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• Windows opaque UserControl not refreshing any graphical changes made on it

  - by Debajyoti Das

  I have created a Windows UserControl. It actually paints a Grid (i.e. vertical and horizontal lines) using Graphics. User can change each cell height and width, and according to that Grid is refreshed. Overriding the OnPaint event I have created the grid. I used SetStyle(ControlStyles.Opaque, true) to make it transparent. I used this control on a form and from there I change the values of the cell height and width but due to Opaque the new grid is overlapping on the previous one and making it clumsy. How do I resolve this? UserControl Code: public partial class Grid : UserControl { public Grid() { InitializeComponent(); SetStyle(ControlStyles.Opaque, true); } private float _CellWidth = 10, _CellHeight = 10; private Color _GridColor = Color.Black; public float CellWidth { get { return this._CellWidth; } set { this._CellWidth = value; } } public float CellHeight { get { return this._CellHeight; } set { this._CellHeight = value; } } public Color GridColor { get { return this._GridColor; } set { this._GridColor = value; } } protected override void OnPaint(PaintEventArgs e) { base.OnPaint(e); Graphics g; float iHeight = this.Height; float iWidth = this.Width; g = e.Graphics; Pen myPen = new Pen(GridColor); myPen.Width = 1; if (this.CellWidth > 0 && this.CellHeight > 0) { for (float X = 0; X <= iWidth; X += this.CellWidth) { g.DrawLine(myPen, X, 0, X, iHeight); } for (float Y = 0; Y <= iHeight; Y += this.CellHeight) { g.DrawLine(myPen, 0, Y, iWidth, Y); } } } public override void Refresh() { base.ResumeLayout(true); base.Refresh(); ResumeLayout(true); } } Form Code: public partial class Form1 : Form { public Form1() { InitializeComponent(); } private void Form1_Load(object sender, EventArgs e) { } private void btnBrowse_Click(object sender, EventArgs e) { try { if (ofdImage.ShowDialog() == System.Windows.Forms.DialogResult.OK) { pbImage.Image = Image.FromFile(ofdImage.FileName); } } catch (Exception ex) { MessageBox.Show(ex.Message); } } private void btnShowGrid_Click(object sender, EventArgs e) { if (grid1.Visible) { grid1.Visible = false; btnShowGrid.Text = "Show"; } else { grid1.Visible = true; btnShowGrid.Text = "Hide"; } } private void btnGridCellMaximize_Click(object sender, EventArgs e) { grid1.CellHeight += 1; grid1.CellWidth += 1; grid1.Refresh(); } private void btnGridCellMinimize_Click(object sender, EventArgs e) { grid1.CellHeight -= 1; grid1.CellWidth -= 1; grid1.Refresh(); } }

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• How to make Generic Object in java implements an interface

  - by ahmed elbagoury

  I want to have a generic object that implements an interface. I mean if i have a class A Class A <E>{ E x; } I want to make sure that x will implement a particular interface(myInterface). In other words, that the type E implements an interface.

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• What is marker interface?

  - by Rajesh

  What is the use of extending marker interface.

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• Graphical Submit button doesn't 'post' in Firefox or IE

  - by Ken

  I've looked all over the net to try and solve this problem, but I can't find any solutions. So far, I've checked 3 different browsers. It does not work on IE or FireFox, but it works just fine in Safari. Here's the problem. When I click the button to submit my form, it forwards me to the 'post' URL page - INSTEAD of posting the data, and going to the thank you page like the script commands. Here's the code I'm working with: <form name="loginfrm" method="post" action="http://www.myaffiliatepowersite.com/members/quick_signup.php" style="margin:0; padding:0;"> " /

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• Why does the check for an interface fail?

  - by chobo

  I have a class that implements an interface. In another area of the code I check if that class instance contains that interface, but it doesn't work. The check to see if the class contains the interface always fails (false) when it should be true. Below is a simple representation of what I am trying to accomplish. Example public interface IModel { bool validate(); } public class SomeModel : IModel { public SomeModel { } public bool Validate() { return true; } } // Dummy method public void Run() { SomeModel model = new SomeModel(); if (model is IModel) { string message = "It worked"; } else { string message = "It failed"; } }

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• Cannot understand the behaviour of dotnet compiler while instantiating a class thru interface(C#)

  - by Newbie

  I have a class that impelemnts an interface. The interface is public interface IRiskFactory { void StartService(); void StopService(); } The class that implements the interface is public class RiskFactoryService : IRiskFactory { } Now I have a console application and one window service. From the console application if I write the following code static void Main(string[] args) { IRiskFactory objIRiskFactory = new RiskFactoryService(); objIRiskFactory.StartService(); Console.ReadLine(); objIRiskFactory.StopService(); } It is working fine. However, when I mwrite the same piece of code in Window service public partial class RiskFactoryService : ServiceBase { IRiskFactory objIRiskFactory = null; public RiskFactoryService() { InitializeComponent(); objIRiskFactory = new RiskFactoryService(); <- ERROR } /// <summary> /// Starts the service /// </summary> /// <param name="args"></param> protected override void OnStart(string[] args) { objIRiskFactory.StartService(); } /// <summary> /// Stops the service /// </summary> protected override void OnStop() { objIRiskFactory.StopService(); } } It throws error: Cannot implicitly convert type 'RiskFactoryService' to 'IRiskFactory'. An explicit conversion exists (are you missing a cast?) When I type casted to the interface type, it started working objIRiskFactory = (IRiskFactory)new RiskFactoryService(); My question is why so? Thanks.(C#)

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• Bind Java program to Cocoa interface

  - by Kevin

  Hi, would it be possible to bind a Java application to a Cocoa graphical interface? I'm working in Eclipse right now, on my mac, and am wondering if Interface Builder could be used to construct a new interface so that I don't have to look at Swing all day. Any ideas/suggestions? Thanks!

