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  • model (3ds) stats & snapshot in linux

    - by acidzombie24
    I want to write an app that takes in a model filename via cmd line, create a list of stats (poly count, scaling, as much as possible or maybe the stats that i would like) and to load the model with its textures (with anything else) and draw it from multiple position to save the images as pngs. How would i do this? are there utils i can use to extract data from models? how about drawing the models? my server does not have a desktop or video card, would no video HW be a problem?

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  • Virtualbox, merging snapshots and base disk

    - by Henrik
    Hi, I have a virtual machine with about 30 snapshots in branches. The current development path is 22 snapshots plus the base disk. The amount of files is seemingly having an impact now on IO and the dev laptop I'm using (don't know if it is host disk performance issues with the 140GB total size over a lot of fragments, or just the fact that it is hitting sectors distributed across a lot of files). I would like to merge the current development branch of snapshots together with the base disk, but I am unsure if the following command would produce the correct outcome. I am not able to boot this disk after the procedure completes (5-6 hours). vboxmanage clonehd "C:\VPC-Storage\.VirtualBox\Machines\CRM\Snapshots\{245b27ac-e658-470a-b978-8e62137c33b1}.vhd" "E:\crm-20100624.vhd" --format VHD --type normal Could anyone confirm if this is the correct approach or not?

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  • why Observable snapshot observer vector

    - by han14466
    In Observable's notifyObservers method, why does the coder use arrLocal = obs.toArray();? Why does not coder iterate vector directly? Thanks public void notifyObservers(Object arg) { Object[] arrLocal; synchronized (this) { /* We don't want the Observer doing callbacks into * arbitrary code while holding its own Monitor. * The code where we extract each Observable from * the Vector and store the state of the Observer * needs synchronization, but notifying observers * does not (should not). The worst result of any * potential race-condition here is that: * 1) a newly-added Observer will miss a * notification in progress * 2) a recently unregistered Observer will be * wrongly notified when it doesn't care */ if (!changed) return; arrLocal = obs.toArray(); clearChanged(); } for (int i = arrLocal.length-1; i>=0; i--) ((Observer)arrLocal[i]).update(this, arg); }

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  • Hidden Periodic Screenshots on a corporate workstation?

    - by ssxuser80
    Can anyone recommend something that allows us to take hidden periodic screenshots of a workstation? We have a user who we believe is abusing his computer privileges. We have our suspicions that he may be playing games, etc. We need to monitor his screen without him being aware of it. Currently, the IT Department here is using Dameware Mini Remote Control to view his login sessions. But there isn't an option to set up automatic periodic screenshots. I'm hoping to find a tool that has this option and can be centrally managed as well. Thank you for your time. Any help would be greatly appreciated.

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  • Netgear NV+ resize volume for snapshots

    - by kurresmack
    Hey! I have a Netgear NV+ that I want to setup snapshots on. As I do not have any space allocated for ths I just wanted to check what will happend to my data if I allocate space for snapshots? I could not find any resource for this on google which makes me to belive that no data is affected but just wanted to make sure that there is no foramt!

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  • Flex sending snapshot without using base64Encode

    - by atd
    var is:ImageSnapshot = myImagesnapshot; var str:String = ImageSnapshot.encodeImageAsBase64(is); As of now, I am sending my jpeg data to the server with the code above. The problem is that it almost doubles the size of the data. Is there a way to send the image data directly without using any encoding.

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  • How do I get my snapshots in Nexus to appear in the m2eclipse dependency search?

