481 lines
16 KiB
C++
481 lines
16 KiB
C++
/*
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ADVICE.hpp - generic loosely coupled interaction guided by symbolic pattern
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Copyright (C) Lumiera.org
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2010, Hermann Vosseler <Ichthyostega@web.de>
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This program is free software; you can redistribute it and/or
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modify it under the terms of the GNU General Public License as
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published by the Free Software Foundation; either version 2 of the
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License, or (at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program; if not, write to the Free Software
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Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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/** @file advice.hpp
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** Expecting Advice and giving Advice: a cross-cutting collaboration of loosely coupled participants.
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** This header exposes the basics of the advice system and the public access points. The advice system
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** is a system wide singleton service, but clients never talk directly to this singleton; rather they
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** use advice::Provision and advice::Request as access point.
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**
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** \par Advice collaboration pattern
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** Advice collaboration is a special pattern of interaction extracted from multiple use cases within
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** Lumiera. Creating this abstraction was partially inspired by aspect oriented programming, especially
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** the idea of cross-cutting the primary dependency hierarchy. Another source of inspiration where the
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** various incarnations of properties with dynamic binding. For defining the actual binding, we rely
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** on predicate notation and matching (planned: unification) as known from rule based systems.
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**
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** <b>Definition</b>: Advice is an optional, mediated collaboration between entities taking on
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** the roles of advisor and advised, thereby passing a custom piece of advice data, managed by
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** the advice support system. The possibility of advice is created by both of the collaborators
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** entering the system, where the advised entity exposes a point of advice, while the advising
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** entity provides an actual advice value.
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**
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** \par Collaborators
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** - the advised entity
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** - the advisor
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** - point of advice
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** - advice system
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** - the binding
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** - the advice
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**
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** Usually, the \em advised entity opens the collaboration by requesting an advice. The \em advice itself
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** is a piece of data of a custom type, which needs to be copyable. Obviously, both the advised and the
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** advisor need to share knowledge about the meaning of this advice data. The actual advice collaboration
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** happens at a \em point-of-advice, which needs to be derived first. To this end, the advised puts up an
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** \em request by providing his \em binding, which is a pattern for matching. An entity about to give advice
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** opens possible advice \em channels by putting up an advisor binding, which similarly is a pattern. The
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** advice \em system as mediator resolves both sides, by matching (which in the most general case could be
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** an unification). This process creates an advice point \em solution -- allowing the advisor to fed the
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** piece of advice into the advice channel, causing it to be placed into the point of advice. After passing
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** a certain (implementation defined) break point, the advice leaves the influence of the advisor and gets
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** exposed to the advised entities. Especially this involves copying the advice data into a location managed
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** by the advice system. In the standard case, the advised entity accesses the advice synchronously and
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** non-blocking. Typically, the advice data type is default constructible and thus there is always a basic
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** form of advice available, thereby completely decoupling the advised entity from the timings related
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** to this collaboration.
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**
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** \par interfaces and implementation
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** Client code is assumed to interface solely through the advice::Request and advice::Provision classes,
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** which both can be instantiated and copied freely, may be used as member or mixed in as baseclass.
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** The AdviceSystem on the other hand is an implementation facility (actually a singleton) and lives
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** in the advice.cpp translation unit. The interface entities inherit protected from AdviceLink,
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** which is implemented in the same scope as the AdviceSystem and thus allowed to talk to it
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** directly. The AdviceSystem in turn uses advice::Index to keep track of the collaboration
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** partners, which, for this purpose, are handled as type-erased PointOfAdvice elements.
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** The latter class contains 4 API functions used by the index to manage solutions.
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**
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** @note as of 6/2010 this is an experimental setup and implemented just enough to work out
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** the interfaces and gain practical usage experiences. Ichthyo expects this collaboration
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** service to play a central role at various places within proc-layer.
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** @todo allow variables in binding patterns
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** @todo use the lumiera MPool instead of heap allocations
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** @todo consider to provide variations of the basic behaviour by policy classes
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** @todo the implementation is generic/defensive, and could be improved and optimised
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**
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** @see AdviceBasics_test usage example
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** @see advice.cpp implementation
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** @see advice::Index index datastructure
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** @see binding.hpp
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**
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*/
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#ifndef LIB_ADVICE_H
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#define LIB_ADVICE_H
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#include "lib/error.hpp"
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#include "lib/null-value.hpp"
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#include "lib/advice/binding.hpp"
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#include "lib/symbol.hpp"
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#include "lib/util.hpp"
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#include <boost/noncopyable.hpp>
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namespace lib {
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namespace advice {
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using lib::Symbol;
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using util::isSameObject;
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/**
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* Basic (abstracted) view of an advice collaboration partner,
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* as used internally by the AdviceSystem to manage the participants.
