Active Files A Mechanism for Integrating Legacy Applications

Active Files:A Mechanism for Integrating Legacy Applications into Distributed Systems

Partha Dasgupta Department of Computer Science Arizona State University

partha@08f99541336c1eb91a375d88 Ayal Itzkovitz and Vijay Karamcheti Department of Computer Science

New York University

ayali,vijayk@08f99541336c1eb91a375d88

Abstract

Despite increasingly distributed internet information sources with diverse storage formats and access-control constraints,most of the end applications(e.g.,?lters and media players)that view and manipulate data from these sources operate against a traditional?le-based interface. These legacy applications need to be rewritten to access re-mote sources,or need to rely upon ad hoc intermediary ap-plications that aggregate the data into a passive?le before executing the legacy application.

This paper presents a simple,elegant,programmable method for allowing natural integration of legacy applica-tions into distributed system infrastructures.The approach called active?les,enables multiple information sources to be encapsulated as a local?le that serves as their logical proxyw.This local?le is accessed though a sentinel pro-cess,which automatically starts when the?le is opened,ag-gregates data from multiple sources,and?lters all access to and from the?le.More importantly,the integration of active ?les into client applications is transparent:an active?le is virtually indistinguishable from a regular?le.Active?les ?nd a variety of applications in both distributed and non-distributed systems.We discuss active?les,their semantics, their usage and their implementations in Windows NT. 1.Introduction

The growth of the internet has been accompanied by an explosion in information sources.These sources are dis-tributed,store data in different formats,are dynamically changing,and may be subject to a variety of constraints in-cluding consistency,privacy,and access-control.However, most of the end applications that view and manipulate data This research was sponsored by DARPA/AFRL-Rome agreements F30602-96-1-0320and F30602-99-1-0517,by NSF award CCR-9876128, and Microsoft.from these sources(e.g.,browser helper applications such as?lters and media players)assume a traditional?le-based interface,treating?les simply as a passive,persistent,un-interpreted sequence of bytes.Consequently,the integra-tion of such legacy applications into distributed systems has required either signi?cant code modi?cation to use a new distributed system-aware API or relies on the ad hoc use of intermediary applications that isolate the end application from the data sources.These intermediaries perform neces-sary operations such as access control,?ltering,and format conversion before aggregating the data into a passive?le that can be handed down to legacy applications.

However,there are signi?cant shortcomings to both these approaches.The?rst,supported by constructs such as dynamic HTML and cgi-bin scripts to couple activity with network-accessed?les,and component frameworks such as DCOM,and CORBA to support object-speci?c access interfaces,has seen restricted use except in a few scenar-ios.Reasons include the vastly different semantics provided by these constructs,their relatively complicated APIs,and their heavyweight implementations.The second approach although more popular,has the disadvantage that the data collected by the intermediary is completely decoupled from both the original sources of the information and the end ap-plication.Consequently,it is unable to track changes in the original sources or be controlled by the end application.For example,an end application that searches through a col-lection of distributed databases cannot see changes in these databases,nor in?uence the progress of the search when an intermediary?rst aggregates data from these databases and presents it to the search application as a?le.

In this paper,we present an elegant,easy to use,and programmable concept,that allows natural integration of legacy applications into distributed system infrastructures. This association is done by a construct we call active?les, which enables distributed sources of information to be en-capsulated in the form of a local?le that serves as their logical proxy.An active?le has the look and feel of a reg-ular?le,but is associated with a sentinel(process)that can

To appear in the Proceedings of International Conference on Distributed Computing Systems, April 2000

act on the streams of data that enter the?le on a write or exit the?le on a read.More importantly,from the perspec-tive of the end-application,active?les are indistinguishable from non-active?les.There is no reprogramming,or re-compilation necessary for using active?les.The sentinel can perform a variety of tasks,including aggregating data from distributed sources,?ltering data entering/exiting the ?le,and performing actions that have external side-effects.

Active?les provide a convenient abstraction for alleviat-ing several shortcomings of intermediary approaches.The sentinel process can control?ow of data between distributed sources and the end application,enforcing consistency,pri-vacy,and access-control constraints required by the former while simultaneously yielding control to the end applica-tion.For example,an active?le can provide the illusion of accessing a single?le even though the?le data is physically located on multiple remote sites with varied authentication and access-control policies.In addition,it can monitor how the application uses this?le,caching only the most fre-quently accessed contents for performance.Moreover,the cache can be kept consistent with any updates performed to its contents at any of the remote sources.Note that all of these behaviors can be expressed without modifying either the end application or the original sources of data.

Active?les can also enhance regular?le functionality in non-distributed systems.Tradi …… 此处隐藏：48155字，全部文档内容请下载后查看。喜欢就下载吧 ……