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• Java REST Interface

  - by Vikram

  I have a PHP web application environment. I am using Slim Framework as REST interface for my application. My application front-end is written using Backbone.js and jQuery. There is a utility (.jar file) which when I use command line makes a remote call (I guess this is a Web Service) which returns me the data. how do I best incorporate this into my webapplication described on top? My application front end will have a Button that should make an AJAX call to the REST Interface and fetch the data as JSON. My approach: PHP-REST interface url is: /api/phprestapi.php exists Add a JAVA-REST interface at url: /api/javarestapi.java (Perhaps) to separate these two Existing Environment: LAMP Stack on Ubuntu How do I achieve this? What is the kind of effort involved? Thanks for your pointers

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• Question about how-to implement an interface

  - by davit-datuashvili

  I have the following interface: public interface MyFunctor { int myFunction(int x); } And I created a class which implements this interface : public class Myfunction1 implements MyFunctor { } But it show me the error : implement all abstract method Please help me.

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• Cannot understand the behaviour of C# compiler while instantiating a class thru interface

  - by Newbie

  I have a class that implements an interface. The interface is public interface IRiskFactory { void StartService(); void StopService(); } The class that implements the interface is public class RiskFactoryService : IRiskFactory { } Now I have a console application and one window service. From the console application if I write the following code static void Main(string[] args) { IRiskFactory objIRiskFactory = new RiskFactoryService(); objIRiskFactory.StartService(); Console.ReadLine(); objIRiskFactory.StopService(); } It is working fine. However, when I mwrite the same piece of code in Window service public partial class RiskFactoryService : ServiceBase { IRiskFactory objIRiskFactory = null; public RiskFactoryService() { InitializeComponent(); objIRiskFactory = new RiskFactoryService(); <- ERROR } /// <summary> /// Starts the service /// </summary> /// <param name="args"></param> protected override void OnStart(string[] args) { objIRiskFactory.StartService(); } /// <summary> /// Stops the service /// </summary> protected override void OnStop() { objIRiskFactory.StopService(); } } It throws error: Cannot implicitly convert type 'RiskFactoryService' to 'IRiskFactory'. An explicit conversion exists (are you missing a cast?) When I type cast to the interface type, it started working objIRiskFactory = (IRiskFactory)new RiskFactoryService(); My question is why so?

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• Cannot convert []string to []interface {}

  - by karlrh

  I'm writing some code, and I need it to catch the arguments and pass them through fmt.Println (I want its default behaviour, to write arguments separated by spaces and followed by a newline). However it takes []interface {} but flag.Args() returns a []string. Here's the code example package main import ( "fmt" "flag" ) func main() { flag.Parse() fmt.Println(flag.Args()...) } This returns the following error: ./example.go:10: cannot use args (type []string) as type []interface {} in function argument Is this a bug? Shouldn't fmt.Println take any array? By the way, I've also tried to do this: var args = []interface{}(flag.Args()) but I get the following error: cannot convert flag.Args() (type []string) to type []interface {} Is there a "Go" way to workaround this?

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• Curing the Database-Application mismatch