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* Each PointOfAdvice is characterised by a binding pattern, used to
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* pair up advice::Request and advice::Provision entries. Moreover,
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* each PointOfAdvice can refer to an existing advice solution
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* provided elsewhere in the system. The specific type of advice
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* (and thus the storage requirements) are abstracted away,
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* as is the distinction between Request and Provision.
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*
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* @see AdviceSystem
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* @see AdviceIndex_test
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*/
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class PointOfAdvice
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{
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Binding::Matcher pattern_;
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PointOfAdvice* resolution_;
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protected:
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/** define or re-define the binding, which
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* specifically labels this attachment to the advice system.
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* @note issuing this on an existing connection is equivalent
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* to re-connecting with the new binding.
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*/
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void setBindingPattern (Binding const& binding)
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{
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pattern_ = binding.buildMatcher();
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}
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public:
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explicit
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PointOfAdvice (Binding const& binding)
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: pattern_(binding.buildMatcher())
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, resolution_(0)
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{ }
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// using default copy/assignment
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/* == Adapter interface for use within the Index == */
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void
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setSolution (PointOfAdvice* solution =0)
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{
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resolution_ = solution;
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}
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const PointOfAdvice*
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getSolution () const
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{
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return resolution_;
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}
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Binding::Matcher
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getMatcher () const
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{
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return pattern_;
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}
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friend HashVal
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hash_value (PointOfAdvice const& entry)
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{
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return hash_value (entry.pattern_);
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}
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};
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/**
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* Advice Collaboration partner, internally connected to the AdviceSystem.
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* Both advice::Request and advice::Provision are linked in this way
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* to an internal index datastructure, which allows to carry out
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* the actual advice exchange and collaboration.
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*/
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class AdviceLink
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: public PointOfAdvice
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{
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protected:
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void publishProvision (PointOfAdvice*);
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void discardSolutions ();
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void publishRequestBindingChange(HashVal);
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void registerRequest();
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void deregisterRequest();
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static void* getBuffer(size_t);
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static void releaseBuffer (void*, size_t);
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typedef void (DeleterFunc)(void*);
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static void manageAdviceData (PointOfAdvice*, DeleterFunc*);
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public:
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explicit
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AdviceLink (Binding const& binding)
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: PointOfAdvice(binding)
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{ }
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// using default copy/assignment
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};
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/**
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* Access point for the advising entity (server).
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* This is the interface intended for client code to set and provide
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* concrete advice information of a specific type AD. Instantiating
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* automatically creates a \em type-guard binding pattern, but client code
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* can (and typically should) provide additional predicates to define the
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* "topic" this advice belongs to. This allows advice::Request entries
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* to attach to the suitable advice "channels" and get the specific
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* piece of advice they're looking for.
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*
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* Any advice::Provision remains inactive and thus invisible, until
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* \link #setAdvice setting the concrete advice data \endlink. After that,
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* the provided data is \em copied into the AdviceSystem and remains available
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* even after the original Provision goes out of scope. Consequently, it isn't
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* possible to \em modify advice data once set. But client code may retract
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* the provision or change the binding pattern.
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*
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* @see AdviceBasics_test usage example
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*/
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template<class AD>
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class Provision
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: protected AdviceLink
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{
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/* == policy definitions == */ ////TODO: extract into policy classes
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void deregistrate() { /* NOP */ }
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public:
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explicit
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Provision (Literal bindingSpec =0)
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: AdviceLink (Binding(bindingSpec).addTypeGuard<AD>())
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{ }
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~Provision()
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{
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this->deregistrate();
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}
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Provision (Provision const& o)
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: AdviceLink(o)
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{
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setSolution ( NULL );
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}
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Provision&
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operator= (Provision const& o)
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{
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if (!isSameObject(*this, o))
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{
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AdviceLink::operator= (o);
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setSolution ( NULL );
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}
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}
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void setAdvice (AD const& pieceOfAdvice)
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{
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publishProvision(
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storeCopy (pieceOfAdvice));
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}
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void retractAdvice()
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{
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discardSolutions();
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}
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void
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defineBinding (Literal topic)
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{
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setBindingPattern (Binding(topic).addTypeGuard<AD>());
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maybe_rePublish();
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}
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private:
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PointOfAdvice* storeCopy (AD const& advice_given);
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static void releaseAdviceData (void*);
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void maybe_rePublish ();
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};
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/**
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* Piece of Advice as incorporated into the AdviceSystem.
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* This holder-object contains a copy of the advice data
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* and is placed into the internal storage buffer; the
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* advice index keeps a (type erased) pointer to serve
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* any requests which happen to match the binding.
