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This document describes GNU Guile-RPC version 0.4. It was last updated in May 2014.
| • Introduction: | Overview of Guile-RPC | |
| • Obtaining and Installing: | Notes on installation | |
| • Quick Start: | Writing an RPC client/server in a glimpse | |
| • API Reference: | Everything about Guile-RPC | |
| • Stand-Alone Tools: | Command-line tools | |
| • References: | Useful documents | |
| • Portability: | Porting Guile-RPC to other Schemes | |
| • GNU Free Documentation License: | The license of this manual | |
| • Concept Index: | Concepts discussed in this document | |
| • Function Index: | Index of Scheme procedures | |
| • Variable Index: | Index of Scheme variables | |
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GNU Guile-RPC is a framework for distributed programming under GNU Guile. It is a pure Scheme implementation of the ONC RPC standard, i.e., the “Open Network Computing” Remote Procedure Call standard. ONC RPC is standardized by the Internet Engineering Task Force (IETF) as RFC 1831. It is based on the External Data Representation standard (XDR), known as RFC 4506 (see References).
Remote procedure calls allow programs on different, potentially remote machines to interact together. A remote procedure call is the invocation of the procedure of a program located on a remote host (the RPC server), as the name implies. Doing so requires the procedure arguments on the client-side to be encoded, or marshalled, i.e., converted to a representation suitable for transfer over the network. On the server-side, upon reception of the RPC, those arguments must be decoded or unmarshalled, i.e., converted back to a form that is directly understandable by the server program—for instance, data using Scheme data types, should the server program be written in Scheme. The value returned by the RPC server must be encoded and decoded similarly.
When using the ONC RPC protocol, the way data items are encoded is dictated by the XDR standard. This encoding has the advantage of being particularly compact, allowing for relatively low bandwidth requirements and fast implementations, especially compared to more verbose RPC protocols such as XML-RPC and SOAP.
The XDR encoding is not self-describing, i.e., it is impossible given an arbitrary XDR encoded sequence to determine the XDR type of the encoded data. This is different from D-Bus, for example, which uses a compact and self-describing encoding. In practice, this is sufficient for a wide range of applications.
GNU Guile-RPC provides an easy access to the ONC RPC protocol for the Guile Scheme programmer. In particular, it allows standard Scheme types to be mapped to XDR data types, so that Scheme objects are easily encoded to or decoded from XDR.
Please send bug reports, comments and patches to the Guile-RPC mailing list.
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The GNU Guile-RPC web page is located at http://www.gnu.org/software/guile-rpc/. You can obtain copy of GNU Guile-RPC from ftp://alpha.gnu.org/gnu/guile-rpc/.
In order to use GNU Guile-RPC, all that is needed is GNU Guile 2.0 or later.
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This section illustrates how ONC RPC clients and servers can be
implemented using Guile-RPC. ONC RPC defines a language to describe
RPCs and the data types they depend on (see RFC
4506, Section 6 and RFC 1831, Section 11). This
language, which we refer to as the XDR/RPC language or simply
RPC language, is essentially an interface description
language (IDL). It resembles the C programming language and borrows
C’s type definition constructs and adds the program and
version constructs that introduce RPC definitions.
Consider the following RPC definition, written using the XDR/RPC language:
struct result_t
{
int integer_part;
unsigned int decimal_part;
};
program ARITHMETIC_PROGRAM
{
version ARITHMETIC_VERSION
{
/* Return the integer part and the 1000th of the
given double-precision floating point number. */
result_t split_number (double) = 1;
} = 0;
} = 80000;
It defines a simple RPC interface named ARITHMETIC
which contains only one procedure called split_number (). The
interface itself has a program number that identifies it (here,
80000). Normally, program numbers below 20000000 (hexadecimal) are
assigned by Sun Microsystems, Inc. and thus should not be used unless
the number has been properly registered (see RFC 1831,
for details). It also has a version number (here, 0) that is
user-defined and should be increased when the interface changes (e.g.,
when procedures are added, modified, etc.). Finally, the procedure
split_number () has a procedure number (here, 1) that allows it
to be distinguished from other procedures.
People vaguely familiar with the C programming language should have guessed by now that this simple interface defines a procedure that takes a double-precision floating-point number and returns a structure that contains two fields.
Client and server creation are two-step. Since the first step—data
type definition—is the same for both, that leaves us with a total of
three steps, described below. Nicely, each of these steps can be
automated using the XDR/RPC language compiler (see grpc-compile).
| • Defining Data Types: | Defining RPC data types | |
| • Creating the Client: | Creating the RPC client | |
| • Creating the Server: | Creating the RPC server |
More details about the XDR type definition as well as client and server creation are available in the API reference (see API Reference).
Next: Creating the Client, Previous: Quick Start, Up: Quick Start [Contents][Index]
Before actually creating a client or server for this interface, one must
define the types it uses. The simplest way to define one or several
data types is to pipe their definition in XDR/RPC language through the
compiler (see grpc-compile):
$ grpc-compile --xdr --constants < input.x > types.scm
Given the description from input.x in RPC language, this command
generates code that provides access to the constants and data types
defined therein. The resulting Scheme code, types.scm, can then
be loaded in other Scheme files (see Loading in The GNU
Guile Reference Manual).
In addition, code in types.scm depends on Guile-RPC modules that it uses at run-time. Thus, it must first import the relevant modules:
(use-modules (rpc xdr)
(rpc xdr types))
Then, the result_t type is defined (this is the code generated by
the compiler but it can also be written “by hand”):
(define result-type
(make-xdr-struct-type (list xdr-integer ;; `integer_part'
xdr-unsigned-integer))) ;; `decimal_part'
Next: Creating the Server, Previous: Defining Data Types, Up: Quick Start [Contents][Index]
Producing a client to invoke split_number () is as simple as
this:
(use-modules (rpc rpc))
(define invoke-split-number
(make-synchronous-rpc-call 80000 0 ;; program and version
1 ;; procedure number
xdr-double ;; argument type
result-type))
Again, this definition, minus the use-modules clause, can
alternatively be generated by the compiler from the RPC description in
XDR/RPC language (see grpc-compile):
$ grpc-compile --client < input.x > client.scm
Once this is done, invoking the procedure is as simple as this:
(invoke-split-number 3.14 #x7777 socket)
The first argument to invoke-split-number is the argument of
split_number (); the second argument is a transaction ID, i.e.,
an arbitrarily chosen number that identifies the remote procedure
call; the third argument should be an output port, typically one
bound to a connection to the RPC server:
(define socket (socket PF_INET SOCK_STREAM 0)) (connect socket AF_INET INADDR_LOOPBACK 6666)
This example creates an IPv4 connection to the local host on port 6666 (see Network Sockets and Communication in The GNU Guile Reference Manual).
