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• Rails running multiple delayed_job - lock tables

  - by pepernik

  Hey. I use delayed_job for background processing. I have 8 CPU server, MySQL and I start 7 delayed_job processes RAILS_ENV=production script/delayed_job -n 7 start Q1: I'm wondering is it possible that 2 or more delayed_job processes start processing the same process (the same record-row in the database delayed_jobs). I checked the code of the delayed_job plugin but can not find the lock directive in a way it should be. I think each process should lock the database table before executing an UPDATE on lock_by column. They lock the record simply by updating the locked_by field (UPDATE delayed_jobs SET locked_by...). Is that really enough? No locking needed? Why? I know that UPDATE has higher priority than SELECT but I think this does not have the effect in this case. My understanding of the multy-threaded situation is: Process1: Get waiting job X. [OK] Process2: Get waiting jobs X. [OK] Process1: Update locked_by field. [OK] Process2: Update locked_by field. [OK] Process1: Get waiting job X. [Already processed] Process2: Get waiting jobs X. [Already processed] I think in some cases more jobs can get the same information and can start processing the same process. Q2: Is 7 delayed_jobs a good number for 8CPU server? Why yes/not. Thx 10x!

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• Would this method work to scale out SQL queries?

  - by David

  I have a database containing a single huge table. At the moment a query can take anything from 10 to 20 minutes and I need that to go down to 10 seconds. I have spent months trying different products like GridSQL. GridSQL works fine, but is using its own parser which does not have all the needed features. I have also optimized my database in various ways without getting the speedup I need. I have a theory on how one could scale out queries, meaning that I utilize several nodes to run a single query in parallel. The idea is to take an incoming SQL query and simply run it exactly like it is on all the nodes. When the results are returned to a coordinator node, the same query is run on the union of the resultsets. I realize that an aggregate function like average need to be rewritten into a count and sum to the nodes and that the coordinator divides the sum of the sums with the sum of the counts to get the average. What kinds of problems could not easily be solved using this model. I believe one issue would be the count distinct function. Edit: I am getting so many nice suggestions, but none have addressed the method.

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• What is the best database structure for this scenario?

  - by Ricketts

  I have a database that is holding real estate MLS (Multiple Listing Service) data. Currently, I have a single table that holds all the listing attributes (price, address, sqft, etc.). There are several different property types (residential, commercial, rental, income, land, etc.) and each property type share a majority of the attributes, but there are a few that are unique to that property type. My question is the shared attributes are in excess of 250 fields and this seems like too many fields to have in a single table. My thought is I could break them out into an EAV (Entity-Attribute-Value) format, but I've read many bad things about that and it would make running queries a real pain as any of the 250 fields could be searched on. If I were to go that route, I'd literally have to pull all the data out of the EAV table, grouped by listing id, merge it on the application side, then run my query against the in memory object collection. This also does not seem very efficient. I am looking for some ideas or recommendations on which way to proceed. Perhaps the 250+ field table is the only way to proceed. Just as a note, I'm using SQL Server 2012, .NET 4.5 w/ Entity Framework 5, C# and data is passed to asp.net web application via WCF service. Thanks in advance.

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• java and threads: very strange behaviour

  - by Derk

  private synchronized Map<Team, StandingRow> calculateStanding() { System.out.println("Calculate standing for group " + getName()); Map<Team, StandingRow> standing = new LinkedHashMap<Team, StandingRow>(); for (Team team : teams) { standing.put(team, new StandingRow(team)); } StandingRow homeTeamRow, awayTeamRow; for (Match match : matches.values()) { homeTeamRow = standing.get(match.getHomeTeam()); awayTeamRow = standing.get(match.getAwayTeam()); System.out.println("Contains key for " + match.getHomeTeam() + ": " + standing.containsKey(match.getHomeTeam())); System.out.println("Contains key for " + match.getAwayTeam() + ": " + standing.containsKey(match.getAwayTeam())); } } This is my code. matches contains 6 elements, but the problem is that after two matches no keys are anymore found in the standing map. The output is for example Contains key for Zuid-Afrika: true Contains key for Mexico: true Contains key for Uruguay: true Contains key for Frankrijk: true Contains key for Zuid-Afrika: false Contains key for Uruguay: false Contains key for Frankrijk: false Contains key for Mexico: false Contains key for Mexico: false Contains key for Uruguay: false Contains key for Frankrijk: false Contains key for Zuid-Afrika: false This is in a threaded environment, but the method is synchronized so I thought that this would not give a problem? I have also a simple unit test for this method and that works well.

