DESIGN AND ARCHITECTURE OF A DIGITAL MUSIC LIBRARY ON THE WEB

Transcription

1 DESIGN AND ARCHITECTURE OF A DIGITAL MUSIC LIBRARY ON THE WEB ABSTRACT In this paper, a Web-based digital music library based on a threetier architecture is presented. The digital library s primary goal is to efficiently manage distributed mp3 file collections. The proposed methodology makes use of the metadata existing inside the ID3 tag of each mp3 file. Such information is extracted from the corresponding files and stored in separate, XML-based data structures, which are part of the overall distributed information retrieval module. The digital library s actual data is kept at the back-end in a distributed fashion. Communication between the layers of the digital library and the various architectural components that reside in the middle layer is facilitated through the employment of the XML standard. 1

2 1. INTRODUCTION-RELATED WORK The ultimate goal of a digital library is to provide ways of organizing and efficiently managing large data collections. When designing a Web-based digital music library, the special features of this environment have to be taken under consideration. Thus, the fact that there is no dominant operating system on the Web indicates that, at least at the client side, the proposed attempt should be based on platform-independent software like the Web browser. It is also important to take advantage of the fact that the MPEG-1 layer 3 format (mp3) is the established way to store and disseminate music information on the Web. The popularity of mp3 files has motivated many software companies to develop applications that are able to manipulate mp3 files in various ways and to convert almost any type of audio files according to the mp3 format. It is therefore reasonable and at the same time of vital importance to consider mp3 files as the cell of a Web-based digital music library. However, applications that are focused on delivering music on the Web must deal with two important issues: network restrictions and the copyright problem. The former arises from the fact that the Web is based on TCP/IP, a protocol that is not designed to efficiently handle multimedia data. Hopefully, current research on the field of network protocols will soon provide solutions to this problem [1]. The latter is a rather complicated issue and refers to the fact that music content cannot be properly accessed and disseminated in the context of a digital music library without addressing the legal issues surrounding the use of copyrightable content [2, 3]. There is a considerable amount of implementations of digital music libraries that differ from the one proposed in this paper in terms of their underlying platforms, the use of proprietary software and their accessibility. Thus, VARIATIONS [4] is a digital music library system that provides online access to over 5,000 sound recordings from Indiana University s library collection by relying on client-server architecture. In VARIATIONS, information retrieval is facilitated through the employment of metadata. Each sound object is accompanied with an enhanced USMARC bibliographic record, which is stored in an ASCII text file. The platform specific nature of the corresponding architecture results in a library with limited access capability. JUKEBOX [5] is an application that provides online access to a distributed collection (according to the 3-tier architectural model) of sound recordings. Although this system relies on the mp3 format for the encoding of its sound archives, access to the collection is once again limited due to the fact that decoding at the client side requires a specific hardware decoder. The PATRON [6] client-server architecture has been designed to provide audio (and video) on demand at the University of Surrey. It is based on a metadata repository that is structured according to the XML specifications. The system is widely available since the user interface works within a Web browser. An innovative approach to sound collections is followed at the New Zealand s digital library of music and audio data [7]. According to this approach, no bibliographic metadata is required for the retrieval of requested files. Instead, a melody index system retrieves music on the basis of a few notes that are sung, hummed or otherwise entered. Related content-based music retrieval techniques are employed for querying and browsing music collections of the SMILE [8] system. More recent systems for music delivery on the Web like Napster [9] and Gnutella [10], rely on peer-to-peer architectures for organizing their mp3 file collections and facilitating information retrieval for their users. Specifically, peer-to-peer is a type of 2