On success, invoke-split-number returns a two-element list
where the first element corresponds to the integer_part field
of the result and the second element correspond to the
decimal_part field of the result, both represented as Scheme
exact integers.
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Creating a TCP server for our RPC interface should be quite easy as well. We can re-use our previous type definitions (see Defining Data Types). Then, all we have to do is to create a definition for our program.
(use-modules (rpc rpc server))
(define (split-number-handler number)
;; Handle a `split-number' request.
(let* ((int (floor number))
(dec (floor (* 1000 (- number int)))))
(list (inexact->exact int)
(inexact->exact dec))))
(define my-rpc-program
;; Define our RPC program.
(let* ((proc (make-rpc-procedure 1 xdr-double result-type
split-number-handler))
(version (make-rpc-program-version 0 (list proc))))
(make-rpc-program 80000 (list version))))
Alternatively, using the compiler allows the above definition of
my-rpc-program to be automatically generated from the XDR/RPC
definition file (see grpc-compile):
$ grpc-compile --server < input.x > server.scm
However, there is a slight difference compared to the above
“hand-written” approach: server.scm does not contain the actual
definition of my-rpc-program since it does not know about your
split-number-handler procedure. Instead, given the RPC/XDR
definition given earlier, it contains a
make-ARITHMETIC-PROGRAM-server procedure; this procedure can be
passed a list associating RPC names to Scheme procedures, and returns
the resulting RPC program object:
(define my-rpc-program
(make-ARITHMETIC-PROGRAM-server
`(("ARITHMETIC_VERSION" ;; specify the target version
;; list all supported RPCs for this version
("split_number" . ,split-number-handler)))))
As can be seen, using the compiler-generated server stub, one doesn’t have to deal explicitly with program, version and RPC numbers, which clarifies the code.
Finally, we can make the server listen for incoming connections and handle client requests, using Guile’s networking primitives in The GNU Guile Reference Manual.
;; Creating a listening TCP socket.
(define server-socket (socket PF_INET SOCK_STREAM 0))
;; Listen for connections on 127.0.0.1, port 6666.
(bind server-socket AF_INET INADDR_LOOPBACK 6666)
(listen server-socket 1024)
;; Go ahead and serve requests.
(run-stream-rpc-server (list (cons server-socket my-rpc-program))
1000000 ;; a one-second timeout
#f ;; we don't care about closed connections
(lambda () ;; our idle thunk
(format #t "one second passed~%")))
And now we’re all set: We have a working TCP client and server for
this wonderful RPC interface! This would work similarly for other
stream-oriented transports such as Unix-domain sockets: only the
socket and bind calls need to be adapted.
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This section first details facilities available to manipulate XDR types. It then describes Scheme procedures that should be used to build ONC RPC clients and servers.
| • Usage Notes: | Preliminary remarks | |
| • Implementation of XDR: | The external data representation | |
| • Implementation of ONC RPC: | Remote procedure calls | |
| • Standard RPC Programs: | Interfaces to common RPC programs | |
| • Compiler: | The XDR/RPC language compiler |
Next: Implementation of XDR, Previous: API Reference, Up: API Reference [Contents][Index]
Guile-RPC makes use of the SRFI-34 exception mechanism along with SRFI-35 error conditions to deal with the various protocol errors1. Thus, users are expected to use these mechanisms to handle the error conditions described in the following sections. Hopefully, in most cases, error conditions raised by Guile-RPC code provide users with detailed information about the error.
Next: Implementation of ONC RPC, Previous: Usage Notes, Up: API Reference [Contents][Index]
This section describes how XDR types are represented in Guile-RPC, as well as how one can encode Scheme values to XDR and decode XDR binary data to Scheme values.
| • XDR Type Representations: | Foundations | |
| • XDR Standard Data Types: | The XDR standard types | |
| • XDR Encoding and Decoding: | Encoding to and decoding from XDR |
Next: XDR Standard Data Types, Previous: Implementation of XDR, Up: Implementation of XDR [Contents][Index]
The XDR standard defines various basic data types and allows for the definition of compound data types (“structs”), fixed-size and variable-size arrays as well as “unions”. Fixed-size arrays and structs can actually be thought as the same type: Their size if known in advance and they are encoded as the succession of the data they contain. Thus, those types can be summarized as 4 great classes: “basic” types, variable-length arrays, structs and unions.
The (rpc xdr) module provides facilities to create Scheme objects
representing XDR data types and to manipulate them. These Scheme
objects, described below, are all immutable, i.e., they cannot be
modified after creation.
This returns an <xdr-basic-type> object describing a type whose
encoding fits on size octets, and which is encoded/decoded using
the encoder and decoder procedures. type-pred
should be a predicate checking the validity of an input Scheme value
for encoding into this type.
Optionally, vector-encoder and vector-decoder can be passed
and should be procedures that efficiently encode/decode sequences of
data items of this type (for instance, the vector decoder could use the
bytevector->int-list procedure of the (rnrs bytevectors)
module to speed up decoding). The vector encoder is invoked as
(vector-encoder type value bv index) while the vector
decoder is invoked as (vector-decoder type count port).
Users should normally not need to define new basic types since all the
basic types defined by XDR are already available in (rpc xdr
types) (see XDR Standard Data Types). Thus, we will not describe
its use further.
Return a new XDR struct type made of a sequence of XDR data items whose types are listed in base-types.
Struct types encode from/decode to Scheme lists whose length is that of base-types.