  - by Phil Factor

  If an application requires access to a database, then you have to be able to deploy it so as to be version-compatible with the database, in phase. If you can deploy both together, then the application and database must normally be deployed at the same version in which they, together, passed integration and functional testing.  When a single database supports more than one application, then the problem gets more interesting. I’ll need to be more precise here. It is actually the application-interface definition of the database that needs to be in a compatible ‘version’.  Most databases that get into production have no separate application-interface; in other words they are ‘close-coupled’.  For this vast majority, the whole database is the application-interface, and applications are free to wander through the bowels of the database scot-free.  If you’ve spurned the perceived wisdom of application architects to have a defined application-interface within the database that is based on views and stored procedures, any version-mismatch will be as sensitive as a kitten.  A team that creates an application that makes direct access to base tables in a database will have to put a lot of energy into keeping Database and Application in sync, to say nothing of having to tackle issues such as security and audit. It is not the obvious route to development nirvana. I’ve been in countless tense meetings with application developers who initially bridle instinctively at the apparent restrictions of being ‘banned’ from the base tables or routines of a database.  There is no good technical reason for needing that sort of access that I’ve ever come across.  Everything that the application wants can be delivered via a set of views and procedures, and with far less pain for all concerned: This is the application-interface.  If more than zero developers are creating a database-driven application, then the project will benefit from the loose-coupling that an application interface brings. What is important here is that the database development role is separated from the application development role, even if it is the same developer performing both roles. The idea of an application-interface with a database is as old as I can remember. The big corporate or government databases generally supported several applications, and there was little option. When a new application wanted access to an existing corporate database, the developers, and myself as technical architect, would have to meet with hatchet-faced DBAs and production staff to work out an interface. Sure, they would talk up the effort involved for budgetary reasons, but it was routine work, because it decoupled the database from its supporting applications. We’d be given our own stored procedures. One of them, I still remember, had ninety-two parameters. All database access was encapsulated in one application-module. If you have a stable defined application-interface with the database (Yes, one for each application usually) you need to keep the external definitions of the components of this interface in version control, linked with the application source,  and carefully track and negotiate any changes between database developers and application developers.  Essentially, the application development team owns the interface definition, and the onus is on the Database developers to implement it and maintain it, in conformance.  Internally, the database can then make all sorts of changes and refactoring, as long as source control is maintained.  If the application interface passes all the comprehensive integration and functional tests for the particular version they were designed for, nothing is broken. Your performance-testing can ‘hang’ on the same interface, since databases are judged on the performance of the application, not an ‘internal’ database process. The database developers have responsibility for maintaining the application-interface, but not its definition,  as they refactor the database. This is easily tested on a daily basis since the tests are normally automated. In this setting, the deployment can proceed if the more stable application-interface, rather than the continuously-changing database, passes all tests for the version of the application. Normally, if all goes well, a database with a well-designed application interface can evolve gracefully without changing the external appearance of the interface, and this is confirmed by integration tests that check the interface, and which hopefully don’t need to be altered at all often.  If the application is rapidly changing its ‘domain model’  in the light of an increased understanding of the application domain, then it can change the interface definitions and the database developers need only implement the interface rather than refactor the underlying database.  The test team will also have to redo the functional and integration tests which are, of course ‘written to’ the definition.  The Database developers will find it easier if these tests are done before their re-wiring  job to implement the new interface. If, at the other extreme, an application receives no further development work but survives unchanged, the database can continue to change and develop to keep pace with the requirements of the other applications it supports, and needs only to take care that the application interface is never broken. Testing is easy since your automated scripts to test the interface do not need to change. The database developers will, of course, maintain their own source control for the database, and will be likely to maintain versions for all major releases. However, this will not need to be shared with the applications that the database servers. On the other hand, the definition of the application interfaces should be within the application source. Changes in it have to be subject to change-control procedures, as they will require a chain of tests. Once you allow, instead of an application-interface, an intimate relationship between application and database, we are in the realms of impedance mismatch, over and above the obvious security problems.  Part of this impedance problem is a difference in development practices. Whereas the application has to be regularly built and integrated, this isn’t necessarily the case with the database.  An RDBMS is inherently multi-user and self-integrating. If the developers work together on the database, then a subsequent integration of the database on a staging server doesn’t often bring nasty surprises. A separate database-integration process is only needed if the database is deliberately built in a way that mimics the application development process, but which hampers the normal database-development techniques.  This process is like demanding a official walking with a red flag in front of a motor car.  In order to closely coordinate databases with applications, entire databases have to be ‘versioned’, so that an application version can be matched with a database version to produce a working build without errors.  There is no natural process to ‘version’ databases.  Each development project will have to define a system for maintaining the version level. A curious paradox occurs in development when there is no formal application-interface. When the strains and cracks happen, the extra meetings, bureaucracy, and activity required to maintain accurate deployments looks to IT management like work. They see activity, and it looks good. Work means progress.  Management then smile on the design choices made. In IT, good design work doesn’t necessarily look good, and vice versa.

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• In Java, is there a gain in using interfaces for complex models?

  - by Gnoupi

  The title is hardly understandable, but I'm not sure how to summarize that another way. Any edit to clarify is welcome. I have been told, and recommended to use interfaces to improve performances, even in a case which doesn't especially call for the regular "interface" role. In this case, the objects are big models (in a MVC meaning), with many methods and fields. The "good use" that has been recommended to me is to create an interface, with its unique implementation. There won't be any other class implementing this interface, for sure. I have been told that this is better to do so, because it "exposes less" (or something close) to the other classes which will use methods from this class, as these objects are referring to the object from its interface (all public method from the implementation being reproduced in the interface). This seems quite strange to me, as it seems like a C++ use to me (with header files). There I see the point, but in Java? Is there really a point in making an interface for such unique implementation? I would really appreciate some clarifications on the topic, so I could justify not following such kind of behavior, and the hassle it creates from duplicating all declarations.

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• In Java, is there a performance gain in using interfaces for complex models?

  - by Gnoupi

  The title is hardly understandable, but I'm not sure how to summarize that another way. Any edit to clarify is welcome. I have been told, and recommended to use interfaces to improve performances, even in a case which doesn't especially call for the regular "interface" role. In this case, the objects are big models (in a MVC meaning), with many methods and fields. The "good use" that has been recommended to me is to create an interface, with its unique implementation. There won't be any other class implementing this interface, for sure. I have been told that this is better to do so, because it "exposes less" (or something close) to the other classes which will use methods from this class, as these objects are referring to the object from its interface (all public methods from the implementation being reproduced in the interface). This seems quite strange to me, as it seems like a C++ use to me (with header files). There I see the point, but in Java? Is there really a point in making an interface for such unique implementation? I would really appreciate some clarifications on the topic, so I could justify not following such kind of behavior, and the hassle it creates from duplicating all declarations. Edit: Plenty of valid points in most answers, I'm wondering if I won't switch this question for a community wiki, so we can regroup these points in more structured answers.