    - by Brabster
    Hi, I've been working through the Nexus guide this weekend and I've got everything set up, to the point that I can publish a snapshot to my local nexus install. I can't seem to work out how to get m2eclipse to see the snapshot and offer it as an option in the Add Dependencies search panel. How do I do that? Thanks! In case it's of any use, my settings.xml is as follows: <settings xmlns="http://maven.apache.org/SETTINGS/1.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/SETTINGS/1.0.0 http://maven.apache.org/xsd/settings-1.0.0.xsd"> <localRepository /> <interactiveMode /> <usePluginRegistry /> <offline /> <pluginGroups /> <servers> <server> <id>localSnap</id> <username>deployment</username> <password>*****</password> </server> </servers> <mirrors> <mirror> <!--This sends everything else to /public --> <id>nexus</id> <mirrorOf>*</mirrorOf> <url>http://localhost:8080/nexus/content/groups/public</url> </mirror> </mirrors> <profiles> <profile> <id>nexus</id> <!--Enable snapshots for the built in central repo to direct --> <!--all requests to nexus via the mirror --> <repositories> <repository> <id>central</id> <url>http://central</url> <releases> <enabled>true</enabled> </releases> <snapshots> <enabled>true</enabled> </snapshots> </repository> </repositories> <pluginRepositories> <pluginRepository> <id>central</id> <url>http://central</url> <releases> <enabled>true</enabled> </releases> <snapshots> <enabled>true</enabled> </snapshots> </pluginRepository> </pluginRepositories> </profile> </profiles> <activeProfiles> <!--make the profile active all the time --> <activeProfile>nexus</activeProfile> </activeProfiles> </settings>

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  • Begin the Clone Wars Have!

