path: root/dcop/Mainpage.dox

/*
	A dummy source file for documenting the library.
	Copied from HOWTO with small syntactic changes.
*/

/**
 \mainpage The DCOP Desktop COmmunication Protocol library

DCOP is a simple IPC/RPC mechanism built to operate over sockets.
Either unix domain sockets or TCP/IP sockets are supported. DCOP is
built on top of the Inter Client Exchange (ICE) protocol, which comes
standard as a part of X11R6 and later. It also depends on Qt, but
beyond that it does not require any other libraries. Because of this,
it is extremely lightweight, enabling it to be linked into all KDE
applications with low overhead.

\section model Model:

The model is simple.  Each application using DCOP is a client.  They
communicate to each other through a DCOP server, which functions like
a traffic director, dispatching messages/calls to the proper
destinations.  All clients are peers of each other.

Two types of actions are possible with DCOP: "send and forget"
messages, which do not block, and "calls," which block waiting for
some data to be returned.

Any data that will be sent is serialized (also referred to as marshalling
in CORBA speak) using the built-in QDataStream operators available in all
of the Qt classes.  This is fast and easy.  In fact it's so little work
that you can easily write the marshalling code by hand. In addition,
there's a simple IDL-like compiler available (dcopidl and dcopidl2cpp)
that generates stubs and skeletons for you. Using the dcopidl compiler
has the additional benefit of type safety.

The manual method is covered first, followed by the automatic IDL method.


\section establish Establishing the Connection:

KApplication has gained a method called \p KApplication::dcopClient()
which returns a pointer to a DCOPClient instance.  The first time this
method is called, the client class will be created.  DCOPClients have
unique identifiers attached to them which are based on what
KApplication::name() returns.  In fact, if there is only a single
instance of the program running, the appId will be equal to
KApplication::name().

To actually enable DCOP communication to begin, you must use
\p DCOPClient::attach().  This will attempt to attach to the DCOP server.
If no server is found or there is any other type of error, 
DCOPClient::attach() will return false. KApplication will catch a dcop 
signal and display an appropriate error message box in that case.

After connecting with the server via DCOPClient::attach(), you need to
register this appId with the server so it knows about you.  Otherwise,
you are communicating anonymously.  Use the
DCOPClient::registerAs(const QCString &name) to do so.  In the simple
case:

\code
appId = client->registerAs(kapp->name());
\endcode

If you never retrieve the DCOPClient pointer from KApplication, the
object will not be created and thus there will be no memory overhead.

You may also detach from the server by calling DCOPClient::detach().
If you wish to attach again you will need to re-register as well.  If
you only wish to change the ID under which you are registered, simply
call DCOPClient::registerAs() with the new name.

KUniqueApplication automatically registers itself to DCOP. If you
are using KUniqueApplication you should not attach or register
yourself, this is already done. The appId is by definition
equal to \p kapp->name(). You can retrieve the registered DCOP client
by calling \p kapp->dcopClient().


\section sending_data Sending Data to a Remote Application:

To actually communicate, you have one of two choices.  You may either
call the "send" or the "call" method.  Both methods require three
identification parameters: an application identifier, a remote object,
a remote function. Sending is asynchronous (i.e. it returns immediately)
and may or may not result in your own application being sent a message at
some point in the future. Then "send" requires one and "call" requires
two data parameters.

The remote object must be specified as an object hierarchy.  That is,
if the toplevel object is called \p fooObject and has the child
\p barObject, you would reference this object as \p fooObject/barObject.
Functions must be described by a full function signature.  If the
remote function is called \p doIt, and it takes an int, it would be
described as \p doIt(int).  Please note that the return type is not
specified here, as it is not part of the function signature (or at
least the C++ understanding of a function signature).  You will get
the return type of a function back as an extra parameter to
DCOPClient::call(). See the section on call() for more details.

In order to actually get the data to the remote client, it must be
"serialized" via a QDataStream operating on a QByteArray. This is how
the data parameter is "built". A few examples will make clear how this
works.

Say you want to call \p doIt as described above, and not block (or wait
for a response).  You will not receive the return value of the remotely
called function, but you will not hang while the RPC is processed either.
The return value of DCOPClient::send() indicates whether DCOP communication 
succeeded or not.

\code
QByteArray data;
QDataStream arg(data, IO_WriteOnly);
arg << 5;
if (!client->send("someAppId", "fooObject/barObject", "doIt(int)",
	          data))
  qDebug("there was some error using DCOP.");
\endcode

OK, now let's say we wanted to get the data back from the remotely
called function.  You have to execute a DCOPClient::call() instead of a 
DCOPClient::send(). The returned value will then be available in the 
data parameter "reply".  The actual return value of call() is still 
whether or not DCOP communication was successful.