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• Diving into OpenStack Network Architecture - Part 2 - Basic Use Cases

  - by Ronen Kofman

    rkofman Normal rkofman 4 138 2014-06-05T03:38:00Z 2014-06-05T05:04:00Z 3 2735 15596 Oracle Corporation 129 36 18295 12.00 Clean Clean false false false false EN-US X-NONE HE /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin-top:0in; mso-para-margin-right:0in; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0in; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:Arial; mso-bidi-theme-font:minor-bidi; mso-bidi-language:AR-SA;} In the previous post we reviewed several network components including Open vSwitch, Network Namespaces, Linux Bridges and veth pairs. In this post we will take three simple use cases and see how those basic components come together to create a complete SDN solution in OpenStack. With those three use cases we will review almost the entire network setup and see how all the pieces work together. The use cases we will use are: 1.       Create network – what happens when we create network and how can we create multiple isolated networks 2.       Launch a VM – once we have networks we can launch VMs and connect them to networks. 3.       DHCP request from a VM – OpenStack can automatically assign IP addresses to VMs. This is done through local DHCP service controlled by OpenStack Neutron. We will see how this service runs and how does a DHCP request and response look like. In this post we will show connectivity, we will see how packets get from point A to point B. We first focus on how a configured deployment looks like and only later we will discuss how and when the configuration is created. Personally I found it very valuable to see the actual interfaces and how they connect to each other through examples and hands on experiments. After the end game is clear and we know how the connectivity works, in a later post, we will take a step back and explain how Neutron configures the components to be able to provide such connectivity.  We are going to get pretty technical shortly and I recommend trying these examples on your own deployment or using the Oracle OpenStack Tech Preview. Understanding these three use cases thoroughly and how to look at them will be very helpful when trying to debug a deployment in case something does not work. Use case #1: Create Network Create network is a simple operation it can be performed from the GUI or command line. When we create a network in OpenStack the network is only available to the tenant who created it or it could be defined as “shared” and then it can be used by all tenants. A network can have multiple subnets but for this demonstration purpose and for simplicity we will assume that each network has exactly one subnet. Creating a network from the command line will look like this: # neutron net-create net1 Created a new network: +---------------------------+--------------------------------------+ | Field                     | Value                                | +---------------------------+--------------------------------------+ | admin_state_up            | True                                 | | id                        | 5f833617-6179-4797-b7c0-7d420d84040c | | name                      | net1                                 | | provider:network_type     | vlan                                 | | provider:physical_network | default                              | | provider:segmentation_id  | 1000                                 | | shared                    | False                                | | status                    | ACTIVE                               | | subnets                   |                                      | | tenant_id                 | 9796e5145ee546508939cd49ad59d51f     | +---------------------------+--------------------------------------+ Creating a subnet for this network will look like this: # neutron subnet-create net1 10.10.10.0/24 Created a new subnet: +------------------+------------------------------------------------+ | Field            | Value                                          | +------------------+------------------------------------------------+ | allocation_pools | {"start": "10.10.10.2", "end": "10.10.10.254"} | | cidr             | 10.10.10.0/24                                  | | dns_nameservers  |                                                | | enable_dhcp      | True                                           | | gateway_ip       | 10.10.10.1                                     | | host_routes      |                                                | | id               | 2d7a0a58-0674-439a-ad23-d6471aaae9bc           | | ip_version       | 4                                              | | name             |                                                | | network_id       | 5f833617-6179-4797-b7c0-7d420d84040c           | | tenant_id        | 9796e5145ee546508939cd49ad59d51f               | +------------------+------------------------------------------------+ We now have a network and a subnet, on the network topology view this looks like this: Now let’s dive in and see what happened under the hood. Looking at the control node we will discover that a new namespace was created: # ip netns list qdhcp-5f833617-6179-4797-b7c0-7d420d84040c   The name of the namespace is qdhcp-<network id> (see above), let’s look into the namespace and see what’s in it: # ip netns exec qdhcp-5f833617-6179-4797-b7c0-7d420d84040c ip addr 1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN     link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00     inet 127.0.0.1/8 scope host lo     inet6 ::1/128 scope host        valid_lft forever preferred_lft forever 12: tap26c9b807-7c: <BROADCAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN     link/ether fa:16:3e:1d:5c:81 brd ff:ff:ff:ff:ff:ff     inet 10.10.10.3/24 brd 10.10.10.255 scope global tap26c9b807-7c     inet6 fe80::f816:3eff:fe1d:5c81/64 scope link        valid_lft forever preferred_lft forever   We see two interfaces in the namespace, one is the loopback and the other one is an interface called “tap26c9b807-7c”. This interface has the IP address of 10.10.10.3 and it will also serve dhcp requests in a way we will see later. Let’s trace the connectivity of the “tap26c9b807-7c” interface from the namespace.  