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• What's a good Java-based Master-Slave communication mechanism?

  - by plecong

  I'm creating a Java application that requires master-slave communication between JVMs, possibly residing on the same physical machine. There will be a "master" server running inside a JEE application server (i.e. JBoss) that will have "slave" clients connect to it and dynamically register itself for communication (that is the master will not know the IP addresses/ports of the slaves so cannot be configured in advance). The master server acts as a controller that will dole work out to the slaves and the slaves will periodically respond with notifications, so there would be bi-directional communication. I was originally thinking of RPC-based systems where each side would be a server, but it could get complicated, so I'd prefer a mechanism where there's an open socket and they talk back and forth. I'm looking for a communication mechanism that would be low-latency where the messages would be mostly primitive types, so no serious serialization is necessary. Here's what I've looked at: RMI JMS: Built-in to Java, the "slave" clients would connect to the existing ConnectionFactory in the application server. JAX-WS/RS: Both master and slave would be servers exposing an RPC interface for bi-directional communication. JGroups/Hazelcast: Use shared distributed data structures to facilitate communication. Memcached/MongoDB: Use these as "queues" to facilitate communication, though the clients would have to poll so there would be some latency. Thrift: This does seem to keep a persistent connection, but not sure how to integrate/embed a Thrift server into JBoss WebSocket/Raw Socket: This would work, but require a lot more custom code than I'd like. Is there any technology I'm missing? Edit: Also looked at: JMX: Have the client connect to JBoss' JMX server and receive JMX notifications for bidirectional comms.

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• Python's asyncore to periodically send data using a variable timeout. Is there a better way?

  - by Nick Sonneveld

  I wanted to write a server that a client could connect to and receive periodic updates without having to poll. The problem I have experienced with asyncore is that if you do not return true when dispatcher.writable() is called, you have to wait until after the asyncore.loop has timed out (default is 30s). The two ways I have tried to work around this is 1) reduce timeout to a low value or 2) query connections for when they will next update and generate an adequate timeout value. However if you refer to 'Select Law' in 'man 2 select_tut', it states, "You should always try to use select() without a timeout." Is there a better way to do this? Twisted maybe? I wanted to try and avoid extra threads. I'll include the variable timeout example here: #!/usr/bin/python import time import socket import asyncore # in seconds UPDATE_PERIOD = 4.0 class Channel(asyncore.dispatcher): def __init__(self, sock, sck_map): asyncore.dispatcher.__init__(self, sock=sock, map=sck_map) self.last_update = 0.0 # should update immediately self.send_buf = '' self.recv_buf = '' def writable(self): return len(self.send_buf) > 0 def handle_write(self): nbytes = self.send(self.send_buf) self.send_buf = self.send_buf[nbytes:] def handle_read(self): print 'read' print 'recv:', self.recv(4096) def handle_close(self): print 'close' self.close() # added for variable timeout def update(self): if time.time() >= self.next_update(): self.send_buf += 'hello %f\n'%(time.time()) self.last_update = time.time() def next_update(self): return self.last_update + UPDATE_PERIOD class Server(asyncore.dispatcher): def __init__(self, port, sck_map): asyncore.dispatcher.__init__(self, map=sck_map) self.port = port self.sck_map = sck_map self.create_socket(socket.AF_INET, socket.SOCK_STREAM) self.bind( ("", port)) self.listen(16) print "listening on port", self.port def handle_accept(self): (conn, addr) = self.accept() Channel(sock=conn, sck_map=self.sck_map) # added for variable timeout def update(self): pass def next_update(self): return None sck_map = {} server = Server(9090, sck_map) while True: next_update = time.time() + 30.0 for c in sck_map.values(): c.update() # <-- fill write buffers n = c.next_update() #print 'n:',n if n is not None: next_update = min(next_update, n) _timeout = max(0.1, next_update - time.time()) asyncore.loop(timeout=_timeout, count=1, map=sck_map)

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• Upload using python script takes very long on one laptop as compared to another