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• How to get the actual address of a pointer in C?

  - by Airjoe

  BACKGROUND: I'm writing a single level cache simulator in C for a homework assignment, and I've been given code that I must work from. In our discussions of the cache, we were told that the way a small cache can hold large addresses is by splitting the large address into the position in the cache and an identifying tag. That is, if you had an 8 slot cache but wanted to store something with address larger than 8, you take the 3 (because 2^3=8) rightmost bits and put the data in that position; so if you had address 22 for example, binary 10110, you would take those 3 rightmost bits 110, which is decimal 5, and put it in slot 5 of the cache. You would also store in this position the tag, which is the remaining bits 10. One function, cache_load, takes a single argument, and integer pointer. So effectively, I'm being given this int* addr which is an actual address and points to some value. In order to store this value in the cache, I need to split the addr. However, the compiler doesn't like when I try to work with the pointer directly. So, for example, I try to get the position by doing: npos=addr%num_slots The compiler gets angry and gives me errors. I tried casting to an int, but this actually got me the value that the pointer was pointing to, not the numerical address itself. Any help is appreciated, thanks!

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• What exactly does entitlemets.plist do ?

  - by sagar

  To execute my application in iPhone first of all I have to add provisioning profile in to my iPhone. Good. But I don't understand the reason behind adding a entitlements.plist in iphone application in xCode under resources. What does that entitlements.plist file do actually ? Why that .plist file have only single Boolean in it ? (get-task-allow) Can't we add that Boolean variable to application-info.plist ? I mean, I can't find the reason behind storing a single Boolean variable in separate plist file. Instead application-info.plist already contains many many application settings. Can't we just add this Boolean variable to application-info.plist. Ok, any way. I know - it won't be possible. Because we have to follow - what apple says. But, here main intention behind all these was "what is the basic need of entitlements.plist ?" or "what is the functionality of entitlements.plist ?" "How provisioning profile, entitlements.plist, application-info.plist & iPhone application compile all together & make verification on iPhone & execute it on iPhone ? "

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• Web page database query optimization

  - by morpheous

  I am putting together a web page which is quite 'expensive' in terms of database hits. I don't want to start optimizing at this stage - though with me trying to hit a deadline, I may end up not optimizing at all. Currently the page requires 18 (that's right eighteen) hits to the db. I am already using joins, and some of the queries are UNIONed to minimize the trips to the db. My local dev machine can handle this (page is not slow) however, I feel if I release this into the wild, the number of queries will quickly overwhelm my database (MySQL). I could always use memcache or something similar, but I would much rather continue with my other dev work that needs to be completed before the deadline - at least retrieving the page works - its simply a matter of optimization now (if required). My question therefore is - is 18 db queries for a single page retrieval completely outrageous - (i.e. I should put everything on hold and optimize the hell of the retrieval logic), or shall I continue as normal, meet the deadline and release on schedule and see what happens? [Edit] Just to clarify, I have already done the 'obvious' things like using (single and composite) indexes for fields used in the queries. What I haven't yet done is to run a query analyzer to see if my indexes etc are optimal.

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• What does the Excel VBA range.Rows property really do?

  - by RBarryYoung

  OK, I am finishing up an add-on project for a legacy Excel-VBA application, and I have once again run up against the conundrum of the mysterious range.Rows(?) and worksheet.Rows properties. Does anyone know what these properties really do and what they are supposed to provide to me? (note: all of this probably applies to the corresponding *.Columns properties also). What I would really like to be able to use it for is to return a range of rows, like this: SET rng = wks.Rows(iStartRow, iEndRow) But I have never been able to get it to do that, even though the Intellisense shows two arguments for it. Instead I have to use one of the two or three other (very kludgy) techniques. The help is very unhelpful (typically so for Office VBA), and googling for "Rows" is not very useful, no matter how many other terms I add to it. The only things that I have been able to use it for are 1) return a single row as a range ( rng.Rows(i) ) and 2) return a count of the rows in a range ( rng.Rows.Count ). Is that it? Is there really nothing else that it's good for? Clarification: I know that it returns a range and that there are other ways to get a range of rows. What I am asking for is specifically what do we get from .Rows() that we do not already get from .Cells() and .Range()? The two things that I know are 1) an easier way to return a range of a single row and 2) a way to count the number of rows in a range. Is there anything else?

