On the Specification of a Component Repository

On the Speci?cation of a Component Repository

Nuno Rodrigues1and Luis Barbosa2

1Sidereus,S.A.,Porto,Portugal

nfr@sidereus.pt

https://www.wendangku.net/doc/8d2855081.html,rm′a tica,Univ.Minho,Braga,Portugal

lsb@di.uminho.pt

Abstract

The lack of a commonly accepted de?nition of a software component,the proliferation of competing‘standards’and component frameworks,is here to stay,raising the fundamental

question in component-based development of how to cope in practice with heterogeneity.

This paper reports on the design of a Component Repository aimed to give at least a partial

answer to the above question.The repository was fully speci?ed in Vdm and a working

prototype is currently being used in an industrial environment.

1Introduction

”I’m sure this problem has already been solved”and”probably,others would like to(re)use my solution”,are common concerns in the everyday life of a programmer.Concerns that the emerg-ing of component-based programming paradigm aims to transform into e?ective programming practises.Actually,all engineering disciplines rely on standard components to design and build their artifacts and development methodologies based on third-party assembly of components—software engineering should not be the exception.

Although the term software component has been around for a long time,component-based pro-gramming has become a buzzword since mid1990’s(see,e.g.,[10,15,14,9,17]).The basic motivation is to replace conventional programming by system’s construction by composition and con?guration of reusable o?-the-shelf units,often regarded as‘abstractions with plugs’.Or, quoting from[15],‘independently deployable unit of composition with contractually de?ned in-terfaces’.The paradigm is often illustrated by the visual metaphor of a palette of computational units,treated as black boxes,and a canvas into which they can be dropped and plugged together through wires,corresponding to some sort of gluing code.

However,as it happened before with object orientation,component programming has grown up to popular technologies before consensual de?nitions and principles,let alone formal foundations,

47

have been put forward.In particular,it has given rise to a number of increasingly popular technologies designed around speci?c interaction paradigms—e.g.,Corba and JavaBeans based on object-like composition,JavaSpaces and TSpaces on top of shared data space models, typical of classical coordination languages,and channel based techniques inherited from process languages,as in IWIM/Manifold or Drawin.Such technologies also di?er at the application level—e.g.acting as source-level languages extensions(such as Corba or JavaSpaces),or as connector frameworks for dynamic(binary-level)units(as in e.g.,Com https://www.wendangku.net/doc/8d2855081.html,).

From an engineering point of view a key issue to the success of the component paradigm is integration.In fact,unless one restricts himself to a particular framework or,even worse,to a speci?c tool within a framework,it is no easy task to identify,select,re-use and compose heterogeneous software components from a virtual‘global market’(as represented,for example, by the Internet).The lack of a commonly accepted de?nition of‘component’,the proliferation of competing‘standards’and component frameworks,is here to stay,raising the question of how to cope in practice with heterogeneity.

This paper reports on a concrete approach to a component integration problem found in the context of an european-wide industrial software development project(the Ikf Eureka E!2235 consortium).The challenge was to build a component repository(of both source code and interface information)able to register and marketing all the components produced by di?erent teams in the project.The repository was also supposed to act as an exchange market:each user being able not only to register its own components,but also to announce its plans to issue speci?c components and even to publicise its needs and associated requirements in the form of collections of APIs.Although such component marketplaces are emerging in the Web,the underlying description techniques remain rather informal,mainly textual,plus an additional, limited,classi?cation in terms the intended business areas.

In a sense API,the acronym for Application Program Interface,emerged as a keyword.But what is in an API,when the component technology used ranges from Java and Corba code to Haskell or Wsdl?Coming back to Szyperski’s de?nition above,what seems crucial to handle is the de?nition of what an interface is.

Initial prospects of implementing the repository as an API database and resorting to text-based retrieve methods for querying,soon reveal impractical given the heterogeneity of the APIs supplied by the di?erent project teams.As the complexity of the individual components and the size of the repository increases,higher levels of abstraction are required,and,for all practical purposes,going abstract means going formal.The Vdm meta-language[5,4]was chosen as the common language to which di?erent sorts of APIs were mapped.A complete,working prototype of the repository,named CompSrc to express the idea of a‘component source indexing space’, has been developed in the Vdm ToolBox complemented with a dedicated web interface for easier access.