  - by Engr Am

  I have a python 2.7 code which uses STORBINARY function for uploading files to an ftp server and RETRBINARY for downloading from this server. However, the issue is the upload is taking a very long time on three laptops from different brands as compared to a Dell laptop. The strange part is when I manually upload any file, it takes the same time on all the systems. The manual upload rate and upload rate with the python script is the same on the Dell Laptop. However, on every other brand of laptop (I have tried with IBM, Toshiba, Fujitsu-Siemens) the python script has a very low upload rate than the manual attempt. Also, on all these other laptops, the upload rate using the python script is the same (1Mbit/s) while the manual upload rate is approx. 8 Mbit/s. I have tried to vary the filesize for the upload to no avail. TCP Optimizer improved the download rate on all the systems but had no effect on the upload rate. Download rate using this script on all the systems is fine and same as the manual download rate. I have checked the server and it has more than 90% free space. The network connection is the same for all the laptops, and I try uploading only with one laptop at a time. All the laptops have almost the same system configurations, same operating system and approximately the same free drive space. If anything the Dell laptop is a little less in terms of processing power and RAM than 2 of the others, but I suppose this has no effect as I have checked many times to see how much was the CPU usage and network usage during these uploads and downloads, and I am sure that no other virus or program has been eating up my bandwidth. Here is the code ('ftp' and 'file_path' are inputs to the function): path,filename=os.path.split(file_path) filesize=os.path.getsize(file_path) deffilesize=(filesize/1024)/1024 f = open(file_path, "rb") upstart = time.clock() print ftp.storbinary("STOR "+filename, f) upende = time.clock()-upstart outname="Upload " f.close() return upende, deffilesize, outname

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• Best practice - logging events (general) and changes (database)

  - by b0x0rz

  need help with logging all activities on a site as well as database changes. requirements: * should be in database * should be easily searchable by initiator (user name / session id), event (activity type) and event parameters i can think of a database design but either it involves a lot of tables (one per event) so i can log each of the parameters of an event in a separate field OR it involves one table with generic fields (7 int numeric and 7 text types) and log everything in one table with event type field determining what parameter got written where (and hoping that i don't need more than 7 fields of a certain type, or 8 or 9 or whatever number i choose)... example of entries (the usual things): [username] login failed @datetime [username] login successful @datetime [username] changed password @datetime, estimated security of password [low/ok/high/perfect] @datetime [username] clicked result [result number] [result id] after searching for [search string] and got [number of results] @datetime [username] clicked result [result number] [result id] after searching for [search string] and got [number of results] @datetime [username] changed profile name from [old name] to [new name] @datetime [username] verified name with [credit card type] credit card @datetime datbase table [table name] purged of old entries @datetime via automated process etc... so anyone dealt with this before? any best practices / links you can share? i've seen it done with the generic solution mentioned above, but somehow that goes against what i learned from database design, but as you can see the sheer number of events that need to be trackable (each user will be able to see this info) is giving me headaches, BUT i do LOVE the one event per table solution more than the generic one. any thoughts? edit: also, is there maybe an authoritative list of such (likely) events somewhere? thnx stack overflow says: the question you're asking appears subjective and is likely to be closed. my answer: probably is subjective, but it is directly related to my issue i have with designing a database / writing my code, so i'd welcome any help. also i tried narrowing down the ideas to 2 so hopefully one of these will prevail, unless there already is an established solution for these kinds of things.

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• Cache data in SQL CE database

  - by user93422

  Background I have an SQL CE database, that is constantly updated (every second). I have a (web) application that allows a user to look at the data in real-time. At some point a user can click "take a snapshot" button, and it will open the snapshot in a different window. And then on that form, there is "print" and "download" buttons that will either generate a page for printing, or will stream the data as CSV file - but same data snapshot has to be used, i.e. I can't go to the DB to get latest data for that. Details SQL CE dabatase is exposed through WCF web service. Snapshot consists of up to 500 records, 10 columns each. Expiration time on the snapshot of 2 hours is sufficient. It is a low-traffic application, so I don't expect more than few (5) connections at the same time. Loosing snapshot is not a big deal, user can simply generate new one. database is accessed by self-hosted WCF web service using Linq-to-SQL. Web site is ASP.NET MVC hosted on UltiDev Cassini. database, and web site are most likely be on the same box, when deployed. The entire app is intranet bound. Problem I need to cache the snapshot of the data at the moment user pressed "take a snapshot" button, so that I can use same data to generate print page, or generate a file for download. Solution 1: Each time there is a need to generate a snapshot, I will create a table in the database. Since there are no temp tables in SQL CE, I will need to clean it up myself. Solution 2: Cache the snapshot in-memory on either DB server, or web server. Question: Is there anything wrong with proposed solutions? Any different solution suggestions?

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• Ogre material scripts; how do I give a technique multiple lod_indexes?