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• Parse numbers in singe textbox for query

  - by Joshua Slocum

  I’ve built a webform in Visual Web Developer Express 2008 to help me with my work. I use a webform to run query requests that are emailed to me. The inputs are in this format 12312 12312 12312 12312 12312 12312 12312 12312 I enter the first number in a textbox and the second number in another textbox and click a button that runs a query and returns the results in a gridview(single row). string strConn, strSQL; strConn = AppConfig.Connection strSQL = 'select fields from table where FirstNum=:FirstNum and SecondNum=:SecondNum'; using (OracleConnection cn = new OracleConnection(strConn)) { OracleCommand cmd = new OracleCommand(strSQL, cn); cmd.Parameters.AddWithValue(":FirstNum", txtFirstNum.Text); cmd.Parameters.AddWithValue(":SeconNum", txtSecondNum.Text); cn.Open(); using (OracleDataReader rdr = cmd.ExecuteReader()) { dgResults.DataSource = rdr; dgResults.DataBind(); } cn.Close(); } I had an idea to help me speed up my work. I’d like to be able to past both numbers in a single textbox ( like this 12312 12312 ) and have the code parse out the nubmers for the query. Or even better would be to past all of them in a multiline textbox like this 12312 12312 12312 12312 12312 12312 12312 12312 And have them all parsed and the query run for each line and the results all output to one gridview. I’m just not sure how to approach this. Any suggestions would be appreciated. Thank you.

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• Message queue proxy in Python + Twisted

  - by gasper_k

  Hi, I want to implement a lightweight Message Queue proxy. It's job is to receive messages from a web application (PHP) and send them to the Message Queue server asynchronously. The reason for this proxy is that the MQ isn't always avaliable and is sometimes lagging, or even down, but I want to make sure the messages are delivered, and the web application returns immediately. So, PHP would send the message to the MQ proxy running on the same host. That proxy would save the messages to SQLite for persistence, in case of crashes. At the same time it would send the messages from SQLite to the MQ in batches when the connection is available, and delete them from SQLite. Now, the way I understand, there are these components in this service: message listener (listens to the messages from PHP and writes them to a Incoming Queue) DB flusher (reads messages from the Incoming Queue and saves them to a database; due to SQLite single-threadedness) MQ connection handler (keeps the connection to the MQ server online by reconnecting) message sender (collects messages from SQlite db and sends them to the MQ server, then removes them from db) I was thinking of using Twisted for #1 (TCPServer), but I'm having problem with integrating it with other points, which aren't event-driven. Intuition tells me that each of these points should be running in a separate thread, because all are IO-bound and independent of each other, but I could easily put them in a single thread. Even though, I couldn't find any good and clear (to me) examples on how to implement this worker thread aside of Twisted's main loop. The example I've started with is the chatserver.py, which uses service.Application and internet.TCPServer objects. If I start my own thread prior to creating TCPServer service, it runs a few times, but the it stops and never runs again. I'm not sure, why this is happening, but it's probably because I don't use threads with Twisted correctly. Any suggestions on how to implement a separate worker thread and keep Twisted? Do you have any alternative architectures in mind?

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• Threading is slow and unpredictable?

  - by Jake

  I've created the basis of a ray tracer, here's my testing function for drawing the scene: public void Trace(int start, int jump, Sphere testSphere) { for (int x = start; x < scene.SceneWidth; x += jump) { for (int y = 0; y < scene.SceneHeight; y++) { Ray fired = Ray.FireThroughPixel(scene, x, y); if (testSphere.Intersects(fired)) sceneRenderer.SetPixel(x, y, Color.Red); else sceneRenderer.SetPixel(x, y, Color.Black); } } } SetPixel simply sets a value in a single dimensional array of colours. If I call the function normally by just directly calling it it runs at a constant 55fps. If I do: Thread t1 = new Thread(() => Trace(0, 1, testSphere)); t1.Start(); t1.Join(); It runs at a constant 50fps which is fine and understandable, but when I do: Thread t1 = new Thread(() => Trace(0, 2, testSphere)); Thread t2 = new Thread(() => Trace(1, 2, testSphere)); t1.Start(); t2.Start(); t1.Join(); t2.Join(); It runs all over the place, rapidly moving between 30-40 fps and sometimes going out of that range up to 50 or down to 20, it's not constant at all. Why is it running slower than it would if I ran the whole thing on a single thread? I'm running on a quad core i5 2500k.

