[senf.git] / Socket / Mainpage.dox

namespace senf {

/** \mainpage The SENF Socket Library

    The Socket library provides a high level and object oriented abstraction of the BSD socket
    API. The abstraction is based on several concepts:

    \li The basic visible interface is a \link handle_group handle object \endlink
    \li The socket interface relies on a \link policy_group policy framework \endlink to configure
        it's functionality
    \li The rest of the socket API is accessible using a classic inheritance hierarchy of \link
        protocol_group protocol classes \endlink

    The handle/body architecture provides automatic reference counted management of socket
    instances, the policy framework provides highly efficient access to the most important socket
    functions (like reading and writing) and the inheritance hierarchy provides convenient access to
    the multitude of special and protocol dependent options.

    \see 
        \ref structure \n
        \ref usage \n
        \ref handle_group \n
        \ref policy_group \n
        \ref protocol_group \n
        \ref addr_group \n
        \ref extend \n
        \ref implementation
 */

/** \page structure Overview of the Socket Library Structure

    \image html Handle.png

    This diagram tries to give a structural overview of the Socket Library, it does \e not directly
    show, how the library is implemented. This will be explained later.

    The outside interface to the library is a Handle object. This is the only object, the library
    user directly interacts with. Every handle references some socket. This is like the ordinary
    POSIX API: the file descriptor (also called file handle, an integer number) references a socket
    structure which lives in kernel space. In this library, the Handle object (which is not a simple
    integer any more but an object) references the Socket (which is part of the
    implementation). Several handles may reference the same Socket. In contrast to the kernel API,
    the library employs reference counting to release a socket when the last Handle to it goes out
    of scope.

    The behavior of a Socket is defined by it's Protocol. It is divided into two parts: the
    <em>policy interface</em> and the <em>protocol interface</em>. Together they provide the
    complete API for a specific type of Socket as defined by the Protocol. The <em>policy
    interface</em> provides highly efficient access to the most frequently used operations whereas
    the <em>protocol interface</em> completes the interface by providing a complete set of all
    protocol specific operations not found in the policy interface. This structure allows us to
    combine the benefits of two design methodologies: The policy interface utilizes a policy based
    design technique and is highly efficient albeit more complex to implement, whereas the protocol
    interface is based on a more common inheritance architecture which is not as optimized for
    performance but much simpler to implement. We reduce the complexity of the implementation by
    reducing the policy interface to a minimal sensible subset of the complete API.

    \section over_policy The Policy Interface
    
    The policy of a Socket consists of several parts, called <em>policy axis</em>. Each axis
    corresponds to one specific interface aspect of the Socket. The exact meaning of the policy axis
    are defined elsewhere (see \ref policy_group). The Protocol will always provide a complete set
    of <em>policy classes</em>, one for each axis.

    This <em>complete socket policy</em> defines the policy interface of the protocol. This
    interface is carried over into the Handle. The socket policy as defined in the Handle however
    may be <em>incomplete</em>. This mans, that the \e accessible interface of the Socket depends on
    the type of Handle used. The inherent interface does not change but the view of this interface
    does if the Handle does not provide the \e complete policy interface. This feature is very
    important. It allows to define generic Handle types. A generic Handle with an incompletely
    defined policy can point to an arbitrary Socket as long as all those policy axis which \e are
    defined match those defined in that Socket's protocol. Using such a generic handle decouples the
    implementation parts using this handle from the other socket aspects (e.g. you may define a
    generic socket handle for TCP based communication leaving the addressingPolicy undefined which
    makes your code independent of the type of addressing, IPv4 or IPv6). 

    This can be described as generalized compile-time polymorphism: A base class reference to some
    derived class will only give access to a reduced interface (the base class interface) of a
    class. The class still is of it's derived type (and inherently has the complete interface) but
    only part of it is accessible via the base class reference. Likewise a generic handle (aka base
    class reference) will only provide a reduced interface (aka base class interface) to the derived
    class instance (aka socket).