  - by BlueNovember

  I have an Ogre material script that defines 4 rendering techniques. 1 using GLSL shaders, then 3 others that just use textures of different resolutions. I want to use the GLSL shader unconditionally if the graphics card supports it, and the other 3 textures depending on camera distance. At the moment my script is; material foo { lod_distances 1600 2000 technique shaders { lod_index 0 lod_index 1 lod_index 2 //various passes here } technique high_res { lod_index 0 //various passes here } technique medium_res { lod_index 1 //various passes here } technique low_res { lod_index 2 //various passes here } Extra information The Ogre manual says; Increasing indexes denote lower levels of detail You can (and often will) assign more than one technique to the same LOD index, what this means is that OGRE will pick the best technique of the ones listed at the same LOD index. OGRE determines which one is 'best' by which one is listed first. Currently, on a machine supporting the GLSL version I am using, the script behaves as follows; Camera 2000 : Shader technique Camera 1600 <= 2000 : Medium Camera <= 1600 : High If I change the lod order in shader technique to { lod_index 2 lod_index 1 lod_index 0 } The behaviour becomes; Camera 2000 : Low Camera 1600 <= 2000 : Medium Camera <= 1600 : Shader implying only the latest lod_index is used. If I change it to lod_index 0 1 2 It shouts at me Compiler error: fewer parameters expected in foo.material(#): lod_index only supports 1 argument So how do I specify a technique to have 3 lod_indexes? Duplication works; technique shaders { lod_index 0 //various passes here } technique shaders1 { lod_index 1 //passes repeated here } technique shaders2 { lod_index 2 //passes repeated here } ...but it's ugly.

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• Getting started with open source

  - by lola

  Hi all, I'm an undergraduate who has decided that he wants to join the open source community and contribute. However, I have come to think that, once you have chosen an open source project, a lot of time is spent in learning the nitty gritties of that project in addition to stuff like subversion,etc which a typical undergraduate isn't exposed to. So, you have to stick with that project for a long time, say a year or two, before moving on to other projects. In this case, choosing the right(for you) initial project is very important since if you choose one,and say, the development in your field of interest(in that project) is a low priority and not exciting enough, you'll lose interest and stop contributing to open source all together. So what I wanted to know was, since there are thousands of open source projects, is all this being documented somewhere with tags,etc so that a beginner can choose his projects. The GSoc 2010 ideas list is a great starting point, but it only covers a handful. Hence, I thought why not ask this at stackoverflow: if you have any pointers as to where to start, when choosing a FOSS project or any other tips related to starting with FOSS. P.S. I'm interested in projects involving mobile ad hoc networks(those using TinyOS, preferably), so pointers related to these will be great. I'm looking through Freifunk and OLPC as of now, needed more ideas.

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• best practice - loging events (general) and changes (database)

  - by b0x0rz

  need help with logging all activities on a site as well as database changes. requirements: * should be in database * should be easily searchable by initiator (user name / session id), event (activity type) and event parameters i can think of a database design but either it involves a lot of tables (one per event) so i can log each of the parameters of an event in a separate field OR it involves one table with generic fields (7 int numeric and 7 text types) and log everything in one table with event type field determining what parameter got written where (and hoping that i don't need more than 7 fields of a certain type, or 8 or 9 or whatever number i choose)... example of entries (the usual things): [username] login failed @datetime [username] login successful @datetime [username] changed password @datetime, estimated security of password [low/ok/high/perfect] @datetime [username] clicked result [result number] [result id] after searching for [search string] and got [number of results] @datetime [username] clicked result [result number] [result id] after searching for [search string] and got [number of results] @datetime [username] changed profile name from [old name] to [new name] @datetime [username] verified name with [credit card type] credit card @datetime datbase table [table name] purged of old entries @datetime etc... so anyone dealt with this before? any best practices / links you can share? i've seen it done with the generic solution mentioned above, but somehow that goes against what i learned from database design, but as you can see the sheer number of events that need to be trackable (each user will be able to see this info) is giving me headaches, BUT i do LOVE the one event per table solution more than the generic one. any thoughts? edit: also, is there maybe an authoritative list of such (likely) events somewhere? thnx stack overflow says: the question you're asking appears subjective and is likely to be closed. my answer: probably is subjective, but it is directly related to my issue i have with designing a database / writing my code, so i'd welcome any help. also i tried narrowing down the ideas to 2 so hopefully one of these will prevail, unless there already is an established solution for these kinds of things.