\code
QByteArray data, replyData;
QCString replyType;
QDataStream arg(data, IO_WriteOnly);
arg << 5;
if (!client->call("someAppId", "fooObject/barObject", "doIt(int)",
                  data, replyType, replyData))
  qDebug("there was some error using DCOP.");
else {
  QDataStream reply(replyData, IO_ReadOnly);
  if (replyType == "QString") {
    QString result;
    reply >> result;
    print("the result is: %s",result.latin1());
  } else
    qDebug("doIt returned an unexpected type of reply!");
}
\endcode


\section receiving_data Receiving Data via DCOP:

Currently the only real way to receive data from DCOP is to multiply
inherit from the normal class that you are inheriting (usually some
sort of QWidget subclass or QObject) as well as the DCOPObject class.
DCOPObject provides one very important method: DCOPObject::process().
This is a pure virtual method that you must implement in order to
process DCOP messages that you receive.  It takes a function
signature, QByteArray of parameters, and a reference to a QByteArray
for the reply data that you must fill in.

Think of DCOPObject::process() as a sort of dispatch agent.  In the
future, there will probably be a precompiler for your sources to write
this method for you.  However, until that point you need to examine
the incoming function signature and take action accordingly.  Here is
an example implementation.

\code
bool BarObject::process(const QCString &fun, const QByteArray &data,
		        QCString &replyType, QByteArray &replyData)
{
  if (fun == "doIt(int)") {
    QDataStream arg(data, IO_ReadOnly);
    int i; // parameter
    arg >> i;
    QString result = self->doIt (i);
    QDataStream reply(replyData, IO_WriteOnly);
    reply << result;
    replyType = "QString";
    return true;
  } else {
    qDebug("unknown function call to BarObject::process()");
    return false;
  }
}
\endcode


\section receiving_calls Receiving Calls and processing them:

If your applications is able to process incoming function calls
right away the above code is all you need. When your application
needs to do more complex tasks you might want to do the processing
out of 'process' function call and send the result back later when
it becomes available.

For this you can ask your DCOPClient for a transactionId. You can
then return from the 'process' function and when the result is
available finish the transaction. In the mean time your application
can receive incoming DCOP function calls from other clients.

Such code could like this:

\code
bool BarObject::process(const QCString &fun, const QByteArray &data,
		        QCString &, QByteArray &)
{
  if (fun == "doIt(int)") {
    QDataStream arg(data, IO_ReadOnly);
    int i; // parameter
    arg >> i;
    QString result = self->doIt(i);

    DCOPClientTransaction *myTransaction;
    myTransaction = kapp->dcopClient()->beginTransaction();

    // start processing...
    // Calls slotProcessingDone when finished.
    startProcessing( myTransaction, i);

    return true;
  } else {
    qDebug("unknown function call to BarObject::process()");
    return false;
  }
}

slotProcessingDone(DCOPClientTransaction *myTransaction, const QString &result)
{
    QCString replyType = "QString";
    QByteArray replyData;
    QDataStream reply(replyData, IO_WriteOnly);
    reply << result;
    kapp->dcopClient()->endTransaction( myTransaction, replyType, replyData );
}
\endcode


\section dcopidl Using the dcopidl compiler:

dcopidl makes setting up a DCOP server easy. Instead of having to implement
the process() method and unmarshalling (retrieving from QByteArray) parameters
manually, you can let dcopidl create the necessary code on your behalf.

This also allows you to describe the interface for your class in a
single, separate header file.

Writing an IDL file is very similar to writing a normal C++ header. An
exception is the keyword 'ASYNC'. It indicates that a call to this
function shall be processed asynchronously. For the C++ compiler, it
expands to 'void'.

Example:

\code
#ifndef MY_INTERFACE_H
#define MY_INTERFACE_H

#include <dcopobject.h>

class MyInterface : virtual public DCOPObject
{
  K_DCOP

  k_dcop:

    virtual ASYNC myAsynchronousMethod(QString someParameter) = 0;
    virtual QRect mySynchronousMethod() = 0;
};

#endif
\endcode

As you can see, you're essentially declaring an abstract base class, which
virtually inherits from DCOPObject.

If you're using the standard KDE build scripts, then you can simply
add this file (which you would call MyInterface.h) to your sources
directory. Then you edit your Makefile.am, adding 'MyInterface.skel'
to your SOURCES list and MyInterface.h to include_HEADERS.

The build scripts will use dcopidl to parse MyInterface.h, converting
it to an XML description in MyInterface.kidl. Next, a file called
MyInterface_skel.cpp will automatically be created, compiled and
linked with your binary.