    - by Antony Reynolds
    Creating a New Virtual Machine from an Existing Virtual Disk In previous posts I described how I set up an OEL6 machine under VirtualBox that can run an 11gR2 database and FMW 11.1.1.5.  That is great if you want the DB and FMW running in the same virtual image and it has served me well for some proof of concepts and also for some testing of different JVMs.  However I also wanted to run some testing of FMW with the database running on a separate physical machine.  So in this post I will show how to take a VirtualBox image and create a new image based on the disks from that original image. What are my Options? There is more than one way to skin a cat, or in this case to create two separate VMs that can run on different hardware.  Some of the options include: Create new virtual disk images for each new VM. Clone the existing disk images and point the new VM at the cloned images. Point the new VM at the existing snapshots. #1 is too much like hard work, install OEL twice, install a database again, install FMW again, run RCU again!  Life is too short! #2 is probably the safest way of doing things.  VirtualBox allows you to clone a disk image for use in a separate machine.  However this of course duplicates the disk and means that it is now occupying 3 times the space, once for the original disk and twice more for the two clones I would need. #3 is the most space efficient way of doing things.  It does mean however that I can only run the new “cloned” images if I have access to the original image because that is where the base snapshots reside.  However this is not a problem for me as long as I remember to keep all threee images together.  So this is the approach we will follow. Snapshot, What Snapshot? As we are going to create new virtual machines based on existing snapshots we need to figure out which snapshot to use.  We do this by opening the “Media Manager” from within VirtualBox and moving the mouse over the snapshot images until we find the snapshots we want – the snapshot name is identified in the “Attached to:” comment.  In my case I wanted the FMW installed snapshot because that had a database configured for FMW alongside the FMW software.  I made a note of the filename of that snapshot (actually I just noted the first 5 characters as that was all that was needed to uniquely identify the snapshot file). When we create the new machines we will point them at the snapshot filename we have just checked. Network or NotWork? Because we want the two new machines to communicate with each other when hosted in different physical machines we can’t use the default NAT networking mode without a lot of hassle.  But at the same time we need them to have fixed IP addresses relative to each other so that they can see each other whilst also being able to see the outside world. To achieve all these requirements I created two network adapters for each machine.  Adapter 1 was a standard NAT mapping.  This will allow each machine to get a dynamic IP address (10.0.2.15 by default) that can be used to access the external world through the VBox provided NAT gateway.  This is the same as the existing configuration. The second adapter I created as a bridged adapter.  This gives the virtual machine direct access to the host network card and by using fixed IP addresses each machine can see the other.  It is important to choose fixed IP addresses that are not routable across your internal network so you don’t get any clashes with other machines on your network.  Of course you could always get proper fixed IP addresses from your network people, but I have serveral people using my images and as long as I don’t have two instances of the same VM on the same network segment this is easier and avoids reconfiguring the network every time someone wants a copy of my VM.  If it is available I would suggest using the 10.0.3.* network as 10.0.2.* is the default NAT network.  You can check availability by pinging 10.0.3.1 and 10.0.3.2 from your host machine.  If it times out then you are probably safe to use that. Creating the New VMs Now that I had collected the data that I needed I went ahead and created the new VMs. When asked for a “Boot Hard Disk” I used the “Choose a virtual hard disk file…” link to find the snapshot I had previously selected and set that to be the existing hard disk.  I chose the previously existing SOA 11.1.1.5 install for both the new DB and FMW machines because that snapshot had the database with the RCU completed that I wanted for my DB machine and it had the SOA software installed which I wanted for my FMW machine. After the initial creation of the virtual machine go into the network setting section and enable a second adapter which will be bridged.  Make a note of the MAC addresses (the last four digits should be sufficient) of the two adapters so that you can later set the bridged adapter to use fixed IP and the NAT adapter to use DHCP. We are now ready to start the VMs and reconfigure Linux. Reconfiguring Linux Because I now have two new machines I need to change their network configuration.  In particular I need to change the hostname, update the hosts file and change the network settings. Changing the Hostname I renamed both hosts by running the hostname command as root: hostname vboxfmw.oracle.com I also edited the /etc/sysconfig file and set the correct hostname in there. HOSTNAME=vboxfmw.oracle.com Changing the Network Settings I needed to change the network configuration to give the bridged network a fixed IP address.  I first explicitly set the MAC addresses of the two adapters, because the order of the virtual adapters in the VirtualBox Manager is not necessarily the same as the order of the adapters in the guest OS.  So I went in to the System->Preferences->Network Connections screen and explicitly set the “Device MAC address” for the two adapters. Having correctly mapped the Linux adapters to the VirtualBox adapters I then set the Bridged adapter to use fixed IP addressing rather than DHCP.  There is no need for additional routing or default gateways because we expect the two machine to be on the same LAN segment. Updating the Hosts File Having renamed the machines and reconfigured the network I then updated the /etc/hosts file to refer to the new machine name add a new line to the hosts file to provide an additional IP address for my server (the new fixed IP address) add a new line for the fixed IP address of the other virtual machine 10.0.3.101      vboxdb.oracle.com       vboxdb  # Added by NetworkManager 10.0.2.15       vboxdb.oracle.com       vboxdb  # Added by NetworkManager 10.0.3.102      vboxfmw.oracle.com      vboxfmw # Added by NetworkManager 127.0.0.1       localhost.localdomain   localhost ::1     vboxdb.oracle.com       vboxdb  localhost6.localdomain6 localhost6 To make sure everything takes effect I restarted the server. Reconfiguring the Database on the DB Machine Because we changed the hostname the listener and the EM console no longer start so I need to modify the listener.ora to use the new hostname and I also need to rebuild the EM configuration because it also relies on the hostname. I edited the $ORACLE_HOME/network/admin/listener.ora and changed the listening address to the new hostname:       (ADDRESS = (PROTOCOL = TCP)(HOST = vboxdb.oracle.com)(PORT = 1521)) After changing the listener.ora I was able to start the listener using: lsnrctl start I also had to reconfigure the EM database control.  I first deconfigured it using the command: emca -deconfig dbcontrol db -repos drop This drops the repository and removes any existing registered dbcontrols. I then re-configured it using the following command: emca -config dbcontrol db -repos create This creates the EM repository and then configures and starts dbcontrol. Now my database machine is ready so I can close it down and take a snapshot. Disabling the Database on the FMW Machine I set up the database to start automatically by creating a service called “dbora”.  On the FMW machine I do not need the database running so I can prevent it auto-starting by running the following command: chkconfig –del dbora Note that because I am using a snapshot it is not a waste of disk space to have the DB installed but not used.  As long as I don’t run it, it won’t cost me anything. I can now close the FMW machine down and take a snapshot. Creating a New Domain The FMW machine is now ready to create a new domain.  When creating the domain I can point it at the second machine which is running the database.  I can potentially run these machines on two separate physical machines as long as I have the original virtual machine available to both of the physical machines. Gotchas in Snapshotting VirtualBox does not support the concept of linked machines in a network like some virtualization technologies so when creating a snapshot it is a good idea to shut both VMs down and then take a snapshot on both of them.  This is because we want to keep the database in sync with the middleware.  One way to make sure that this happens would be to place all the domain configuration files on the database server via an NFS share, this would mean that all we would need to snapshot would be the database machine because that would hold all the state and configuration. The Sky’s the Limit We have covered a simple case of having just two machines.  I have a more complicated configuration in which two machine run a RAC database off the same base OS image, and two more machines run a SOA cluster based on the same OS image.  Just remember what machine holds state and what are the consequences of taking a snapshot.