The paper is organised as follows:the CompSrc architecture and the generic component model underlying the whole system are described in the following two sections.Section4discusses some of the composition patterns automatically detected by CompSrc;an example is brie?y

discussed in section5.We conclude with some prospect of future work in section6.

2The CompSrc Architecture

The CompSrc repository is built around a broad characterisation of what an interface to com-ponent source code is,speci?ed in the Vdm meta-language in the early project stages.This is called,in the context of this system,the abstract API.The component source code database is indexed by such abstract APIs which are‘extracted’from the actual component APIs.This is done by extractor functions speci?ed for the typical target languages used in the project.Such extractors correspond to the incoming arrows in Figure1.

In the centre of the structure diagram is the interface repository indexing the actual component source data base.The repository supports the usual management operations—e.g.,registering, removing,updating,etc.The most relevant feature,however,is the support of a component calculator which makes it possible to compute new services based on the automatic composition of old functionality,according to more or less strict interaction patterns,and,then,to build new APIs and new components which would act as if the actual source code has been merged.The interaction patterns available are di?erent kinds of pipelining(linear,?attened or monadic,as described below)as well as multiplicative and additive aggregation,corresponding to component parallel composition and choice,respectively.The underlying component calculus is described in detail in[2,1].Once a new component is built,by the calculator,the new abstract API is registered in the system and the component‘combined’code is supplied,in the form of a conditional compiling script in https://www.wendangku.net/doc/8d2855081.html, notation.This corresponds to the outgoing arrows in the diagram.

3Abstract Interfaces

The‘quest’for a common abstract API format started from a reverse speci?cation of the(static information part of)di?erent sorts of component APIs used in the project.These ranged from the‘object-oriented?avour’,emphasising hierarchical class structures,to‘functional modules’described by service signatures and,eventually,datatype constraints.The formal analysis of typical API examples in Java,Corba,Microsoft Visual Basic,Haskell and Wsdl,which is documented in[11],lead to the description of a component as an indexed collection of services (referred to as modules in the sequel)and mirroring the overall component architecture described in Figure2.

Each module is,then,de?ned as an ordered tuple of information including the following aspects:

1.Desc-a text description of the current module

Figure1:Component Space Architecture

2.INH-a set of imported modules

3.Ex-a map enumerating examples concerning the module usage

4.Func-the set of functions(?ne-grain services)de?ned in the module

5.State-the implementation state of the module which is one of:

?Implemented-the module is implemented and ready to use

?In Implementation-the module is currently in implementation process

?Requested-someone is interested in this module and is willing to pay for it

?To Convert-the module is speci?ed in a language but needs to be implemented in

a di?erent platform

?Generated-generated by the‘component calculator’in CompSrc

6.Platform-the target implementation platform

7.StatDep-the static dependencies set,i.e.,environment variables,runtime environments,

...

8.Protocols-a brief description of the communication protocols that the module imple-

ments

9.SpecAPI-speci?c(platform-dependent)module data for each kind of API,basically

distinguishing purely functional interfaces from state-based ones(as found in object-orientation)

Figure2:Structure of a Component

Figure3:Component DataType

4Component Assembly

For each API submitted to CompSrc a corresponding abstract API(i.e.,a value of the respec-tive Vdm datatype)is built by the(language-dependent)interface extractors mentioned above. Such abstract APIs provide the basis for classifying,locating and retrieving components from the underlying repository.Furthermore they become the‘raw material’used by the component calculator to generate new components.

Such generation is done in two di?erent,but complementary,ways:aggregation and wiring. Component aggregation is achieved by the application of a small set of operators acting on the abstract interfaces as a whole1.In particular,they model:

?Interface restriction to a set of modules or even,within a particular module,to a set of 1see[2]for an overview of the corresponding calculus partially implemented in CompSrc.

Figure4:Module DataType

services.

?Additive aggregation,in which di?erent modules coming from two di?erent interfaces are selected and packaged into a new one.If both interfaces are indexing actual code written in compatible implementation languages(i.e.,related by an embedding)the code generation process is activated.

?Multiplicative aggregation,corresponding to the synchronous execution of modules in two di?erent components.

