RPC::XML::Server - A sample server implementation based on RPC::XML
SYNOPSIS
use RPC::XML::Server;
...
$srv = RPC::XML::Server->new(port => 9000);
# Several of these, most likely:
$srv->add_method(...);
...
$srv->server_loop; # Never returns
DESCRIPTION
This is a sample XML-RPC server built upon the RPC::XML data classes, and
using HTTP::Daemon and HTTP::Response for the communication layer.
USAGE
Use of the RPC::XML::Server is based on an object model. A server is
instantiated from the class, methods (subroutines) are made public by adding
them through the object interface, and then the server object is responsible
for dispatching requests (and possibly for the HTTP listening, as well).
Static Methods
These methods are static to the package, and are used to provide external
access to internal settings:
INSTALL_DIR
Returns the directory that this module is installed into. This is used by
methods such as "add_default_methods" to locate the XPL files that are
shipped with the distribution.
version
Returns the version string associated with this package.
product_tokens
This returns the identifying string for the server, in the format
"NAME/VERSION" consistent with other applications such as Apache and
LWP. It is provided here as part of the compatibility with HTTP::Daemon
that is required for effective integration with Net::Server.
Methods
The following are object (non-static) methods. Unless otherwise explicitly
noted, all methods return the invoking object reference upon success, and a
non-reference error string upon failure.
See ``Content Compression'' below for details of how the server class manages
gzip-based compression and expansion of messages.
new(OPTIONS)
Creates a new object of the class and returns the blessed reference. Depending
on the options, the object will contain some combination of an HTTP listener,
a pre-populated HTTP::Response object, a RPC::XML::Parser object, and
a dispatch table with the set of default methods pre-loaded. The options that
new accepts are passed as a hash of key/value pairs (not a hash reference).
The accepted options are:
no_http
If passed with a "true" value, prevents the creation and storage of the
HTTP::Daemon object. This allows for deployment of a server object in other
environments. Note that if this is set, the server_loop method described
below will silently attempt to use the Net::Server module.
no_default
If passed with a "true" value, prevents the loading of the default methods
provided with the RPC::XML distribution. These may be later loaded using
the add_default_methods interface described later. The methods themselves
are described below (see ``The Default Methods Provided'').
path
host
port
queue
These four are specific to the HTTP-based nature of the server. The path
argument sets the additional URI path information that clients would use to
contact the server. Internally, it is not used except in outgoing status and
introspection reports. The host, port and queue arguments are passed
to the HTTP::Daemon constructor if they are passed. They set the hostname,
TCP/IP port, and socket listening queue, respectively. They may also be used
if the server object tries to use Net::Server as an alternative server
core.
xpl_path
If you plan to add methods to the server object by passing filenames to the
"add_method" call, this argument may be used to specify one or more
additional directories to be searched when the passed-in filename is a
relative path. The value for this must be an array reference. See also
add_method and xpl_path, below.
timeout
Specify a value (in seconds) for the HTTP::Daemon server to use as a
timeout value when reading request data from an inbound connection. The
default value is 10 seconds. This value is not used except by HTTP::Daemon.
auto_methods
If specified and set to a true value, enables the automatic searching for a
requested remote method that is unknown to the server object handling the
request. If set to ``no'' (or not set at all), then a request for an unknown
function causes the object instance to report an error. If the routine is
still not found, the error is reported. Enabling this is a security risk, and
should only be permitted by a server administrator with fully informed
acknowledgement and consent.
auto_updates
If specified and set to a ``true'' value, enables the checking of the
modification time of the file from which a method was originally loaded. If
the file has changed, the method is re-loaded before execution is handed
off. As with the auto-loading of methods, this represents a security risk, and
should only be permitted by a server administrator with fully informed
acknowledgement and consent.
parser
If this parameter is passed, the value following it is expected to be an
array reference. The contents of that array are passed to the new method
of the RPC::XML::Parser object that the server object caches for its use.
See the RPC::XML::Parser manual page for a list of recognized parameters
to the constructor.
message_file_thresh
If this key is passed, the value associated with it is assumed to be a
numerical limit to the size of in-memory messages. Any out-bound request that
would be larger than this when stringified is instead written to an anonynous
temporary file, and spooled from there instead. This is useful for cases in
which the request includes RPC::XML::base64 objects that are themselves
spooled from file-handles. This test is independent of compression, so even
if compression of a request would drop it below this threshhold, it will be
spooled anyway. The file itself is unlinked after the file-handle is created,
so once it is freed the disk space is immediately freed.
message_temp_dir
If a message is to be spooled to a temporary file, this key can define a
specific directory in which to open those files. If this is not given, then
the "tmpdir" method from the File::Spec package is used, instead.