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• how to clear XFixes regions

  - by ~buratinas

  Hi, I'm writing some low level code for X11 platform. To achieve best data copying performance I use XFixes/XDamage extensions. How can I clear the contents of XFixes region after one refresh cycle? Or do they clean themselves after I use XFixesSetPictureClipRegion? My code is something like that: Display xdpy; XShamPixmap pixmap_; XFixesRegion region_; damage_event_callback(damage_geometry_t geometry, XDamage damage,...) { unsigned curr_region = XFixesCreateRegion(xdpy, 0, 0); XDamageSubtract(xdpy, damage, None, curr_region); XFixesTranslateRegion( xdpy, curr_region, geometry.left(), geometry.top() ); XFixesUnionRegion (xdpy, region_, region_, curr_region); } process_damage_events(...) { XFixesSetPictureClipRegion( xdpy, pixmap_, 0, 0, region_); XCopyArea (xdpy, window_->id(), pixmap_, XDefaultGC(xdpy, XDefaultScreen(xdpy)), 0,0,width(),height(),0,0); /*Should clear region_ here */ ... } Currently I clear the region by deleting and recreating, but I guess it's not the best way to do that.

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• architecture and tools for a remote control application?

  - by slothbear

  I'm working on the design of a remote control application. From my iPhone or a web browser, I'll send a few commands. Soon my home computer will perform the commands and send back results. I know there are remote desktop apps, but I want something programmable, something simpler, and something that I wrote. My current direction is to use Amazon Simple Queue Service (SQS) as the message bus. The iPhone places some messages in a queue. My local Java/JRuby program notices the messages on the queue, performs the work and sends back status via a different queue. This will be a very low-volume application. At $1.00 for a million requests (plus a handful of data transfer charges), Amazon SQS looks a lot more affordable than having my own server of any type. And super reliable, that's important for me too. Are there better/standard toolkits or architectures for this kind of remote control? Cost is not a big issue, but I prefer the tons I learn by doing it myself. I'm moderately concerned about security, but doubt it will be a problem. The list of commands recognized will be very short, and only recognized in specific contexts. No "erase hard drive" stuff. update: I'll probably distribute these programs to some other people who want the same function, but who don't have Amazon SQS accounts. For now, they'll use anonymous access to my queues, with random 80-character queue names.

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• Online file storage similar to Amazon S3

  - by Joel G

  I am looking to code a file storage application in perl similar to amazon s3. I already have a amazon s3 clone that I found online called parkplace but its in ruby and is old also isn't built for high loads. I am not really sure what modules and programs I should use so id like some help picking them out. My requirements are listed below (yes I know there are lots but I could start simple then add more once I get it going): Easy API implementation for client side apps. (maybe RESTful but extras like mkdir and cp (?) Centralized database server for the USERDB (maybe PostgreSQL (?). Logging of all connections, bandwidth used, well pretty much everything to a centralized server (maybe PostgreSQL again (?). Easy server side configuration (config file(s) stored on the servers). Web based control panel for admin(s) and user(s) to show logs. (could work just running queries from the databases) Fast High Uptime Low memory usage Some sort of load distribution/load balancer (maybe a dns based or pound or perlbal or something else (?). Maybe a cache of some sort (memcached or parlbal or something else (?). Thanks in advance

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• TicTacToe strategic reduction

  - by NickLarsen

  I decided to write a small program that solves TicTacToe in order to try out the effect of some pruning techniques on a trivial game. The full game tree using minimax to solve it only ends up with 549,946 possible games. With alpha-beta pruning, the number of states required to evaluate was reduced to 18,297. Then I applied a transposition table that brings the number down to 2,592. Now I want to see how low that number can go. The next enhancement I want to apply is a strategic reduction. The basic idea is to combine states that have equivalent strategic value. For instance, on the first move, if X plays first, there is nothing strategically different (assuming your opponent plays optimally) about choosing one corner instead of another. In the same situation, the same is true of the center of the walls of the board, and the center is also significant. By reducing to significant states only, you end up with only 3 states for evaluation on the first move instead of 9. This technique should be very useful since it prunes states near the top of the game tree. This idea came from the GameShrink method created by a group at CMU, only I am trying to avoid writing the general form, and just doing what is needed to apply the technique to TicTacToe. In order to achieve this, I modified my hash function (for the transposition table) to enumerate all strategically equivalent positions (using rotation and flipping functions), and to only return the lowest of the values for each board. Unfortunately now my program thinks X can force a win in 5 moves from an empty board when going first. After a long debugging session, it became apparent to me the program was always returning the move for the lowest strategically significant move (I store the last move in the transposition table as part of my state). Is there a better way I can go about adding this feature, or a simple method for determining the correct move applicable to the current situation with what I have already done?