The wiring process,on the other hand,is based on the search for composition possibilities among the collections of functions of a speci?ed set of components.Such search can be systematic, exposing all possible connections arising from a given set of components,or user-oriented in which each possible composition is validated or discarded by the user.In any case the problem is to identify pairs of functions,in di?erent components,whose range and domain match,according to some matching criteria detailed below.Note that the collection of functions in a component abstract API models the available?ne-grain services.

As shown above in the respective speci?cation(Fig.4),a module records interface information for each function in its function set.Such information amounts basically to the signature,i.e., a type declaration of its arguments and result.It becomes clear that a uniform speci?cation of the datatypes used in all components registered in CompSrc is a key issue in the design of the repository.Therefore,when an abstract API for a submitted component is built,each extractor engine analysis the available type information to build a correspondent abstract representation in the form of(instances of)polynomial functors.A brief explanation is now in order.

A function

f:I?→O

models a computational process as a transformation rule between two structures I and O,i.e., as a recipe(a tool,a technology)to build‘gnus’from‘gnats’.

Types I and O may be‘primitive’(i.e.,de?ned as such at the programming language level). Often,however,such is not the case.For example,one may know how to produce‘gnus’from ‘gnats’but not in all cases.This is expressed by observing the output of f in a more re?ned context:O is replaced by1+O and f is said to be a partial function.In other situations one may recognise that there is some environmental(or context)information about‘gnats’that,for some reason,should be hidden from input.It may be the case that such information is too extensive to be supplied to f by its user,or that it is shared by other functions as well.It might also be the case that building gnus would eventually modify the environment,thus in?uencing latter production of more‘gnus’.For U a denotation of such context information,the signature of f becomes

f:I?→(O×U)U

In both cases f can be typed as

f:I?→F O

for F X=X+1and F X=(X×U)U,https://www.wendangku.net/doc/8d2855081.html,rmally,F can be thought of as a type transformer providing a shape for the output of f.Technically,F is a functor2The notation used above(exponentiation and+)stand for some basic datatype(and functor)constructors which express the ways in which‘types’(information,in general)can be composed.Such basic constructors are

?Cartesian product(A×B)for aggregation in the spatial axis;

?sum(A+B),for choice(i.e.,aggregation in the temporal axis);

?exponentiation,or function space,(A B)for functional dependence;

?constants,like the exception type1or,in general,any primitive type;and

?powerset(P A)and sequences(A?)related to non deterministic and deterministic collec-tions of data,respectively.

These constructors can be found almost directly in high-level languages(such as,e.g.,Haskell or the Vdm meta-language)and inferred in a systematic way from other programming nota-tions.Functors built from(and closed by)such constructors plus functor composition are called (extended)polynomial and extensively used in the repository to record functions’signatures. In general,each function in the abstract API is speci?ed as

F I→

G O

2A concept borrowed from category theory(see e.g.,[7,8]or[3]for a computer science perspective)capturing a uniform transformation of both‘types’and‘type-preserving’operations.

where I and O are the import(respectively,export)datatypes embedded in a behavioural context F(respectively,G)represented as a polynomial functor.For example a non deterministic service may be modelled by a function serv:I?→P O,where P stands for the(?nite)powerset functor.Similarly a service in the form of a partial transducer may take the form of machine: I?→1+(O×I)K.

This sort of embedding through a functor entails the need to equip CompSrc with the ability to compare functors involved in the datatypes of function signature’s.Functor(structural)equality and functor instantiation order are achieved by the comparison functions listed in appendices A and B.

Based on such functions the repository is able to perform di?erent types of‘functional’compo-sition,besides the obvious one between functions sharing the domain of one with the codomain of the other.In particular the following‘extra’pipelining composition patterns are considered:?Curry insensitive,which allows to perform curry or uncurry on a pair of otherwise no composable services.

?Monadic,whenever the context information is captured by a monad(which,as shown in

[1]is often the case)3.Monadic composition includes both the usual Kleisli composition

of monadic functions used in functional programming,and a simpler scheme amounting to the monadic embedding of a‘plain’function.Both cases are illustrated in Figure5.

Kleisli composition of functions f and g is achieved by?attening(with the corresponding multiplicationμ)the result of the top line composition scheme.The bottom line scheme represents monadic embedding.

