Hello, again!
Consider what happens now if you attempt to load MyBlog in your browser. The controller class MyBlog is handed the request. Because no method was named explicitly, the index method is invoked. And because index doesn’t produce any output, the my_blog/index.rhtml template is returned to the user. Finally, let’s take advantage of our template’s dynamic properties to set a value in the controller and pass that along to the template. We modify our index method to read: def index @now = Time.now.to_s end Notice how we have used an @ character at the beginning of the variable @now. I found this to be a little confusing at first, as @ normally is used as a prefix for instance variables in Ruby. But it becomes fairly natural and logical after a little time. Finally, we modify our template such that it incorporates the string value contained in @now:Hello, world!
11 is now <%= @now %>.
Once again, you can retrieve the page even without restart¬ ing Ruby. You should see the date and time as kept on the serv¬ er, updated each time you refresh the page. Conclusion Ruby on Rails is, without a doubt, one of the most talked-about Web technologies to emerge in the past few years. This month, we saw how straightforward it is to create a new Rails applica¬ tion, to create a controller and a view and to integrate them using a combination of naming conventions and relatively stan¬ dard template syntax. However, we did not discuss views, par¬ ticularly those associated with a relational database. Next month, we will do exactly that, connecting Rails to the PostgreSQL database. I believe doing so will begin to show why people are so excited about Rails and why it might be a good tool for many Web developers to learn. Resources for this article: www.linuxjournal.com/article/ 8457.0 Reuven M. Lerner, a longtime Web/database con¬ sultant and developer, now is a graduate student in the Learning Sciences program at Northwestern University. His Weblog is at altneuland.lerner.co.il, and you can reach him at reuven@lerner.co.il. 201 OCTOBER 2005 WWW.LINUXJOURNAL.COM MORE SPACE. LESS MONEY. Unlimited Affordable Network Storage Everybody needs more space. And they need to spend less money. What if you can both have more space and spend less money? What if you could put IV 2 terabytes in only 3 rack units? What if that IV 2 terabytes cost less than $10,000? In¬ cluding the SATA disk drives. Imagine if you could glue it all together with a RAID appliance into one system. What if you could add as much storage as you wanted, one shelf at a time, and never have to 'fork-lift’ anything? Coraid’s new SATA EtherDrive Storage allows you to do just that. Using industry standard SATA disk drives, EtherDrive Storage connects disks directly to your Ethernet network. Each disk appears as a local drive to any Linux, FreeBSD or Solaris system using our open ATA-over-Eth- ernet (AoE) protocol. Since the disks just appear as local drives you already know how to use them. The EtherDrive® SATA Storage Shelf is a 3U rack- mount network appliance that contains 15 SATA drive slots. Its triple redundant power supply protects you from your most likely failure. Its dual Gb Ethernet interfaces allow your data to go fast; 200MB per second. And at a very af¬ fordable price. List price for the EtherDrive Storage Shelf, without disks, is only $3,995. Our companion product, the RAIDBlade RAID controller, allows a virtually unlimited number of Storage Shelves to be combined into a set of logical AoE storage de¬ vices. Now you can have unlimited storage at a very af¬ fordable price. For complete information, visit our website at www.coraid.com, or call, toll-free, 1-877-548-7200. And we’ll show how we’ve made network storage so affordable, you can have all the space you want. www.coraid.com info@coraid.com 1.706.548.7200 CO RAID Network Programming in the Kernel Take a tour of the kernel's networking functionality by writing a network client that runs in kernel space. BY PRADEEP PAOALA AND RAVI PARIMI 11 Linux distributions provide a wide range of network appli¬ cations—from daemons that provide a variety of services such as WWW, mail and SSH to client programs that access one or more of these services. These programs are written in user mode and use the system calls pro¬ vided by the kernel to perform various operations like network read and write. Although this is the traditional method of writing programs, there is another inter¬ esting way to develop these applications by implementing them in the kernel. The TUX Web server is a good example of an application that runs inside the kernel and serves static content. In this article, we explain the basics of writing network applications within the kernel and their advantages and disadvantages. As an example, we explain the implementation of an in-kemel FTP client. Advantages and Disadvantages of In-Kernel Implementations Why would one want to implement applications within the kernel? Here are a few advantages: ■ When a user-space program makes a system call, there is some overhead associated in the user-space/kernel- space transition. By programming all functionality