First stop is OVS, we see that the interface connects to bridge  “br-int” on OVS: # ovs-vsctl show 8a069c7c-ea05-4375-93e2-b9fc9e4b3ca1     Bridge "br-eth2"         Port "br-eth2"             Interface "br-eth2"                 type: internal         Port "eth2"             Interface "eth2"         Port "phy-br-eth2"             Interface "phy-br-eth2"     Bridge br-ex         Port br-ex             Interface br-ex                 type: internal     Bridge br-int         Port "int-br-eth2"             Interface "int-br-eth2"         Port "tap26c9b807-7c"             tag: 1             Interface "tap26c9b807-7c"                 type: internal         Port br-int             Interface br-int                 type: internal     ovs_version: "1.11.0"   In the picture above we have a veth pair which has two ends called “int-br-eth2” and "phy-br-eth2", this veth pair is used to connect two bridge in OVS "br-eth2" and "br-int". In the previous post we explained how to check the veth connectivity using the ethtool command. It shows that the two are indeed a pair: # ethtool -S int-br-eth2 NIC statistics:      peer_ifindex: 10 . .   #ip link . . 10: phy-br-eth2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000 . . Note that “phy-br-eth2” is connected to a bridge called "br-eth2" and one of this bridge's interfaces is the physical link eth2. This means that the network which we have just created has created a namespace which is connected to the physical interface eth2. eth2 is the “VM network” the physical interface where all the virtual machines connect to where all the VMs are connected. About network isolation: OpenStack supports creation of multiple isolated networks and can use several mechanisms to isolate the networks from one another. The isolation mechanism can be VLANs, VxLANs or GRE tunnels, this is configured as part of the initial setup in our deployment we use VLANs. When using VLAN tagging as an isolation mechanism a VLAN tag is allocated by Neutron from a pre-defined VLAN tags pool and assigned to the newly created network. By provisioning VLAN tags to the networks Neutron allows creation of multiple isolated networks on the same physical link.  The big difference between this and other platforms is that the user does not have to deal with allocating and managing VLANs to networks. The VLAN allocation and provisioning is handled by Neutron which keeps track of the VLAN tags, and responsible for allocating and reclaiming VLAN tags. In the example above net1 has the VLAN tag 1000, this means that whenever a VM is created and connected to this network the packets from that VM will have to be tagged with VLAN tag 1000 to go on this particular network. This is true for namespace as well, if we would like to connect a namespace to a particular network we have to make sure that the packets to and from the namespace are correctly tagged when they reach the VM network. In the example above we see that the namespace interface “tap26c9b807-7c” has vlan tag 1 assigned to it, if we examine OVS we see that it has flows which modify VLAN tag 1 to VLAN tag 1000 when a packet goes to the VM network on eth2 and vice versa. We can see this using the dump-flows command on OVS for packets going to the VM network we see the modification done on br-eth2: #  ovs-ofctl dump-flows br-eth2 NXST_FLOW reply (xid=0x4):  cookie=0x0, duration=18669.401s, table=0, n_packets=857, n_bytes=163350, idle_age=25, priority=4,in_port=2,dl_vlan=1 actions=mod_vlan_vid:1000,NORMAL  cookie=0x0, duration=165108.226s, table=0, n_packets=14, n_bytes=1000, idle_age=5343, hard_age=65534, priority=2,in_port=2 actions=drop  cookie=0x0, duration=165109.813s, table=0, n_packets=1671, n_bytes=213304, idle_age=25, hard_age=65534, priority=1 actions=NORMAL   For packets coming from the interface to the namespace we see the following modification: #  ovs-ofctl dump-flows br-int NXST_FLOW reply (xid=0x4):  cookie=0x0, duration=18690.876s, table=0, n_packets=1610, n_bytes=210752, idle_age=1, priority=3,in_port=1,dl_vlan=1000 actions=mod_vlan_vid:1,NORMAL  cookie=0x0, duration=165130.01s, table=0, n_packets=75, n_bytes=3686, idle_age=4212, hard_age=65534, priority=2,in_port=1 actions=drop  cookie=0x0, duration=165131.96s, table=0, n_packets=863, n_bytes=160727, idle_age=1, hard_age=65534, priority=1 actions=NORMAL   To summarize we can see that when a user creates a network Neutron creates a namespace and this namespace is connected through OVS to the “VM network”. OVS also takes care of tagging the packets from the namespace to the VM network with the correct VLAN tag and knows to modify the VLAN for packets coming from VM network to the namespace. Now let’s see what happens when a VM is launched and how it is connected to the “VM network”. Use case #2: Launch a VM Launching a VM can be done from Horizon or from the command line this is how we do it from Horizon: Attach the network: And Launch Once the virtual machine is up and running we can see the associated IP using the nova list command : # nova list +--------------------------------------+--------------+--------+------------+-------------+-----------------+ | ID                                   | Name         | Status | Task State | Power State | Networks        | +--------------------------------------+--------------+--------+------------+-------------+-----------------+ | 3707ac87-4f5d-4349-b7ed-3a673f55e5e1 | Oracle Linux | ACTIVE | None       | Running     | net1=10.10.10.2 | +--------------------------------------+--------------+--------+------------+-------------+-----------------+ The nova list command shows us that the VM is running and that the IP 10.10.10.2 is assigned to this VM. Let’s trace the connectivity from the VM to VM network on eth2 starting with the VM definition file. The configuration files of the VM including the virtual disk(s), in case of ephemeral storage, are stored on the compute node at/var/lib/nova/instances/<instance-id>/. Looking into the VM definition file ,libvirt.xml,  we see that the VM is connected to an interface called “tap53903a95-82” which is connected to a Linux bridge called “qbr53903a95-82”: <interface type="bridge">       <mac address="fa:16:3e:fe:c7:87"/>       <source bridge="qbr53903a95-82"/>       <target dev="tap53903a95-82"/>     </interface>   Looking at the bridge using the brctl show command we see this: # brctl show bridge name     bridge id               STP enabled     interfaces qbr53903a95-82          8000.7e7f3282b836       no              qvb53903a95-82                                                         tap53903a95-82    The bridge has two interfaces, one connected to the VM (“tap53903a95-82 “) and another one ( “qvb53903a95-82”) connected to “br-int” bridge on OVS: # ovs-vsctl show 83c42f80-77e9-46c8-8560-7697d76de51c     Bridge "br-eth2"         Port "br-eth2"             Interface "br-eth2"                 type: internal         Port "eth2"             Interface "eth2"         Port "phy-br-eth2"             Interface "phy-br-eth2"     Bridge br-int         Port br-int             Interface br-int                 type: internal         Port "int-br-eth2"             Interface "int-br-eth2"         Port "qvo53903a95-82"             tag: 3             Interface "qvo53903a95-82"     ovs_version: "1.11.0"   As we showed earlier “br-int” is connected to “br-eth2” on OVS using the veth pair int-br-eth2,phy-br-eth2 and br-eth2 is connected to the physical interface eth2. The whole flow end to end looks like this: VM è tap53903a95-82 (virtual interface)è qbr53903a95-82 (Linux bridge) è qvb53903a95-82 (interface connected from Linux bridge to OVS bridge br-int) è int-br-eth2 (veth one end) è phy-br-eth2 (veth the other end) è eth2 physical interface. The purpose of the Linux Bridge connecting to the VM is to allow security group enforcement with iptables. Security groups are enforced at the edge point which are the interface of the VM, since iptables nnot be applied to OVS bridges we use Linux bridge to apply them. In the future we hope to see this Linux Bridge going away rules.  VLAN tags: As we discussed in the first use case net1 is using VLAN tag 1000, looking at OVS above we see that qvo41f1ebcf-7c is tagged with VLAN tag 3. The modification from VLAN tag 3 to 1000 as we go to the physical network is done by OVS  as part of the packet flow of br-eth2 in the same way we showed before. To summarize, when a VM is launched it is connected to the VM network through a chain of elements as described here. During the packet from VM to the network and back the VLAN tag is modified. Use case #3: Serving a DHCP request coming from the virtual machine In the previous use cases we have shown that both the namespace called dhcp-<some id> and the VM end up connecting to the physical interface eth2  on their respective nodes, both will tag their packets with VLAN tag 1000.We saw that the namespace has an interface with IP of 10.10.10.3. Since the VM and the namespace are connected to each other and have interfaces on the same subnet they can ping each other, in this picture we see a ping from the VM which was assigned 10.10.10.2 to the namespace: The fact that they are connected and can ping each other can become very handy when something doesn’t work right and we need to isolate the problem. In such case knowing that we should be able to ping from the VM to the namespace and back can be used to trace the disconnect using tcpdump or other monitoring tools. To serve DHCP requests coming from VMs on the network Neutron uses a Linux tool called “dnsmasq”,this is a lightweight DNS and DHCP service you can read more about it here. If we look at the dnsmasq on the control node with the ps command we see this: dnsmasq --no-hosts --no-resolv --strict-order --bind-interfaces --interface=tap26c9b807-7c --except-interface=lo --pid-file=/var/lib/neutron/dhcp/5f833617-6179-4797-b7c0-7d420d84040c/pid --dhcp-hostsfile=/var/lib/neutron/dhcp/5f833617-6179-4797-b7c0-7d420d84040c/host --dhcp-optsfile=/var/lib/neutron/dhcp/5f833617-6179-4797-b7c0-7d420d84040c/opts --leasefile-ro --dhcp-range=tag0,10.10.10.0,static,120s --dhcp-lease-max=256 --conf-file= --domain=openstacklocal The service connects to the tap interface in the namespace (“--interface=tap26c9b807-7c”), If we look at the hosts file we see this: # cat  /var/lib/neutron/dhcp/5f833617-6179-4797-b7c0-7d420d84040c/host fa:16:3e:fe:c7:87,host-10-10-10-2.openstacklocal,10.10.10.2   If you look at the console output above you can see the MAC address fa:16:3e:fe:c7:87 which is the VM MAC. This MAC address is mapped to IP 10.10.10.2 and so when a DHCP request comes with this MAC dnsmasq will return the 10.10.10.2.If we look into the namespace at the time we initiate a DHCP request from the VM (this can be done by simply restarting the network service in the VM) we see the following: # ip netns exec qdhcp-5f833617-6179-4797-b7c0-7d420d84040c tcpdump -n 19:27:12.191280 IP 0.0.0.0.bootpc > 255.255.255.255.bootps: BOOTP/DHCP, Request from fa:16:3e:fe:c7:87, length 310 19:27:12.191666 IP 10.10.10.3.bootps > 10.10.10.2.bootpc: BOOTP/DHCP, Reply, length 325   To summarize, the DHCP service is handled by dnsmasq which is configured by Neutron to listen to the interface in the DHCP namespace. Neutron also configures dnsmasq with the combination of MAC and IP so when a DHCP request comes along it will receive the assigned IP. Summary In this post we relied on the components described in the previous post and saw how network connectivity is achieved using three simple use cases. These use cases gave a good view of the entire network stack and helped understand how an end to end connection is being made between a VM on a compute node and the DHCP namespace on the control node. One conclusion we can draw from what we saw here is that if we launch a VM and it is able to perform a DHCP request and receive a correct IP then there is reason to believe that the network is working as expected. We saw that a packet has to travel through a long list of components before reaching its destination and if it has done so successfully this means that many components are functioning properly. In the next post we will look at some more sophisticated services Neutron supports and see how they work. We will see that while there are some more components involved for the most part the concepts are the same. @RonenKofman

