_PU = "KogaAlphaPU"; public static final boolean DEBUG = true; private static final EmProvider singleton = new EmProvider(); private EntityManagerFactory emf; private EmProvider() {} public static EmProvider getInstance() { return singleton; } public EntityManagerFactory getEntityManagerFactory() { if(emf == null) { emf = Persistence.createEntityManagerFactory(DB_PU); } if(DEBUG) { System.out.println("factory created on: " + new Date()); } return emf; } public void closeEmf() { if(emf.isOpen() || emf != null) { emf.close(); } emf = null; if(DEBUG) { System.out.println("EMF closed at: " + new Date()); } } }//end class And my method using EmProvider: public String foo() { EntityManager em = null; List<Object[]> out = null; try { em = EmProvider.getInstance().getEntityManagerFactory().createEntityManager(); Query query = em.createNativeQuery(JPQL_JOIN); //just some random query out = query.getResultList(); } catch(Exception e) { //handle error.... } finally { if(em != null) { em.close(); //make sure to close EntityManager } } I made sure to close EntityManager (em) within method level as suggested. But when should EntityManagerFactory be closed then? And why EMF has to be singleton so bad??? I read about concurrency issues but as I am not experienced multi-thread-grammer, I can't really be clear on this idea.

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• Start developing a database application using Oracle + Net Beans

  - by Ranhiru

  I have thought of creating my first database application for one of my projects using Oracle and Java. I have chosen Netbeans as my development environment. I have a few questions to getting started. Please bare with me as I'm a complete beginner to Oracle + Netbeans This will be a data intensive (yet still for a college project) database application. I do not need 1000 user concurrency or any other very advanced features but basic stuff such as triggers, stored procedures etc. Will the 11g "Express" (XE) suffice for my requirements? Do i need any Java to Oracle bridge (database connectivity driver eg. ODBC etc) for Netbeans to connect to the oracle database? If yes, what are they? Does Netbeans support Oracle databases natively? Any easy to follow guide on how do i connect to the database and insert/retrieve/display data on a J2SE application? (I know that i should Google this but if there's any guide previously followed by anyone and is considered easy, it would be greatly appreciated.)

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• How to change Hibernate´s auto persistance strategy

  - by Kristofer Borgstrom

  I just noted that my hibernate entities are automatically persisted to the database (or at least to cache) before I call any save() or update() method. To me this is a pretty strange default behavior but ok, as long as I can disable it, it´s fine. The problem I have is I want to update my entity´s state (from 1 to 2) only if the entity in the database still has the state it had when I retrieved [1] (this is to eliminate concurrency issues when another server is updating this same object). For this reason I have created a custom NamedQuery that will only update the entity if state is 1. So here is some pseudo-code: //Get the entity Entity item = dao.getEntity(); item.getState(); //==1 //Update the entity item.setState(2); //Here is the problem, this effectively changes the state of my entity braking my query that verifies that state is still == 1. dao.customUpdate(item); //Returns 0 rows changes since state != 1. So, how do I make sure the setters don´t change the state in cache/db? Thanks, Kristofer

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• Test the sequentiality of a column with a single SQL query

  - by LauriE

  Hey, I have a table that contains sets of sequential datasets, like that: ID set_ID some_column n 1 'set-1' 'aaaaaaaaaa' 1 2 'set-1' 'bbbbbbbbbb' 2 3 'set-1' 'cccccccccc' 3 4 'set-2' 'dddddddddd' 1 5 'set-2' 'eeeeeeeeee' 2 6 'set-3' 'ffffffffff' 2 7 'set-3' 'gggggggggg' 1 At the end of a transaction that makes several types of modifications to those rows, I would like to ensure that within a single set, all the values of "n" are still sequential (rollback otherwise). They do not need to be in the same order according to the PK, just sequential, like 1-2-3 or 3-1-2, but not like 1-3-4. Due to the fact that there might be thousands of rows within a single set I would prefer to do it in the db to avoid the overhead of fetching the data just for verification after making some small changes. Also there is the issue of concurrency. The way locking in InnoDB (repeatable read) works (as I understand) is that if I have an index on "n" then InnoDB also locks the "gaps" between values. If I combine set_ID and n to a single index, would that eliminate the problem of phantom rows appearing? Looks to me like a common problem. Any brilliant ideas? Thanks! Note: using MySQL + InnoDB

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• Business Tier | client state in desktop application- way around Stateful Session Beans?