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• Looping login with Facebook JS SDK and Rails

  - by nafe

  I'm using the Facebook JS SDK for single-sign with my rails app. I translated the php code from the Facebook example (at the bottom of the page under "Single Sign-on with the JavaScript SDK") into ruby. This appeared to be working great but I've noticed one user that gets continual redirects when trying to login. The logs look like: Processing UsersController#login (for X.X.X.X at 2010-05-22 17:25:55) [GET] Redirected to http://myapp.com/ Completed in 0ms (DB: 0) | 302 Found [http://myapp.com/login] (times as many entries as my unfortunate user leaves the browser redirecting in a loop). My client side code includes a callback with the "auth.sessionChange": FB.Event.subscribe('auth.sessionChange', function(response) { if (response.session) { // A user has logged in, and a new cookie has been saved window.location = "/login"; } else { // The user has logged out, and the cookie has been cleared window.location = "/logout"; } }); So it seems to me like this event is continually firing. Although, I can't test this theory because I can't recreate this scenario locally. I don't think it's the rails controller. The code here is just: def login # if first time user create db entry # now redirect back to where the user came from begin redirect_to :back rescue ActionController::RedirectBackError redirect_to root_url end end Does anyone have any idea on what's going on?

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• Use of Syntactic Sugar / Built in Functionality

  - by Kyle Rozendo

  I was busy looking deeper into things like multi-threading and deadlocking etc. The book is aimed at both pseudo-code and C code and I was busy looking at implementations for things such as Mutex locks and Monitors. This brought to mind the following; in C# and in fact .NET we have a lot of syntactic sugar for doing things. For instance (.NET 3.5): lock(obj) { body } Is identical to: var temp = obj; Monitor.Enter(temp); try { body } finally { Monitor.Exit(temp); } There are other examples of course, such as the using() {} construct etc. My question is when is it more applicable to "go it alone" and literally code things oneself than to use the "syntactic sugar" in the language? Should one ever use their own ways rather than those of people who are more experienced in the language you're coding in? I recall having to not use a Process object in a using block to help with some multi-threaded issues and infinite looping before. I still feel dirty for not having the using construct in there. Thanks, Kyle

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• Database nesting layout confusion

  - by arzon

  I'm no expert in databases and a beginner in Rails, so here goes something which kinda confuses me... Assuming I have three classes as a sample (note that no effort has been made to address any possible Rails reserved words issue in the sample). class File < ActiveRecord::Base has_many :records, :dependent => :destroy accepts_nested_attributes_for :records, :allow_destroy => true end class Record < ActiveRecord::Base belongs_to :file has_many :users, :dependent => :destroy accepts_nested_attributes_for :users, :allow_destroy => true end class User < ActiveRecord::Base belongs_to :record end Upon entering records, the database contents will appear as such. My issue is that if there are a lot of Files for the same Record, there will be duplicate record names. This will also be true if there will be multiple Records for the same user in the the Users table. I was wondering if there is a better way than this so as to have one or more files point to a single Record entry and one or more Records will point to a single User. BTW, the File names are unique. Files table: id name 1 name1 2 name2 3 name3 4 name4 Records table: id file_id record_name record_type 1 1 ForDaisy1 ... 2 2 ForDonald1 ... 3 3 ForDonald2 ... 4 4 ForDaisy1 ... Users table: id record_id username 1 1 Daisy 2 2 Donald 3 3 Donald 4 4 Daisy Is there any way to optimize the database to prevent duplication of entries, or this should really the correct and proper behavior. I spread out the database into different tables to be able to easily add new columns in the future.

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• highlight the text of the DOM range element,

  - by ganapati hegde

  I select some text on the html page(opened in firefox) using mouse,and using javascript functions, i create/get the rangeobject corresponding to the selected text. userSelection =window.getSelection(); var rangeObject = getRangeObject(userSelection); Now i want to highlight all the text which comes under the rangeobject.I am doing it like this, var span = document.createElement("span"); rangeObject.surroundContents(span); span.style.backgroundColor = "yellow"; Well,this works fine, only when the rangeobject(startpoint and endpoint) lies in the same textnode,then it highlights the corresponding text.Ex <p>In this case,the text selected will be highlighted properly, because the selected text lies under a single textnode</p> But if the rangeobject covers more than one textnode, then it is not working properlay, It highlights only the texts which lie in the first textnode,Ex <p><h3>In this case</h3>, only the text inside the header(h3) will be highlighted, not any text outside the header</p> Any idea how can i make, all the texts which comes under rangeobject,highlighted,independent of whether range lies in a single node or multiple node? Thanks....