Return an object describing an XDR variable-length array of elements
of types base-type (again, an XDR type object). If
max-element-count is a number, then it describes the maximum
number of items of type base-type that are allow in actual
arrays of this type. If base-type is #f, then arrays of
this type may contain up to 2^32 - 1 items of type base-type.
Vector types are encoded from arrays, which includes Scheme
vectors, SRFI-4 vectors or strings (see Arrays in The GNU Guile Reference Manual). By default, vector types decode to
vectors, but any other kind of array can be used: it only
needs to be specified as the vector-decoder argument of
make-xdr-basic-type for the corresponding base type. Of course,
SRFI-4 vectors, for example, may only be used when an XDR integer vector
with a matching integer range is expected.
If max-element-count is specified and a vector to be encoded
contains more than max-element-count elements, then an
&xdr-vector-size-exceeded-error error condition is raised.
Likewise, if XDR data to be decoded describes vectors larger than
max-element-count, this error condition is raised.
Return a new XDR discriminated union type, using discr-type as the
discriminant type (which must be a 32-bit basic type) and
discr/type-alist to select the “arm” type depending on the
discriminant value. If no suitable value is found in discr/type-alist
and default-type is not #f, then default type is used as the arm
type.
Union types encode from/decode to Scheme pairs whose car is the
discriminant’s value and whose cdr is the actual union value.
Return the type that should be used for union’s arm given discriminant (a Scheme value).
Sometimes, one may want to define recursive types, i.e., types that refer to themselves. This is particularly useful to implement lists. For example, in XDR language, a list of integers can be defined as follows:
struct integer_list_t
{
int x;
integer_list_t *next;
};
This notation is a shortcut for the following structure:
struct integer_list_t
{
int x;
union switch (bool opted)
{
case TRUE:
integer_list_t value;
case FALSE:
void;
} next;
};
The integer_list_t type references itself. Defining it using our
API seems impossible at first: one cannot pass a self-reference to
make-xdr-struct-type (since the object is not yet created!), and
the self-reference cannot be added after the fact since objects returned
by make-xdr-struct-type are immutable.
The API addresses this problem by allowing thunks (zero-argument
procedures) to be used as types. Together with Scheme’s letrec
recursive binding construct or a top-level define (see letrec in Revised^5 Report on the
Algorithmic Language Scheme), it makes it possible to create such
recursive types:
(letrec ((integer-list
(make-xdr-struct-type
(list xdr-integer
(make-xdr-union-type xdr-boolean
`((TRUE . ,(lambda ()
integer-list))
(FALSE . ,xdr-void))
#f)))))
integer-list)
The trick here is that using the thunk effectively defers the evaluation of the self-reference2.
It is often useful to know the size in octets it takes to encode a given value according to an XDR type. However, as we just saw, the size of some XDR types (variable-length arrays and unions) cannot be known in advance: The encoding size depends on the actual value to encode. The following procedure allow the computation of the size of the XDR representation of some value.
Return the size (in octets) of type when applied to value. type must be an XDR type object returned by one of the above procedures, while value should be a Scheme value suitable for encoding with type.
The following section lists the standard XDR data types.
Next: XDR Encoding and Decoding, Previous: XDR Type Representations, Up: Implementation of XDR [Contents][Index]
All the basic data types defined by XDR are defined in the (rpc
xdr types) module.
XDR’s 32-bit and 64-bit signed and unsigned integer types. This type decodes to/encodes from Scheme exact numbers.
32-bit and 64-bit IEEE-754 floating point numbers. This type decodes to/encodes from Scheme inexact numbers. Note that XDR also defines a “quadruple-precision floating point type” (i.e., 128-bit long) that is currently not available (FIXME).
The “void” type that yields zero bits. Any Scheme value is suitable
as an input when encoding with this type. When decoding this type,
the %void value (which may be compared via eq?) is
returned.
XDR provides support for “enumerations”, similar to that found in C. An enumeration type maps symbols to integers and are actually encoded as 32-bit integers.
Return an enumeration type that obeys the symbol-integer mapping
provided in enum-alist which should be a list of symbol-integer
pairs. The returned type decodes to/encodes from Scheme symbols, as
provided in enum-alist. Upon decoding/encoding of an
enumeration, an &xdr-enumeration-error is raised if an
incorrect value (i.e., one not listed in enum-alist) is found.
XDR’s boolean type which is an enumeration. It encodes to/decodes
from Scheme symbols TRUE and FALSE.
Several fixed-size and variable-size are predefined in the standard.
Return a fixed-length “opaque” array of size octets. An opaque array is simply a sequence of octets.
The returned XDR type object is actually an <xdr-struct-type>
object. Thus, it encodes from/decodes to Scheme lists of exact
integers. Conversion to a Scheme string, if needed, is left to the
user.
Return a variable-length opaque array. As for
make-xdr-vector-type (see XDR Type Representations),
limit can be either a number specifying the maximum number of
elements that can be held by the created type, or #f in which
case the variable-length array can hold up to 2^32 - 1 octets.
The returned XDR type object is actually an <xdr-vector-type>
object. Thus, it encodes from/decodes to Scheme vectors of exact
integers.
This is a synonym of make-xdr-variable-length-opaque-array
since XDR’s string type actually represents ASCII strings, i.e.,
sequences of octets.
These convenience variables contain the unlimited variable-length opaque array.
Previous: XDR Standard Data Types, Up: Implementation of XDR [Contents][Index]
The following primitives are exported by the (rpc xdr) module.
They implement the encoding of Scheme values to XDR data types, and
the decoding of binary XDR data to Scheme values. The exact mapping
between XDR data types and Scheme data types has been discussed
earlier.
Encode value (a suitable Scheme value), using XDR type type, into bytevector bv at index. Return the index where encoding ended.
bv should be an R6RS bytevector large enough to hold the XDR
representation of value according to type. To that end, users
may rely on xdr-type-size (see XDR Type Representations).
Error conditions sub-classing &xdr-error may be raised during
encoding. The exact exceptions that may be raised are type-dependent
and have been discussed in the previous sections.
