High Reliability Intranets for Document Management

Transcription

1 High Reliability Intranets for Document Management Simon Cleary and Andrew Jennings CSE,RMIT Introduction The dramatic growth of IP based applications has been driven by a publication model of web publishing, enabled by the wide availability of web viewing software. Given the continued development of this model the issues of reliability tend to be overshadowed. This is typical of the technology phase where fascination with the capabilities can obscure difficulties in actual implementation. Company Intranets largely follow the same model, but their direction of development is slightly different. It is natural to begin to embed Intranets into mainstream work practices, and not just use the Web as a mechanism for diffusion of information. When we instead adopt Intranets as a mainstream system component, then we must be much more concerned with reliability of the Intranet approach. Typically reliability for Web servers is expressed simply as survivability: if the Intranet delivers documents then it is taken as a reliable system. However this is quite unacceptable for users. They wish to interact with the system in a reasonable time frame, and may factor in responses to web requests as part of another information system. So we are in a process of evolution from a publication model to a system model. These published hyper linked documents can be thought of as a document databases. As such, web documents can suffer from integrity errors and version control problems especially if these documents are replicated on a large scale distributed network. A simple process of making sure that hypertext links are validated during their generation will give some level of integrity. Another difficulty is how to determine the closest correct document link in the case of replicated document databases. If the document database is to be searched it may be necessary to pre-index the document tree but we should ensure that the index is synchronized with the real document tree. Other issues associated with managing access control for authors and access control for clients may also be difficult to solve. These issues concern managing and storing user attributes and determine what methods are to be used to provide user authentication. Commercial organisations such as Netscape have been attempting to solve some of these problems by employing solutions based on adapting existing version control software and developing their own internal publishing tools to work with their tightly control publishing architecture. These issues can be complicated by the inclusion of both server side processing and browser based extensions. Often there is a need to develop different versions of the server processes to run on the different platforms used by the web servers. Netscape uses a variety of platforms to serve their web documents, these include SGI, AIX, Solaris, DEC Unix and NT boxes. Therefore they need to maintain a variety of binaries for their server applications.

2 Standardisation on one server platform would solve this but as most developers are finding there is still a requirement that these systems support legacy equipment and software systems. The issues associated with the use of Java applications or Active X executables and maintaining version control for these applications can be solved by the use of update technology that tells the client to download the new version next time they visit a particular link. The Netscape browser uses a Java archive class (JAR) and signed objects to implement a secure model for download of new versions. Similar mechanisms are in place for Active X. It is assumed that developers of such applications thoroughly test updates before distribution but that may depend on the variety of software and hardware configuration used by the end users and the ability to test these applications on these configurations. The user behaviour model for publishing information via web base clients and servers has been undergoing some change. Early implementation of such systems relied on the user being proactive on finding new or updated information. Now with the use of new technologies such as netcasting (Marimba s Castinet) and dynamic HTML it is possible that a user to just register an interest and automatically receive updated messages when new information is available. If we consider only intranets this is quite an efficient mechanism of distributed information within an organisation to only those that are interested. Many intranets and extranets are now considered 3-tier architectures. The first tier is the thin client, the second tier, a middleware server and the final (3rd tier) an established database server (it may be a legacy system or may not). We cannot assume that such a system will scale relatively easily. As the demand rises for a particular service then so should the number of servers providing that service. Design of such system should be given some thought, as it may be possible to create a bottleneck in the second tier of such a system. It is desirable that a flexible scalable mechanism for identifying or discovering necessary resources on the network be provided to eliminate such bottlenecks. Our motivation is to explore the role of mobile code and data when establishing more reliable Intranets. Is it useful to migrate applications freely amongst machines to improve reliability? Clustering To deal with a high volume of web requests, the most common method of addressing the volume is machine clustering. A group of servers effectively acts as a single server, and the load of requests is evenly distributed amongst the clustered machines. client router server 1 server 2 server 3

3 Damanji et al provide a comprehensive review of the methods of hosting a web service on a cluster of machines. The most difficult issue is how to create the presence of a single IP address, by distributing requests amongst the servers in the cluster: Direct http redirection. Using the provision in the http protocol, an alternative server is directly specified by a busy server. Server side DNS approach. When a DNS request is received, it delivers an IP address of the cluster taking each server in turn. Server side single IP image approach. The router address is publicised and the router takes care of distribution to the correct server. Server software based solution (Damanji) et al take advantage of the ifconfig alias option to allow attachment of multiple domain names to a single network interface. The software based approach provides for the possibility of dynamic re-configuration of the network, and this is a potential advantage. The nominal configuration can be that of a dispatcher servicing a number of servers: dispatcher client router server 1 server 2 server 3 In normal configuration the dispatcher can attempt to evenly distribute load amongst the servers. For cases where the dispatching machine becomes heavily loaded, we are investigating the dynamic reconfiguration of the cluster, with the ability for any server in the cluster to take on the role of dispatcher. Load Balancing In the publication model, every server is identical and the problem of load balancing reduces to the problem of ensuring that every server receives its equal share of requests. Experience with heavily loaded servers such as the Netscape host indicate that this is a non-trivial problem. Requests come in bunches, and as a consequence can still impact heavily on servers. However heavily loaded Intranets face a more variable environment, and may have to be more versatile. In the case where requests are linked to applications hosted on the servers, we may find that a small number of requests causes a heavy load on the server. We are investigating the capability to migrate applications dynamically according to load. With continuous monitoring of Web server performance we can isolate areas of difficulty and if