  - by arthur

  I read positive inputs on the following posts concerning client state management: Stateful EJBs in web application?, here, and here. I want to know how implement such client state management for desktop applications (Swing, AWT, SWT, and other). let's assume there are n desktop clients supposed to use some (remote) services provided on an Application Server. How to maintain a separate and permanent data state for each client , distinct from the other without have to use using stateful session Beans (SFSB) ? is that even possible on this application type ? With Webapp(Servlets / JsSF and JSP) some can avoid using SFSB by HttpSession object and/or coupling it with stateless session beans (SLSB). HttpSession object would keep information for each (Web)client separate and the SLSB would play the business logic music. But HttpSession objects aren't present on desktop application and I go stuck with only SFSB and SLSB. Knowing the problems(Concurrency, Error Handling, usw ) of SFSB, I would have not wanted to use it. What would be the other options? is there only SFSB available? Thanks in advances

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• Strange results about C++11 memory model (Relaxed ordering)

  - by Dancing_bunny

  I was testing the example in the memory model of the Anthony Williams's book "C++ Concurrency" #include<atomic> #include<thread> #include<cassert> std::atomic_bool x,y; std::atomic_int z; void write_x_then_y() { x.store(true, std::memory_order_relaxed); y.store(true, std::memory_order_relaxed); } void read_y_then_x() { while(!y.load(std::memory_order_relaxed)); if(x.load(std::memory_order_relaxed)) { ++z; } } int main() { x = false; y = false; z = 0; std::thread a(write_x_then_y); std::thread b(read_y_then_x); a.join(); b.join(); assert(z.load()!=0); } According to the explanation, relaxed operations on difference variables (here x and y) can be freely reordered. However, I repeated running the problem for more than several days. I never hit the situation that the assertion (assert(z.load()!=0);) fires. I just use the default optimization and compile the code using g++ -std=c++11 -lpthread dataRaceAtomic.cpp Does anyone actually try it and hit the assertion? Could anyone give me an explanation about my test results? BTW, I also tried the version without using the atomic type, I got the same result. Currently, both programs are running healthily. Thanks.

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• Trouble with go tour crawler exercise

  - by David Mason

  I'm going through the go tour and I feel like I have a pretty good understanding of the language except for concurrency. On slide 71 there is an exercise that asks the reader to parallelize a web crawler (and to make it not cover repeats but I haven't gotten there yet.) Here is what I have so far: func Crawl(url string, depth int, fetcher Fetcher, ch chan string) { if depth <= 0 { return } body, urls, err := fetcher.Fetch(url) if err != nil { ch <- fmt.Sprintln(err) return } ch <- fmt.Sprintf("found: %s %q\n", url, body) for _, u := range urls { go Crawl(u, depth-1, fetcher, ch) } } func main() { ch := make(chan string, 100) go Crawl("http://golang.org/", 4, fetcher, ch) for i := range ch { fmt.Println(i) } } The issue I have is where to put the close(ch) call. If I put a defer close(ch) somewhere in the Crawl method, then I end up writing to a closed channel in one of the spawned goroutines, since the method will finish execution before the spawned goroutines do. If I omit the call to close(ch), as is shown in my example code, the program deadlocks after all the goroutines finish executing but the main thread is still waiting on the channel in the for loop since the channel was never closed.