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• Best way of learning Python + GUI when coming from .NET

  - by Oscar Mederos

  I've been developing applications in C# / VB.NET for about 3-4 years (.NET Framework v2.0, 3.5, 4). I have also developed some command-line applications or scripts in C, and Python under Linux. Sometimes I need to develop my applications in another languages, like Python, but the problem thing is that lots of those applications require a GUI. Maybe not a too complex one, but it does require some windows with buttons, text boxes, list boxes,... What books/tips/tutorials do you suggest me to start working with that language and be able to deploy my deliverables not only in .NET? Note: Learning python is not the big deal here, because I already know the basic of it. I just want to focus on the GUI. Maybe this question should be on UI instead of here? If so, please, migrate it :)

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• User roles in GWT applications

  - by csaffi

  Hi everybody, I'm wondering if you could suggest me any way to implement "user roles" in GWT applications. I would like to implement a GWT application where users log in and are assigned "roles". Based on their role, they would be able to see and use different application areas. Here are two possible solution I thought: 1) A possible solution could be to make an RPC call to the server during onModuleLoad. This RPC call would generate the necessary Widgets and/or place them on a panel and then return this panel to the client end. 2) Another possible solution could be to make an RPC call on login retrieving from server users roles and inspecting them to see what the user can do. What do you think about? Thank you very much in advance for your help!

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• Data validation best practices: how can I better construct user feedback?

  - by Cory Larson

  Data validation, whether it be domain object, form, or any other type of input validation, could theoretically be part of any development effort, no matter its size or complexity. I sometimes find myself writing informational or error messages that might seem harsh or demanding to unsuspecting users, and frankly I feel like there must be a better way to describe the validation problem to the user. I know that this topic is subjective and argumentative. I've migrated this question from StackOverflow where I originally asked it with little response. Basically, I'm looking for good resources on data validation and user feedback that results from it at a theoretical level. Topics and questions I'm interested in are: Content Should I be describing what the user did correctly or incorrectly, or simply what was expected? How much detail can the user read before they get annoyed? (e.g. Is "Username cannot exceed 20 characters." enough, or should it be described more fully, such as "The username cannot be empty, and must be at least 6 characters but cannot exceed 30 characters."?) Grammar How do I decide between phrases like "must not," "may not," or "cannot"? Delivery This can depend on the project, but how should the information be delivered to the user? Should it be obtrusive (e.g. JavaScript alerts) or friendly? Should they be displayed prominently? Immediately (i.e. without confirmation steps, etc.)? Logging Do you bother logging validation errors? Internationalization Some cultures prefer or better understand directness over subtlety and vice-versa (e.g. "Don't do that!" vs. "Please check what you've done."). How do I cater to the majority of users? I may edit this list as I think more about the topic, but I'm genuinely interested in proper user feedback techniques. I'm looking for things like research results, poll results, etc. I've developed and refined my own techniques over the years that users seem to be okay with, but I work in an environment where the users prefer to adapt to what you give them over speaking up about things they don't like. I'm interested in hearing your experiences in addition to any resources to which you may be able to point me.

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• What are the pros (and cons) of using “Sign in with Twitter/Facebook” for a new website?

  - by Paul D. Waite

  Myself and a friend are looking to launch a little forum site. I’m considering using the “Sign in with Facebook/Twitter” APIs, possibly exclusively (a la e.g. Lanyrd), for user login. I haven’t used either of these before, nor run a site with user logins at all. What are the pros (and cons) of these APIs? Specifically: What benefits do I get as a developer from using them? What drawbacks are there? Do end users actually like/dislike them? Have you experienced any technical/logistical issues with these APIs specifically? Here are the pros and cons I’ve got so far: Pros More convenient for the user (“register” with two clicks, sign in with one) Possibly no need to maintain our own login system  Cons No control over our login process Exclude Facebook/Twitter users who are worried about us having some sort of access to their accounts Users’ accounts on our site are compromised if their Facebook/Twitter accounts are compromised. And if we don’t maintain our own alternative login system: Dependency on Facebook/Twitter for our login system Exclude non-Facebook/non-Twitter users from our site

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• What are the pros and cons of using “Sign in with Twitter/Facebook” for a new website?

  - by Paul D. Waite

  Myself and a friend are looking to launch a little forum site. I’m considering using the “Sign in with Facebook/Twitter” APIs, possibly exclusively, for user login.I haven’t used either of these before, nor run a site with user logins at all. What are the pros and cons of these APIs? Specifically: Is the idea of using them, and/or using them exclusively (i.e. having no login system other than one or both of these), any good? What benefits do I get as a developer from using them? Do end users actually like/dislike them? Have you experienced any technical/logistical issues with these APIs specifically? Here are the pros and cons I’ve got so far: Pros More convenient for the user (“register” with two clicks, sign in with one) Possibly no need to maintain our own login system  Cons No control over our login process Exclude non-Facebook/non-Twitter users (if we don’t maintain our own login system) Exclude Facebook/Twitter users who are worried about us having some sort of access to their accounts Users’ accounts on our site are compromised if their Facebook/Twitter accounts are compromised.

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• Will there be a difference between Unity "Desktop" and Unity "Netbook" interface?

  - by bash

  What has me slightly confused from all the different reports about Unity becoming the default shell in the 11.04 desktop edition: Some claim and this is how most people seem to see it, that what currently is the UNE interface will be taken 1:1 (including any changes made during the Natty cycle) and used as the interface for the desktop version - meaning that the interface will be exactly the same for 11.04 UNE and UDE. Some other reports talk more along the lines a "desktop unity", making it sound, that while both will be using Unity, the Desktop edition will get it's own Unity, tailored specifically towards the Desktop formfactor. So both editions will be using Unity, but with slight differences in the layout or interface. So my question is, will there be any difference between 11.04 Unity "Desktop" and Unity "Netbook" interface?