The speci?cation of the wiring process is based on the structural comparison functions mentioned above to relate,up to the intended composition pattern,functions input/output contexts.

5An Example

The example shown in this section illustrates the identi?cation of composable functions in two rather di?erent components,developed in the context of the Ikf project,by di?erent groups and using di?erent technologies.This example is fully documented in[12],to which the interested reader is referred to.

The?rst component is a Vdm-SL speci?cation of a robot which manages box storing inside a generic warehouse.The component provides functionality to,e.g.,?nd the best?t of a box

3A monad is also a concept borrowed from category https://www.wendangku.net/doc/8d2855081.html,putationally,it stands for a functor,repre-senting some sort of‘computational e?ect’,equipped with an embedding function(η:X?→F X)to‘see’simple values as F-computations and a multiplication(μ:FF X?→F X)to‘?atten’computational e?ects(see,eg,[16])

Figure5:Monadic Composition

inside the warehouse storing space,remove a box,rearrange the warehouse in order to get the biggest amount of free space,etc.

The second component is a web-publisher generator from any sort of information organised as a leaf tree(i.e.,a binary tree with all information stored on the leafs),developed in Haskell. This component provides,in particular,a function y2html to generate the Html representation of a leaf tree value.

In appendix C part of the abstract interfaces generated for CompSrc from the sources of these two components are shown.As a remark note the type information derived,in the second component,from the following Haskell declaration:

data Y a b i=Leaf(Unit a b)|Node(Mode i,(Y a b i,Y a b i))

deriving Show

data Mode i=Hr i|Hl i|Vt i|Vb i deriving Show

data Frame i=Frame i i deriving Show

data Sheet a b i=Rect(Frame i)(Y a b i)deriving Show

Applying some of the composition test suites de?ned in the CompSrc leads to the identi?cation of several possible compositions between(the abstract interfaces of)these two distinct modules.The repository is able,in particular,to determine,in an automatic way,that the functors underlying type Space in the robot component and type Y in the web-publisher one are structurally identical.This fact opens the possibility of composing function y2html in the latter component with any function returning values of type Space in the former.Such is the case,e.g.,of functions freeSpace,defragment and whichBoxes.The composition of y2html with any of these functions provides,for free,generators of Html interfaces for the warehouse Vdm prototype.

Such wiring possibility is detected by the application of function compareFunctor(in appendix B)whose result in the Vdm ToolBox syntax,is

{mk_("y2html",{"freeSpace","defragment","whichBoxes"})}

This identi?es a functional wiring scheme between y2html,on one hand,and functions freeSpace,de-fragment and whichBoxes,on the other.It also tells that the order of application for this interaction is y2html following freeSpace,defragment or whichBoxes.

6Conclusions and Further Work

Software development by component assembly is most likely to become the main stream in software engineering in the near future.This will lead to a broader understanding of what a software component is(virtually any sort of content can be encapsulated in a reusable entity with well-de?ned interfaces and able to be connected at runtime)and to the emergence of standardised component frameworks able to integrate heterogeneous components and deal with other such systems in a cooperative manner.The ability to deal with‘non native’components has been recognised as the hallmark of the so-called‘second-generation’component systems[13],a step ahead of what is currently achieved in Corba,JavaBeans or Objectspace Voyager.

The basic lesson learnt from the development of CompSrc is the potential of formal,model-oriented, methods in guiding the design of such platforms.We believe this exercise can be further extended to cope with some issues not covered in the present version,namely,dynamic instantiation of components, location and mobility.Those are fundamental issues for modelling distributed component frameworks.

The CompSrc prototype has been used,not only in the context of the project in which it was originally developed,but also to organise software components arising from a massive re-engineering e?ort of legacy code undertaken by the software company to which the?rst author is a?liated.

Such an architectural re-engineering e?ort aimed to identify service components orthogonal to the basic development layers considered in the design practice(i.e.,database de?nition,middleware and GUI). For each identi?ed component an abstract interface,as described above,has been written and directly submitted to CompSrc together with https://www.wendangku.net/doc/8d2855081.html, script to navigate in the(monolithic)legacy code(which remains unchanged)and generate the actual executable code corresponding to the new abstract API.In a subsequent stage new software products incorporating the recovered components have been generated within CompSrc.

