The Process Facet

An activation service, such as an IceGrid node, needs a reliable way to gracefully deactivate a server. One approach is to use a platform-specific mechanism, such as POSIX signals. This works well on POSIX platforms when the server is prepared to intercept signals and react appropriately. On Windows platforms, it works less reliably for C++ servers, and not at all for Java servers. For these reasons, the Process facet provides an alternative that is both portable and reliable.

Be aware of the security considerations associated with enabling the Process facet.

The Process Interface

The Slice interface Ice::Process allows an activation service to request a graceful shutdown of the program:

module Ice
{
    interface Process
    {
        void shutdown();
        void writeMessage(string message, int fd);
    }
}

When shutdown is invoked, the object implementing this interface is expected to initiate the termination of its process. The activation service may expect the program to terminate within a certain period of time, after which it may terminate the program abruptly.

The writeMessage operation allows remote clients to print a message to the program's standard output (fd == 1) or standard error (fd == 2) channels.

Obtaining the Local Process Facet

We already showed how to obtain a proxy for a remote administrative facet, but suppose you want to interact with the facet in your local address space. The code below shows the necessary steps:

process = communicator.findAdminFacet("Process")
if process is not None:
    assert isinstance(propertiesAdmin, Ice.Process)
    ...  

Application Requirements for the Process Facet

The default implementation of the Process facet requires cooperation from an application in order to successfully terminate a process. Specifically, the facet invokes shutdown on its communicator and assumes that the application uses this event as a signal to commence its termination procedure. For example, an application typically uses a thread (often the main thread) to call the communicator operation waitForShutdown, which blocks the calling thread until the communicator is shut down or destroyed. After waitForShutdown returns, the calling thread can initiate a graceful shutdown of its process.

Replacing the Process Facet

You can replace the default Process facet if your application requires a different scheme for gracefully shutting itself down. To define your own facet, create a servant that implements the Ice::Process interface. As an example, the C++ servant definition shown below duplicates the functionality of the default Process facet:

class MyProcess final : public Ice::Process
{
public:
    MyProcess(Ice::CommunicatorPtr communincator) : 
        _communicator{std::move(communicator)}
    {
    }

    void shutdown(const Ice::Current&) final
    {
        _communicator->shutdown();
    }
    
    void writeMessage(
        std::string message, 
        std::int32_t fd, 
        const Ice::Current&) final
    {
        switch (fd)
        {
            case 1:
            {
                cout << message << endl;
                break;
            }
            case 2:
            {
                cerr << message << endl;
                break;
            }
        }
    }

private:
    const Ice::CommunicatorPtr _communicator;
};

As you can see, the default implementation of shutdown simply shuts down the communicator, which initiates an orderly termination of the Ice runtime's server-side components and prevents object adapters from dispatching any new requests. You can add your own application-specific behavior to the shutdown method to ensure that your program terminates in a timely manner.

A servant must not call destroy on its communicator while dispatching a request.

To avoid the risk of a race condition, the recommended strategy for replacing the Process facet is to delay creation of the administrative facets until after communicator initialization, so that your application has a chance to replace the facet:

# Delay admin object creation for admin object hosted in the Ice.Admin object adapter
Ice.Admin.DelayCreation=1

With Ice.Admin.DelayCreation enabled, the application can safely remove the default Process facet and install its own:

communicator = ...
communicator.removeAdminFacet("Process")
myProcessFacet = MyProcess(...)
communicator.addAdminFacet(myProcessFacet, "Process")

If you host the admin object in the Ice.Admin object adapter, the final step is to create the admin object by calling getAdmin on the communicator. And if you host the admin object in your own object adapter, the final set is to create the admin object with createAdmin.

Integrating the Process Facet with an Activation Service

If the Ice.Admin.ServerId and Ice.Default.Locator properties are defined, the communicator performs the following steps after creating the admin object:

• Obtains proxies for the Process facet and the default locator

• Invokes getRegistry on the locator proxy to obtain a proxy for the locator registry

• Invokes setServerProcessProxy on the locator registry and supplies the value of Ice.Admin.ServerId along with a proxy for the Process facet

The identifier specified by Ice.Admin.ServerId must uniquely identify the process within the locator registry.

In the case of IceGrid, IceGrid defines the Ice.Admin.ServerId and Ice.Default.Locator properties for each deployed server. IceGrid also supplies a value for Ice.Admin.Endpoints if neither this property nor Ice.Admin.Enabled are defined by the server.

See Also

• Communicator

• Versioning

• Security Considerations for Administrative Facets

• Object Adapters

• Creating the admin Object

• Ice.Admin.*

• IceGrid