3 file-sharing mechanism that allows users under the same network (i.e. Internet) to access the contents of a specified area on each other's hard drives. Such systems are widely accepted from the Web community since they rely on Web technologies and therefore require no specific operating system. However, the future of such systems is uncertain, since they are accused of violating copyright laws by the music industry. Consequently, the legal issues that arise when delivering music on the Web seem to be the most crucial problems in the development of effective, Web-based digital music libraries. In this paper, the architecture of a Web-based digital music library is presented. The proposed architecture is based on the three-tier model, which is common for distributed applications and builds upon the well-established Web technology. Music data at the back-end is stored according to the mp3 format in a distributed fashion and retrieved by employing a distributed information retrieval module. It is the middle layer s responsibility to create and broadcast streams of audio data according to playlists that reflect users preferences. Communication between the various components of the architecture is facilitated through the employment of the emerging XML standard. Finally, the user interface is provided by common Web browsers that interact with the middle layer s Web server. The rest of this paper is organized as follows: Section 2 identifies fundamental requirements for building a digital music library on the Web. Section 3 underlines the importance of metadata for the development of efficient information retrieval techniques in such systems. Based on the conclusions that derived from the previous sections, section 4 presents the digital music library in terms of architecture and functionality. Finally, we conclude by summarizing the work presented throughout the paper in section REQUIREMENTS OF A DIGITAL MUSIC LIBRARY ON THE WEB As it has been previously stated, a digital music library on the Web that contains copyrightable content, should consistently respect rightsholders legal rights. One way to achieve such a goal is to employ broadcasting applications that are based on streaming servers like Icecast [11] or Shoutcast [12]. This way, music files won t be copied without control to multiple destinations. Instead, one or more continuous audio streams will be broadcasted to the connected workstations. Such a scenario is followed by Web radio applications and seems to be accepted from the music industry since it resembles traditional radio, for which a robust legal framework has already been established. On the other hand, broadcasting applications provide less control over music data to their users since the original music files remain at the server side at all times. Apart from the aforementioned legal issues, a number of technical requirements should be taken under consideration for the development of a digital music library on the Web. Thus, the repository structure of such an application should be able to handle huge numbers of mp3 files in a consistent, scalable and efficient way. Such files could overlap or be encoded in various bit rates. At the same time, the growth of the repository should have minimal effect on the overall performance of the system. In order to provide efficient information retrieval to the users of the digital library, the employment of distributed solutions seem to have a number of advantages over the employment of centralized ones. Thus, distributed search engines scale much better than centralized engines, while at the same time successfully preserve local 3

4 autonomy. Moreover, index construction takes less bandwidth since other servers need not be probed during this procedure [13]. An important requirement of a digital library that provides distributed storage and retrieval is platform independence. Users should be able to access the digital library from the Web independently of the operating system their workstation relies on. Similarly, repository nodes should be able to work with various operating systems. Exploiting the Web as a universal operating system and employing standards like XML for the dissemination of information between the various architectural components of the digital library, seems to be a solution that satisfies such requirements [14]. 3. METADATA One of the features that distinguishes digital libraries to the Web is their ability to store data in a consistent and at the same time organized manner [6]. In the context of well-structured data repositories, documents within digital libraries are most frequently accompanied by metadata. Metadata are defined as data that are used to describe such documents. In the case of a digital music library like the one described here, documents are essentially music files encoded according to the mp3 format. Such files are characterized from the fact that metadata are included in the body of each file. According to the mp3 specifications, metadata like artist, genre and album exist within the ID3 (and/or ID3v2) tag of the mp3 file. More specifically, the ID3 is a chunk of data prepended to the binary audio data. Each ID3 tag holds one or more smaller chunks of information, called frames. These frames can contain various types of information about the corresponding mp3 song such as title, album, performer, website, lyrics, equalizer presets, pictures etc [15]. Fig. 1 illustrates the layout of a typical ID3 (v2) tagged mp3 file. MP3 Audio data ID3 tag Title Artist... Genre Fig. 1. Internal layout of an ID3 (v1.1) tagged mp3 file It is fair to expect that a digital music library consisting of mp3 files, will exploit their corresponding ID3 tag for the development of more efficient information retrieval techniques. So far, existing digital music libraries of mp3 files on the Web like Napster [9] and Gnutela [10], seem to rely only on the filename of each mp3 file to extract metadata information and to build the corresponding indices. However, such strategy is subject to ambiguous results since there is no standard way to name mp3 files and, furthermore, there is only room for just a few metadata elements in the name of such files. On the other hand, relying on the ID3 tag to retrieve metadata from mp3 files implies that a valid ID3 tag exists for every mp3 file of the collection. Otherwise, the entire 4

5 information retrieval module of the digital library would be at the very least inaccurate and misleading. It is therefore necessary to accommodate the digital music library with software that will ensure the integrity of its contents, as far as the ID3 tag is concerned. For performance reasons, such information should be replicated and kept separately from the body of the mp3 files. 4. DIGITAL MUSIC LIBRARY 4.1 Motivation The proposed work is strongly motivated from the fact that there is lack of applications on the Web capable of efficiently handling audio data that is encoded under the popular mp3 format. Moreover, ongoing efforts like Napster or Gnutella do not take advantage of the descriptive information (i.e. metadata forming the ID3 tag), which is embedded in the body of such files. Another argument in favor of the proposed work is the need to provide a framework for properly managing copyrighted information (i.e. mp3 files consisting of copyrighted songs) on the Web in a way that copyrighted laws are followed, while retaining as much functionality as possible. Having the above thoughts in mind, we introduce a digital music library capable of efficiently handling audio files, which are encoded under the mp3 format in a way that respects rightsholders legal rights. This is achieved by exploiting the ID3 tag of the mp3 files for the development of content-sensitive information retrieval techniques. 4.2 Underlying Architecture The proposed digital music library relies on a three-tier architecture, typical for distributed Web-based applications, as it is illustrated in fig. 2. It is an XML-based architecture that handles mp3 file collections forming a unified back-end repository. Data requested and retrieved from the repository is manipulated in various ways at the middle layer (middleware), which also encapsulates application-specific logic. The requested data (i.e. mp3 files), as received from the middleware (in the form of an audio stream), is broadcasted to traditional Web browsers that serve as "thin clients" [16], in the sense that there is no client-specific software installed. It is the browsers responsibility to manipulate the audio stream with appropriate software (in Client Middleware Back-end Audio stream XML Fig. 2. The digital library s three-tier architecture 5