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• What are the pros (and cons) of using “Sign in with Twitter/Facebook” for a new website?

  - by Paul D. Waite

  Myself and a friend are looking to launch a little forum site. I’m considering using the “Sign in with Facebook/Twitter” APIs, possibly exclusively (a la e.g. Lanyrd), for user login. I haven’t used either of these before, nor run a site with user logins at all. What are the pros (and cons) of these APIs? Specifically: What benefits do I get as a developer from using them? What drawbacks are there? Do end users actually like/dislike them? Have you experienced any technical/logistical issues with these APIs specifically? Here are the pros and cons I’ve got so far: Pros More convenient for the user (“register” with two clicks, sign in with one) Possibly no need to maintain our own login system  Cons No control over our login process Exclude Facebook/Twitter users who are worried about us having some sort of access to their accounts Users’ accounts on our site are compromised if their Facebook/Twitter accounts are compromised. And if we don’t maintain our own alternative login system: Dependency on Facebook/Twitter for our login system Exclude non-Facebook/non-Twitter users from our site

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• Modified Strategy Design Pattern

  - by Samuel Walker

  I've started looking into Design Patterns recently, and one thing I'm coding would suit the Strategy pattern perfectly, except for one small difference. Essentially, some (but not all) of my algorithms, need an extra parameter or two passed to them. So I'll either need to pass them an extra parameter when I invoke their calculate method or store them as variables inside the ConcreteAlgorithm class, and be able to update them before I call the algorithm. Is there a design pattern for this need / How could I implement this while sticking to the Strategy Pattern? I've considered passing the client object to all the algorithms, and storing the variables in there, then using that only when the particular algorithm needs it. However, I think this is both unwieldy, and defeats the point of the strategy pattern. Just to be clear I'm implementing in Java, and so don't have the luxury of optional parameters (which would solve this nicely).

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• Data validation best practices: how can I better construct user feedback?

  - by Cory Larson

  Data validation, whether it be domain object, form, or any other type of input validation, could theoretically be part of any development effort, no matter its size or complexity. I sometimes find myself writing informational or error messages that might seem harsh or demanding to unsuspecting users, and frankly I feel like there must be a better way to describe the validation problem to the user. I know that this topic is subjective and argumentative. StackOverflow might not be the proper channel for diving into this subject, but like I've mentioned, we all run into this at some point or another. There are so many StackExchange sites now; if there is a better one, feel free to share! Basically, I'm looking for good resources on data validation and user feedback that results from it at a theoretical level. Topics and questions I'm interested in are: Content Should I be describing what the user did correctly or incorrectly, or simply what was expected? How much detail can the user read before they get annoyed? (e.g. Is "Username cannot exceed 20 characters." enough, or should it be described more fully, such as "The username cannot be empty, and must be at least 6 characters but cannot exceed 30 characters."?) Grammar How do I decide between phrases like "must not," "may not," or "cannot"? Delivery This can depend on the project, but how should the information be delivered to the user? Should it be obtrusive (e.g. JavaScript alerts) or friendly? Should they be displayed prominently? Immediately (i.e. without confirmation steps, etc.)? Logging Do you bother logging validation errors? Internationalization Some cultures prefer or better understand directness over subtlety and vice-versa (e.g. "Don't do that!" vs. "Please check what you've done."). How do I cater to the majority of users? I may edit this list as I think more about the topic, but I'm genuinely interest in proper user feedback techniques. I'm looking for things like research results, poll results, etc. I've developed and refined my own techniques over the years that users seem to be okay with, but I work in an environment where the users prefer to adapt to what you give them over speaking up about things they don't like. I'm interested in hearing your experiences in addition to any resources to which you may be able to point me.

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• NCurses, scrolling of multiline items, "current item" pointer and "selected items"

  - by mjf

  I am looking for hints/ideas for the best (most effective) way on how to scroll multi-line items as well as emphasizing of the "current item" and "selected items" such as: 1 FOO ITEM 1 Foo sub-item 2 Foo sub-item 3 Foo sub-item 2 BAR ITEM 1 Bar sub-item 3 BAZ ITEM 1 Baz sub-item 2 Baz sub-item 4 RAB ITEM 5 ZZZ ITEM 1 Zzz sub-item 2 Zzz sub-item 3 Zzz sub-item 4 Zzz sub-item using NCurses (some combination of windows, sub-windows, pads, copywin? Uff! In fact, the lines could exceed the stdscr's width so that possibility to scroll left/right would be also nice - pads?)... The whole items (including the sub-items) are supposed to be emphasized as full-width window/pad areas. The "current item" (including it's set of lines) should be emphasized (i.e. using A_BOLD), selected set of items of choice (including the set of lines for each the selected item) should be emphasized in another way (i.e. using A_REVERSE). What would you choose to cope with it the most effective NCurses way? (The less redrawals/refreshes the better and terminal is supposed to have the ability to change it's size - such as XTerm running under "floating window" management.) Thank you for your ideas (or perhaps some piece of code where something similar is already solved - I was not able to find anything helpful on the Internet. I mean I am not going to copy/paste foreign code but programming NCurses properly is still somehow difficult to me). P.S.: Would you suggest to "smooth-scroll" +1/-1 screen line or rather "jump-scroll" +lines/-lines of the items? (I personally prefer the latter one.) Sincerely, -- mjf

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