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• looking to streamline my RSS feed mashup

  - by Mark Cejas

  Hello crafty developers, I have aggregated RSS feeds from various sources with RSSowl, fetching directly from the social mention API. The RSS feeds are categorized into the following major categories: blogs, news, twitter, Q&A and social networking sites. Each major category is nested with a common group of RSS feeds that represent a particular client/brand ontology. Merging these feeds into the RSSowl reader application, allows me to conduct and save refined search queries (from the aggregated data) into a single file - that I can then tag and further segment for analysis. This scheme is utilized for my own research needs and has helped me considerably. However, I find this RSS mashup scheme kinda clumsy, it requires quite a bit of time to initially organize all of the feeds and I would like to be able to do further natural language processing to the data as well as eventually be able to rank the collected list of URL's into some order of media prominence - right I don't want to pay the ridiculous radian6 web analytics fees, when my intuition is telling me that with a bit of 'elbow grease' I can maybe leverage some available resources online to develop a functional low scale web mining application and get some good intelligence from it. I am now starting to learn a little about computer science - my background is in physical science/statistics so is my thinking in the right track? So, I guess I am imagining an application that allows me to query in a refined manner. A manner that allows me to search for keyword combinations, applying AND/OR operators, selectively focus my queries into particular sources - like a collection of blogs or twitter, or social networking communities, then save the results of my queries into a structured format that can then be manipulated and explored. Am I dreaming? I just had to get all of this out. any bit of advice and insight would be hugely appreciated. my best, Mark

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• Webserver sending corrupt or corrupting served files

  - by NotIan

  EDIT: Looks like the problem was a rootkit that corrupted a bunch of low level linux commands, including top, ps, ifconfig, netstat and others. The problem was resolved by taking all web files off the server and wiping it. A dedicated server we operate is having a strange issue. Files are not be sent complete or are showing up with garbage data. Example: http://sustainablefitness.com/images/banner_bootcamps.jpg To make matters more confusing this corruption does NOT happen when the files are served as https, (I would post a link, but I don't have enough rep points, just add an 's' after http in the link above.) When I throw load at the server, I get dozens of (swapd)s in top this is the only thing that really jumps out. I can't post images but ( imgur.com / ZArSq.png ) is a screenshot of top. I have tried a lot of stuff so far, I am willing to try anything that I can. A dedicated server we operate is having a strange issue. Files are not be sent complete or are showing up with garbage data. Example: http://sustainablefitness.com/images/banner_bootcamps.jpg To make matters more confusing this corruption does NOT happen when the files are served as https, (I would post a link, but I don't have enough rep points, just add an 's' after http in the link above.) When I throw load at the server, I get dozens of (swapd)s in top this is the only thing that really jumps out. I can't post images but ( imgur.com / ZArSq.png ) is a screenshot of top. I have tried a lot of stuff so far, I am willing to try anything that I can.

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• How to find an embedded platform?

  - by gmagana

  I am new to the locating hardware side of embedded programming and so after being completely overwhelmed with all the choices out there (pc104, custom boards, a zillion option for each board, volume discounts, devel kits, ahhh!!) I am asking here for some direction. Basically, I must find a new motherboard and (most likely) re-implement the program logic. Rewriting this in C/C++/Java/C#/Pascal/BASIC is not a problem for me. so my real problem is finding the hardware. This motherboard will have several other devices attached to it. Here is a summary of what I need to do: Required: 2 RS232 serial ports (one used all the time for primary UI, the second one not continuous) 1 modem (9600+ baud ok) [Modem will be in simultaneous use with only one of the serial port devices, so interrupt sharing with one serial port is OK, but not both] Minimum permanent/long term storage: Whatever O/S requires + 1 MB (executable) + 512 KB (Data files) RAM: Minimal, whatever the O/S requires plus maybe 1MB for executable. Nice to have: USB port(s) Ethernet network port Wireless network Implementation languages (any O/S I will adapt to): First choice Java/C# (Mono ok) Second choice is C/Pascal Third is BASIC Ok, given all this, I am having a lot of trouble finding hardware that will support this that is low in cost. Every manufacturer site I visit has a lot of options, and it's difficult to see if their offering will even satisfy my must-have requirements (for example they sometimes list 3 "serial ports", but it appears that only one of the three is RS232, for example, and don't mention what the other two are). The #1 constraint is cost, #2 is size. Can anyone help me with this? This little task has left me thinking I should have gone for EE and not CS :-). EDIT: A bit of background: This is a system currently in production, but the original programmer passed away, and the current hardware manufacturer cannot find hardware to run the (currently) DOS system, so I need to reimplement this in a modern platform. I can only change the programming and the motherboard hardware.