Future work on the the CompSrc prototype is foreseen in two main directions:

?The scaling up CompSrc to act over the web in a transparent way,instead of relying on a localised component source database,as well as making it able to produce new component connection at runtime,is a main challenge from a technological point of view.

?Conceptually more demanding is the addition of facilities to cope with heterogeneity not only at the‘linguistic’and component-style levels(e.g.,the integration of object and functional models),

but also at the level of the interaction style(aiming at the integration of,e.g.,method invocation, data?ow stream processing and event-based interaction).Some preliminary work on a similar topic is documented in[6].

Acknowledgements.

The work reported in this paper was supported by the Ikf project(Information and Knowledge Fusion), IKF-IPTG-CW(ComponentWare)under contract E!2235.

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A CompareFunctor Function

B EqualsLessFunctor Function

C Component Source Values

Figure6:VDM-SL Warehouse Storing Robot DataTypes

Figure7:Warehouse Storing Robot

Figure8:Web-publisher Generator

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英语造句

一般过去式 时间状语：yesterday just now (刚刚) the day before three days ag0 a week ago in 1880 last month last year 1. I was in the classroom yesterday. I was not in the classroom yesterday. Were you in the classroom yesterday. 2. They went to see the film the day before. Did they go to see the film the day before. They did go to see the film the day before. 3. The man beat his wife yesterday. The man didn’t beat his wife yesterday. 4. I was a high student three years ago. 5. She became a teacher in 2009. 6. They began to study english a week ago 7. My mother brought a book from Canada last year. 8.My parents build a house to me four years ago . 9.He was husband ago. She was a cooker last mouth. My father was in the Xinjiang half a year ago. 10.My grandfather was a famer six years ago. 11.He burned in 1991

学生造句--Unit 1

●I wonder if it’s because I have been at school for so long that I’ve grown so crazy about going home. ●It is because she wasn’t well that she fell far behind her classmates this semester. ●I can well remember that there was a time when I took it for granted that friends should do everything for me. ●In order to make a difference to society, they spent almost all of their spare time in raising money for the charity. ●It’s no pleasure eating at school any longer because the food is not so tasty as that at home. ●He happened to be hit by a new idea when he was walking along the riverbank. ●I wonder if I can cope with stressful situations in life independently. ●It is because I take things for granted that I make so many mistakes. ●The treasure is so rare that a growing number of people are looking for it. ●He picks on the weak mn in order that we may pay attention to him. ●It’s no pleasure being disturbed whena I settle down to my work. ●I can well remember that when I was a child, I always made mistakes on purpose for fun. ●It’s no pleasure accompany her hanging out on the street on such a rainy day. ●I can well remember that there was a time when I threw my whole self into study in order to live up to my parents’ expectation and enter my dream university. ●I can well remember that she stuck with me all the time and helped me regain my confidence during my tough time five years ago. ●It is because he makes it a priority to study that he always gets good grades. ●I wonder if we should abandon this idea because there is no point in doing so. ●I wonder if it was because I ate ice-cream that I had an upset student this morning. ●It is because she refused to die that she became incredibly successful. ●She is so considerate that many of us turn to her for comfort. ●I can well remember that once I underestimated the power of words and hurt my friend. ●He works extremely hard in order to live up to his expectations. ●I happened to see a butterfly settle on the beautiful flower. ●It’s no pleasure making fun of others. ●It was the first time in the new semester that I had burned the midnight oil to study. ●It’s no pleasure taking everything into account when you long to have the relaxing life. ●I wonder if it was because he abandoned himself to despair that he was killed in a car accident when he was driving. ●Jack is always picking on younger children in order to show off his power. ●It is because he always burns the midnight oil that he oversleeps sometimes. ●I happened to find some pictures to do with my grandfather when I was going through the drawer. ●It was because I didn’t dare look at the failure face to face that I failed again. ●I tell my friend that failure is not scary in order that she can rebound from failure. ●I throw my whole self to study in order to pass the final exam. ●It was the first time that I had made a speech in public and enjoyed the thunder of applause. ●Alice happened to be on the street when a UFO landed right in front of her. ●It was the first time that I had kept myself open and talked sincerely with my parents. ●It was a beautiful sunny day. The weather was so comfortable that I settled myself into the

英语句子结构和造句

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