in the kernel, we can make gains in performance. ■ The data corresponding to any appli¬ cation that sends or receives packets is copied from user mode to kernel mode and vice versa. By implement¬ ing network applications within the kernel, it is possible to reduce such overhead and increase efficiency by not copying data to user mode. ■ In specific research and high-perfor¬ mance computing environments, there is a need for achieving data transfers at great speeds. Kernel applications are useful in such situations. On the other hand, in-kernel imple¬ mentations have certain disadvantages: ■ Security is a primary concern within the kernel, and a large class of user¬ mode applications are not suitable to be run directly in the kernel. Consequently, special care needs to be taken while designing in-kernel applications. For example, reading and writing to files within the kernel is usually a bad idea, but most appli¬ cations require some kind of file I/O. ■ Large applications cannot be imple¬ mented in the kernel due to memory constraints. Network Programming Basics Network programming is usually done with sockets. A socket serves as a com¬ munication end point between two pro¬ cesses. In this article, we describe net¬ work programming with TCP/IP sockets. Server programs create sockets, bind to well-known ports, listen and accept connections from clients. Servers are usu¬ ally designed to accept multiple connec¬ tions from clients—they either fork a new process to serve each client request (con¬ current servers) or completely serve one request before accepting more connec¬ tions (iterative servers). Client programs, on the other hand, create sockets to con¬ nect to servers and exchange information. FTP Client-Server Interaction Let’s take a quick look at how an FTP client and server are implemented in user mode. We discuss only active FTP in this article. The differences between active and passive FTP are not relevant to our discussion of network programming here. Socket Programming Basics Here is a brief explanation of the design of an FTP client and server. The server program creates a socket using the socket () system call. It then binds on a well-known port using bi nd () and waits for connections from clients using the 1 i sten () system call. The server then accepts incoming requests from clients using accept () and forks a new process (or thread) to serve each incom¬ ing client request. The client program creates a control socket using socket () and next calls connect () to establish a connection with the server. It then creates a separate socket for data transfer using socket () and binds to an unprivileged port client server Figure 1. The FTP protocol uses two sockets: one for control messages and one for data. Flere's how the first connection, used for commands, gets set up. 221 OCTOBER 2005 WWW.LINUXJOURNAL.COM The bolA7 craiL Easy, high-performance clustering. For years, many searched, but none could find it. Some said it didn't exist. But not the Penguin. Penguin Computing® made easy, high-performance clustering a quest. Now you can find Linux hardware and software solutions, configured to order, driven by Scyld's commercially supported, industry-leading Linux clustering software. For the turnkey clusters you need to run even your most important applica¬ tions, come to Penguin Computing. Penguin Computing's dedicated experts, who are 100% focused on Linux, are waiting to serve. O d) Powerful, easy clustering. It's the once and future thing. Love what you do (^) CL E o .o www.penguin □□□□ mmnn SCYLD registered trademark of Penguin Computing, Inc. Scyld, Scyld Software, Scyld Beowulf, and the Scyld Block Logo are trademarks of Scyld Software, Inc. Linux is a registered trademark of Linus Torvalds. Other names are for informational purposes only and may be trademarks of their respective owners. (>1024) using bi nd (). The client now 1 i sten () s on this port for data transfer from the server. The server now has enough knowledge to honor a data transfer request from the client. Finally, the client uses accept () to accept connections from the server to send and receive data. For sending and receiving data, the client and server use the wri te () and read() or sendmsgQ and recvmsgO system calls. The client issues close () on all open sockets to tear down its connection to the server. Figure 1 sums it up. FTP Commands Here is a list of a few FTP commands we used. Because our program provides only a basic implementation of the protocol, we discuss only the relevant commands: a TYPE I\r\n command to the server to request this. Figure 2 is a diagram that shows a few FTP commands and their responses from the server. Socket Programming in the Kernel Writing programs in the kernel is different from doing the same in user space. We explain a few issues concerned with writing a network application in the kernel. Refer to Greg Kroah-Hartman’s arti¬ cle “Things You Never Should Do in the Kernel” (see the on¬ line Resources). First, let’s examine how a system call in user space completes its task. For example, look at the socket () system call: ■ The client sends a USER