Provided by: librpc-xml-perl_0.82-1_all 
NAME
RPC::XML::Procedure - Object encapsulation of server-side RPC procedures
SYNOPSIS
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 { ... });
DESCRIPTION
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.
CLASSES
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
function.
SUBROUTINES/METHODS
The following methods are provided by this class:
new(FILE|HASHREF|LIST)
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.
clone
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.
name
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.
namespace
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).
code([NEW])
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.
signature([NEW])
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.
help([NEW])
Returns or sets the help-text for the method. As with code, the previous value is
lost.
hidden([NEW])
Returns or sets the hidden status of the method. Setting it loses the previous value.
version([NEW])
Returns or sets the version string for the method (overwriting as with the other
accessors).
add_signature(LIST)
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.
delete_signature(LIST)
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.
match_signature(SIGNATURE)
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.
reload
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
operation.
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:
file
When the method was loaded from a file, this key contains the path to the file used.
namespace
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".
mtime
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.
called
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.
NAMESPACES
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.
DIAGNOSTICS
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.
CAVEATS
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.
BUGS
Please report any bugs or feature requests to "bug-rpc-xml at rt.cpan.org", or through the
web interface at <http://rt.cpan.org/NoAuth/ReportBug.html?Queue=RPC-XML>. I will be
notified, and then you'll automatically be notified of progress on your bug as I make
changes.
SUPPORT
• RT: CPAN's request tracker
<http://rt.cpan.org/NoAuth/Bugs.html?Dist=RPC-XML>
• AnnoCPAN: Annotated CPAN documentation
<http://annocpan.org/dist/RPC-XML>
• CPAN Ratings
<http://cpanratings.perl.org/d/RPC-XML>
• Search CPAN
<http://search.cpan.org/dist/RPC-XML>
• MetaCPAN
<https://metacpan.org/release/RPC-XML>
• Source code on GitHub
<http://github.com/rjray/rpc-xml>
LICENSE AND COPYRIGHT
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
(<http://www.opensource.org/licenses/artistic-license-2.0.php>) or the GNU LGPL 2.1
(<http://www.opensource.org/licenses/lgpl-2.1.php>).
CREDITS
The XML-RPC standard is Copyright (c) 1998-2001, UserLand Software, Inc. See
<http://www.xmlrpc.com> for more information about the XML-RPC specification.
SEE ALSO
RPC::XML::Server, make_method
AUTHOR
Randy J. Ray "<rjray@blackperl.com>"