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  • INSERT and transaction serialization in PostreSQL

    - by Alexander
    I have a question. Transaction isolation level is set to serializable. When the one user opens a transaction and INSERTs or UPDATEs data in "table1" and then another user opens a transaction and tries to INSERT data to the same table, does the second user need to wait 'til the first user commits the transaction?

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  • INSERT and transaction searilization in PostreSQL

    - by Alexander
    Hello! I have a question. Transaction isolation level set to serializable. When the one user open transaction and INSERT or UPDATE data in "table1" and then another user open transaction and try to INSERT data to the same table is second user need to wait 'til the first user commits the transaction?

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  • How do i set the Transaction Isolation in EJB?

    - by Nitesh Panchal
    Hello, I am not able to find a way to set TransactionIsolation in ejb. Can anybody tell me how do i set it? I am using persistence. I have looked the following classes : EntityManager , EntityManagerFactory, UserTransaction. None of them seems to have any method like setTransactionIsolation or such. Do we need to change persistence.xml? I just read a book named Mastering EJB 3.0 4th edition. They gave a full 10 page theory about Isolation level that this problems occur and that occurs and such things but at the end they gave this paragraph :- "As we now know, the EJB standard does not deal with isolation levels directly, and rightly so. EJB is a component specification. It defines the behavior and contracts of a business component with clients and middleware infrastructure (containers) such that the component can be rendered as various middleware services properly. EJBs therefore are transactional components that interact with resource managers, such as the JDBC resource manager or JMS resource manager, via JTS, as part of a transaction. They are not, hence, resource components in themselves. Since isolation levels are very specific to the behavior and capabilities of the underlying resources, they should therefore be specified at the resource API levels. " What exactly does it mean? What is meant by resource level APIs? Please help me. If persistence has no way to set Isolation Level then why do they give such huge theory in an EJB book and make it heavy in weight unnecessarily :(

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  • easiest way to convert virtualbox snapshots to tree view

    - by amir beygi
    HI all My virtual box snapshot view is like this Name: Snapshot 2 (UUID: cb45aef4-54d4-4c4e-ad3e-dd7cccb6103a) Name: s131 (UUID: 8ec30c82-7796-4e51-8161-979f1b95fb0f) Name: s131 (UUID: 42066f33-969b-41f3-a779-7f6e2c45ea2c) Name: s131 (UUID: d71b9bc5-b862-46b5-ae4d-f88d3dd9756d) Name: s131 (UUID: 681896a9-7e61-4b5a-90bc-cb1bd785c6fc) Name: s131 (UUID: d7bf8593-d218-442d-b23b-4ee16e74087d) Name: s131 (UUID: e8b16fd2-7add-4294-b908-34c4e6dc79dc) Name: s131 (UUID: 57c3f5d7-d4ed-4a62-a7b8-5594f819e08e) Name: Snapshot 3 (UUID: 4a684149-9dd6-4bb2-baf5-5f590e91a344) Name: Snapshot 4 (UUID: d4cbaa7c-ae78-41e0-9962-46c587a9c667) Name: Snapshot 5 (UUID: 81567b6e-eea9-49a6-b3b8-a07f0be337d8) * and i want to convert this text to a tree like this Name: Snapshot 2 (UUID: cb45aef4-54d4-4c4e-ad3e-dd7cccb6103a) +--Name: s131 (UUID: 8ec30c82-7796-4e51-8161-979f1b95fb0f) +--Name: s131 (UUID: 42066f33-969b-41f3-a779-7f6e2c45ea2c) +--Name: s131 (UUID: d71b9bc5-b862-46b5-ae4d-f88d3dd9756d) +--Name: s131 (UUID: 681896a9-7e61-4b5a-90bc-cb1bd785c6fc) | +--Name: s131 (UUID: d7bf8593-d218-442d-b23b-4ee16e74087d) | +--Name: s131 (UUID: e8b16fd2-7add-4294-b908-34c4e6dc79dc) | +--Name: s131 (UUID: 57c3f5d7-d4ed-4a62-a7b8-5594f819e08e) +--Name: Snapshot 3 (UUID: 4a684149-9dd6-4bb2-baf5-5f590e91a344) +--Name: Snapshot 4 (UUID: d4cbaa7c-ae78-41e0-9962-46c587a9c667) +--Name: Snapshot 5 (UUID: 81567b6e-eea9-49a6-b3b8-a07f0be337d8) * or even an array that contents line number and parent's line number. My environment is linux, programming language is C, and i got this results from this shell command VBoxManage snapshot s2000 showvminfo s|grep Name|grep UUID