Decode from port (a binary input port) a value of XDR type type. Return the decoded value.
Error conditions sub-classing &xdr-error may be raised during
encoding. The exact exceptions that may be raised are type-dependent
and have been discussed in the previous sections.
Next: Standard RPC Programs, Previous: Implementation of XDR, Up: API Reference [Contents][Index]
This section describes facilities available for the creation of ONC RPC clients and servers, as well as lower-level details about raw RPC messages.
| • Building an RPC Client: | Sending RPC requests | |
| • Building an RPC Server: | Processing RPC requests | |
| • ONC RPC Message Types: | Low-level details | |
| • Record Marking Standard: | Standard encoding for ONC RPC over TCP |
Next: Building an RPC Server, Previous: Implementation of ONC RPC, Up: Implementation of ONC RPC [Contents][Index]
Basic building blocks for the creation of RPC clients are provided by
the (rpc rpc) module. The easiest way to build an RPC client
is through make-synchronous-rpc-call.
Return a procedure that may be applied to a list of arguments, transaction ID (any unsigned number representable on 32 bits), and I/O port, to make a synchronous RPC call to the remote procedure numbered procedure in program, version version. On success, the invocation result is eventually returned. Otherwise, an error condition is raised. arg-type and result-type should be XDR type objects (see XDR Type Representations).
Error conditions that may be raised include those related to XDR
encoding and decoding (see XDR Encoding and Decoding), as well as
RPC-specific error conditions inheriting from &rpc-error (i.e.,
conditions that pass the rpc-error? predicate). These are
detailed in assert-successful-reply.
For an example, see Creating the Client.
It is also possible to create “one-way” calls, i.e., RPC calls that do
not expect a reply (i.e., no return value, not even void). This
is useful, for instance, to implement batched calls where clients do not
wait for the server to reply (see Section 7.4.1 of RFC
1831). Asynchronous calls can be implemented in terms of this, too.
Similar to make-synchronous-rpc-call, except that the returned
procedure does not wait for a reply.
Next: ONC RPC Message Types, Previous: Building an RPC Client, Up: Implementation of ONC RPC [Contents][Index]
The (rpc rpc server) module provides helpful facilities for
building an ONC RPC server. In particular, it provides tools to
decode RPC call messages, as well as an event loop mechanisms that
allows RPC calls to be automatically dispatched to the corresponding
Scheme handlers.
Return an <rpc-call> object that denotes the procedure call
requested in call-msg (the result of an (xdr-decode
rpc-message port) operation). If call-msg is not an
appropriate RPC call message, an error condition is raised.
The error condition raised may be either
onc-rpc-version-mismatch-error? or
rpc-invalid-call-message-error?.
The returned object can be queried using the rpc-call-
procedures described below.
Return the transaction ID (“xid” for short) of call.
Return the program, version or procedure number of call.
Return the credentials and verifier provided by the client for call. FIXME: As of version 0.4, this information is not usable.
The following procedures allow the description of RPC “programs”. Such descriptions can then be readily used to produced a full-blown RPC processing loop.
Return a new object describing the RPC program identified by number and consisting of the versions listed in versions.
Return a new object describing the RPC program version identified by number and consisting of the procedures listed in procedures.
Return a new object describing a procedure whose number is number, whose argument type is argument-xdr-type and whose result type is result-xdr-type (see XDR Type Representations). handler should be a procedure that will be invoked upon reception of an RPC call for that procedure.
If synchronous RPC processing is used, i.e., through
serve-one-stream-request, then handler is passed the
decoded argument and should return a result type that may be encoded as
result-xdr-type. If asynchronous processing is used, i.e.,
through serve-one-stream-request/asynchronous, then handler
is passed the decoded argument along with a continuation that must be
invoked to actually return the result.
If one-way? is passed and is true, then the returned procedure is
marked as “one-way” (see make-one-way-rpc-call). For one-way procedures,
run-stream-rpc-server and similar procedures ignores the return
value of handler and don’t send any reply when procedure
number is invoked.
Once a program, its versions and procedures have been defined, an RPC server for that program (and possibly others) can be run using the following procedures.
Run a full-blown connection-oriented (i.e., SOCK_STREAM, be it
PF_UNIX or PF_INET) RPC server for the given listening
sockets and RPC programs. sockets+rpc-programs should be a list
of listening socket-RPC program pairs (where “RPC programs” are
objects as returned by make-rpc-program). timeout should
be a number of microseconds that the loop should wait for input; when no
input is available, idle-thunk is invoked, thus at most every
timeout microseconds. If close-connection-proc is a
procedure, it is called when a connection is being closed is passed the
corresponding <stream-connection> object.
While an RPC server is running over a stream-oriented transport such as
TCP using run-stream-rpc-server, its procedure handlers can get
information about the current connection and client:
This procedure returns a <stream-connection> object describing the
current TCP connection (when within a run-stream-rpc-server
invocation). This object can be queried with the procedures described
below.
Return #t if obj is a <stream-connection> object.
Return the I/O port (not the TCP port) for connection.
Return the IP address of the peer/client of connection (see Network Socket Address in The GNU Guile Reference Manual).
Return the RPC program object corresponding to connection.
For a complete RPC server example, Creating the Server.
The run-stream-rpc-server mechanism is limited to servers
managing only RPC connections, and only over stream-oriented
transports. Should your server need to handle other input sources, a
more geneneral event handling mechanism is available. This works by
first creating a set of I/O managers and then passing
run-input-event-loop a list of I/O manager-file descriptor pairs
to actually handle I/O events.
Return an I/O manager. When data is available for reading, read-handler will be called and passed a port to read from; when an exception occurs on a port, exception-handler is called and passed the failing port.
Return #t if obj is an I/O manager.
Return, respectively, the exception handler and the read handler of manager.
Run an input event loop based on fd+manager-list, a list of pairs of input ports (or file descriptors) and I/O managers. I/O managers are invoked are invoked and passed the corresponding port when data becomes readable or when an exception occurs. I/O manager handlers can:
#f, in which case the port and I/O manager are removed
from the list of watched ports;
When timeout (a number of microseconds) is reached, idle-thunk is
invoked. If timeout is #f, then an infinite timeout is assumed and
idle-thunk is never run. The event loop returns when no watched port
is left.