6 the form of plug-ins or built-in functionality). By taking a closer look at the back-end, it can be seen that it consists of distributed mp3 file collections that correspond to the various repository nodes of the digital library. Considering the distributed nature of this layer, each file within the digital library should have a unique name/identifier. Thus, the digital library s nodes are wrapped around Web servers that assign unique names to their contents and make them available to the middleware. Every file of each node resides in a predefined file system hierarchy. The root folder of such hierarchies corresponds to the document root folder of the Web server. The actual content of the node (i.e. mp3 files) resides under the folder Collection, which in turn, resides under the Web server s root folder. Fig. 3 illustrates the file system hierarchy of each node. Document root folder/collection/song1.mp3 song2.mp3 songn.mp3 Fig. 3. The digital library s file system hierarchy According to the described naming conventions, the unique name/id of an mp3 file residing in a node that employs a Web server named e.g. thalis.com would be thalis.com/collection/mysong.mp3. For each node of the digital library, an XML repository hierarchy is defined. It is structured in XML format and consists of sub-trees of elements that correspond to information extracted from the ID3 tag of the node s mp3 files. According to the size of the corresponding file collection, such a hierarchy can be mapped to a number of data structures varying from a single file to complex Relational Database Management Systems (RDBMSs). An example of a hierarchy mapped to a single XML file (i.e. repository.xml file) is demonstrated in fig. 4. <?xml version="1.0" encoding="iso "?> <songs> <song entry="november 13, 1999" counter="23" title="lotus" id=" " artist="rem genre= Rock album= The Greatest 99 /> <song entry="november 23, 1998" counter="564" title="catch" id=" 3" artist="the cure genre= Rock album= the cure /> <song entry="november 13, 1999" counter="566" title="with or without you" id=" 3" artist="u2 genre= Rock album= U2 /> </songs> Fig. 4. A typical XML repository hierarchy file 6

7 The attribute counter monitors the number of times the corresponding mp3 file has been accessed from the users of the digital library. It is employed to measure the popularity of this file in the digital library and increases incrementally each time a user selects the specified file. Such information may be exploited from the middleware to rank search result lists according to the popularity of their items. The rest of the attributes illustrated in fig. 4 are essentially metadata included in the ID3 tag of the mp3 file. As it has already been stated, only mp3 files with a completed ID3 tag are allowed to be entered in the digital library. In the case of a user trying to upload an mp3 file with a missing ID3 tag, the user is prompted to complete the subset of the ID3 tag that is required from the system (i.e. Title, Artist, Year, Genre). Uppon acceptance of the file, it is routed to an appropriate node of the digital library. Adequate software modules update the corresponding XML repository hierarchy structure with the new entry by mapping the ID3 tag to the equivalent element (as illustrated in fig. 4). Consistency between the repository hierarchy and the actual content of a repository node is provided from a set of software modules that are periodically executed locally in each node. Such modules update the local repository hierarchy with modifications that occurr to the contents of the corresponding node. Finally, each node of the digital library maintains a search engine that queries the local XML repository hierarchy. The nodes search engines are invoked from the Mediator module residing at the middleware, as it will be described in the next section. 4.3 Provided Functionality The core functionality of the proposed architecture is encapsulated at the middleware. It is the middleware s responsibility to produce the audio stream(s) that will be ultimately broadcasted to the client side. According to the context in which the proposed digital library architecture operates, a number of possible scenarios may be followed for the construction of the resulting audio stream(s), varying from Web radio stations to University music libraries. In this section, the employment of the proposed architecture to a Web radio station will be presented. Following this scenario, the middleware manipulates requests from users and accordingly acquires the corresponding mp3 files from the back-end to produce the resulting audio stream. The proposed digital library architecture as applied to a prototype Web radio station is presented in fig. 5. 7