Any other keys in the options hash not explicitly used by the constructor are
copied over verbatim onto the object, for the benefit of sub-classing this
class. All internal keys are prefixed with "__" to avoid confusion. Feel
free to use this prefix only if you wish to re-introduce confusion.
url
This returns the HTTP URL that the server will be responding to, when it is in
the connection-accept loop. If the server object was created without a
built-in HTTP listener, then this method returns "undef".
requests
Returns the number of requests this server object has marshalled. Note that in
multi-process environments (such as Apache or Net::Server::PreFork) the value
returned will only reflect the messages dispatched by the specific process
itself.
response
Each instance of this class (and any subclasses that do not completely
override the "new" method) creates and stores an instance of
HTTP::Response, which is then used by the HTTP::Daemon or Net::Server
processing loops in constructing the response to clients. The response object
has all common headers pre-set for efficiency. This method returns a reference
to that object.
started([BOOL])
Gets and possibly sets the clock-time when the server starts accepting
connections. If a value is passed that evaluates to true, then the current
clock time is marked as the starting time. In either case, the current value
is returned. The clock-time is based on the internal time command of Perl,
and thus is represented as an integer number of seconds since the system
epoch. Generally, it is suitable for passing to either localtime or to the
"time2iso8601" routine exported by the RPC::XML package.
timeout(INT)
You can call this method to set the timeout of new connections after
they are received. This function returns the old timeout value. If
you pass in no value then it will return the old value without
modifying the current value. The default value is 10 seconds.
add_method(FILE | HASHREF | OBJECT)
add_proc(FILE | HASHREF | OBJECT)
This adds a new published method or procedure to the server object that
invokes it. The new method may be specified in one of three ways: as a
filename, a hash reference or an existing object (generally of either
RPC::XML::Procedure or RPC::XML::Method classes).
If passed as a hash reference, the following keys are expected:
name
The published (externally-visible) name for the method.
version
An optional version stamp. Not used internally, kept mainly for informative
purposes.
hidden
If passed and evaluates to a "true" value, then the method should be hidden
from any introspection API implementations. This parameter is optional, the
default behavior being to make the method publically-visible.
code
A code reference to the actual Perl subroutine that handles this method. A
symbolic reference is not accepted. The value can be passed either as a
reference to an existing routine, or possibly as a closure. See
``How Methods are Called'' for the semantics the referenced subroutine must
follow.
signature
A list reference of the signatures by which this routine may be invoked. Every
method has at least one signature. Though less efficient for cases of exactly
one signature, a list reference is always used for sake of consistency.
help
Optional documentation text for the method. This is the text that would be
returned, for example, by a system.methodHelp call (providing the server
has such an externally-visible method).
If a file is passed, then it is expected to be in the XML-based format,
described in the RPC::XML::Procedure manual (see RPC::XML::Procedure).
If the name passed is not an absolute pathname, then the file will be searched
for in any directories specified when the object was instantiated, then in the
directory into which this module was installed, and finally in the current
working directory. If the operation fails, the return value will be a
non-reference, an error message. Otherwise, the return value is the object
reference.
The add_method and add_proc calls are essentialy identical unless called
with hash references. Both files and objects contain the information that
defines the type (method vs. procedure) of the funtionality to be added to the
server. If add_method is called with a file that describes a procedure, the
resulting addition to the server object will be a RPC::XML::Procedure
object, not a method object.
For more on the creation and manipulation of procedures and methods as
objects, see RPC::XML::Procedure.
delete_method(NAME)
delete_proc(NAME)
Delete the named method or procedure from the calling object. Removes the
entry from the internal table that the object maintains. If the method is
shared across more than one server object (see ``share_methods''), then the
underlying object for it will only be destroyed when the last server object
releases it. On error (such as no method by that name known), an error string
is returned.
The delete_proc call is identical, supplied for the sake of symmetry. Both
calls return the matched object regardless of its underlying type.
list_methods
list_procs
This returns a list of the names of methods and procedures the server current
has published. Note that the returned values are not the method objects, but
rather the names by which they are externally known. The ``hidden'' status of a
method is not consulted when this list is created; all methods and procedures
known are listed. The list is not sorted in any specific order.