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  • How to isolate data per customer, Django powered website

    - by Sawwy
    I have recently started learning python and django and working on a project that includes building a website for collecting information from customers. I am currently trying to figure out best way to isolate the customer data (collected information is sensitive and should only be accessible by customer and the service provider). I found this post Postgresql - one database for everyone, or one-database per customer and my question is that can I automate the model inheritance with customer creation via admin? To be specific, when save() is called for adding customer via django admin, this should create the customer specific tables (create a new set of tables with 'company_name' -prefix). For more information of the environment, I have extended the basic user registration with custom UserProfile adding 'company' and 'role' fields for each user. Upon login, the 'company' of the user will be checked to filter out tables without the 'company_name' prefix. 'Role' will further filter the which company-specific tables and set rights (view, edit). will appreciate any suggestions if more elegant methods could be used to solve the data isolation problem than model inheritance.

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  • Ivy and Snapshots (Nexus)

    - by Uberpuppy
    Hey folks, I'm using ant, ivy and nexus repo manager to build and store my artifacts. I managed to get everything working: dependency resolution and publishing. Until I hit a problem... (of course!). I was publishing to a 'release' repo in nexus, which is locked to 'disable redeploy' (even if you change the setting to 'allow redeploy' (really lame UI there imo). You can imagine how pissed off I was getting when my changes weren't updating through the repo before I realised that this was happening. Anyway, I now have to switch everything to use a 'Snapshot' repo in nexus. Problem is that this messes up my publish. I've tried a variety of things, including extensive googling, and haven't got anywhere whatsoever. The error I get is a bad PUT request, error code 400. Can someone who has got this working please give me a pointer on what I'm missing. Many thanks, Alastair fyi, here's my config: Note that I have removed any attempts at getting snapshots to work as I didn't know what was actually (potentially) useful and what was complete guff. This is therefore the working release-only setup. Also, please note that I've added the XXX-API ivy.xml for info only. I can't even get the xxx-common to publish (and that doesn't even have dependencies). Ant task: <target name="publish" depends="init-publish"> <property name="project.generated.ivy.file" value="${project.artifact.dir}/ivy.xml"/> <property name="project.pom.file" value="${project.artifact.dir}/${project.handle}.pom"/> <echo message="Artifact dir: ${project.artifact.dir}"/> <ivy:deliver deliverpattern="${project.generated.ivy.file}" organisation="${project.organisation}" module="${project.artifact}" status="integration" revision="${project.revision}" pubrevision="${project.revision}" /> <ivy:resolve /> <ivy:makepom ivyfile="${project.generated.ivy.file}" pomfile="${project.pom.file}"/> <ivy:publish resolver="${ivy.omnicache.publisher}" module="${project.artifact}" organisation="${project.organisation}" revision="${project.revision}" pubrevision="${project.revision}" pubdate="now" overwrite="true" publishivy="true" status="integration" artifactspattern="${project.artifact.dir}/[artifact]-[revision](-[classifier]).[ext]" /> </target> Couple of ivy files to give an idea of internal dependencies: XXX-Common project: <ivy-module version="2.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="http://ant.apache.org/ivy/schemas/ivy.xsd"> <info organisation="com.myorg.xxx" module="xxx_common" status="integration" revision="1.0"> </info> <publications> <artifact name="xxx_common" type="jar" ext="jar"/> <artifact name="xxx_common" type="pom" ext="pom"/> </publications> <dependencies> </dependencies> </ivy-module> XXX-API project: <ivy-module version="2.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="http://ant.apache.org/ivy/schemas/ivy.xsd"> <info organisation="com.myorg.xxx" module="xxx_api" status="integration" revision="1.0"> </info> <publications> <artifact name="xxx_api" type="jar" ext="jar"/> <artifact name="xxx_api" type="pom" ext="pom"/> </publications> <dependencies> <dependency org="com.myorg.xxx" name="xxx_common" rev="1.0" transitive="true" /> </dependencies> </ivy-module> IVY Settings.xml: <ivysettings> <properties file="${ivy.project.dir}/project.properties" /> <settings defaultResolver="chain" defaultConflictManager="all" /> <credentials host="${ivy.credentials.host}" realm="Sonatype Nexus Repository Manager" username="${ivy.credentials.username}" passwd="${ivy.credentials.passwd}" /> <caches> <cache name="ivy.cache" basedir="${ivy.cache.dir}" /> </caches> <resolvers> <ibiblio name="xxx_publisher" m2compatible="true" root="${ivy.xxx.publish.url}" /> <chain name="chain"> <url name="xxx"> <ivy pattern="${ivy.xxx.repo.url}/com/myorg/xxx/[module]/[revision]/ivy-[revision].xml" /> <artifact pattern="${ivy.xxx.repo.url}/com/myorg/xxx/[module]/[revision]/[artifact]-[revision].[ext]" /> </url> <ibiblio name="xxx" m2compatible="true" root="${ivy.xxx.repo.url}"/> <ibiblio name="public" m2compatible="true" root="${ivy.master.repo.url}" /> <url name="com.springsource.repository.bundles.release"> <ivy pattern="http://repository.springsource.com/ivy/bundles/release/[organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /> <artifact pattern="http://repository.springsource.com/ivy/bundles/release/[organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /> </url> <url name="com.springsource.repository.bundles.external"> <ivy pattern="http://repository.springsource.com/ivy/bundles/external/[organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /> <artifact pattern="http://repository.springsource.com/ivy/bundles/external/[organisation]/[module]/[revision]/[artifact]-[revision].[ext]" /> </url> </chain> </resolvers> </ivysettings>