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• Passing data structures to different threads

  - by Robb

  I have an application that will be spawning multiple threads. However, I feel there might be an issue with threads accessing data that they shouldn't be. I'm relatively new to threading so bare with me. Here is the structure of the threaded application (sorry for the crudeness): MainThread / \ / \ / \ Thread A Thread B / \ / \ / \ / \ / \ / \ Thread A_1 Thread A_2 Thread B_1 Thread B_2 Under each lettered thread (which could be many), there will only be two threads and they are fired of sequentially. The issue i'm having is I'm not entirely sure how to pass in a datastructure into these threads. So, the datastructure is created in MainThread, will be modified in the lettered thread (Thread A, etc) specific to that thread and then a member variable from that datastructure is sent to Letter_Numbered threads. Currently, the lettered thread class has a member variable and when the class is constructed, the datastructure from mainthread is passed in by reference, invoking the copy constructor so the lettered thread has it's own copy to play with. The lettered_numbered thread simply takes in a string variable from the data structure within the lettered thread. My question is, is this accceptable? Is there a much better way to ensure each lettered thread gets its own data structure to play with? Sorry for the somewhat poor explanation, please leave comments and i'll try to clarify.

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• JBoss Clustered Service that sends emails from txt file

  - by michael lucas

  I need a little push in the right direction. Here's my problem: I have to create an ultra-reliable service that sends email messages to clients whose addresses are stored in txt file on FTP server. Single txt file may contain unlimited number of entries. Most often the file contains about 300,000 entries. Service exposes interface with just two simple methods: TaskHandle sendEmails(String ftpFilePath); ProcessStatus checkProcessStatus(TaskHandle taskHandle); Method sendEmails() returns TaskHandle by which we can ask for ProcessStatus. For such a service to be reliable clustering is necessary. Processing single txt file might take a long time. Restarting one node in a cluster should have no impact on sending emails. We use JBoss AS 4.2.0 which comes with a nice HASingletonController that ensure one instance of service is running at given time. But once a fail-over happens, the second service should continue work from where the first one stopped. How can I share state between nodes in a cluster in such a way that leaves no possibility of sending some emails twice?

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• Why does Tex/Latex not speed up in subsequent runs?

  - by Debilski

  I really wonder, why even recent systems of Tex/Latex do not use any caching to speed up later runs. Every time that I fix a single comma*, calling Latex costs me about the same amount of time, because it needs to load and convert every single picture file. (* I know that even changing a tiny comma could affect the whole structure but of course, a well-written cache format could see the impact of that. Also, there might be situations where 100% correctness is not needed as long as it’s fast.) Is there something in the language of Tex which makes this complicated or impossible to accomplish or is it just that in the original implementation of Tex, there was no need for this (because it would have been slow anyway on those large computers)? But then on the other hand, why doesn’t this annoy other people so much that they’ve started a fork which has some sort of caching (or transparent conversion of Tex files to a format which is faster to parse)? Is there anything I can do to speed up subsequent runs of Latex? Except from putting all the stuff into chapterXX.tex files and then commenting them out?

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• Thread-safe data structure design

  - by Inso Reiges

  Hello, I have to design a data structure that is to be used in a multi-threaded environment. The basic API is simple: insert element, remove element, retrieve element, check that element exists. The structure's implementation uses implicit locking to guarantee the atomicity of a single API call. After i implemented this it became apparent, that what i really need is atomicity across several API calls. For example if a caller needs to check the existence of an element before trying to insert it he can't do that atomically even if each single API call is atomic: if(!data_structure.exists(element)) { data_structure.insert(element); } The example is somewhat awkward, but the basic point is that we can't trust the result of "exists" call anymore after we return from atomic context (the generated assembly clearly shows a minor chance of context switch between the two calls). What i currently have in mind to solve this is exposing the lock through the data structure's public API. This way clients will have to explicitly lock things, but at least they won't have to create their own locks. Is there a better commonly-known solution to these kinds of problems? And as long as we're at it, can you advise some good literature on thread-safe design? EDIT: I have a better example. Suppose that element retrieval returns either a reference or a pointer to the stored element and not it's copy. How can a caller be protected to safely use this pointer\reference after the call returns? If you think that not returning copies is a problem, then think about deep copies, i.e. objects that should also copy another objects they point to internally. Thank you.

