Provided by: librpc-xml-perl_0.80-2_all bug


       RPC::XML::Procedure - Object encapsulation of server-side RPC procedures


           require RPC::XML::Procedure;

           $procedure = RPC::XML::Procedure->new({ name => 'system.identity',
                                                   code => sub { ... },
                                                   signature => [ 'string' ] });
           $method    = RPC::XML::Method->new('/path/to/status.xpl');
           $function  = RPC::XML::Function->new(name => 'add',
                                                code => sub { ... });


       The RPC::XML::Procedure package is designed primarily for behind-the-scenes use by the
       RPC::XML::Server class and any subclasses of it. It is documented here in case a project
       chooses to sub-class it for their purposes (which would require setting the "method_class"
       attribute when creating server objects, see RPC::XML::Server).

       This package grew out of the increasing need to abstract the operations that related to
       the methods a given server instance was providing. Previously, methods were passed around
       simply as hash references. It was a small step then to move them into a package and allow
       for operations directly on the objects themselves. In the spirit of the original hashes,
       all the key data is kept in clear, intuitive hash keys (rather than obfuscated as the
       other classes do). Thus it is important to be clear on the interface here before sub-
       classing this package.


       This module provides three classes, representing the three types of procedures that
       servers can use:

       Methods (RPC::XML::Method)
           Code that is considered a "method" by the server is called as though it were, in fact,
           a method in that class. The first argument in the list is the server object itself,
           with the arguments to the call making up the rest of the list.  The server checks the
           signature of the method against the arguments list before the call is made. See below
           ("How Procedures Are Called") for more on the invocation of code as methods.

       Procedures (RPC::XML::Procedure)
           Code that is considered a "procedure" by the server is called like a normal (non-
           method) subroutine call. The server object is not injected into the arguments list.
           The signature of the procedure is checked again the list of arguments before the call
           is made, as with methods.

       Functions (RPC::XML::Function)
           Lastly, code that is considered a "function" is the simplest of the three: it does not
           have the server object injected into the arguments list, and no check of signatures is
           done before the call is made. It is the responsibility of the function to properly
           understand the arguments list, and to return a value that the caller will understand.

       There is (currently) no version that is called like a method but ignores signatures like a


       The following methods are provided by this class:

           Creates a new object of the class, and returns a reference to it. The arguments to the
           constructor are variable in nature, depending on the type:

           FILE    If there is exactly on argument that is not a reference, it is assumed to be a
                   filename from which the method is to be loaded. This is presumed to be in the
                   XPL format described below (see "XPL File Structure"). If the file cannot be
                   opened, or if once opened cannot be parsed, an error is raised.

           HASHREF If there is exactly one argument that is a reference, it is assumed to be a
                   hash with the relevant information on the same keys as the object itself uses.
                   This is primarily to support backwards-compatibility to code written when
                   methods were implemented simply as hash references.

           LIST    If there is more than one argument in the list, then the list is assumed to be
                   a sort of "ersatz" hash construct, in that one of the keys ("signature") is
                   allowed to "stack" if it occurs multiple times. Otherwise, any keys that occur
                   multiple times overwrite the previous value:

                   name        The name of the method, as it will be presented to clients

                   code        A reference to a subroutine, or an anonymous subroutine, that will
                               receive calls for the method

                   signature   Provides one calling-signature for the method, as either a space-
                               separated string of types or a list-reference

                   help        The help-text for a method, which is generally used as a part of
                               the introspection interface for a server

                   version     The version number/string for the method

                   hidden      A boolean (true or false) value indicating whether the method
                               should be hidden from introspection and similar listings

                   Note that all of these correspond to the values that can be changed via the
                   accessor methods detailed later.

           If any error occurs during object creation, an error message is returned in lieu of
           the object reference.

           Create a copy of the calling object, and return the new reference. All elements are
           copied over cleanly, except for the code reference stored on the "code" hash key. The
           clone will point to the same code reference as the original. Elements such as
           "signature" are copied, so that changes to the clone will not impact the original.

           Returns the name by which the server is advertising the method. Unlike the next few
           accessors, this cannot be changed on an object. In order to streamline the management
           of methods within the server classes, this must persist. However, the other elements
           may be used in the creation of a new object, which may then be added to the server, if
           the name absolutely must change.

           If the procedure object was created from a file, or if the instantiation included
           namespace information, this accessor will return the namespace that the underlying
           code executes in. Otherwise, it returns an empty string. This cannot be altered (even
           if the code method is used to replace the code routine).