The event loop provided by run-input-event-loop should cover a
wide range of applications. However, it will turn out to be
insufficient in situations where tasks must be executed at specific
times, and where the interval between consecutive executions varies over
the program’s lifetime.
Finally, a lower-level mechanism is available to handle a single incoming RPC:
Serve one RPC for program, reading the RPC from port (using
the record-marking protocol) and writing the reply to port. If
port is closed or the end-of-file was reached, an
&rpc-connection-lost-error is raised.
Same as serve-one-stream-request except that the RPC is to be
handled in an asynchronous fashion.
Concretely, the procedure handler passed to make-rpc-procedure is
called with two arguments instead of one: the first one is the actual
procedure argument, and the second one is a one-argument procedure that
must be invoked to return the procedure’s result—in other words,
procedure call processing is decoupled from procedure call return using
continuation-passing style.
Next: Record Marking Standard, Previous: Building an RPC Server, Up: Implementation of ONC RPC [Contents][Index]
The (rpc rpc types) module provides a representation of the
various XDR types defined in the standard to represent RPC messages
(see References). We only describe the most important ones as
well as procedures from the (rpc rpc) module that help use it.
This variable contains a XDR struct type representing all possible RPC
messages—the rpc_msg struct type defined in RFC 1831. By
“rpc message” we mean the header that is transmitted before the
actual procedure argument to describe the procedure call being made.
Roughly, this header contains a transaction ID allowing clients to match call/reply pairs, plus information describing either the call or the reply being made. Calls essentially contain a program, version and procedure numbers. Replies, on the other hand, can be more complex since they can describe a large class of errors.
This variable is bound to an XDR enumeration. Its two possible values
are CALL and REPLY (both represented in Scheme using
symbols), denoting a procedure call and a reply to a procedure call,
respectively.
Return an rpc-message datum. type should be either
CALL or REPLY (the two values of the
rpc-message-type enumeration). The arguments args are
message-type-specific. For example, a message denoting a procedure
call to procedure number 5 of version 1 of program 77 can be created
as follows:
(define my-call-msg
(make-rpc-message #x123 ;; the transaction ID
'CALL ;; the message type
77 1 5))
It can then be encoded in the usual way (see XDR Encoding and Decoding):
(let* ((size (xdr-type-size rpc-message my-call-msg))
(bv (make-bytevector size)))
(xdr-encode! bv 0 rpc-message my-call-msg)
;;; ...
)
Likewise, a reply message denoting a successful RPC call can be produced as follows:
(make-rpc-message xid 'REPLY 'MSG_ACCEPTED 'SUCCESS)
It is worth noting that in practice, “messages” of type
rpc-message are followed by additional data representing either
the procedure call arguments (if the message is a CALL message)
or the procedure return value (if the message is a REPLY
message).
Return true if rpc-msg (an RPC message as returned by a previous
(xdr-decode rpc-message port) call) is a valid reply for the
invocation labeled with transaction ID xid indicating that it
was accepted. If xid is #t, any reply transaction ID is
accepted and it is returned (provided the rest of the message denotes
an accepted message). On failure, an appropriate error condition is
raised.
The error conditions that may be raised obey rpc-error? and
rpc-call-error?. More precisely, error conditions include the
following:
rpc-program-unavailable-error?If rpc-msg denotes the fact that the program requested by the corresponding RPC call is not available.
rpc-program-mismatch-error?If the corresponding RPC call requested a program version that is not
available. The procedures
rpc-program-mismatch-error:low-version and
rpc-program-mismatch-error:high-version return, respectively,
the lowest and highest version numbers supported by the remote server.
rpc-procedure-unavailable-error?If the corresponding RPC call requested a procedure that is not available.
rpc-garbage-arguments-error?If the remote server failed to decode the procedure arguments.
rpc-system-error?If the remote server failed to allocate enough memory for argument decoding, for instance.
Previous: ONC RPC Message Types, Up: Implementation of ONC RPC [Contents][Index]
The ONC RPC standard defines a record-marking protocol for
stream-oriented transport layers such as TCP whereby (1) each RPC
message is sent out as a single record and (2) where records may
be split into several fragments. This allows implementations to
“delimit one message from another in order to detect and possibly
recover from protocol errors” (see RFC
1831, Section 10).
This protocol is implemented by the (rpc rpc transports)
module. It is automatically used by the high-level client and server
facilities, namely make-synchronous-rpc-call and run-stream-rpc-server. The available facilities are described
below.
Send the RPC message of len octets encoded at offset offset in bv (a bytevector) to port. This procedure sends the len octets of the record without fragmenting them.
This procedure is a generalization of send-rpc-record.
Return a procedure that sends data according to the record marking standard, chopping its input bytevector into fragments of size fragment-len octets.
Return a binary input port that proxies port in order to implement decoding of the record marking standard (RFC 1831, Section 10).
Next: Compiler, Previous: Implementation of ONC RPC, Up: API Reference [Contents][Index]
GNU Guile-RPC provides client-side and/or server-side of some commonly found ONC RPC program, which are described below. Currently, this is limited to the portmapper interface, but other interfaces (e.g., “mount”, NFSv2) may follow.
| • The Portmapper Program: | A directory of instances of RPC programs |
Previous: Standard RPC Programs, Up: Standard RPC Programs [Contents][Index]
The (rpc rpc portmap) module implements the widespread
portmapper RPC program defined in RFC 1833 (see RFC
1833, Section 3). As the name suggests, the portmapper
interface allows servers to be queried for the association between an
RPC service and the port it is listening to. It also allows clients to
query the list of services registered.
In practice, most machines run a system-wide portmap daemon on
port 111 (TCP or UDP), and it is this server that is queried for
information about locally hosted RPC programs. The grpc-rpcinfo
program is a portmapper client that can be used to query a portmapper
server (see the grpc-rpcinfo tool)
Note that registering RPC programs with the portmapper is optional: it is basically a directory mechanism that allows servers to be located quite easily, but other existing mechanisms could be used for that purpose, e.g., decentralized service discovery (see service discovery with DNS-SD in Guile-Avahi in Using Avahi in Guile Scheme Programs).