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• What information should a SVN/Versioned file commit comment contain?

  - by RenderIn

  I'm curious what kind of content should be in a versioned file commit comment. Should it describe generally what changed (e.g. "The widget screen was changed to display only active widgets") or should it be more specific (e.g. "A new condition was added to the where clause of the fetchWidget query to retrieve only active widgets by default") How atomic should a single commit be? Just the file containing the updated query in a single commit (e.g. "Updated the widget screen to display only active widgets by default"), or should that and several other changes + interface changes to a screen share the same commit with a more general description like ("Updated the widget screen: A) display only active widgets by default B) added button to toggle showing inactive widgets") I see subversion commit comments being used very differently and was wondering what others have had success with. Some comments are as brief as "updated files", while others are many paragraphs long, and others are formatted in a way that they can be queried and associated with some external system such as JIRA. I used to be extremely descriptive of the reason for the change as well as the specific technical changes. Lately I've been scaling back and just giving a general "This is what I changed on this page" kind of comment.

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• Putting a C++ Vector as a Member in a Class that Uses a Memory Pool

  - by Deep-B

  Hey, I've been writing a multi-threaded DLL for database access using ADO/ODBC for use with a legacy application. I need to keep multiple database connections for each thread, so I've put the ADO objects for each connection in an object and thinking of keeping an array of them inside a custom threadInfo object. Obviously a vector would serve better here - I need to delete/rearrange objects on the go and a vector would simplify that. Problem is, I'm allocating a heap for each thread to avoid heap contention and stuff and allocating all my memory from there. So my question is: how do I make the vector allocate from the thread-specific heap? (Or would it know internally to allocate memory from the same heap as its wrapper class - sounds unlikely, but I'm not a C++ guy) I've googled a bit and it looks like I might need to write an allocator or something - which looks like so much of work I don't want. Is there any other way? I've heard vector uses placement-new for all its stuff inside, so can overloading operator new be worked into it? My scant knowledge of the insides of C++ doesn't help, seeing as I'm mainly a C programmer (even that - relatively). It's very possible I'm missing something elementary somewhere. If nothing easier comes up - I might just go and do the array thing, but hopefully it won't come to that. I'm using MS-VC++ 6.0 (hey, it's rude to laugh! :-P ). Any/all help will be much appreciated.

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• MySQL Gurus: How to pull a complex grid of data from MySQL database with one query?

  - by iopener

  Hopefully this is less complex than I think. I have one table of companies, and another table of jobs, and a third table with that contains a single entry for each employee in each job from each company. NOTE: Some companies won't have employees in some jobs, and some companies will have more than one employee in some jobs. The company table has a companyid and companyname field, the job table has a jobid and jobtitle field, and the employee table has employeeid, companyid, jobid and employeename fields. I want to build a table like this: +-----------+-----------+-----------+ | Company A | Company B | Company C | ------+-----------+-----------+-----------+ Job A | Emp 1 | Emp 2 | | ------+-----------+-----------+-----------+ Job B | Emp 3 | | Emp 4 | | | | Emp 5 | ------+-----------+-----------+-----------+ Job C | | Emp 6 | | | | Emp 7 | | | | Emp 8 | | ------+-----------+-----------+-----------+ I had previously been looping through a result set of jobs, and for each job, looping through a result set of each company, and for each company, looping through each employee and printing it in a table (gross, but performance was not supposed to be a consideration). The app has grown in popularity, and now we have 100 companies and hundreds of jobs, and the server is crapping out (all the id fields are indexed). Any suggestions on how to write a single query to get this data? I don't need the company names or job titles (obviously), but I do need some way to identify where each row from the result should be printed. I'm imagining a result set that just contained a long list of joined employees, and I could write a loop to use the companyid and employeeid values to tell me when to create a new cell or table row. This works as long as there aren't ZERO employees; I would need a NULL employee name for that I think? Am I completely on the wrong track? Thanks in advance for any ideas!

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• Private Java class properties mysteriously reset between method calls....