    \section over_protocol The Protocol Interface

    The protocol interface is provided by a set of <em>protocol facets</em>. Each facet provides a
    part of the interface. Whereas the policy interface is strictly defined (the number and type of
    policy axis is fixed and also the possible members provided by the policy interface are fixed),
    the protocol interface is much more flexible. Any member needed to provide a complete API for
    the specific protocol may be defined, the number and type of facets combined to provide the
    complete interface is up to the Protocol implementor. This flexibility is necessary to provide a
    complete API for every possible protocol.

    However this flexibility comes at a cost: To access the protocol interface the user must know
    the exact protocol of the socket. With other words, the protocol is only accessible if the
    handle you use is a <em>protocol specific</em> handle. A protocol specific Handle differs from a
    generic Handle in two ways: It always has a complete policy and it knows the exact protocol type
    of the socket (which generic handles don't). This allows to access to the complete protocol
    interface.

    \section over_impl Implementation of the Socket Libarary Structure

    In the Implementation, the socket policy is identified by an instance of the senf::SocketPolicy
    template. The Socket representation is internally represented in a senf::SocketBody which is not
    outside visible. The Handle is provided by a hierarchy of handle templates. Each Handle template
    uses template arguments for the policy and/or protocol as needed (see \ref handle_group).

    The Handle hierarchy divides the interface into two separate strains: the client interface
    (senf::ClientSocketHandle and senf::ProtocolClientSocketHandle) provides the interface of a
    client socket whereas the server interface (senf::ServerSocketHandle and
    senf::ProtocolServerSocketHandle) provides the interface as used by server sockets.

    The protocol interface is implemented using inheritance: The Protocol class inherits from each
    protocol facet using multiple (virtual public) inheritance. The Protocol class therefore
    provides the complete protocol API in a unified (see \ref protocol_group).
 */

/** \page usage Using the Socket Library

    Whenever you use the socket library, what you will be dealing with are FileHandle derived
    instances. The socket library relies on reference counting to automatically manage the
    underlying socket representation. This frees you of having to manage the socket lifetime
    explicitly.

    \section usage_create Creating a Socket Handle

    To create a new socket handle (opening a socket), you will need to use
    ProtocolClientSocketHandle or ProtocolServerSocketHandle. You will probably not use these
    templates as is but use proper typedefs (for example TCPv4ClientSocketHandle or
    PacketSocketHandle). The documentation for these socket handles are found in the protocol class
    (for example TCPv4SocketProtocol or PacketProtocol).

    \section usage_reusable Writing Reusable Components

    To make your code more flexible, you should not pass around your socket in this form. Most of
    your code will be using only a small subset of the ProtocolClientSocketHandle or
    ProtocolServerSocketHandle API.
    
    If instead of using the fully specified handle type you use a more incomplete type, you allow
    your code to be used with all sockets which fulfill the minimal requirements of your code. These
    types are based on the ClientSocketHandle and ServerSocketHandle templates which implement the
    policy interface without providing the concrete protocol interface.  To use those templates you
    may define a special reduced policy or handle for your code. By giving only an incomplete policy
    you thereby reduce the interface to that required by your module:

    \code
      typedef ClientSocketHandle<
          MakeSocketPolicy<
              ReadablePolicy,
              StreamFramingPolicy,
              ConnectedCommunicationPolicy > > MyReadableHandle;

    \endcode

    This defines \c MyReadableHandle as a ClientSocketHandle which will have only read
    functionality. Your code expects a stream interface (in contrast to a packet or datagram based
    interface). You will not have \c write or \c readfrom members. \c write will be disabled since
    the WritePolicy is unknown, \c readfrom will be disabled since a socket with the
    ConnectedCommunicationPolicy does not have a \c readfrom member.

    \see
        \ref policy_group \n
        \ref handle_group \n
        \ref protocol_group
 */

/** \page extend Extending the Library

    There are two layers, on which the socket library can be extended: On the protocol layer and on
    the policy layer. Extending the protocol layer is quite simple and works as long as the desired
    protocol does use the same BSD API used by the standard internet protocols as implemented in the
    standard policies (i.e. it uses ordinary read() and write() or rcvfrom() or sendto() calls and
    so on).

    If however the implementation of a policy feature needs to be changed, a new policy class has to
    be written. This also is not very complicated however the integration is more complex.

