The main Program

The main entry point to the Ice run time is represented by the local interface Ice::Communicator. You must initialize the Ice run time by calling Ice::initialize before you can do anything else in your program. Ice::initialize returns a reference to an instance of an Ice:::Communicator:

require 'Ice'

status = 0
ic = nil
begin
    ic = Ice::initialize(ARGV)
    # ...
rescue => ex
    puts ex
    status = 1
end

# ...

Ice::initialize accepts the argument list that is passed to the program by the operating system. The function scans the argument list for any command-line options that are relevant to the Ice run time; any such options are removed from the argument list so, when Ice::initialize returns, the only options and arguments remaining are those that concern your application. If anything goes wrong during initialization, initialize throws an exception.

Before leaving your program, you must call Communicator::destroy. The destroy operation is responsible for finalizing the Ice run time. In particular, destroy ensures that any outstanding threads are joined with and reclaims a number of operating system resources, such as file descriptors and memory. Never allow your program to terminate without calling destroy first; doing so has undefined behavior.

require 'Ice'

status = 0
ic = nil
begin
    ic = Ice::initialize(ARGV)
    # ...
rescue => ex
    puts ex
    status = 1
end

if ic
    begin
        ic.destroy()
    rescue => ex
        puts ex
        status = 1
    end
end

exit(status)

Note that the code places the call to Ice::initialize into a begin block and takes care to return the correct exit status to the operating system. Also note that an attempt to destroy the communicator is made only if the initialization succeeded.

22.16.1 The Ice::Application Class

The preceding program structure is so common that Ice offers a class, Ice::Application, that encapsulates all the correct initialization and finalization activities. The synopsis of the class is as follows (with some detail omitted for now):

module Ice
    class Application
        def main(args, configFile=nil, initData=nil)

        def run(args)

        def Application.appName()

        def Application.communicator()
    end
end

The intent of this class is that you specialize Ice::Application and implement the abstract run method in your derived class. Whatever code you would normally place in your main program goes into run instead. Using Ice::Application, our program looks as follows:

require 'Ice'

class Client < Ice::Application
    def run(args)
        # Client code here...
        return 0
    end
end

app = Client.new()
status = app.main(ARGV)
exit(status)

If you prefer, you can also reopen Ice::Application and define run directly:

require 'Ice'

class Ice::Application
    def run(args)
        # Client code here...
        return 0
    end
end

app = Ice::Application.new()
status = app.main(ARGV)
exit(status)

The Application.main function does the following:

1. It installs an exception handler. If your code fails to handle an exception, Application.main prints the exception information before returning with a non-zero return value.

2. It initializes (by calling Ice::initialize) and finalizes (by calling Communicator.destroy) a communicator. You can get access to the communicator for your program by calling the static communicator accessor.

3. It scans the argument list for options that are relevant to the Ice run time and removes any such options. The argument list that is passed to your run method therefore is free of Ice-related options and only contains options and arguments that are specific to your application.

4. It provides the name of your application via the static appName member function. The return value from this call is the first element of the argument vector passed to Application.main, so you can get at this name from anywhere in your code by calling Ice::Application::appName (which is usually required for error messages).

Using Ice::Application ensures that your program properly finalizes the Ice run time, whether your program terminates normally or in response to an exception or signal. We recommend that all your programs use this class; doing so makes your life easier. In addition Ice::Application also provides features for signal handling and configuration that you do not have to implement yourself when you use this class.

Catching Signals

A program typically needs to perform some cleanup work before terminating, such as flushing database buffers or closing network connections. This is particularly important on receipt of a signal or keyboard interrupt to prevent possible corruption of database files or other persistent data.

To make it easier to deal with signals, Ice::Application encapsulates Ruby’s signal handling capabilities, allowing you to cleanly shut down on receipt of a signal:

class Application
    def Application.destroyOnInterrupt()

    def Application.ignoreInterrupt()

    def Application.callbackOnInterrupt()

    def Application.holdInterrupt()

    def Application.releaseInterrupt()

    def Application.interrupted()

    def interruptCallback(sig):
        # Default implementation does nothing.
    end
    # ...
end

The methods behave as follows:

This function configures Ice::Application to invoke interruptCallback when a signal occurs, thereby giving the subclass responsibility for handling the signal.

This method restores signal delivery to the previous disposition. Any signal that arrives after holdInterrupt was called is delivered when you call releaseInterrupt.

This method returns True if a signal caused the communicator to shut down, False otherwise. This allows us to distinguish intentional shutdown from a forced shutdown that was caused by a signal. This is useful, for example, for logging purposes.

A subclass implements this function to respond to signals. The function may be called concurrently with any other thread and must not raise exceptions.

By default, Ice::Application behaves as if destroyOnInterrupt was invoked, therefore our program requires no change to ensure that the program terminates cleanly on receipt of a signal. (You can disable the signal-handling functionality of Ice::Application by passing the constant NoSignalHandling to the constructor. In that case, signals retain their default behavior, that is, terminate the process.) However, we add a diagnostic to report the occurrence of a signal, so our program now looks like:

require 'Ice'

class MyApplication < Ice::Application
    def run(args)
        # Client code here...

        if Ice::Application::interrupted()
            print Ice::Application::appName() + ": terminating"
        end

        return 0
    end
end

app = MyApplication.new()
status = app.main(ARGV)
exit(status)

Ice::Application and Properties

Apart from the functionality shown in this section, Ice::Application also takes care of initializing the Ice run time with property values. Properties allow you to configure the run time in various ways. For example, you can use properties to control things such as the thread pool size or the trace level for diagnostic output. The main method of Ice::Application accepts an optional second parameter allowing you to specify the name of a configuration file that will be processed during initialization. We discuss Ice properties in more detail in Chapter 26.

Limitations of Ice::Application

Ice::Application is a singleton class that creates a single communicator. If you are using multiple communicators, you cannot use Ice::Application. Instead, you must structure your code as we saw in Chapter 3 (taking care to always destroy the communicator).

22.16 The main Program

22.16.1 The Ice::Application Class

Catching Signals

Ice::Application and Properties

Limitations of Ice::Application