           Returns or sets the code-reference that will receive calls as marshalled by the
           server. The existing value is lost, so if it must be preserved, then it should be
           retrieved prior to the new value being set.

           Return a list reference containing the signatures, or set it. Each element of the list
           is a string of space-separated types (the first of which is the return type the method
           produces in that calling context). If this is being used to set the signature, then an
           array reference must be passed that contains one or more strings of this nature.
           Nested list references are not allowed at this level. If the new signatures would
           cause a conflict (a case in which the same set of input types are specified for
           different output types), the old set is silently restored.

           Returns or sets the help-text for the method. As with code, the previous value is

           Returns or sets the hidden status of the method. Setting it loses the previous value.

           Returns or sets the version string for the method (overwriting as with the other

           Add one or more signatures (which may be a list reference or a string) to the internal
           tables for this method. Duplicate signatures are ignored. If the new signature would
           cause a conflict (a case in which the same set of input types are specified for
           different output types), the old set is restored and an error message is returned.

           Deletes the signature or signatures (list reference or string) from the internal
           tables. Quietly ignores any signature that does not exist. If the new signature would
           cause a conflict (a case in which the same set of input types are specified for
           different output types), the old set is restored and an error message is returned.

           Check that the passed-in signature is known to the method, and if so returns the type
           that the method should be returning as a result of the call. Returns a zero (0)
           otherwise. This differs from other signature operations in that the passed-in
           signature (which may be a list-reference or a string) does not include the return
           type. This method is provided so that servers may check a list of arguments against
           type when marshalling an incoming call. For example, a signature of 'int int' would be
           tested for by calling "$M->match_signature('int')" and expecting the return value to
           be "int".

       call(SERVER, PARAMLIST)
           Execute the code that this object encapsulates, using the list of parameters passed in
           PARAMLIST. The SERVER argument should be an object derived from the RPC::XML::Server
           class. For some types of procedure objects, this becomes the first argument of the
           parameter list to simulate a method call as if it were on the server object itself.
           The return value should be a data object (possibly a RPC::XML::fault), but may not
           always be pre-encoded. Errors trapped in $@ are converted to fault objects. This
           method is generally used in the "dispatch" method of the server class, where the
           return value is subsequently wrapped within a RPC::XML::response object.

           Instruct the object to reload itself from the file it originally was loaded from,
           assuming that it was loaded from a file to begin with. Returns an error if the method
           was not originally loaded from a file, or if an error occurs during the reloading

   Additional Hash Data
       In addition to the attributes managed by the accessors documented earlier, the following
       hash keys are also available for use. These are also not strongly protected, and the same
       care should be taken before altering any of them:

           When the method was loaded from a file, this key contains the path to the file used.

           If the code is loaded from a file, this hash key will reflect what namespace the code
           executes in. If the file specified a namespace, that is the value you will get (any
           occurrence of "." in the specified namespace will have been converted to "::"). If no
           explicit namespace was provided, the namespace of the class you called new from will
           be used. See "Namespaces".

           When the method was loaded from a file, this key contains the modification-time of the
           file, as a UNIX-style "time" value. This is used to check for changes to the file the
           code was originally read from.

           When the method is being used by one of the server classes provided in this software
           suite, this key is incremented each time the server object dispatches a request to the
           method. This can later be checked to provide some indication of how frequently the
           method is being invoked.

   XPL File Structure
       This section focuses on the way in which methods are expressed in these files, referred to
       here as "XPL files" due to the "*.xpl" filename extension (which stands for "XML Procedure
       Layout"). This mini-dialect, based on XML, is meant to provide a simple means of
       specifying method definitions separate from the code that comprises the application
       itself. Thus, methods may theoretically be added, removed, debugged or even changed
       entirely without requiring that the server application itself be rebuilt (or, possibly,
       without it even being restarted).

       The XML-based file structure
           The XPL Procedure Layout dialect is a very simple application of XML to the problem of
           expressing the method in such a way that it could be useful to other packages than
           this one, or useful in other contexts than this one.

           The lightweight DTD for the layout can be summarized as:

               <!ELEMENT  proceduredef  (name, namespace?, version?, hidden?,
                                         signature+, help?, code)>
               <!ELEMENT  methoddef     (name, namespace?, version?, hidden?,
                                         signature+, help?, code)>
               <!ELEMENT  functiondef   (name, namespace?, version?, hidden?,
                                         signature+, help?, code)>
               <!ELEMENT  name       (#PCDATA)>
               <!ELEMENT  namespace  (#PCDATA)>
               <!ELEMENT  version    (#PCDATA)>
               <!ELEMENT  hidden     EMPTY>
               <!ELEMENT  signature  (#PCDATA)>
               <!ELEMENT  help       (#PCDATA)>
               <!ELEMENT  code       (#PCDATA)>
               <!ATTLIST  code       language (#PCDATA)>

           The containing tag is always one of "<methoddef>", "<proceduredef>" or
           "<functiondef>". The tags that specify name, signatures and the code itself must
           always be present. Some optional information may also be supplied. The "help" text, or
           what an introspection API would expect to use to document the method, is also marked
           as optional.  Having some degree of documentation for all the methods a server
           provides is a good rule of thumb, however.