    \section extend_protocol Writing a new protocol class

    Most protocols can be implemented by just implementing a new protocol class. The protocol class
    must be derived from ConcreteSocketProtocol and takes the socket policy (as created by
    MakeSocketPolicy) as a template argument. See the documentation of this class for the interface.

    \attention You may want to use multiple inheritance as it is used in the implementation of the
    standard protocols (See \ref protocol_group). You must however be extra careful to ensure, that
    every class ultimately has SocketPolicy as a public \e virtual base.

    After the protocol class has been defined, you will probably want to provide typedefs for the
    new protocol sockets. If the new protocol is connection oriented, this will be like
    \code
    typedef ProtocolClientSocketHandle<MyProtocolClass> MyProtocolClientSocketHandle;
    typedef ProtocolServerSocketHandle<MyProtocolClass> MyProtocolServerSocketHandle;
    \endcode

    \section extend_policy Extending the policy framework

    If you have to extend the policy framework, you will need to be aware of some important
    limitations of the socket library:

    \li When you define a new policy for some axis, this new policy <em>must not</em> be derived
        from one of the existing concrete policy classes (except of course the respective policy
        axis base class). This is important since the policy type is \e not polymorphic. The policy
        to be used is selected by the compiler using the \e static type, which is exactly what is
        desired, since this allows calls to be efficiently inlined.

    \li Therefore, extending the policy framework will make the new socket probably \e incompatible
        with generic code which relies on the policy axis which is extended. Example: If you write a
        new write policy because your protocol does not use ordinary write() system calls but some
        protocol specific API, Then any generic function relying on WritablePolicy will \e not work
        with the new socket, since the socket does \e not have this policy, it has some other kind
        of write policy.

    Therefore you need to be careful of what you are doing. The first step is to find out, which
    policy you will have to implement. For this, find the ClientSocketHandle and/or
    ServerSocketHandle members you want to change (see \ref ClientSocketHandle and \ref
    ServerSocketHandle).  Not all policy axis directly contribute to the SocketHandle
    interface. However, some policy members additionally depend on other policy axis (example:
    AddressingPolicy::connect is only defined if the communication policy is
    ConnectedCommunication).

    \see policy_group
 */

/** \page glossary Glossary

    <table class="glossary">

    <tr><td>policy</td> <td>collection of policy classes, one for each policy axis, instantiation of
    the SocketPolicy template</td></tr>

    <tr><td>policy axis</td> <td>one aspect defined in the socket policy, typedef and member of the
    SocketPolicy template</td></tr>

    <tr><td>policy class</td> <td>implementation of a single policy axis, class derived from the
    axis base class</td></tr>

    <tr><td>complete policy</td> <td>socket policy where each axis is specified completely</td></tr>

    <tr><td>incomplete policy</td> <td>socket policy, where at least one axis is not fully
    specified</td></tr>

    <tr><td>protocol class</td> <td>definition of a protocol as a class, class inheriting from
    ConcreteSocketProtocol.</td></tr>

    <tr><td>protocol facet</td> <td>a class providing some subset of the protocol interface, class
    derived from SocketProtocol but not from ConcreteSocketProtocol</td></tr>

    <tr><td>policy interface</td> <td>interface directly provided by
    ClientSocketHandle/ServerSocketHandle and defined through the policy</td>

    <tr><td>protocol interface</td> <td>interface provided by the protocol class and accessible via
    the ProtocolClientSocketHandle::protocol()/ProtocolServerSocketHandle::protocol()
    member</td></tr>

    </table>
 */

/** \page implementation Implementation notes

    \section class_diagram Class Diagram

    \image html SocketLibrary-classes.png

    \section impl_notes Arbitrary Implementation Notes

    \li The implementation tries to isolate the library user as much as possible from the system
        header files since those headers define a lot of define symbols and introduce a host of
        symbols into the global namespace. This is, why some classes define their own \c enum types
        to replace system defined define constants. This also precludes inlining some functionality.

    \li To reduce overhead, template functions/members which are more than one-liners are often
        implemented in terms of a non-template function/member. This is also used to further the
        isolation from system headers as defined above (template code must always be included into
        every compilation unit together with all headers need for the implementation).
 */

}

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