  - by Michael Jones

  I have a very odd problem. A class property is mysteriously reset between method calls. The following code is executed so the constructor is called, then the parseConfiguration method is called. Finally, processData is called. The parseConfiguration method sets the "recursive" property to "true". However, as soon as it enters "processData", "recursive" becomes "false". This problem isn't isolated to a single class -- I have several examples of this in my code. How can this possibly be happening? I've tried initialising properties when they're declared outside any methods, I've tried initialising them in constructors... nothing works. The only complication I can think of here is that this class is invoked by an object that runs in a thread -- but here is one instance per thread, so surely no chance that threads are interfering. I've tried setting both methods to "synchronized", but this still happens. Please help! /** * This class or its superclasses are NOT threaded and don't extend Thread */ public class DirectoryAcquirer extends Manipulator { /** * @var Whether to recursively scan directories */ private boolean recursive = false; /** * Constructor */ public DirectoryAcquirer() { } /** * Constructor that initialises the configuration * * @param config * @throws InvalidConfigurationException */ public DirectoryAcquirer(HierarchicalConfiguration config) throws InvalidConfigurationException { super(config); } @Override protected void parseConfiguration() throws InvalidConfigurationException { // set whether to recurse into directories or not if (this.config.containsKey("recursive")) { // this.recursive gets set to "true" here this.recursive = this.config.getBoolean("recursive"); } } @Override public EntityCollection processData(EntityCollection data) { // here this.recursive is "false" this.logger.debug("processData: Entered method"); } }

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• Ordering by a max or a min from another table

  - by Paul Tomblin

  I have a table that consists of a unique id, and a few other attributes. It holds "schedules". Then I have another table that holds a list of all the times each schedule has or will "fire". This isn't the exact schema, but it's close: create table schedule ( id varchar(40) primary key, attr1 int, attr2 varchar(20) ); create table schedule_times ( id varchar(40) foreign key schedule(id), fire_date date ); I want to query the schedule table, getting the attributes and the next and previous fire_dates, in Java, sometimes ordering on one of the attributes, but sometimes ordering on either previous fire_date or the next fire_date. Ordering by the attributes is easy, I just stick an "order by" into the string while I'm building my prepared statement. I'm not even sure how to go about selecting the last fire_date and the next one in a single query - I know that I can find the next fire_date for a given id by doing a SELECT min(fire_date) FROM schedule_times WHERE id = ? AND fire_date > sysdate; and the similar thing for previous fire_date using max() and fire_date < sysdate. I'm just drawing a blank on how to incorporate that into a single select from the schedule so I can get both next and previous fire_date in one shot, and also how to order by either of those attributes.

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• activemessaging with stomp and activemq.prefetchSize=1

  - by Clint Miller

  I have a situation where I have a single activemq broker with 2 queues, Q1 and Q2. I have two ruby-based consumers using activemessaging. Let's call them C1 and C2. Both consumers subscribe to each queue. I'm setting activemq.prefetchSize=1 when subscribing to each queue. I'm also setting ack=client. Consider the following sequence of events: 1) A message that triggers a long-running job is published to queue Q1. Call this M1. 2) M1 is dispatched to consumer C1, kicking off a long operation. 3) Two messages that trigger short jobs are published to queue Q2. Call these M2 and M3. 4) M2 is dispatched to C2 which quickly runs the short job. 5) M3 is dispatched to C1, even though C1 is still running M1. It's able to dispatch to C1 because prefetchSize=1 is set on the queue subscription, not on the connection. So the fact that a Q1 message has already been dispatched doesn't stop one Q2 message from being dispatched. Since activemessaging consumers are single-threaded, the net result is that M3 sits and waits on C1 for a long time until C1 finishes processing M1. So, M3 is not processed for a long time, despite the fact that consumer C2 is sitting idle (since it quickly finishes with message M2). Essentially, whenever a long Q1 job is run and then a whole bunch of short Q2 jobs are created, exactly one of the short Q2 jobs gets stuck on a consumer waiting for the long Q1 job to finish. Is there a way to set prefetchSize at the connection level rather than at the subscription level? I really don't want any messages dispatched to C1 while it is processing M1. The other alternative is that I could create a consumer dedicated to processing Q1 and then have other consumers dedicated to processing Q2. But, I'd rather not do that since Q1 messages are infrequent--Q1's dedicated consumers would sit idle most of the day tying up memory.

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