17.2 Implementing a File System Server

We have now seen enough of the server-side C# mapping to implement a server for the file system we developed in Chapter 5. (You may find it useful to review the Slice definition for our file system in Section 5.4 before studying the source code.)

Our server is composed of three source files:

• Server.cs

This file contains the server main program.

• DirectoryI.cs

This file contains the implementation for the Directory servants.

• FileI.cs

This file contains the implementation for the File servants.

17.2.1 The Server Main Program

Our server main program, in the file Server.cs, uses the Ice.Application class we discussed in Section 16.3.1. The run method installs a signal handler, creates an object adapter, instantiates a few servants for the directories and files in the file system, and then activates the adapter. This leads to a Main method as follows:

using Filesystem;
using System;

public class Server
{
    class App : Ice.Application
    {
        public override int run(string[] args)
        {
            // Terminate cleanly on receipt of a signal
            //
            shutdownOnInterrupt();

            // Create an object adapter (stored in the _adapter
            // static members)
            //
            Ice.ObjectAdapter adapter
                = communicator().createObjectAdapterWithEndpoints(
                      "SimpleFilesystem", "default ‑p 10000");
            DirectoryI._adapter = adapter;
            FileI._adapter = adapter;

            // Create the root directory (with name "/" and no
            // parent)
            //
            DirectoryI root = new DirectoryI("/", null);

            // Create a file called "README" in the root directory
            //
            File file = new FileI("README", root);
            string[] text;
            text = new string[]{ "This file system contains "
                                 + "a collection of poetry." };
            try {
                file.write(text);
            } catch (GenericError e) {
                Console.Error.WriteLine(e.reason);
            }

            // Create a directory called "Coleridge"
            // in the root directory
            //
            DirectoryI coleridge =
                new DirectoryI("Coleridge", root);

            // Create a file called "Kubla_Khan"
            // in the Coleridge directory
            //
            file = new FileI("Kubla_Khan", coleridge);
            text = new string[]{
                "In Xanadu did Kubla Khan",
                "A stately pleasure‑dome decree:",
                "Where Alph, the sacred river, ran",
                "Through caverns measureless to man",
                "Down to a sunless sea." };
            try {
                file.write(text);
            } catch (GenericError e) {
                Console.Error.WriteLine(e.reason);
            }

            // All objects are created, allow client requests now
            //
            adapter.activate();

            // Wait until we are done
            //
            communicator().waitForShutdown();

            if (interrupted())
                Console.Error.WriteLine(
                    appName() + ": terminating");

            return 0;
        }
    }

    public static void Main(string[] args)
    {
        App app = new App();
        Environment.Exit(app.main(args));
    }
}

The code uses a using directive for the Filesystem namespace. This avoids having to continuously use fully-qualified identifiers with a Filesystem. prefix.

The next part of the source code is the definition of the Server class, which includes a nested class that derives from Ice.Application and contains the main application logic in its run method. Much of this code is boiler plate that we saw previously: we create an object adapter, and, towards the end, activate the object adapter and call waitForShutdown.

The interesting part of the code follows the adapter creation: here, the server instantiates a few nodes for our file system to create the structure shown in Figure 17.1

Figure 17.1. A small file system.

As we will see shortly, the servants for our directories and files are of type DirectoryI and FileI, respectively. The constructor for either type of servant accepts two parameters, the name of the directory or file to be created and a reference to the servant for the parent directory. (For the root directory, which has no parent, we pass a null parent.) Thus, the statement

DirectoryI root = new DirectoryI("/", null);

creates the root directory, with the name "/" and no parent directory.

Here is the code that establishes the structure in Figure 17.1:

        // Create the root directory (with name "/" and no parent)
        //
        DirectoryI root = new DirectoryI("/", null);

        // Create a file called "README" in the root directory
        //
        File file = new FileI("README", root);
        string[] text;
        text = new string[]{ "This file system contains "
                             + "a collection of poetry." };
        try {
            file.write(text);
        } catch (GenericError e) {
            Console.Error.WriteLine(e.reason);
        }

        // Create a directory called "Coleridge"
        // in the root directory
        //
        DirectoryI coleridge = new DirectoryI("Coleridge", root);

        // Create a file called "Kubla_Khan"
        // in the Coleridge directory
        //
        file = new FileI("Kubla_Khan", coleridge);
        text = new string[]{ "In Xanadu did Kubla Khan",
                             "A stately pleasure‑dome decree:",
                             "Where Alph, the sacred river, ran",
                             "Through caverns measureless to man",
                             "Down to a sunless sea." };
        try {
            file.write(text);
        } catch (GenericError e) {
            Console.Error.WriteLine(e.reason);
        }

We first create the root directory and a file README within the root directory. (Note that we pass a reference to the root directory as the parent when we create the new node of type FileI.)

The next step is to fill the file with text:

        string[] text;
        text = new string[]{ "This file system contains "
                             + "a collection of poetry." };
        try {
            file.write(text);
        } catch (GenericError e) {
            Console.Error.WriteLine(e.reason);
        }

Recall from Section 14.7.3 that Slice sequences by default map to C# arrays. The Slice type Lines is simply an array of strings; we add a line of text to our README file by initializing the text array to contain one element.

