man RPC::XML::Procedure (3): 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

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

AUTHOR

Randy J. Ray "<[email protected]>"