Adaptability in Multi-tier Information Systems
Multi-tier architectures are now the standard for advanced information
systems. The information system exports its functionality via business
methods that, e.g., maintain a shopping cart, perform purchase
transactions, or search for specific product information. Once a client
has connected to the information system, it can call these methods as
needed. The method execution takes place on an application server (also
called middle-tier). All critical data is stored in a database system.
Whenever a method has to access such data (retrieval or update), the
application server makes the corresponding calls to the database
system. The database system is called the backend-tier. In the current,
web-centric world, a web-server is often placed between the client and
the application server. In this case, clients are often web-browsers,
and the web-server is responsible to accept the http requests,
transform them to calls to the application server, and return the
responses of the application server to the client in form of
dynamically generated web-pages. A web-server is also considered a
In the commercial world, J2EE, .NET, and CORBA are specifications for
the middle-tiers of such multi-tier architectures. Application servers
provide far reaching functionality and services that help in the
development of information systems. Important services are various
communication mechanisms (remote procedure calls, messaging systems,
persistent queues, etc.), transaction management, security, access
control, persistence, and many more.
The fact that application
servers introduce an additional layer of complexity, and that both web
and application servers use a wide range of new technology
leaves us with many challenging problems.
Our research looks at several issues in regard to such multi-tier
- Fault-Tolerance: 7/24
availability is crucial for many of these information systems. Hence,
such systems must be able to handle the crash of individual components,
e.g., the crash of the web-server, the applicatin server, or the
database system. Ideally, such crash is transparent to the client,
i.e., the client always receives exactly one response for each request
it sends to the system. Furthermore, the crash of a component may not
leave the system in an inconsistent state. Replication is widely used
to provide such fault-tolerance. The idea is that servers are
replicated and if one replica crashes the work assigned to this replica
can failover to another replica. In principle, each tier can be
replicated. We have several projects on database
replication. Furthermore, we have developed replication strategies
for application servers. Our work in this area consists of several
- Since an application server is not the last component in the
multi-tier architecture but makes calls to the database backend-tier,
any solution for application server replication must take the
interaction between application server and database system, and the
behavior of the database system into account. We analyze what
interaction patterns exist between these components. In particular, we
look at transaction management, and how the actions of transactions can
affect both application server and database system.
- We have to define what correctness means. This must be done
from the perspective of the client, e.g., that he/she always receives
an appropriate response to a request, and from the perspective of the
application server and database system, e.g., the state they maintain
should be consistent.
- Based on the model and the correctness criteria we have
developed algorithms that provide fault-tolerance in an efficient
manner. Requests are executed by a primary server, and state changes
are propagated to backup servers which can take over if the primary
crashes. Execution at the clients, the primary, and the backups is
carefully coordinated to provide the degree of correctness that is
- We have implemented our algorithms witin a concrete application
server architecture, namely the J2EE specification. Our replication
tool is open-source and can be accessed via SourceForge.
- Load Balancing:
Replication cannot only be used for fault-tolerance. Since the work to
be performed at backups is not as time consuming as the work at the
primary, the backups can be primary servers for other clients. Hence,
by adding new server replicas to the system we are able to handle more
client requests. There are two main issues to handle. First, how to
distribute clients over the server to optimally distribute the load.
The second is how to maintain the system if replicas leave the system
due to crashes or necessary maintenance, or join the system after
recovery or as new replicas in order to increase system capacity. Such
system configurations have to be performed in a transparent manner
without interrupting ongoing processing in the system.
- Fault-tolerance across the
entire system: Even if each individual tier provides
fault-tolerance mechanisms, e.g., through replication, it is not clear
that the server system in its entirety provides fault-tolerance and
correctness. We have analyzed how we can reason about the
correctness of the entire system given fault-tolerant properties of the
individual tiers. From there, we have derived interaction patterns that
guarantee system-wide fault-tolerance.
- Showing Correctness of a Replication Algorithm in a Component based System. H. Wu, B. Kemme. IDEAS
Conf., September 2008.
- Replication in Service Oriented Architectures. M. Ameling, M. Roy, B. Kemme. Int. Conf. on Software and Data Technologies. July 2008.
- Consistent and Scalable Cache Replication for Multi-Tier J2EE Applications. F. Perez-Sorrosal, M.Patiño-Martínez, R. Jiménez-Peris, B. Kemme. ACM/IFIP/USENIX Int. Middleware Conf., Nov. 2007.
Toolkit for J2EE Application Servers. H. Wu, B. Kemme, A.
and S. Patarin. Software Demonstration at the
ACM/IFIP/USENIX Conf. on Middleware, Grenoble, France, December
- Fault-tolerance for Stateful
Servers in the Presence of Advanced Transactions Patterns. H. Wu,
B. Kemme. Proc. of the IEEE Symp. on Reliable Distributed Systems
(SRDS), Orlando, Florida, October 2005.
- The ADAPT Framework for
Composable Web Services. A. Bartoli, R. Jiménez-Peris, B.
Kemme, C. Pautasso, S. Patarin,
S. Wheater and S. Woodman. IEEE Distributed Systems Online - Web
Systems Section. Invited Paper. Sept. 2005.
Multi-tier Architecture. B. Kemme, M.
Patiño-Martínez, R. Jiménez-Peris, J. Salas. VLDB
Workshop on Design, Implementation, and Deployment of Database
Replication, Trondheim, Norway, August 2005.
- ADAPT: Middleware Technologies
Adaptive and Composable Distributed
Components. A. Bartoli, R. Jiménez-Peris, B. Kemme, C.
Pautasso, S. Patarin, S. Weater, S. Woodman. Europ. Dependable
Computing Conference (EDCC), Project Track, April 2005.
- Eager Replication for Stateful
Servers. H. Wu, B. Kemme, V. Maverick. Int. Symposium on
Distributed Objects and Applications (DOA), Agia Napa, Cyprus, October
of UDDI Registry Replication Strategies. C.
Sun, Y. Lin, B. Kemme. IEEE Int. Conference on Web Services
Diego, California, July 2004.
Part of the work has been performed in the context of the Adapt project
(Middleware Technologies for Adaptive and Composable Distributed
Components). Adapt was a RTD project funded by the Information Sociaty
Technologies Programme of the European Commision under FP5, and the
Programme de soutien à la recherche (PSR) of the
Ministère du Développement économique, de
l'innovation et de l'exportation (MDEIE) du Québec, Canada.