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• Celery Received unregistered task of type (run example)

  - by Echeg

  I'm trying to run example from Celery documentation. I run: celeryd --loglevel=INFO /usr/local/lib/python2.7/dist-packages/celery/loaders/default.py:64: NotConfigured: No 'celeryconfig' module found! Please make sure it exists and is available to Python. "is available to Python." % (configname, ))) [2012-03-19 04:26:34,899: WARNING/MainProcess] -------------- celery@ubuntu v2.5.1 ---- **** ----- --- * *** * -- [Configuration] -- * - **** --- . broker: amqp://guest@localhost:5672// - ** ---------- . loader: celery.loaders.default.Loader - ** ---------- . logfile: [stderr]@INFO - ** ---------- . concurrency: 4 - ** ---------- . events: OFF - *** --- * --- . beat: OFF -- ******* ---- --- ***** ----- [Queues] -------------- . celery: exchange:celery (direct) binding:celery tasks.py: # -*- coding: utf-8 -*- from celery.task import task @task def add(x, y): return x + y run_task.py: # -*- coding: utf-8 -*- from tasks import add result = add.delay(4, 4) print (result) print (result.ready()) print (result.get()) In same folder celeryconfig.py: CELERY_IMPORTS = ("tasks", ) CELERY_RESULT_BACKEND = "amqp" BROKER_URL = "amqp://guest:guest@localhost:5672//" CELERY_TASK_RESULT_EXPIRES = 300 When I run "run_task.py": on python console eb503f77-b5fc-44e2-ac0b-91ce6ddbf153 False errors on celeryd server [2012-03-19 04:34:14,913: ERROR/MainProcess] Received unregistered task of type 'tasks.add'. The message has been ignored and discarded. Did you remember to import the module containing this task? Or maybe you are using relative imports? Please see http://bit.ly/gLye1c for more information. The full contents of the message body was: {'retries': 0, 'task': 'tasks.add', 'utc': False, 'args': (4, 4), 'expires': None, 'eta': None, 'kwargs': {}, 'id': '841bc21f-8124-436b-92f1-e3b62cafdfe7'} Traceback (most recent call last): File "/usr/local/lib/python2.7/dist-packages/celery/worker/consumer.py", line 444, in receive_message self.strategies[name](message, body, message.ack_log_error) KeyError: 'tasks.add' Please explain what's the problem.

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• JavaOne Latin America 2012 is a wrap!

  - by arungupta

  Third JavaOne in Latin America (2010, 2011) is now a wrap! Like last year, the event started with a Geek Bike Ride. I could not attend the bike ride because of pre-planned activities but heard lots of good comments about it afterwards. This is a great way to engage with JavaOne attendees in an informal setting. I highly recommend you joining next time! JavaOne Blog provides a a great coverage for the opening keynotes. I talked about all the great set of functionality that is coming in the Java EE 7 Platform. Also shared the details on how Java EE 7 JSRs are willing to take help from the Adopt-a-JSR program. glassfish.org/adoptajsr bridges the gap between JUGs willing to participate and looking for areas on where to help. The different specification leads have identified areas on where they are looking for feedback. So if you are JUG is interested in picking a JSR, I recommend to take a look at glassfish.org/adoptajsr and jump on the bandwagon. The main attraction for the Tuesday evening was the GlassFish Party. The party was packed with Latin American JUG leaders, execs from Oracle, and local community members. Free flowing food and beer/caipirinhas acted as great lubricant for great conversations. Some of them were considering the migration from Spring -> Java EE 6 and replacing their primary app server with GlassFish. Locaweb, a local hosting provider sponsored a round of beer at the party as well. They are planning to come with Java EE hosting next year and GlassFish would be a logical choice for them ;) I heard lots of positive feedback about the party afterwards. Many thanks to Bruno Borges for organizing a great party! Check out some more fun pictures of the party! Next day, I gave a presentation on "The Java EE 7 Platform: Productivity and HTML 5" and the slides are now available: With so much new content coming in the plaform: Java Caching API (JSR 107) Concurrency Utilities for Java EE (JSR 236) Batch Applications for the Java Platform (JSR 352) Java API for JSON (JSR 353) Java API for WebSocket (JSR 356) And JAX-RS 2.0 (JSR 339) and JMS 2.0 (JSR 343) getting major updates, there is definitely lot of excitement that was evident amongst the attendees. The talk was delivered in the biggest hall and had about 200 attendees. Also spent a lot of time talking to folks at the OTN Lounge. The JUG leaders appreciation dinner in the evening had its usual share of fun. Day 3 started with a session on "Building HTML5 WebSocket Apps in Java". The slides are now available: The room was packed with about 150 attendees and there was good interaction in the room as well. A collaborative whiteboard built using WebSocket was very well received. The following tweets made it more worthwhile: A WebSocket speek, by @ArunGupta, was worth every hour lost in transit. #JavaOneBrasil2012, #JavaOneBr @arungupta awesome presentation about WebSockets :) The session was immediately followed by the hands-on lab "Developing JAX-RS Web Applications Utilizing Server-Sent Events and WebSocket". The lab covers JAX-RS 2.0, Jersey-specific features such as Server-Sent Events, and a WebSocket endpoint using JSR 356. The complete self-paced lab guide can be downloaded from here. The lab was planned for 2 hours but several folks finished the entire exercise in about 75 mins. The wonderfully written lab material and an added incentive of Java EE 6 Pocket Guide did the trick ;-) I also spoke at "The Java Community Process: How You Can Make a Positive Difference". It was really great to see several JUG leaders talking about Adopt-a-JSR program and other activities that attendees can do to participate in the JCP. I shared details about Adopt a Java EE 7 JSR as well. The community keynote in the evening was looking fun but I had to leave in between to go through the peak Sao Paulo traffic time :) Enjoy the complete set of pictures in the album:

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• Agile Database Techniques: Effective Strategies for the Agile Software Developer – book review

  - by DigiMortal

     Agile development expects mind shift and developers are not the only ones who must be agile. Every chain is as strong as it’s weakest link and same goes also for development teams. Agile Database Techniques: Effective Strategies for the Agile Software Developer by Scott W. Ambler is book that calls also data professionals to be part of agile development. Often are DBA-s in situation where they are not part of application development and later they have to survive large set of applications that all use databases different way. Of course, only some of these applications are not problematic when looking what database server has to do to serve them. I have seen many applications that rape database servers because developers have no clue what is going on in database (~3K queries to database per web application request – have you seen something like this? I have…) Agile Database Techniques covers some object and database design technologies and gives suggestions to development teams about topics they need help or assistance by DBA-s. The book is also good reading for DBA-s who usually are not very strong in object technologies. You can take this book as bridge between these two worlds. I think teams that build object applications that use databases should buy this book and try at least one or two projects out with Ambler’s suggestions. Table of contents Foreword by Jon Kern. Foreword by Douglas K. Barry. Acknowledgments. Introduction. About the Author. Part One: Setting the Foundation. Chapter 1: The Agile Data Method. Chapter 2: From Use Cases to Databases — Real-World UML. Chapter 3: Data Modeling 101. Chapter 4: Data Normalization. Chapter 5: Class Normalization. Chapter 6: Relational Database Technology, Like It or Not. Chapter 7: The Object-Relational Impedance Mismatch. Chapter 8: Legacy Databases — Everything You Need to Know But Are Afraid to Deal With. Part Two: Evolutionary Database Development. Chapter 9: Vive L’ Évolution. Chapter 10: Agile Model-Driven Development (AMDD). Chapter 11: Test-Driven Development (TDD). Chapter 12: Database Refactoring. Chapter 13: Database Encapsulation Strategies. Chapter 14: Mapping Objects to Relational Databases. Chapter 15: Performance Tuning. Chapter 16: Tools for Evolutionary Database Development. Part Three: Practical Data-Oriented Development Techniques. Chapter 17: Implementing Concurrency Control. Chapter 18: Finding Objects in Relational Databases. Chapter 19: Implementing Referential Integrity and Shared Business Logic. Chapter 20: Implementing Security Access Control. Chapter 21: Implementing Reports. Chapter 22: Realistic XML. Part Four: Adopting Agile Database Techniques. Chapter 23: How You Can Become Agile. Chapter 24: Bringing Agility into Your Organization. Appendix: Database Refactoring Catalog. References and Suggested Reading. Index.