The list_procs call is provided for symmetry. Both calls list all published
routines on the calling server object, regardless of underlying type.
xpl_path([LISTREF])
Get and/or set the object-specific search path for "*.xpl" files (files that
specify methods) that are specified in calls to add_method, above. If a
list reference is passed, it is installed as the new path (each element of the
list being one directory name to search). Regardless of argument, the current
path is returned as a list reference. When a file is passed to add_method,
the elements of this path are searched first, in order, before the
installation directory or the current working directory are searched.
get_method(NAME)
get_proc(NAME)
Returns a reference to an object of the class RPC::XML::Method or
RPC::XML::Procedure, which is the current binding for the published method
NAME. If there is no such method known to the server, then "undef" is
returned. The object is implemented as a hash, and has the same key and value
pairs as for "add_method", above. Thus, the reference returned is suitable
for passing back to "add_method". This facilitates temporary changes in what
a published name maps to. Note that this is a referent to the object as stored
on the server object itself, and thus changes to it could affect the behavior
of the server.
The get_proc interface is provided for symmetry.
server_loop(HASH)
Enters the connection-accept loop, which generally does not return. This is
the "accept()"-based loop of HTTP::Daemon if the object was created with
an instance of that class as a part. Otherwise, this enters the run-loop of
the Net::Server class. It listens for requests, and marshalls them out via
the "dispatch" method described below. It answers HTTP-HEAD requests
immediately (without counting them on the server statistics) and efficiently
by using a cached HTTP::Response object.
Because infinite loops requiring a "HUP" or "KILL" signal to terminate are
generally in poor taste, the HTTP::Daemon side of this sets up a localized
signal handler which causes an exit when triggered. By default, this is
attached to the "INT" signal. If the Net::Server module is being used
instead, it provides its own signal management.
The arguments, if passed, are interpreted as a hash of key/value options (not
a hash reference, please note). For HTTP::Daemon, only one is recognized:
signal
If passed, should be the traditional name for the signal that should be bound
to the exit function. If desired, a reference to an array of signal names may
be passed, in which case all signals will be given the same handler. The user
is responsible for not passing the name of a non-existent signal, or one that
cannot be caught. If the value of this argument is 0 (a "false" value) or the
string "NONE", then the signal handler will not be installed, and the
loop may only be broken out of by killing the running process (unless other
arrangements are made within the application).
The options that Net::Server responds to are detailed in the manual pages
for that package. All options passed to "server_loop" in this situation are
passed unaltered to the "run()" method in Net::Server.
dispatch(REQUEST)
This is the server method that actually manages the marshalling of an incoming
request into an invocation of a Perl subroutine. The parameter passed in may
be one of: a scalar containing the full XML text of the request, a scalar
reference to such a string, or a pre-constructed RPC::XML::request object.
Unless an object is passed, the text is parsed with any errors triggering an
early exit. Once the object representation of the request is on hand, the
parameter data is extracted, as is the method name itself. The call is sent
along to the appropriate subroutine, and the results are collated into an
object of the RPC::XML::response class, which is returned. Any non-reference
return value should be presumed to be an error string.
The dispatched method may communicate error in several ways. First, any
non-reference return value is presumed to be an error string, and is encoded
and returned as an RPC::XML::fault response. The method is run under an
"eval()", so errors conveyed by $@ are similarly encoded and returned. As
a special case, a method may explicitly "die()" with a fault response, which
is passed on unmodified.
add_default_methods([DETAILS])
This method adds all the default methods (those that are shipped with this
extension) to the calling server object. The files are denoted by their
"*.xpl" extension, and are installed into the same directory as this
Server.pm file. The set of default methods are described below (see
``The Default Methods Provided'').
If any names are passed as a list of arguments to this call, then only those
methods specified are actually loaded. If the "*.xpl" extension is absent on
any of these names, then it is silently added for testing purposes. Note that
the methods shipped with this package have file names without the leading
"status." part of the method name. If the very first element of the list of
arguments is "except" (or "-except"), then the rest of the list is
treated as a set of names to not load, while all others do get read. The
Apache::RPC::Server module uses this to prevent the loading of the default
"system.status" method while still loading all the rest of the defaults. (It
then provides a more Apache-centric status method.)