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  • SQL SERVER – Concurrency Basics – Guest Post by Vinod Kumar

    - by pinaldave
    This guest post is by Vinod Kumar. Vinod Kumar has worked with SQL Server extensively since joining the industry over a decade ago. Working on various versions from SQL Server 7.0, Oracle 7.3 and other database technologies – he now works with the Microsoft Technology Center (MTC) as a Technology Architect. Let us read the blog post in Vinod’s own voice. Learning is always fun when it comes to SQL Server and learning the basics again can be more fun. I did write about Transaction Logs and recovery over my blogs and the concept of simplifying the basics is a challenge. In the real world we always see checks and queues for a process – say railway reservation, banks, customer supports etc there is a process of line and queue to facilitate everyone. Shorter the queue higher is the efficiency of system (a.k.a higher is the concurrency). Every database does implement this using checks like locking, blocking mechanisms and they implement the standards in a way to facilitate higher concurrency. In this post, let us talk about the topic of Concurrency and what are the various aspects that one needs to know about concurrency inside SQL Server. Let us learn the concepts as one-liners: Concurrency can be defined as the ability of multiple processes to access or change shared data at the same time. The greater the number of concurrent user processes that can be active without interfering with each other, the greater the concurrency of the database system. Concurrency is reduced when a process that is changing data prevents other processes from reading that data or when a process that is reading data prevents other processes from changing that data. Concurrency is also affected when multiple processes are attempting to change the same data simultaneously. Two approaches to managing concurrent data access: Optimistic Concurrency Model Pessimistic Concurrency Model Concurrency Models Pessimistic Concurrency Default behavior: acquire locks to block access to data that another process is using. Assumes that enough data modification operations are in the system that any given read operation is likely affected by a data modification made by another user (assumes conflicts will occur). Avoids conflicts by acquiring a lock on data being read so no other processes can modify that data. Also acquires locks on data being modified so no other processes can access the data for either reading or modifying. Readers block writer, writers block readers and writers. Optimistic Concurrency Assumes that there are sufficiently few conflicting data modification operations in the system that any single transaction is unlikely to modify data that another transaction is modifying. Default behavior of optimistic concurrency is to use row versioning to allow data readers to see the state of the data before the modification occurs. Older versions of the data are saved so a process reading data can see the data as it was when the process started reading and not affected by any changes being made to that data. Processes modifying the data is unaffected by processes reading the data because the reader is accessing a saved version of the data rows. Readers do not block writers and writers do not block readers, but, writers can and will block writers. Transaction Processing A transaction is the basic unit of work in SQL Server. Transaction consists of SQL commands that read and update the database but the update is not considered final until a COMMIT command is issued (at least for an explicit transaction: marked with a BEGIN TRAN and the end is marked by a COMMIT TRAN or ROLLBACK TRAN). Transactions must exhibit all the ACID properties of a transaction. ACID Properties Transaction processing must guarantee the consistency and recoverability of SQL Server databases. Ensures all transactions are performed as a single unit of work regardless of hardware or system failure. A – Atomicity C – Consistency I – Isolation D- Durability Atomicity: Each transaction is treated as all or nothing – it either commits or aborts. Consistency: ensures that a transaction won’t allow the system to arrive at an incorrect logical state – the data must always be logically correct.  Consistency is honored even in the event of a system failure. Isolation: separates concurrent transactions from the updates of other incomplete transactions. SQL Server accomplishes isolation among transactions by locking data or creating row versions. Durability: After a transaction commits, the durability property ensures that the effects of the transaction persist even if a system failure occurs. If a system failure occurs while a transaction is in progress, the transaction is completely undone, leaving no partial effects on data. Transaction Dependencies In addition to supporting all four ACID properties, a transaction might exhibit few other behaviors (known as dependency problems or consistency problems). Lost Updates: Occur when two processes read the same data and both manipulate the data, changing its value and then both try to update the original data to