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Re: moving along on rules language
2003-09-02 08:44:40
Alex - I'd suggest you publish this as individual ID right away rather
than just having a URL as reference. Would give us all a formal
reference for the document.
All - please have a look at Alex proposal and at the IRML draft (Andre
is working on an updated version addressing Alex comments, but he's on
vacation this week, and the general approach behind IRML should be
clear from the current draft-beck-opes-irml-03.txt).
We need to decide on how to move forward and whether to adopt one of
the documents as WG draft. Please comment on the list.
Thanks,
Markus
Alex Rousskov wrote:
On Fri, 22 Aug 2003, Marshall Rose wrote:
great. if your confident with an alternative approach, put an
initial document together and submit it. i don't want to forestall
alternatives, but we need to be wrapping things up.
An alternative draft is attached in plain text. HTML rendering is at
http://www.measurement-factory.com/tmp/opes/snapshots/current/p.html
As we discussed, this draft is not as detailed or as polished as the
IRML draft. The draft purpose is to illustrate the "killer idea"
behind the alternative approach: a minimalistic and efficient
configuration language designed specifically for the problem domain,
but with a room to grow. See draft introduction for more detailed
treatment of design goals.
I hope this draft is sufficient to pick one direction over the other,
but I would be happy to clarify or add as needed, of course. I do
realize that there has been a lot more work put into IRML already. I
wish I knew about its existence earlier. If nothing else, this draft
is probably the ultimate form of "constructive comments" IRML folks
are asking for :-).
On the positive side, many IRML ideas/developments can be easily
reused if an alternative approach is chosen. The reverse is probably
not as true because P capitalizes on being a language rather than a
hierarchical collection of attributes (but it is all relative and
equivalent from Turing point of view, of course).
HTH,
Alex.
P.S. If "P" is already taken as a language name, please let me know.
My quick search yielded no matches. "PL" does not count, IMHO.
------------------------------------------------------------------------
Open Pluggable Edge Services A. Rousskov
Internet-Draft The Measurement Factory
Expires: February 29, 2004 August 31, 2003
P: Message Processing Language
draft-rousskov-opes-rules
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
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The list of current Internet-Drafts can be accessed at http://
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The list of Internet-Draft Shadow Directories can be accessed at
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This Internet-Draft will expire on February 29, 2004.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
P is a simple configuration language designed for efficient and
compact specification of message processing instructions at
application proxies. P can be used to instruct an intermediary how to
manipulate the application message being proxied. Such instructions
needed in an Open Pluggable Edge Services (OPES) context.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Language elements . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2 Statements . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.3 Expressions . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.4 Assignments . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. OPES Services . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 11
7. Compliance . . . . . . . . . . . . . . . . . . . . . . . . . . 12
A. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
B. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Normative References . . . . . . . . . . . . . . . . . . . . . 15
Informative References . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . 17
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1. Introduction
The Open Pluggable Edge Services (OPES) architecture
[I-D.ietf-opes-architecture], enables cooperative application
services (OPES services) between a data provider, a data consumer,
and zero or more OPES processors. The application services under
consideration analyze and possibly transform application-level
messages exchanged between the data provider and the data consumer.
OPES processors need to be told what services are to be applied to
what application messages. P language can be used for this
configuration task.
In other words, P language primary objective is to express statements
similar to:
if message meets criteria C,
then apply service S;
Figure 1
Thus, P programs mostly deal with how formulating message-dependent
conditions and executing services.
P design attempts to satisfy several conflicting goals:
flexibility: OPES intermediaries deal with a wide range of
applications and protocols (SMTP, HTTP, RTSP, IM, etc.). The
language must be able to accommodate virtually all known tasks in
selecting a desired adaptation service for a message of a known
application protocol (and conceivable future applications).
efficiency: Language interpretation must be efficient enough to be
comparable with other message processing overheads at a typical
application proxy (e.g., interpreting HTTP headers to determine
response cachability).
simplicity: Typical configurations must be easy to write and
understand for a typical OPES system administrator.
correctness: Many message handling configurations are written without
direct access to intermediaries that will use those
configurations. The extent of off-line (compile-time) correctness
checks should catch all syntax errors and many common semantic
errors such as undefined values and type conflicts.
compactness: It is possible that some processing instructions will be
piggybacked as headers/metadata to messages they refer to, placing
stringent size requirements on language code.