Note that there is no symmetric call in this case. The provided API is
implemented as methods, and thus only this interface is provided.
add_methods_in_dir(DIR [, DETAILS])
add_procs_in_dir(DIR [, DETAILS])
This is exactly like add_default_methods above, save that the caller
specifies which directory to scan for "*.xpl" files. In fact, the
add_default_methods routine simply calls this routine with the installation
directory as the first argument. The definition of the additional arguments is
the same as above.
add_procs_in_dir is provided for symmetry.
share_methods(SERVER, NAMES)
share_procs(SERVER, NAMES)
The calling server object shares the methods and/or procedures listed in
NAMES with the source-server passed as the first object. The source must
derive from this package in order for this operation to be permitted. At least
one method must be specified, and all are specified by name (not by object
refernce). Both objects will reference the same exact RPC::XML::Procedure
(or Method, or derivative thereof) object in this case, meaning that
call-statistics and the like will reflect the combined data. If one or more of
the passed names are not present on the source server, an error message is
returned and none are copied to the calling object.
Alternately, one or more of the name parameters passed to this call may be
regular-expression objects (the result of the qr operator). Any of these
detected are applied against the list of all available methods known to the
source server. All matching ones are inserted into the list (the list is pared
for redundancies in any case). This allows for easier addition of whole
classes such as those in the "system.*" name space (via "qr/^system\./"),
for example. There is no substring matching provided. Names listed in the
parameters to this routine must be either complete strings or regular
expressions.
The share_procs interface is provided for symmetry.
copy_methods(SERVER, NAMES)
copy_procs(SERVER, NAMES)
This behaves like the method share_methods above, with the exception that
the calling object is given a clone of each method, rather than referencing
the same exact method as the source server. The code reference part of the
method is shared between the two, but all other data are copied (including a
fresh copy of any list references used) into a completely new
RPC::XML::Procedure (or derivative) object, using the "clone()" method
from that class. Thus, while the calling object has the same methods
available, and is re-using existing code in the Perl runtime, the method
objects (and hence the statistics and such) are kept separate. As with the
above, an error is flagged if one or more are not found.
This routine also accepts regular-expression objects with the same behavior
and limitations. Again, copy_procs is simply provided for symmetry.
Specifying Server-Side Remote Methods
Specifying the methods themselves can be a tricky undertaking. Some packages
(in other languages) delegate a specific class to handling incoming requests.
This works well, but it can lead to routines not intended for public
availability to in fact be available. There are also issues around the access
that the methods would then have to other resources within the same running
system.
The approach taken by RPC::XML::Server (and the Apache::RPC::Server
subclass of it) require that methods be explicitly published in one of the
several ways provided. Methods may be added directly within code by using
"add_method" as described above, with full data provided for the code
reference, signature list, etc. The "add_method" technique can also be used
with a file that conforms to a specific XML-based format (detailed in the
manual page for the RPC::XML::Procedure class, see RPC::XML::Procedure).
Entire directories of files may be added using "add_methods_in_dir", which
merely reads the given directory for files that appear to be method
definitions.
How Methods Are Called
When a routine is called via the server dispatcher, it is called with the
arguments that the client request passed. Depending on whether the routine is
considered a ``procedure'' or a ``method'', there may be an extra argument at the
head of the list. The extra argument is present when the routine being
dispatched is part of a RPC::XML::Method object. The extra argument is a
reference to a RPC::XML::Server object (or a subclass thereof). This is
derived from a hash reference, and will include two special keys:
method_name
This is the name by which the method was called in the client. Most of the
time, this will probably be consistent for all calls to the server-side
method. But it does not have to be, hence the passing of the value.
signature
This is the signature that was used, when dispatching. Perl has a liberal
view of lists and scalars, so it is not always clear what arguments the client
specifically has in mind when calling the method. The signature is an array
reference containing one or more datatypes, each a simple string. The first
of the datatypes specifies the expected return type. The remainder (if any)
refer to the arguments themselves.
Note that by passing the server object reference first, method-classed
routines are essentially expected to behave as actual methods of the server
class, as opposed to ordinary functions. Of course, they can also discard the
initial argument completely.
The routines should not make (excessive) use of global variables, for obvious
reasons. When the routines are loaded from XPL files, the code is created as a
closure that forces execution in the RPC::XML::Procedure package. If the
code element of a procedure/method is passed in as a direct code reference by
one of the other syntaxes allowed by the constructor, the package may well be
different. Thus, routines should strive to be as localized as possible,
independant of specific namespaces. If a group of routines are expected to
work in close concert, each should explicitly set the namespace with a
"package" declaration as the first statement within the routines themselves.