8 Thin clients Middleware Back-end playlist. xml Node 1 Repository Streaming server Decider Local S.E. Mp3s Election.xml Crawler.xml Node 2 Repository Web browser with audio playback capabilities Web server with audio streaming capabilities Cache DB Mediator Registry.xml Local S.E. Mp3s Node n Repository Local S.E. Mp3s Fig. 5. The digital music library architecture Users are initially requested to vote for the songs they would like to listen from the Web radio station. If a voted mp3 file exists, an entry is added to the election.xml file. Such a file is structured in XML format and contains the name and location of the mp3 files that users have voted for, during a specified time period. When the time period expires, the Decider module decides which files will be selected for broadcasting, copies their location (as provided from the election.xml file) to the playlist.xml file (acting as a playlist) and empties the election.xml file. The deriving playlist is fed to the streaming server, which in turn broadcasts the produced stream to the connected clients. The Decider takes under consideration the participation of songs in recent playlists in order to avoid repeated broadcasting of the same songs in a relatively small time interval. Moreover, the Decider makes sure that in a sequence of successive time intervals (the total duration of these intervals resembles the program of a traditional radio station) only songs of related categories will be broadcasted. In order to locate mp3 files in the digital library, a similar information retrieval technique to the one proposed by Dushay et al. in [17] is followed. Specifically, users enter the name and, optionally, other metadata of the song they wish to hear, as illustrated in fig. 6. 8

9 Fig. 6. Searching the digital music library The Mediator module receives the request and translates the query to a machine understandable format. Then, according to the information contained in the registry.xml and crawler.xml files, the Mediator decides which search engines located at the back-end will be invoked. More specifically, the registry.xml file contains information about the location of all the participating nodes of the digital library and the crawler.xml file contains information about the content of each node of the digital library (i.e. which categories and artists are hosted in each node). Then, the translated query is addressed to the chosen nodes. Each node performs a local search to its contents and returns the search results in XML format to the Mediator. The Mediator merges the incoming results, deletes duplicates and ranks them according to their relevance and popularity. The deriving search results list is then transformed to HTML with the employment of an XSLT processor [18] and is transmitted to the connected workstations. If the desired song appears in the list, the user may select it and submit it to the middleware. A corresponding element will then be added to the election.xml file and the song will be considered for broadcasting from the Web radio station. Finally, in order to maintain good performance for the system, a cache database needs to be preserved at the middleware. This way, access to back-end data will be reduced and requests from users will be satisfied quicker. 4.4 Interacting with the Digital Library Users interact with the digital music library from thin client workstations. Such interaction is accomplished with the help of a typical Web browser. The Web browser accepts and forwards user requests to the middleware through adequate user interface functions that reside at the Web server. For example, users may ask the system whether a specific song exists at the back-end by feeding the adequate data 9

10 in the form illustrated in fig. 6. If adequate mp3 files exist in the digital music library, a search results list (as it can be observed in fig. 7) is presented to the user from which she/he can vote for the song she/he wishes to hear from the Web radio station. It is the streaming server s responsibility to cooperate with the Web server in order to broadcast the audio stream(s) back to the connected Web browsers. At the client side, a corresponding playback client that is attached to the Web browser receives the audio stream and delivers it to the workstation s sound card. 5. CONCLUSIONS Fig. 7. Presenting the search results In this paper, the architecture of a Web-based digital library that handles mp3 files in a distributed fashion has been presented. The proposed architecture relies on a three-tier model for the storage and retrieval of its contents. The motivation for this work was the lack of adequate tools and applications for organizing, managing and disseminating mp3 files on the Web in an efficient and broadly acceptable way. Broadcasting continuous audio streams is an alternative way of delivering music to the Web without the legal implications that arise from current approaches that facilitate uncontrolled distribution of audio files between Web users. The proposed architecture is suitable for the development of digital music libraries of many kinds varying from automated Web radio stations to complex library systems that manage many hours of sound recordings. At the University of Piraeus in Greece, we are currently in the process of implementing the proposed digital library in the context of an automated Web radio station. Although the distributed nature of the proposed architecture is capable of efficiently handling large numbers of mp3 files, certain problems arise as far as network availability is concerned. The number of users that can be simultaneously connected to the digital library depends on the bandwidth capacity of the corresponding network. Fortunately, emerging network protocols like ATM over IP and Ipv6, promise to provide better support for multimedia applications on the Web like the one that has been presented in this paper. 6. ACKNOWLEDGMENTS The research described here was funded in part by the Greek General Secretariat on Research and Technology under a PENED grant. We would also like to thank our colleagues, Elena Mplanta and Padelis Giannitsios for their constructive remarks during the implementation phase of the described work. 10

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