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security: It should be difficult if not impossible to write malicious
code that would result in security vulnerability of compliant
language interpreter.
P design is based on a minimal useful subset of features from several
programming languages such as R (S) and Smalltalk. Technically
speaking, P is a single-assignment, lazy evaluation, strongly typed
functional programming language.
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2. Syntax
P syntax is defined by the following Augmented Backus-Naur Form
(ABNF) [RFC2234]:
code = *(statement ";")
statement = assignment / function-call / if-statement
assignment = identifier ":=" expression
if-statement = "if" "(" expression ")" "{" code "}"
expression =
name / function-call / "{" code "}"
... ; more to be defined (logical and arithmetic expressions)
name = identifier *( "." identifier)
function-call = name "(" [params] ")"
params = expression *( "," expression)
identifier = ALPHA *(ALPHA / DIGIT / "_")
... ; more primitives to be defined as needed
Figure 2
XXX: add /* comments */ and // comments.
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3. Language elements
3.1 Objects
P is centered around the concept of an "object" that is similar to
objects from other object-oriented languages. An object is a
collection of object members: attributes and methods. Attributes are
named slots that store other objects. Methods are named pieces of
code that manipulate the object they belong to or other objects. P
objects are identified by their names (e.g., Http or tmp). Attributes
and methods are accessed via their names using the dot (".") operator
applied to a named object. For example, Http.message.headers
expression accesses (names) headers inside a message inside the HTTP
module. Everything that can be named in P is an object. Some objects
may not have any members.
P does not have facilities for describing objects. When writing a P
program, only objects supported by the interpretor can be used and no
new objects can be added. P supports loadable modules that can be
used to add objects to support new application protocols. In fact, P
core supports no application protocols directly. Instead, modules
like "Http" can be used to process messages depending on application
protocol being proxied.
All P objects have types and no default (silent) type conversion is
supported. However, explicit type conversion (casting) is rarely
needed because many object methods are polymorphic (accept several
types).
service := Services.findOne("http://iana.org/opes/services/example";);
Figure 3
3.2 Statements
Objects are manipulated using if-statements and function-calls.
if (Http.request.method == "GET") {
Core.execService(service);
}
Figure 4
3.3 Expressions
P expressions are used in if-statements to specify the condition for
the if-statement body to be interpreted.
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if (Http.request.method == "GET" and time.current() > time.noon) {
...
}
Figure 5
Binary operators such as "==" or "+" are not global special symbols
but are passed to the object on the left for interpretation, along
with the expression on the right. Applying a binary operator is
semantically equivalent to calling an object method. For example, the
following two expressions are equivalent:
a + b + c
(a.+(b)) + c
(a.+(b)).+(c)
Figure 6
The "a + b + c" form is preferred for purely visual reasons. Core P
module provides basic objects and operators for them (e.g., boolean
and integer). Application-specific modules usually provide
applications-specific objects; those objects usually have
application-specific methods and may not have methods to support
operations common for basic types. For example, an Http module
supplies an HTTP header object that does not have a "*" method.
XXX: define operator precedence, if any.
3.4 Assignments
Most procedural programming languages use variables to store
intermediate processing results. In such languages, a variable is
essentially a named piece of memory that can be assigned a value and
can be updated with new values as needed. P does not have such
variables. Instead, P uses a "single assignment" approach: an
expression can be tagged with a name and that name can be reused many
times in the program. On the surface, this is equivalent to having
all "traditional" variables declared as "constant". The following two
if-statements are semantically equivalent in P:
if (Http.request.headers.have(Http.makeHeader("Client-IP"))) {...}
h := Http.makeHeader("Client-IP");
hs := Http.request.headers();
if (hs.have(h) {...}
Figure 7
If the expression changes, a new name must be used to tag the new
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expression. After an assignment statement, the value of the name is
not the value of the expression, but the expression itself. Thus,
the following two code fragments are equivalent and make no sense in
P (the first fragment would make sense in languages such as C++):
h := Http.makeHeader("Client-IP");
h := Http.makeHeader("Server-IP");
h := Http.makeHeader("Client-IP");
Http.makeHeader("Client-IP") := Http.makeHeader("Server-IP");
Figure 8
The interpreter can but does not have to evaluate the expression
named in the assignment statement until the name is actually used in
an expression that requires evaluation (e.g., as a parameter of a
function call statement). This allows for optional performance
optimizations where only used expressions are evaluated.