4 needed create new servers of the desired identity. We may begin with servers of identical type, and grow more servers of the type needed by applications. client router server 1 (type A) server 2 (type B) server 3 (type C) In a commercial web environment it should be possible to estimate the potential population for a particular web based application and hence it should be possible to estimate potential transactions placed on the system. A major problem with specifying throughput of a web server is actually determining the capabilities of web servers. It would be nice if the server platforms supporting particular environments had a method of benchmarking its potential throughput and processing speeds on web based traffic. Often a company will install a web server, run the applications and monitor its load, raise the load until the server falls over, and then run this server at half the acceptable peak load. From an engineering perspective this approach is rather suspect. New Services: Warehouses, Data Mining Data warehousing is a particular good example of an application that is suitable for a distributed web model. Often the data used in a data warehouse by its very nature static, the data being searched is most likely the result of the previous day s work (or previous week) and as such can readily be distributed without suffering from replication integrity problems. Interrogation of the data could be managed via intelligent client software which talks to individual brokers or a broker layer consisting of many brokers to determine location of data or particular service to look at the data. Data mining is the processing of discovering patterns in information contain within large volumes of data and finding things that people did not realize. It has it roots in AI pattern extraction and discovery programming and is suited to background processing. As such an intranet could make use of data mining to distribute the processing for pattern discovery. Similar to the processing of trawling the internet with spiders or worm programs. In this context the data mining process would interrogate databases and information stores within the intranet. Directions A new area of intranet publishing involves the development of Enterpise Applications which are based upon the Internet model. These applications are over considered to be multi-tier applications in that they employ a three layered model of the application. These present a presentation model, business model and data access model. The resulting components enable the developers of such applications to hid the implementation detail of what is being presented

5 and to create a level of abstraction that enables these models to talk to each other. The infrastructure used by the developers provides the resources, security and communication components for the system. These systems have the ability to be distributed and enable the developer to use a combination of network components such as IIOP (Corba), RPC (DCOM), ActiveX and Java. These type of systems pose their own problems in terms of implementation but enable the developer to concentrate of the distributed nature of their process without having to concentrate on the transport or naming mechanisms. Developers must keep in mind the distributed nature of their applications and not unnecessarily constrain middle layer applications. They should ensure adequate resource discovery mechanisms and support for their final applications. Borland are in the process of developing and managing this middle layer, providing redundancy and load monitoring through their Entera product. Conclusions Intranets are in a process of evolution from a simple information distribution model to a system component model. For simple information distribution we can assume that each item for distribution is similar in size, analogous to a newspaper distribution. However when we embed Intranets as part of applications this requires new approaches. Reliability is more than survivability: it is not enough to ensure that the network remains alive: it must respond to user requests within a reasonable time frame. We propose an approach to clustering and load balancing that begins to address this need for reliable Intranets. One of the motivations of this work was to address the real advantages of Java. Simply being able to port code easily from one platform to another is only a small motivation to adopt Java. Is the ability to dynamically move both code and data to any machine on a network useful? Our developments indicate that the pressing need for more reliable Intranet services may be addressed by Java based applications on machine clusters. In the past the adoption of multiple processor machines was driven by the need for reliable transaction processing systems. The widespread adoption of dual homed systems was critical to the development of the on-line transaction processing industry. We see a direct analogy here in the adoption of mobile data and mobile code. It can provide for extremely high levels of reliability in network processing systems. References Java One conference paper on RMI ( Damani, O., Chung, P., Huang,Y.,Kintala,C., Wang,Y. ONE-IP: techniques for hosting a service on a cluster of machines Sixth International World Wide Web Conference, Hyper Proceedings

6 Davidson, J. The Role of a DHCP Server in an Enterprise Network Network Telesystems White Paper, Kristian Paul Bubendorfer Resource Based Policies for Load Distribution Thesis, Victoria University of Wellington August 31, Mosedale, D., Foss, W., McCool, R. Lessons Learned Administering Netscape s Internet Site IEEE Internet Computing, March/April 1997 Vol No. 2

7 Andrew Jennings received the PhD degree from Monash University in He worked for Telstra Research Labs in Melbourne until 1989, leading research projects in networking and software development. From 1989 to 1990 he was a research fellow at the Kansai Advanced Research Centre in Kobe, Japan. He is now Professor of Computer Systems at RMIT in Melbourne. Current projects include the development of high reliability web servers and new software architectures for networked applications. Simon Cleary received his Masters Of Engineering degree from RMIT in He is main interests lie in the development of distributed applications in computing environments. He has consulted for several organisations during his time as a Lecturer, ASC, Supreme Court of Victoria, Telstra and Victorian State Government. Current projects include Course Information Warehouse and WWW online enrollements.