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• Sudoku Recursion Issue (Java)

  - by SkylineAddict

  I'm having an issue with creating a random Sudoku grid. I tried modifying a recursive pattern that I used to solve the puzzle. The puzzle itself is a two dimensional integer array. This is what I have (By the way, the method doesn't only randomize the first row. I had an idea to randomize the first row, then just decided to do the whole grid): public boolean randomizeFirstRow(int row, int col){ Random rGen = new Random(); if(row == 9){ return true; } else{ boolean res; for(int ndx = rGen.nextInt() + 1; ndx <= 9;){ //Input values into the boxes sGrid[row][col] = ndx; //Then test to see if the value is valid if(this.isRowValid(row, sGrid) && this.isColumnValid(col, sGrid) && this.isQuadrantValid(row, col, sGrid)){ // grid valid, move to the next cell if(col + 1 < 9){ res = randomizeFirstRow(row, col+1); } else{ res = randomizeFirstRow( row+1, 0); } //If the value inputed is valid, restart loop if(res == true){ return true; } } } } //If no value can be put in, set value to 0 to prevent program counting to 9 setGridValue(row, col, 0); //Return to previous method in stack return false; } This results in an ArrayIndexOutOfBoundsException with a ridiculously high or low number (+- 100,000). I've tried to see how far it goes into the method, and it never goes beyond this line: if(this.isRowValid(row, sGrid) && this.isColumnValid(col, sGrid) && this.isQuadrantValid(row, col, sGrid)) I don't understand how the array index goes so high. Can anyone help me out?

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• Two radically different queries against 4 mil records execute in the same time - one uses brute force.

  - by IanC

  I'm using SQL Server 2008. I have a table with over 3 million records, which is related to another table with a million records. I have spent a few days experimenting with different ways of querying these tables. I have it down to two radically different queries, both of which take 6s to execute on my laptop. The first query uses a brute force method of evaluating possibly likely matches, and removes incorrect matches via aggregate summation calculations. The second gets all possibly likely matches, then removes incorrect matches via an EXCEPT query that uses two dedicated indexes to find the low and high mismatches. Logically, one would expect the brute force to be slow and the indexes one to be fast. Not so. And I have experimented heavily with indexes until I got the best speed. Further, the brute force query doesn't require as many indexes, which means that technically it would yield better overall system performance. Below are the two execution plans. If you can't see them, please let me know and I'll re-post then in landscape orientation / mail them to you. Brute-force query: Index-based exception query: My question is, based on the execution plans, which one look more efficient? I realize that thing may change as my data grows.

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• Centralized Exception handling for Eclipse plug-in

  - by Svilen

  Hello, At first I thought this would be question often asked, however trying (and failing) to look up info on this proved me wrong. Is there a mechanism in Eclipse platform for centralized exception handling of exceptions? For example... You have plug-in project which connects to a DB and issues queries, results of which are used to populate some e.g. views. This is like the most common example ever. :) Queries are executed almost for any user action, from every UI control the plug-in provides. Most likely the DB Query API will have some specific to the DB SomeDBGeneralException declared as being thrown by it. That's OK, you can handle those according to whatever your software design is. But how about unchecked exceptions which are likely to occur, e.g. , when communication with DB suddenly breaks for some network related reason? What if in such case one would like to catch those exceptions in a central place and for example provide user friendly message to the user (rather than the low level communication protocol api messages) and even some possible actions the user could execute in order to deal with the specific problem? Thinking in Eclipse platform context, the question may be rephrased as "Is there an extension point like "org.eclipse.ExceptionHandler" which allows to declare exception handlers for specific (some kind of filtering support) exceptions giving a lot of flexibility with the actual handling?"