P does not have user-defined functions. However, some code reuse is
possible because P code is a valid expression and, hence, can be
named and reused:
code := { ... complicated service action ... };
if (condition1) { code; };
...
if (condition2) { code; };
Figure 9
XXX: document whether expression has to be evaluated in the
assignment context or use context. Document name scope.
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4. Modules
Application-specific support is available in P via modules. Basic P
primitives such as integer types and boolean operations comprise the
Core module. Module is an object. The Core modules supplies the
following methods to manipulate other modules:
Core.import("M"): load a module called "M" and return it as the
result.
Core.lookup(M): start looking up unresolved attributes and method
identifiers in a previously loaded module M.
The Core module is assumed to be loaded (and being looked up) before
the interpretation starts.
XXX: document lookup conflict resolution.
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5. OPES Services
Services module contains basic attributes and methods for searching
and executing OPES services:
Services.findOne(URI): returns a service object that corresponds to
the specified URI. Fails if no corresponding object exists.
Services.applyOne(service, ...): applies the specified service to the
current application message and optionally supplies
service-specific application parameters.
Here is a service application example for a German to French
translation service:
Http := import("Http");
if (Http.response.language_is("german")) {
service := Services.find("opes://services/tran/german/french");
service.toDialect("southern");
Services.apply(service, Http.request.headers);
}
Figure 10
XXX: explain how failures are propagated and can be handled
XXX: add Core.interpreter.stop and Core.interpreter.restart methods.
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6. Security Considerations
XXX: document non-obvious vulnerabilities: too many names, too deep
nesting, invalid math, too much error logging; execution of
unauthorized services, unauthorized exposure of sensitive information
to authorized services.
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7. Compliance
XXX: define what a compliant interpreter is.
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Appendix A. Examples
This appendix contains half-baked examples to illustrate P usage in
common OPES environments. Example themes are taken from
[I-D.beck-opes-irml] to ease the comparison with IRML.
Here is a data provider example:
interpreter.languageVersion("1.0"); // fails if incompatible
Http := import("Http");
lookup(Http);
// Is the requested web document our home page?
isHome := request.uri.looksLikeHome();
// Does the user send us a specific cookie?
cookie := makeHeader("Cookie", "sew=23");
haveCookie := request.headers.have(cookie);
if (isHome and haveCookie) {
Services := import("Services");
service := Services.findOne("opes://local.net/add-lcl-content");
service.clientIp(request.clientIp);
Services.apply(service);
}
Figure 11
Here is a data consumer example:
Services := import("Services");
service := Services.findOne("opes://privacy.net/priv-serv");
service.action("remove-referer");
Services.apply(service);
Figure 12
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Appendix B. Change Log
Internal WG revision control ID: $Id: iab-cons.xml,v 1.19 2003/08/28
03:48:32 rousskov Exp $
Initial revision.
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Normative References
[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[I-D.ietf-opes-end-comm]
Barbir, A., "OPES processor and end points
communications", draft-ietf-opes-end-comm-00 (work in
progress), June 2003.
[I-D.ietf-opes-architecture]
Barbir, A., "An Architecture for Open Pluggable Edge
Services (OPES)", draft-ietf-opes-architecture-04 (work in
progress), December 2002.
[I-D.ietf-opes-scenarios]
Barbir, A., "OPES Use Cases and Deployment Scenarios",
draft-ietf-opes-scenarios-01 (work in progress), August
2002.
[RFC3238] Floyd, S. and L. Daigle, "IAB Architectural and Policy
Considerations for Open Pluggable Edge Services", RFC
3238, January 2002.
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Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[I-D.beck-opes-irml]
Beck, A. and M. Hofmann, "IRML: A Rule Specification
Language for Intermediary Services",
draft-beck-opes-irml-03 (work in progress), June 2003.
Author's Address
Alex Rousskov
The Measurement Factory
EMail: rousskov(_at_)measurement-factory(_dot_)com
URI: http://www.measurement-factory.com/
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