The module exports client-side procedures, as returned by
make-synchronous-rpc-call (see Building an RPC Client), for
the various portmapper procedures. They are listed below.
Invoke the null RPC over port, ignoring arg, and
return %void.
Invoke the set RPC over port with argument arg. The
invoked server should register the RPC program specified by arg,
where arg must be an XDR struct (i.e., a Scheme list) containing
these four elements: the RPC program number, its version number, its
protocol and its port. The protocol number should be one of
IPPROTO_TCP or IPPROTO_UDP (see Network Sockets and
Communication in The GNU Guile Reference Manual). An XDR
boolean is returned, indicating whether the request successful.
Invoke the unset RPC over port with argument arg.
The invoked server should unregister the RPC program specified by
arg, where arg must have the same form as for
portmapper-set. Again, an XDR boolean is returned, indicating
whether the request was successful.
Invoke the get-port RPC over port with argument arg,
which must have the same form as previously mentioned (except that its
port number is ignored). The invoked server returns an unsigned integer
indicating the port of that RPC program.
Invoke the dump RPC over port, ignoring arg. The
invoked server should return a list of 4-element lists describing the
registered RPC programs. Those four element list are the same as for
portmapper-set and portmapper-get, namely the RPC program
number and version, its protocol and its port.
Invoke the call-it procedure over port. Quoting RFC 1833,
this procedure “allows a client to call another remote procedure on the
same machine without knowing the remote procedure’s port number”.
Concretely, it makes the portmapper invoke over UDP the procedure of the
program matching the description in arg, where arg is an XDR
struct (i.e., a Scheme list) containing an RPC program and version
number, a procedure number, and an opaque array denoting the procedure
arguments (an xdr-variable-length-opaque-array).
On success, it returns a struct consisting of the port number of the matching program and an opaque array representing the RPC reply. On failure, it does not return. Therefore, this synchronous call version may be inappropriate. We recommend that you do not use it.
The portmap module also provides convenience functions to
retrieve the symbolic name associated with common RPC program numbers.
The association between program numbers and their name is usually stored
in /etc/rpc on Unix systems and it can be parsed using the
read-rpc-service-list procedure.
Return a list of name-program pairs read from port (e.g., the /etc/rpc file), showing the connection between an RPC program human-readable name and its program number.
Lookup RPC program numbered program in service-list (a list as
returned by read-rpc-service-list) and return its human-readable
name.
Lookup RPC program named program in service-list (a list as
returned by read-rpc-service-list) and return its RPC program
number.
Previous: Standard RPC Programs, Up: API Reference [Contents][Index]
This section describes the compiler’s programming interface.
Most of the time, its command-line interface is all is needed; it is
described in grpc-compile. This
section is intended for users who need more flexibility than is provided
by grpc-compile.
The compiler consists of a lexer, a parser and two compiler
back-ends. The lexer separates input data into valid XDR/RPC language
tokens; the parser then validates the input syntax and produces an
abstract syntax tree of the input. Finally, the back-ends are
responsible for actually “compiling” the input into something usable
by the programmer. The back-end used by the grpc-compile
command is the code generation back-end. In addition, an
experimental run-time compiler back-end is available, making it
possible to compile dynamically definitions in the XDR/RPC language
at run-time; this technology paves the way for a wide range of
crazy distributed applications, the programmer’s imagination being the
only real limitation3.
| • Parser: | Parser API | |
| • Code Generation Compiler Back-End: | Code generation API | |
| • Run-Time Compiler Back-End: | The incredible run-time compiler |
Next: Code Generation Compiler Back-End, Previous: Compiler, Up: Compiler [Contents][Index]
The parser is available under the (rpc compiler parser) module.
The main procedure, rpc-language->sexp, reads XDR/RPC language
descriptions and returns the abstract syntax tree (AST) in the form of
an S-expression. The AST can be shown using the --intermediate
option of the grpc-compile command-line tool (see Invoking grpc-compile). Below is an illustration of the mapping between the
XDR/RPC language and the S-exp representation.
const SIZE = 10;
struct foo
{
int x;
enum { NO = 0, YES = 1 } y;
float z[SIZE];
};
... yields:
(define-constant "SIZE" 10)
(define-type
"foo"
(struct
("x" "int")
("y" (enum ("NO" 0) ("YES" 1)))
("z" (fixed-length-array "float" "SIZE"))))
Read a specification written in the XDR Language from port and
return the corresponding sexp-based representation. This procedure can
raise a &compiler-error exception (see below).
The behavior of the parser can be controlled using the *parser-options* parameter object:
This SRFI-39
parameter object must be a list of symbols or the empty list. Each
symbol describes an option. For instance, allow-unsigned
instructs the parser to recognize unsigned as if it were
unsigned int (see Sun XDR/RPC language
extensions).
Source location information is attached to the S-expressions returned by
rpc-language->sexp. It can be queried using the procedures
below. Note that not-only top-level S-expressions (such as
define-type or define-constant expressions) can be
queried, but also sub-expressions, e.g., the enum S-expression
above.
Return the source location associated with sexp or #f if no
source location information is available.
Return the line number, column number or file name from location
loc, an object returned by sexp-location.
In case of parse errors or other compiler errors, a
&compiler-error error condition (or an instance of a sub-type
thereof) may be raise.
The “compiler error” error condition type.
Return #t if c is a compiler error.
Return the source location information associated with c, or
#f if that information is not available.
Next: Run-Time Compiler Back-End, Previous: Parser, Up: Compiler [Contents][Index]
The code generation back-end is provided by the (rpc compiler)
module. Given an XDR/RPC description, it returns a list of
S-expressions, each of which is a top-level Scheme expression
implementing an element of the input description. These expressions are
meant to be dumped to a Scheme file; this is what the command-line
interface of the compiler does (see grpc-compile).