The next thing you have to do is to choose which of your classes will
implement the interface described in MyInterface.h. Alter the inheritance
of this class such that it virtually inherits from MyInterface. Then
add declarations to your class interface similar to those on MyInterface.h,
but virtual, not pure virtual.

Example:

\code
class MyClass: public QObject, virtual public MyInterface
{
  Q_OBJECT

  public:
    MyClass();
    ~MyClass();

    ASYNC myAsynchronousMethod(QString someParameter);
    QRect mySynchronousMethod();
};
\endcode
\note (Qt issue) Remember that if you are inheriting from QObject, you must
place it first in the list of inherited classes.

In the implementation of your class' ctor, you must explicitly initialize
those classes from which you are inheriting from. This is, of course, good
practice, but it is essential here as you need to tell DCOPObject the name of
the interface which your are implementing.

Example:

\code
MyClass::MyClass()
  : QObject(),
    DCOPObject("MyInterface")
{
  // whatever...
}
\endcode


Now you can simply implement the methods you have declared in your interface,
exactly the same as you would normally.

Example:

\code
void MyClass::myAsynchronousMethod(QString someParameter)
{
  qDebug("myAsyncMethod called with param `" + someParameter + "'");
}
\endcode

It is not necessary (though very clean) to define an interface as an
abstract class of its own, like we did in the example above. We could
just as well have defined a k_dcop section directly within MyClass:

\code
class MyClass: public QObject, virtual public DCOPObject
{
  Q_OBJECT
  K_DCOP

  public:
    MyClass();
    ~MyClass();

  k_dcop:
    ASYNC myAsynchronousMethod(QString someParameter);
    QRect mySynchronousMethod();
};
\endcode

In addition to skeletons, dcopidl2cpp also generate stubs. Those make
it easy to call a DCOP interface without doing the marshalling
manually. To use a stub, add MyInterface.stub to the SOURCES list of
your Makefile.am. The stub class will then be called MyInterface_stub.


\section iuc Inter-user communication:

Sometimes it might be interesting to use DCOP between processes
belonging to different users, e.g. a frontend process running
with the user's id, and a backend process running as root.

To do this, two steps have to be taken:

a) both processes need to talk to the same DCOP server
b) the authentication must be ensured

For the first step, you simply pass the server address (as
found in .DCOPserver) to the second process. For the authentication,
you can use the ICEAUTHORITY environment variable to tell the
second process where to find the authentication information.
(Note that this implies that the second process is able to
read the authentication file, so it will probably only work
if the second process runs as root. If it should run as another
user, a similar approach to what kdesu does with xauth must
be taken. In fact, it would be a very good idea to add DCOP
support to kdesu!)

For example

ICEAUTHORITY=~user/.ICEauthority kdesu root -c kcmroot -dcopserver `cat ~user/.DCOPserver`

will, after kdesu got the root password, execute kcmroot as root, talking
to the user's dcop server.


NOTE: DCOP communication is not encrypted, so please do not
pass important information around this way.

\section protocol DCOP Protocol description:

A DCOPSend message does not expect any reply.
\code
data: << fromId << toId << objId << fun << dataSize + data[dataSize]
\endcode

A DCOPCall message can get a DCOPReply, a DCOPReplyFailed
or a DCOPReplyWait message in response.
\code
data: << fromId << toId << objId << fun << dataSize + data[dataSize]
\endcode

DCOPReply is the successful reply to a DCOPCall message
\code
data: << fromId << toId << replyType << replyDataSize + replyData[replyDataSize]
\endcode

DCOPReplyFailed indicates failure of a DCOPCall message
\code
data: << fromId << toId
\endcode

DCOPReplyWait indicates that a DCOPCall message is successfully
being processed but that response will come later.
\code
data: << fromId << toId << transactionId
\endcode

DCOPReplyDelayed is the successful reply to a DCOPCall message
after a DCOPReplyWait message.
\code
data: << fromId << toId << transactionId << replyType << replyData
\endcode

DCOPFind is a message much like a "call" message. It can however
be send to multiple objects within a client. If a function in a
object that is being called returns a boolean with the value "true",
a DCOPReply will be send back containing the DCOPRef of the object
who returned "true".

All c-strings (fromId, toId, objId, fun and replyType), are marshalled with
their respective  length as 32 bit unsigned integer first:
\code
data: length + string[length]
\endcode
\note This happens automatically when using QCString on a QDataStream.

\section Deadlock protection and reentrancy

When a DCOP call is made, the dcop client will be monitoring the
dcop connection for the reply on the call. When an incoming call is
received in this period, it will normally not be processed but queued
until the outgoing call has been fully handled.