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• Artifacts when trying to draw background grid without anti-aliasing in a QGraphicsScene

  - by estan

  Hi folks, I'm trying to draw a background grid in the drawBackground() function of my QGraphicsScene subclass: void Scene::drawBackground(QPainter *painter, const QRectF &rect) { const int gridSize = 50; const int realLeft = static_cast<int>(std::floor(rect.left())); const int realRight = static_cast<int>(std::ceil(rect.right())); const int realTop = static_cast<int>(std::floor(rect.top())); const int realBottom = static_cast<int>(std::ceil(rect.bottom())); // Draw grid. const int firstLeftGridLine = realLeft - (realLeft % gridSize); const int firstTopGridLine = realTop - (realTop % gridSize); QVarLengthArray<QLine, 100> lines; for (qreal x = firstLeftGridLine; x <= realRight; x += gridSize) lines.append(QLine(x, realTop, x, realBottom)); for (qreal y = firstTopGridLine; y <= realBottom; y += gridSize) lines.append(QLine(realLeft, y, realRight, y)); //painter->setRenderHint(QPainter::Antialiasing); painter->setPen(QPen(QColor(220, 220, 220), 0.0)); painter->drawLines(lines.data(), lines.size()); // Draw axes. painter->setPen(QPen(Qt::lightGray, 0.0)); painter->drawLine(0, realTop, 0, realBottom); painter->drawLine(realLeft, 0, realRight, 0); } However, unless I turn on anti-aliasing, moving items around will sometimes leave artifacts in the grid (areas where it's not drawn). It seems it mostly happens at low zoom levels, when the view is zoomed out a bit. Any ideas what I might be doing wrong here? I'd really don't want to turn anti-aliasing on since the lines are strictly horizontal and vertical, and I'd like them to be as crisp as possible. Any help is much appriciated, Regards, Elvis

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• how to set a fixed color bar for pcolor in python matplotlib?

  - by user248237

  I am using pcolor with a custom color map to plot a matrix of values. I set my color map so that low values are white and high values are red, as shown below. All of my matrices have values between 0 and 20 (inclusive) and I'd like 20 to always be pure red and 0 to always be pure white, even if the matrix has values that don't span the entire range. For example, if my matrix only has values between 2 and 7, I don't want it to plot 2 as white and 7 as red, but rather color it as if the range is still 0 to 20. How can I do this? I tried using the "ticks=" option of colorbar but it did not work. Here is my current code (assume "my_matrix" contains the values to be plotted): cdict = {'red': ((0.0, 1.0, 1.0), (0.5, 1.0, 1.0), (1.0, 1.0, 1.0)), 'green': ((0.0, 1.0, 1.0), (0.5, 1.0, 1.0), (1.0, 0.0, 0.0)), 'blue': ((0.0, 1.0, 1.0), (0.5, 1.0, 1.0), (1.0, 0.0, 0.0))} my_cmap = matplotlib.colors.LinearSegmentedColormap('my_colormap', cdict, 256) colored_matrix = plt.pcolor(my_matrix, cmap=my_cmap) plt.colorbar(colored_matrix, ticks=[0, 5, 10, 15, 20]) any idea how I can fix this to get the right result? thanks very much.

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• Algorithm for assigning a unique series of bits for each user?

  - by Mark

  The problem seems simple at first: just assign an id and represent that in binary. The issue arises because the user is capable of changing as many 0 bits to a 1 bit. To clarify, the hash could go from 0011 to 0111 or 1111 but never 1010. Each bit has an equal chance of being changed and is independent of other changes. What would you have to store in order to go from hash - user assuming a low percentage of bit tampering by the user? I also assume failure in some cases so the correct solution should have an acceptable error rate. I would an estimate the maximum number of bits tampered with would be about 30% of the total set. I guess the acceptable error rate would depend on the number of hashes needed and the number of bits being set per hash. I'm worried with enough manipulation the id can not be reconstructed from the hash. The question I am asking I guess is what safe guards or unique positioning systems can I use to ensure this happens.

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• New desktop GUI developer; can choose any platform...