Here is an example XDR/RPC description and the resulting client code, as
obtained, e.g., with grpc-compile --xdr --constants --client:
const max = 010;
struct foo
{
int x;
float y<max>;
};
=>
(define max 8)
(define foo
(make-xdr-struct-type
(list xdr-integer
(make-xdr-vector-type xdr-float max))))
As can be seen here, the generated code uses the run-time support routines described earlier (see Implementation of XDR); an optimization would consist in generating specialized code that does not depend on the run-time support, but it is not implemented yet.
This front-end consists of two procedures:
These procedures return a list of top-level Scheme expressions implementing input for an RPC client or, respectively, a server.
input can be either an input port, a string, or an AST as returned
by rpc-language->sexp (see Parser). If type-defs? is
#t, then type definition code is produced; if
constant-defs? is #t, then constant definition code is
produced.
Both procedures can raise error conditions having a sub-type of
&compiler-error.
Previous: Code Generation Compiler Back-End, Up: Compiler [Contents][Index]
The run-time compiler back-end is also provided by the (rpc
compiler) module. It reads XDR/RPC definitions and returns data
structures readily usable to deal with the XDR data types or RPC
programs described, at run-time. Actually, as of version
0.4, it does not have an API to deal with RPC programs, only
with XDR data types.
Read XDR type definitions from input and return an alist; element
of the returned alist is a pair whose car is a string naming an
XDR data type and whose cdr is an XDR data type object
(see XDR Type Representations). input can be either an input
port, a string, or an AST as returned by rpc-language->sexp
(see Parser).
This procedure can raise error conditions having a sub-type of
&compiler-error.
Here is an example of two procedures that, given XDR type definitions, decode (respectively encode) an object of that type:
(use-modules (rpc compiler)
(rpc xdr)
(rnrs bytevectors)
(rnrs io ports))
(define (decode-data type-defs type-name port)
;; Read binary data from PORT as an object of type
;; TYPE-NAME whose definition is given in TYPE-DEFS.
(let* ((types (rpc-language->xdr-types type-defs))
(type (cdr (assoc type-name types))))
(xdr-decode type port)))
(define (encode-data type-defs type-name object)
;; Encode OBJECT as XDR data type named TYPE-NAME from
;; the XDR type definitions in TYPE-DEFS.
(let* ((types (rpc-language->xdr-types type-defs))
(type (cdr (assoc type-name types)))
(size (xdr-type-size type object))
(bv (make-bytevector size)))
(xdr-encode! bv 0 type object)
(open-bytevector-input-port bv)))
These procedures can then be used as follows:
(let ((type-defs (string-append "typedef hyper chbouib<>;"
"struct foo { "
" int x; float y; chbouib z;"
"};"))
(type-name "foo")
(object '(1 2.0 #(3 4 5))))
(equal? (decode-data type-defs type-name
(encode-data type-defs type-name
object))
object))
=>
#t
Note that in this example type-defs contains two type definitions, which is why the type-name argument is absolutely needed.
Next: References, Previous: API Reference, Up: Top [Contents][Index]
GNU Guile-RPC comes with stand-alone tools that can be used from the command-line.
| • Invoking grpc-compile: | Using the XDR/RPC compiler | |
| • Invoking grpc-rpcinfo: | Querying the portmapper | |
| • Invoking grpc-nfs-export: | Playing with the toy NFS server |
Next: Invoking grpc-rpcinfo, Previous: Stand-Alone Tools, Up: Stand-Alone Tools [Contents][Index]
grpc-compileThe grpc-compile command provides a simple command-line
interface to the XDR/RPC language compiler (see Compiler). It reads
a RPC definitions written in the XDR/RPC language on the standard input
and, depending on the options, write Scheme code containing client,
server, data type or constant definitions on the standard output.
Print a summary of the command-line options and exit.
Print the version number of GNU Guile-RPC and exit.
Compile XDR type definitions.
Compile XDR constant definitions.
Compile client RPC stubs.
Compile server RPC stubs.
Use strict XDR standard compliance per RFC 4506,
Section 6. By default, the compiler recognizes extensions implemented
by Sun Microsystems, Inc., and also available in the GNU C Library’s
rpcgen. These extensions include:
% line comments; these are actually treated
as special directives by rpcgen but they are simply ignored by
grpc-compile;
char type, equivalent to int;
unsigned type, equivalent to
unsigned int;
struct in type specifiers;
string as the type specifier of a
procedure parameter.
Also note that some XDR/RPC definition files (.x files)
originally designed to be used in C programs with rpcgen include
C preprocessor directives. Unlike rpcgen, which automatically
invokes cpp, such input files need to be piped through
cpp -P before being fed to grpc-compile.
Output the intermediate form produced by the parser (see Parser).
Code generation options can be combined. For instance, the command line below writes data type and constant definitions as well as client stubs in a single file:
$ grpc-compile --xdr --constants --client < input.x > client-stubs.scm
The various pieces of generated code can also be stored in separate files. The following example shows how to create one file containing constant and type definitions, another one containing client stubs, and a third one containing server stubs. Since the two last files depend on the first one, care must be taken to load them beforehand.
$ grpc-compile --xdr --constants < input.x > types+constants.scm $ echo '(load "types+constants.scm")' > client-stubs.scm $ grpc-compile --client < input.x >> client-stubs.scm $ echo '(load "types+constants.scm")' > server-stubs.scm $ grpc-compile --server < input.x >> server-stubs.scm
In the future, there may be additional --use-module and
--define-module options to make it easier to use Guile’s module
system in generated code.
Next: Invoking grpc-nfs-export, Previous: Invoking grpc-compile, Up: Stand-Alone Tools [Contents][Index]
grpc-rpcinfoThis program is equivalent to the rpcinfo program available on
most Unix systems and notably provided by the GNU C Library. In is a
client of the portmapper RPC program (see The Portmapper Program).
Among the options supported by rpcinfo, only a few of them are
supported at this moment:
Print a summary of the command-line options and exit.
Print the version number of GNU Guile-RPC and exit.
Query the portmapper and list the registered RPC services.
Unregister the RPC program with the given RPC program and version numbers from the portmapper.