However, the above scenario would cause deadlock if the incoming call
was directly or indirectly a result of the outgoing call and the reply
on the outgoing call is waiting for the result of the incoming call.
(E.g. a circular call such as client A calling client B, with client B
calling client A)

To prevent deadlock in this case, DCOP has a call tracing mechanism that
detects circular calls. When it detects an incoming circular call that
would otherwise be queued and as a result cause deadlock, it will handle
the incoming call immediately instead of queueing it. This means that the
incoming call may be processed at a point in the code where an outgoing
DCOP call is made. An application should be aware of this kind of
reentrancy. A special case of this is when a DCOP client makes a call
to itself, such calls are always handled directly.

Call tracing works by appending a key to each outgoing call. When a client
receives an incoming call while waiting for a response on an outgoing call,
it will check if the key of the incoming call is equal to the key used for
the last outgoing call. If the keys are equal a circular call has been 
detected.

The key used by clients is 0 if they have not yet received any key. In this
case the server will send them back a unique key that they should use in
further calls. If a client makes an outgoing call in response to an incoming
call it will use the key of the incoming call for the outgoing call instead
of the key that was received from the server.

A key value of 1 has a special meaning and is used for non-call messages
such as DCOPSend, DCOPReplyFailed and DCOP signals.

A key value of 2 has a special meaning and is used for priority calls.
When a dcop clien is in priority call mode, it will only handle incoming
calls that have a key value of 2.

NOTE: If client A and client B would call each other simultaneously there
is still a risk of deadlock because both calls would have unique keys and
both clients would decide to queue the incoming call until they receive
a response on their outgoing call.

\section dcop_signals DCOP Signals:

Sometimes a component wants to send notifications via DCOP to other
components but does not know which components will be interested in these
notifications. One could use a broadcast in such a case but this is a very
crude method. For a more sophisticated method DCOP signals have been invented.

DCOP signals are very similair to Qt signals, there are some differences 
though. A DCOP signal can be connected to a DCOP function. Whenever the DCOP
signal gets emitted, the DCOP functions to which the signal is connected are
being called. DCOP signals are, just like Qt signals, one way. They do not
provide a return value. For declaration of dcop signals, the keyword 
\p k_dcop_signals is provided. A declaration looks like this:

\code
class Example : virtual public DCOPClient
{
  K_DCOP

  k_dcop:
    // some ordinary dcop methods here
  ...
  k_dcop_signals:
    // our dcop signal
    void clientDied(pid_t pid);
  ...
}
\endcode

A DCOP signal originates from a DCOP Object/DCOP Client combination (sender). 
It can be connected to a function of another DCOP Object/DCOP Client 
combination (receiver).

\note There are two major differences between connections of Qt signals and 
connections of DCOP signals. In DCOP, unlike Qt, a signal connections can
have an anonymous sender and, unlike Qt, a DCOP signal connection can be
non-volatile.

With DCOP one can connect a signal without specifying the sending DCOP Object 
or DCOP Client. In that case signals from any DCOP Object and/or DCOP Client
will be delivered. This allows the specification of certain events without
tying oneself to a certain object that implementes the events.

Another DCOP feature are so called non-volatile connections. With Qt signal
connections, the connection gets deleted when either sender or receiver of
the signal gets deleted. A volatile DCOP signal connection will behave the
same. However, a non-volatile DCOP signal connection will not get deleted 
when the sending object gets deleted. Once a new object gets created with 
the same name as the original sending object, the connection will be restored.
There is no difference between the two when the receiving object gets deleted,
in that case the signal connection will always be deleted.

A receiver can create a non-volatile connection while the sender doesn't (yet)
exist. An anonymous DCOP connection should always be non-volatile.

The following example shows how KLauncher emits a signal whenever it notices
that an application that was started via KLauncher terminates:

\code
QByteArray params;
QDataStream stream(params, IO_WriteOnly);
stream << pid;
kapp->dcopClient()->emitDCOPSignal("clientDied(pid_t)", params);
\endcode

The task manager of the KDE panel connects to this signal. It uses an 
anonymous connection (it doesn't require that the signal is being emitted
by KLauncher) that is non-volatile:

\code
connectDCOPSignal(0, 0, "clientDied(pid_t)", "clientDied(pid_t)", false);
\endcode

It connects the clientDied(pid_t) signal to its own clientDied(pid_t) DCOP
function. In this case the signal and the function to call have the same name.
This isn't needed as long as the arguments of both signal and receiving function
match. The receiving function may ignore one or more of the trailing arguments
of the signal. E.g. it is allowed to connect the clientDied(pid_t) signal to
a clientDied(void) DCOP function.


\section conclusion Conclusion:

Hopefully this document will get you well on your way into the world of
inter-process communication with KDE!  Please direct all comments and/or
suggestions to the KDE Core Developers List \<kde-core-devel@kde.org\>.

*/