  - by alexantd

  I'm planning a client-server product for a tiny, low-volume, high-cost vertical market. One of the components of the product will be a desktop application, simple to moderate in complexity, for data entry and uploading to a central server from remote PCs and/or Macs via SOAP. The server is a Java web app. Customers will be choosing their platform (Windows or Mac) based on what the client app runs on, so my options are wide-open here. However, I will be developing on a Mac and have a strong allergy to MS-specific technologies (sorry). The app will not need to run on any non-desktop-computer devices and I have total freedom to say it will support X but not Y or Z without any negative consequences (quite the luxury, to be sure). I have a lot of experience in server-side development but very little in desktop GUI stuff, and am evaluating my options on the client - basically what do I want to commit to learning over the next 6+ months. I have server-side Java experience as well as a brief dabble in iPhone development, which went OK. Overall I'm looking for: Ease of learning & development IDE support Healthy surrounding ecosystem (libraries, tools, help, etc.) Quality documentation My options as I see them, in rough order of how I'm currently mentally ranking them: Java Swing Cocoa Java SWT JavaFX Adobe AIR XULRunner Am I leaving anything out?

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• Unit testing an MVC action method with a Cache dependency?

  - by Steve

  I’m relatively new to testing and MVC and came across a sticking point today. I’m attempting to test an action method that has a dependency on HttpContext.Current.Cache and wanted to know the best practice for achieving the “low coupling” to allow for easy testing. Here's what I've got so far... public class CacheHandler : ICacheHandler { public IList<Section3ListItem> StateList { get { return (List<Section3ListItem>)HttpContext.Current.Cache["StateList"]; } set { HttpContext.Current.Cache["StateList"] = value; } } ... I then access it like such... I'm using Castle for my IoC. public class ProfileController : ControllerBase { private readonly ISection3Repository _repository; private readonly ICacheHandler _cache; public ProfileController(ISection3Repository repository, ICacheHandler cacheHandler) { _repository = repository; _cache = cacheHandler; } [UserIdFilter] public ActionResult PersonalInfo(Guid userId) { if (_cache.StateList == null) _cache.StateList = _repository.GetLookupValues((int)ELookupKey.States).ToList(); ... Then in my unit tests I am able to mock up ICacheHandler. Would this be considered a 'best practice' and does anyone have any suggestions for other approaches? Thanks in advance. Cheers

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• Flushing writes in buffer of Memory Controller to DDR device

  - by Rohit

  At some point in my code, I need to push the writes in my code all the way to the DIMM or DDR device. My requirement is to ensure the write reaches the row,ban,column of the DDR device on the DIMM. I need to read what I've written to the main memory. I do not want caching to get me the value. Instead after writing I want to fetch this value from main memory(DIMM's). So far I've been using Intel's x86 instruction wbinvd(write back and invalidate cache). However this means the caches and TLB are flushed. Write-back requests go to the main memory. However, there is a reasonable amount of time this data might reside in the write buffer of the Memory Controller( Intel calls it integrated memory controller or IMC). The Memory Controller might take some more time depending on the algorithm that runs in the Memory Controller to handle writes. Is there a way I force all existing or pending writes in the write buffer of the memory controller to the DRAM devices ?? What I am looking for is something more direct and more low-level than wbinvd. If you could point me to right documents or specs that describe this I would be grateful. Generally, the IMC has a several registers which can be written or read from. From looking at the specs for that for the chipset I could not find anything useful. Thanks for taking the time to read this.

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• How do I serialise a graph in Java without getting StackOverflowException?

  - by Tim Cooper

  I have a graph structure in java, ("graph" as in "edges and nodes") and I'm attempting to serialise it. However, I get "StackOverflowException", despite significantly increasing the JVM stack size. I did some googling, and apparently this is a well known limitation of java serialisation: that it doesn't work for deeply nested object graphs such as long linked lists - it uses a stack record for each link in the chain, and it doesn't do anything clever such as a breadth-first traversal, and therefore you very quickly get a stack overflow. The recommended solution is to customise the serialisation code by overriding readObject() and writeObject(), however this seems a little complex to me. (It may or may not be relevant, but I'm storing a bunch of fields on each edge in the graph so I have a class JuNode which contains a member ArrayList<JuEdge> links;, i.e. there are 2 classes involved, rather than plain object references from one node to another. It shouldn't matter for the purposes of the question). My question is threefold: (a) why don't the implementors of Java rectify this limitation or are they already working on it? (I can't believe I'm the first person to ever want to serialise a graph in java) (b) is there a better way? Is there some drop-in alternative to the default serialisation classes that does it in a cleverer way? (c) if my best option is to get my hands dirty with low-level code, does someone have an example of graph serialisation java source-code that can use to learn how to do it?

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