Note that the host where the portmapper lives can be specified as the
last argument to grpc-rpcinfo:
# Query the portmapper at host `klimt'. $ grpc-rpcinfo -p klimt program vers proto port name 100000 2 tcp 111 portmapper 100000 2 udp 111 portmapper $ grpc-rpcinfo -d 100000 2 klimt ERROR: `portmapper-unset' failed FALSE
Previous: Invoking grpc-rpcinfo, Up: Stand-Alone Tools [Contents][Index]
grpc-nfs-exportGuile-RPC comes with an example NFS (Network File System) server,
provided by the grpc-nfs-export command. More precisely, it
implements NFS version 2, i.e., the NFS_PROGRAM RPC program
version 2 along with the MOUNTPROG program version 1, which are
closely related (see RFC 1094). It is a TCP server.
Enough technical details. The important thing about
grpc-nfs-export is this: although it’s of little use in one’s
everyday life, this NFS server is nothing less than life-changing. It’s
different from any file system you’ve seen before. It’s the ultimate
debugging aid for any good Guile hacker.
The “file hierarchy” served by grpc-nfs-export is—guess
what?—Guile’s module hierarchy! In other words, when mounting the
file system exported by grpc-nfs-export, the available files
are bindings, while directories represent modules
(see The Guile module system in The GNU Guile Reference
Manual). The module hierarchy can also be browsed from the REPL using
Guile’s nested-ref procedure. Here’s a sample session:
$ ./grpc-nfs-export & $ sudo mount -t nfs -o nfsvers=2,tcp,port=2049 localhost: /nfs/ $ ls /nfs/%app/modules/ guile/ guile-rpc/ guile-user/ ice-9/ r6rs/ rpc/ srfi/ $ ls /nfs/%app/modules/rpc/rpc/portmap/%module-public-interface/ lookup-rpc-service-name %portmapper-port lookup-rpc-service-number %portmapper-program-number portmapper-call-it portmapper-set portmapper-dump portmapper-unset portmapper-get-port %portmapper-version-number portmapper-null read-rpc-service-list $ cat /nfs/%app/modules/rpc/xdr/xdr-decode #<procedure xdr-decode (type port)> $ cat /nfs/%app/modules/rpc/xdr/%xdr-endianness big
Here is the option reference:
Print a summary of the command-line options and exit.
Print the version number of GNU Guile-RPC and exit.
Listen for NFS connections on port (default: 2049).
Listen for mount connections on mount-port (default: 6666).
Produce debugging messages.
In addition, grpc-nfs-export can be passed the name of a
Scheme source file, in which case it will load that file in a separate
thread while still serving NFS and mount requests. This allows
the program’s global variables to be monitored via the NFS mount.
As of version 0.4, this toy server exhibits poor
performance, notably when running ls (which translates into a
few readdir and many lookup RPCs, the latter being costly)
in directories containing a lot of files. This is probably partly due
to the use of TCP, and partly due to other inefficiencies that we hope
to fix soon.
Next: Portability, Previous: Stand-Alone Tools, Up: Top [Contents][Index]
RFC 1831R. Srinivasan et al., “RPC: Remote Procedure Call Protocol Specification Version 2”, August 1995.
RFC 4506M. Eisler et al., “XDR: External Data Representation Standard”, May 2006.
RFC 1833R. Srinivasan et al., “Binding Protocols for ONC RPC Version 2”, August 2005.
RFC 1094B. Nowicki, “NFS: Network File System Protocol Specification”, March 1989.
Next: GNU Free Documentation License, Previous: References, Up: Top [Contents][Index]
This appendix is about Guile-RPC’s portability. Of course, Guile-RPC can be ported to any OS/architecture Guile runs on. What this section deals with is portability among Scheme implementations.
Although implemented on top of GNU Guile, Guile-RPC uses mostly portable APIs such as SRFIs. Thus, it should be relatively easy to port to other Scheme implementations or to systems like Snow. Below are a few notes on portability, listing APIs and tools Guile-RPC depends on.
define-module and use-module clauses to some other
Scheme.
(rnrs bytevectors) and
(rnrs io ports) modules of Guile-R6RS-Libs (also included in
Guile 2.x). MzScheme, Larceny, Ikarus (among others) provide these
APIs.
(rpc xdr) module uses Guile’s
generalized vectors API (see Generalized Vectors in The GNU
Guile Reference Manual). This allows applications to use regular
vectors, SRFI-4 homogeneous vectors, arrays, etc., to represent XDR
variable-length arrays (see make-xdr-vector-type). On Scheme implementations that do not
support generalized vectors, regular vectors can be used instead.
(rpc compiler) module uses
Andrew K. Wright’s pattern matcher, known as (ice-9 match) in
Guile. This pattern matcher is portable and available in many Scheme
implementations; alternative, compatible pattern matchers are also
available sometimes, e.g., in MzScheme.
(rpc compiler parser) module uses
Dominique Boucher’s LALR parser
generator, known as (system base lalr) in
Guile 2.0. This package is
available on most Scheme implementations and as a “snowball”.
(rpc compiler lexer) module was automatically
generated using Danny Dubé’s
SILex, a portable lexer generator.
(rpc compiler parser) uses Guile’s “source
property” API, which allows such information to be attached to pairs
(see Source Properties in The GNU Guile Reference Manual).
Behind the scenes, source properties are implemented using a weak hash
table, so it should be easy to port them to other Scheme
implementations.
(rpc rpc server), which contains a server event loop. This part
of the module would need porting to the target system, but it would be
quite easy to isolate the few features it depends on.
Portability patches can be posted to the Guile-RPC mailing list where they will be warmly welcomed!
Next: Concept Index, Previous: Portability, Up: Top [Contents][Index]
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Guile
1.8 provides an implementation of the former in the (srfi
srfi-34) module, while the latter is currently provided by the
guile-lib package.
This idea was inspired by Oleg Kiselyov’s description of thunked parent pointers in SXML, which may be found at http://okmij.org/ftp/Scheme/parent-pointers.txt.
Finding useful applications leveraging the flexibility offered by the run-time compiler back-end is left as an exercise to the reader.