the new value. The second process might overwrite the first update completely. Dirty Reads: Occurs when a process reads uncommitted data. If one process has changed data but not yet committed the change, another process reading the data will read it in an inconsistent state. Non-repeatable Reads: A read is non-repeatable if a process might get different values when reading the same data in two reads within the same transaction. This can happen when another process changes the data in between the reads that the first process is doing. Phantoms: Occurs when membership in a set changes. It occurs if two SELECT operations using the same predicate in the same transaction return a different number of rows. Isolation Levels SQL Server supports 5 isolation levels that control the behavior of read operations. Read Uncommitted All behaviors except for lost updates are possible. Implemented by allowing the read operations to not take any locks, and because of this, it won’t be blocked by conflicting locks acquired by other processes. The process can read data that another process has modified but not yet committed. When using the read uncommitted isolation level and scanning an entire table, SQL Server can decide to do an allocation order scan (in page-number order) instead of a logical order scan (following page pointers). If another process doing concurrent operations changes data and move rows to a new location in the table, the allocation order scan can end up reading the same row twice. Also can happen if you have read a row before it is updated and then an update moves the row to a higher page number than your scan encounters later. Performing an allocation order scan under Read Uncommitted can cause you to miss a row completely – can happen when a row on a high page number that hasn’t been read yet is updated and moved to a lower page number that has already been read. Read Committed Two varieties of read committed isolation: optimistic and pessimistic (default). Ensures that a read never reads data that another application hasn’t committed. If another transaction is updating data and has exclusive locks on data, your transaction will have to wait for the locks to be released. Your transaction must put share locks on data that are visited, which means that data might be unavailable for others to use. A share lock doesn’t prevent others from reading but prevents them from updating. Read committed (snapshot) ensures that an operation never reads uncommitted data, but not by forcing other processes to wait. SQL Server generates a version of the changed row with its previous committed values. Data being changed is still locked but other processes can see the previous versions of the data as it was before the update operation began. Repeatable Read This is a Pessimistic isolation level. Ensures that if a transaction revisits data or a query is reissued the data doesn’t change. That is, issuing the same query twice within a transaction cannot pickup any changes to data values made by another user’s transaction because no changes can be made by other transactions. However, this does allow phantom rows to appear. Preventing non-repeatable read is a desirable safeguard but cost is that all shared locks in a transaction must be held until the completion of the transaction. Snapshot Snapshot Isolation (SI) is an optimistic isolation level. Allows for processes to read older versions of committed data if the current version is locked. Difference between snapshot and read committed has to do with how old the older versions have to be. It’s possible to have two transactions executing simultaneously that give us a result that is not possible in any serial execution. Serializable This is the strongest of the pessimistic isolation level. Adds to repeatable read isolation level by ensuring that if a query is reissued rows were not added in the interim, i.e, phantoms do not appear. Preventing phantoms is another desirable safeguard, but cost of this extra safeguard is similar to that of repeatable read – all shared locks in a transaction must be held until the transaction completes. In addition serializable isolation level requires that you lock data that has been read but also data that doesn’t exist. Ex: if a SELECT returned no rows, you want it to return no. rows when the query is reissued. This is implemented in SQL Server by a special kind of lock called the key-range lock. Key-range locks require that there be an index on the column that defines the range of values. If there is no index on the column, serializable isolation requires a table lock. Gets its name from the fact that running multiple serializable transactions at the same time is equivalent of running them one at a time. Now that we understand the basics of what concurrency is, the subsequent blog posts will try to bring out the basics around locking, blocking, deadlocks because they are the fundamental blocks that make concurrency possible. Now if you are with me – let us continue learning for SQL Server Locking Basics. Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Performance, SQL Query, SQL Server, SQL Tips and Tricks, T SQL, Technology Tagged: Concurrency