           The default methods that this package provides are turned into XPL files by the
           make_method tool (see make_method). The final forms of these may serve as examples of
           what the file should look like.

       Information used only for book-keeping
           Some of the information in the XPL file is only for book-keeping: the version stamp of
           a method is never involved in the invocation. The server also keeps track of the last-
           modified time of the file the method is read from, as well as the full directory path
           to that file. The "<hidden />" tag is used to identify those methods that should not
           be exposed to the outside world through any sort of introspection/documentation API.
           They are still available and callable, but the client must possess the interface
           information in order to do so.

       The information crucial to the method
           The name, signatures and code must be present for obvious reasons. The "<name>" tag
           tells the server what external name this procedure is known by. The "<signature>" tag,
           which may appear more than once, provides the definition of the interface to the
           function in terms of what types and quantity of arguments it will accept, and for a
           given set of arguments what the type of the returned value is. Lastly is the "<code>"
           tag, without which there is no procedure to remotely call.

       Why the <code> tag allows multiple languages
           Note that the "<code>" tag is the only one with an attribute, in this case "language".
           This is designed to allow for one XPL file to provide a given method in multiple
           languages. Why, one might ask, would there be a need for this?

           It is the hope behind this package that collections of RPC suites may one day be made
           available as separate entities from this specific software package.  Given this hope,
           it is not unreasonable to suggest that such a suite of code might be implemented in
           more than one language (each of Perl, Python, Ruby and Tcl, for example). Languages
           which all support the means by which to take new code and add it to a running process
           on demand (usually through an ""eval"" keyword or something similar). If the file
           A.xpl is provided with implementations in all four of the above languages, the name,
           help text, signature and even hidden status would likely be identical. So, why not
           share the non-language-specific elements in the spirit of re-use?

   The "make_method" Utility
       The utility script "make_method" is provided as a part of this software suite. It allows
       for the automatic creation of XPL files from either command-line information or from
       template files. It has a wide variety of features and options, and is out of the scope of
       this particular manual page. The package Makefile.PL features an example of engineering
       the automatic generation of XPL files and their delivery as a part of the normal Perl
       module build process. Using this tool is highly recommended over managing XPL files
       directly. For the full details, see make_method.


       As default behavior, Perl code that is passed to "eval" when a XPL file is loaded gets put
       into the same namespace as the package used to load the XPL.  It is not an issue when you
       create your own RPC::XML::Procedure (or ::Method or ::Function) objects, as the code is
       already instantiated into a given namespace.  This can be important if your code expects
       to call routines in other loaded packages, utilize package-level globals, etc.

       To give developers control over the namespace in XPL code, a new optional tag
       "<namespace>" was added in the 0.65 release. If this tag is present in the XPL being read,
       it defines the namespace that the "<code>" block is evaluated in.

       The value of the namespace tag is a string providing the namespace in either the Perl-
       style of hierarchy parts separated by "::", or the style used by Java, Perl6, etc., in
       which the parts are separated by ".". The latter form is converted to Perl style for the
       evaluation of the code. If there is no namespace declaration in a XPL file, the namespace
       of the class that loads the XPL is used.


       Unless otherwise noted in the individual documentation sections, all methods return the
       object reference on success, or a (non-reference) text string containing the error message
       upon failure.


       Moving the method management to a separate class adds a good deal of overhead to the
       general system. The trade-off in reduced complexity and added maintainability should
       offset this.


       Please report any bugs or feature requests to "bug-rpc-xml at", or through the
       web interface at <>. I will be
       notified, and then you'll automatically be notified of progress on your bug as I make


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       This file and the code within are copyright (c) 2011 by Randy J. Ray.

       Copying and distribution are permitted under the terms of the Artistic License 2.0
       (<>) or the GNU LGPL 2.1


       The XML-RPC standard is Copyright (c) 1998-2001, UserLand Software, Inc.  See
       <> for more information about the XML-RPC specification.


       RPC::XML::Server, make_method


       Randy J. Ray "<>"