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*
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* @note the ptr-to-solution in the inherited PointOfAdvice
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* is currently (5/10) not used, because this \em is
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* already the solution.
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*/
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template<class AD>
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class ActiveProvision
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: public PointOfAdvice
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, boost::noncopyable
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{
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AD theAdvice_;
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public:
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AD const&
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getAdvice() const
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{
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return theAdvice_;
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}
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protected:
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ActiveProvision (PointOfAdvice const& refPoint, AD const& advice_given)
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: PointOfAdvice(refPoint)
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, theAdvice_(advice_given)
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{
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this->setSolution (this); // not used currently (5/10)
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}
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friend class Provision<AD>;
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};
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/* ==== memory management for Provision data ===== */
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/** @internal function to copy advice into an internal buffer.
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@return type erased pointer to the data holder created
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@throw error::Fatal on allocation problems, plus anything
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the advice data may throw during copy construction. */
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template<class AD>
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inline PointOfAdvice*
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Provision<AD>::storeCopy (AD const& advice_given)
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{
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typedef ActiveProvision<AD> Holder;
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void* storage = getBuffer(sizeof(Holder));
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try
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{
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Holder* storedProvision = new(storage) Holder (*this, advice_given);
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manageAdviceData (storedProvision, &releaseAdviceData);
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return storedProvision;
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}
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catch(...)
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{
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Symbol errID = lumiera_error();
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releaseBuffer (storage, sizeof(Holder));
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throw lumiera::error::Fatal ("Failure to store advice data", errID);
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}
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}
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/** @internal assist the AdviceSystem with deallocating buffer storage.
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* Problem is we need to know the exact size of the advice value holder,
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* which information is available only here, in the fully typed context.
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*/
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template<class AD>
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inline void
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Provision<AD>::releaseAdviceData (void* entry)
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{
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typedef ActiveProvision<AD> Holder;
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if (entry)
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{
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Holder* obj = static_cast<Holder*> (entry);
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obj->~Holder();
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releaseBuffer (entry, sizeof(Holder));
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}
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}
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/** @internal in case we've already published this provision,
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* we temporarily need a new provision entry, to allow the
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* AdviceSystem implementation to rewrite the internal index
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*/
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template<class AD>
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inline void
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Provision<AD>::maybe_rePublish ()
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{
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typedef const ActiveProvision<AD> AdviceProvision;
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AdviceProvision* solution = static_cast<AdviceProvision*> (getSolution());
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if (solution) // create copy of the data holder, using the new binding
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publishProvision(
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storeCopy (solution->getAdvice()));
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}
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/**
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* Access point for the advised entity (client).
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* This is the interface intended for client code to request advice
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* of a specific type and optionally limited to a special topic (binding).
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* Instantiating an \c Request<AD> object automatically entails a registration
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* with the AdviceSystem behind the scenes, and deleting it causes deregistration.
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* Request objects may be instantiated and copied freely, and the binding pattern
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* may be changed. The actual advice is accessed through the member function
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* #getAdvice, which might return a default-constructed piece of advice data
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* in case no specific advice has been provided yet. Accessing advice can be
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* considered a lightweight operation, while creating/destroying advice
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* causes an index operation and thus requires a lock.
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*
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* Creating an Request and changing the binding might fail, while the dtor
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* is protected against failure (as you'd expect). Accessing advice can also
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* be considered safe, given the concrete advice type can be default constructed
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* without failure in case there isn't any advice data provided yet.
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*/
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template<class AD>
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class Request
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: protected AdviceLink
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{
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typedef const ActiveProvision<AD> AdviceProvision;
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/* == policy definitions == */ ////TODO: extract into policy classes
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AD const&
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handleMissingSolution() const ///< @warning might segfault when used during shutdown
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{
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return NullValue<AD>::get();
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}
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public:
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explicit
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Request (Literal bindingSpec =0)
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: AdviceLink (Binding(bindingSpec).addTypeGuard<AD>())
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{
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registerRequest();
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}
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~Request()
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{
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deregisterRequest();
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}
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// copying Requests is allowed, using default
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AD const&
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getAdvice() const
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{
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AdviceProvision* solution = static_cast<AdviceProvision*> (this->getSolution());
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if (!solution)
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return this->handleMissingSolution();
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else
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return solution->getAdvice();
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}
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void
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defineBinding (Literal topic)
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{
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HashVal previous_bindingKey (hash_value(*this));
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setBindingPattern (Binding(topic).addTypeGuard<AD>());
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publishRequestBindingChange (previous_bindingKey);
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}
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};
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}} // namespace lib::advice
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#endif
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