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  • What sql server isolation level should I choose to prevent concurrent reads?

    - by Brian Bolton
    I have the following transaction: SQL inserts a 1 new record into a table called tbl_document SQL deletes all records matching a criteria in another table called tbl_attachment SQL inserts multiple records into the tbl_attachment Until this transaction finishes, I don't want others users to be aware of the (1) new records in tbl_document, (2) deleted records in tbl_attachment, and (3) modified records in tbl_attachment. Would Read Committed Isolation be the correct isolation level?

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  • How to start a major iPhone app update in Xcode

    - by Eric
    I have an app in the iPhone app store and have released several minor updates to it. I want to begin work on some major feature additions and reorganization, but don't want to lose the source code of my most recent version in case everything goes horribly wrong. Should I start a new Xcode project from scratch and copy my existing source in? If I do this will I be able to submit the build from this new project as an update or will Apple complain that the build comes from a different Xcode project? I've seen (but not used) Xcode's "Snapshots" and "Source Control" features - are these what I'm looking for? Any help or direction greatly appreciated.

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  • xcode snapshots

    - by norskben
    hi guys What exactly does xcode do with project snapshots after the xcode project is moved or renamed. I have noticed they completely disapear...Where should I look? Defintely not in ~/Library/Application Support/Developer/Shared/SnapshotRepository.sparseimage

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  • maven multi-module versioning

    - by eugenn
    I have a multi-module project. parent POM (1.0-SNAPSHOT) |-- module1 (1.0-SNAPSHOT) |-- module2 (1.0-SNAPSHOT) `-- module3 (1.0-SNAPSHOT) When I execute mvn release:prepare it verify that parent POM has a SNAPSHOT version and all dependent modules don't have a SNAPSHOT version. How automatically update all child modules from SNAPSHOT to the next release version? I would like automatically increment version for all modules.

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  • emacs local version control

    - by aaa
    hello. I am wondering if there is local version control/snapshots for emacs independent of VC? let me clarify: every time I save buffer, I would like to be able to keep track of changes of each save in session. I know I can do something similar with backup files, but they are not automated like VC and a somewhat cumbersome. I have searched Google, but did not find the solution. Perhaps my query string was not good. Thanks

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  • Height of a webpage for snapshots in PHP

    - by gAMBOOKa
    We have an application that takes snapshots of certain web pages. It's quite tightly integrated into the code, so we're not ready to incorporate another library. But we don't have a way of being able to calculate the web page height, so we end up taking snaps of 8000px height. Which is now proving troublesome when inserted into PDFs. Is there a way to find the height of the webpage in PHP?

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  • How to port VirtualBox machines with the recent snapshot?

    - by Liran Orevi
    I've ported a 'hard disk' using the 'clonehd' advice given on How to easily port VirtualBox machines? But it's not at the latest state, it's probably before any 'snapshot' was taken. I have only copied the 'hard disk', I haven't copied any 'snapshot' mainly because there are so many. I'm interested to transfer the 'hard disk' at it's latest state. How can this be done? (easily and not by coping the entire snapshots, machine etc...)

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