Finally, we call the Slice write operation on our FileI servant by simply writing:

file.write(text);

This statement is interesting: the server code invokes an operation on one of its own servants. Because the call happens via a reference to the servant (of type FileI) and not via a proxy (of type FilePrx), the Ice run time does not know that this call is even taking place—such a direct call into a servant is not mediated by the Ice run time in any way and is dispatched as an ordinary C# method call.

In similar fashion, the remainder of the code creates a subdirectory called Coleridge and, within that directory, a file called Kubla_Khan to complete the structure in Figure 17.1.

17.2.2 The FileI Servant Class

Our FileI servant class has the following basic structure:

using Filesystem;
using System;

public class FileI : FileDisp_
{
    // Constructor and operations here...

    public static Ice.ObjectAdapter _adapter;
    private string _name;
    private DirectoryI _parent;
    private string[] _lines;
}

The class has a number of data members:

• _adapter

This static member stores a reference to the single object adapter we use in our server.

• _name

This member stores the name of the file incarnated by the servant.

• _parent

This member stores the reference to the servant for the file’s parent directory.

• _lines

This member holds the contents of the file.

The _name and _parent data members are initialized by the constructor:

    public FileI(string name, DirectoryI parent)
    {
        _name = name;
        _parent = parent;

        Debug.Assert(_parent != null);

        // Create an identity
        //
        Ice.Identity myID = new Ice.Identity();
        myId.name = Ice.Util.generateUUID();

        // Add the identity to the object adapter
        //
        _adapter.add(this, myID);

        // Create a proxy for the new node and
        // add it as a child to the parent
        //
        NodePrx thisNode = NodePrxHelper.uncheckedCast(
                                _adapter.createProxy(myID));
        _parent.addChild(thisNode);
    }

After initializing the _name and _parent members, the code verifies that the reference to the parent is not null because every file must have a parent directory. The constructor then generates an identity for the file by calling Ice.Util.generateUUID and adds itself to the servant map by calling ObjectAdapter.add. Finally, the constructor creates a proxy for this file and calls the addChild method on its parent directory. addChild is a helper function that a child directory or file calls to add itself to the list of descendant nodes of its parent directory. We will see the implementation of this function on page 555.

The remaining methods of the FileI class implement the Slice operations we defined in the Node and File Slice interfaces:

    // Slice Node::name() operation

    public override string name(Ice.Current current)
    {
        return _name;
    }

    // Slice File::read() operation

    public override string[] read(Ice.Current current)
    {
        return _lines;
    }

    // Slice File::write() operation

    public override void write(string[] text, Ice.Current current)
    {
        _lines = text;
    }

The name method is inherited from the generated Node interface (which is a base interface of the _FileDisp class from which FileI is derived). It simply returns the value of the _name member.

The read and write methods are inherited from the generated File interface (which is a base interface of the _FileDisp class from which FileI is derived) and simply return and set the _lines member.

17.2.3 The DirectoryI Servant Class

The DirectoryI class has the following basic structure:

using Filesystem;
using System;
using System.Collections;

public class DirectoryI : DirectoryDisp_
{
    // Constructor and operations here...

    public static Ice.ObjectAdapter _adapter;
    private string _name;
    private DirectoryI _parent;
    private ArrayList _contents = new ArrayList();
}

As for the FileI class, we have data members to store the object adapter, the name, and the parent directory. (For the root directory, the _parent member holds a null reference.) In addition, we have a _contents data member that stores the list of child directories. These data members are initialized by the constructor:

    public DirectoryI(string name, DirectoryI parent)
    {
        _name = name;
        _parent = parent;

        // Create an identity. The
        // parent has the fixed identity "RootDir"
        //
        Ice.Identity myID = new Ice.Identity();
        myID.name = _parent != null ? Ice.Util.generateUUID() : "RootDir";

        // Add the identity to the object adapter
        //
        _adapter.add(this, myID);

        // Create a proxy for the new node and
        // add it as a child to the parent
        //
        NodePrx thisNode = NodePrxHelper.uncheckedCast(
                                _adapter.createProxy(myID));
        if (_parent != null)
            _parent.addChild(thisNode);
    }

The constructor creates an identity for the new directory by calling Ice.Util.generateUUID. (For the root directory, we use the fixed identity "RootDir".) The servant adds itself to the servant map by calling ObjectAdapter.add and then creates a proxy to itself and passes it to the addChild helper function.

addChild simply adds the passed reference to the _contents list:

    public void addChild(NodePrx child)
    {
        _contents.Add(child);
    }

The remainder of the operations, name and list, are trivial:

    public override string name(Ice.Current current)
    {
        return _name;
    }

    public override NodePrx[] list(Ice.Current current)
    {
        return (NodePrx[])_contents.ToArray(typeof(NodePrx));
    }

Note that the _contents member is of type System.Collections.ArrayList, which is convenient for the implementation of the addChild method. However, this requires us to convert the list into a C# array in order to return it from the list operation.