The Default Methods Provided
The following methods are provided with this package, and are the ones
installed on newly-created server objects unless told not to. These are
identified by their published names, as they are compiled internally as
anonymous subroutines and thus cannot be called directly:
system.identity
Returns a string value identifying the server name, version, and possibly a
capability level. Takes no arguments.
system.introspection
Returns a series of struct objects that give overview documentation of one
or more of the published methods. It may be called with a string
identifying a single routine, in which case the return value is a
struct. It may be called with an array of string values, in which
case an array of struct values, one per element in, is returned. Lastly,
it may be called with no input parameters, in which case all published
routines are documented. Note that routines may be configured to be hidden
from such introspection queries.
system.listMethods
Returns a list of the published methods or a subset of them as an array of
string values. If called with no parameters, returns all (non-hidden)
method names. If called with a single string pattern, returns only those
names that contain the string as a substring of their name (case-sensitive,
and this is not a regular expression evaluation).
system.methodHelp
Takes either a single method name as a string, or a series of them as an
array of string. The return value is the help text for the method, as
either a string or array of string value. If the method(s) have no
help text, the string will be null.
system.methodSignature
As above, but returns the signatures that the method accepts, as array of
string representations. If only one method is requests via a string
parameter, then the return value is the corresponding array. If the parameter
in is an array, then the returned value will be an array of array of
string.
system.multicall
This is a simple implementation of composite function calls in a single
request. It takes an array of struct values. Each struct has at least
a "methodName" member, which provides the name of the method to call. If
there is also a "params" member, it refers to an array of the parameters
that should be passed to the call.
system.status
Takes no arguments and returns a struct containing a number of system
status values including (but not limited to) the current time on the server,
the time the server was started (both of these are returned in both ISO 8601
and UNIX-style integer formats), number of requests dispatched, and some
identifying information (hostname, port, etc.).
In addition, each of these has an accompanying help file in the "methods"
sub-directory of the distribution.
These methods are installed as "*.xpl" files, which are generated from files
in the "methods" directory of the distribution using the make_method tool
(see make_method). The files there provide the Perl code that implements
these, their help files and other information.
Content Compression
The RPC::XML::Server class now supports compressed messages, both incoming
and outgoing. If a client indicates that it can understand compressed content,
the server will use the Compress::Zlib (available from CPAN) module, if
available, to compress any outgoing messages above a certain threshhold in
size (the default threshhold is set to 4096 bytes). The following methods are
all related to the compression support within the server class:
compress
Returns a false value if compression is not available to the server object.
This is based on the availability of the Compress::Zlib module at start-up
time, and cannot be changed.
compress_thresh([MIN_LIMIT])
Return or set the compression threshhold value. Messages smaller than this
size in bytes will not be compressed, even when compression is available, to
save on CPU resources. If a value is passed, it becomes the new limit and the
old value is returned.
Spooling Large Messages
If the server anticipates handling large out-bound messages (for example, if
the hosted code returns large Base64 values pre-encoded from file handles),
the "message_file_thresh" and "message_temp_dir" settings may be used in a
manner similar to RPC::XML::Client. Specifically, the threshhold is used to
determine when a message should be spooled to a filehandle rather than made
into an in-memory string (the RPC::XML::base64 type can use a filehandle,
thus eliminating the need for the data to ever be completely in memory). An
anonymous temporary file is used for these operations.
Note that the message size is checked before compression is applied, since the
size of the compressed output cannot be known until the full message is
examined. It is possible that a message will be spooled even if its compressed
size is below the threshhold, if the uncompressed size exceeds the threshhold.
message_file_thresh
message_temp_dir
These methods may be used to retrieve or alter the values of the given keys
as defined earlier for the "new" method.
DIAGNOSTICS
Unless explicitly stated otherwise, all methods return some type of reference
on success, or an error string on failure. Non-reference return values should
always be interpreted as errors unless otherwise noted.
CAVEATS
This began as a reference implementation in which clarity of process and
readability of the code took precedence over general efficiency. It is now
being maintained as production code, but may still have parts that could be
written more efficiently.
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. A helpful patch was sent in by Tino Wuensche to fix problems
in the signal-setting and signal-catching code in server_loop().