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• SQL SERVER – List of All the Samples Database Available to Download for FREE

  - by Pinal Dave

  It is pretty much very common to have a sample database for any database product. Different companies keep on improving their product and keep on coming up with innovation in their product. To demonstrate the capability of their new enhancements they need the sample database. Microsoft have various sample database available for free download for their SQL Server Product. I have collected them here in a single blog post. Download an AdventureWorks Database The AdventureWorks OLTP database supports standard online transaction processing scenarios for a fictitious bicycle manufacturer (Adventure Works Cycles). Scenarios include Manufacturing, Sales, Purchasing, Product Management, Contact Management, and Human Resources. Coconut Dal Coconut Dal is a lightweight data access layer, for use in projects where the Entity Framework cannot be used or Microsoft’s Enterprise Library Data Block is unsuitable. Anyone who is handwriting ADO.NET should use a library instead and Coconut Dal might be the answer.  DataBooster – Extension to ADO.NET Data Provider The dbParallel DataBooster library is a high-performance extension to ADO.NET Data Provider, includes two aspects: 1) A slimmed down API encapsulation which simplified the most common data access operations (DbConnection -> DbCommand -> DbParameter -> DbDataReader) into a single class DbAccess, to help application with a clean DAL, avoid over-packing and redundant-copy of data transfer. 2) A booster for writing mass data onto database. Base on a rational utilization of database concurrency and a effective utilization of network bandwidth. Tabular AMO 2012 The sample is made of two project parts. The first part is a library of functions to manage tabular models -AMO2Tabular V2-. The second part is a sample to build a tabular model -AdventureWorks Tabular AMO 2012- using the AMO2Tabular library; the created model is similar to the ‘AdventureWorks Tabular Model 2012. SQL Server Analysis Services Product Samples SQL Server Analysis Services provides, a unified and integrated view of all your business data as the foundation for all of your traditional reporting, online analytical processing (OLAP) analysis, Key Performance Indicator (KPI) scorecards, and data mining. Analysis Services Samples for SQL Server 2008 R2 This release is dedicated to the samples that ship for Microsoft SQL Server 2008R2. For many of these samples you will also need to download the AdventureWorks family of databases. SQL Server Reporting Services Product Samples This project contains Reporting Services samples released with Microsoft SQL Server product. These samples are in the following five categories: Application Samples, Extension Samples, Model Samples, Report Samples, and Script Samples. If you are interested in contributing Reporting Services samples, please let us know by posting in the developers’ forum. Reporting Services Samples for SQL Server 2008 R2 This release is dedicated to the samples that ship for Microsoft SQL Server 2008 R2 PCU1. For many of these samples you will also need to download the AdventureWorks family of databases. SQL Server Integration Services Product Samples This project contains Integration Services samples released with Microsoft SQL Server product. These samples are in the following two categories: Package Samples and Programming Samples. If you are interested in contributing Integration Services samples, please let us know by posting in the developers’ forum. Integration Services Samples for SQL Server 2008 R2 This release is dedicated to the samples that ship for Microsoft SQL Server 2008R2. For many of these samples you will also need to download the AdventureWorks family of databases. Windows Azure SQL Reporting Admin Sample The SQLReportingAdmin sample for Windows Azure SQL Reporting demonstrates the usage of SQL Reporting APIs, and manages (add/update/delete) permissions of SQL Reporting users. Windows Azure SQL Reporting ReportViewer-SOAP API usage sample These sample projects demonstrate how to embed a Microsoft ReportViewer control that points to reports hosted on SQL Reporting report servers and how to use SQL Reporting SOAP APIs in your Windows Azure Web application. Enterprise Library 5.0 – Integration Pack for Windows Azure This NuGet package contains a zip file with the source code for the Enterprise Library Integration Pack for Windows Azure.  Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Download, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology Tagged: SQL Sample Database

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