Incorporating Fault Tolerance and Reliability in Software Architectures

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Incorporating Fault Tolerance and Reliabilityin
Software Architectures

• Ingrid Buckley
• 01/15/09

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Agenda

• Introduction
• Service Oriented Architecture
• Motivation
• Problem
• Related Work Approaches
• RobustBPEL
• Using Fault Tolerance measures in software
  Architecture
• Autonomous functionality/ Behavior in Web
  Services
• Challenges
• Conclusion
• Future Work
• Recommendations
• References

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Introduction

• The Software Architecture (SA) of a program or
  computing system is the structure or structures
  of the system, which comprise software
  components, the externally visible properties of
  those components, and the relationships between
  them.
• A service is a function or method that does some
  particular action.
• Web services are software components defined by
  their interfaces that can be accessed on the
  Internet and incorporated into applications.
• The communication can involve either simple data
  passing or it could involve two or more services
  coordinating some activity. Some means of
  connecting services to each other is required.
• Web services are a realization of a more abstract
  architectural style called Service-Oriented
  Architecture (SOA).
• Service-Oriented Architecture (SOA) has been
  considered to be the new phase in the evolution
  of distributed system applications.
• We define SOA as an architectural style in which
  a system is composed from a set of loosely
  coupled services that interact with each other by
  sending messages.

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Service Oriented Architecture

• In order to interoperate, each service publishes
  its description, which defines its interface and
  expresses constraints and policies that must be
  respected in order to interact with it.
• Discovery enables agents to retrieve Web
  service-related resource descriptions.

SOA roles
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Service Oriented Architecture
SOA architectural layers.
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Motivation

• SOA could enable the design and realization of
  flexible and extensible applications which span
  across multiple organizations.
• SOA has promised to provide enterprises with
  modular, reusable and easily extensible
  architectures that would enable them to adapt
  their applications easily so that they remain
  competitive and compliant.
• Even though there is a common acceptance of this
  concept, a real problem hinders the widespread
  use of SOA
• A methodology to design and build secure
  service-oriented applications is needed.

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Problem

• How to achieve greater usage of the Service
  Oriented Architecture by making it more
  dependable.
• How to achieve reliable and fault tolerant web
  services
• The following properties must be added to the SOA
  
• Reliability
• Fault Tolerance

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Using Fault Tolerance tactics in Software
Architecture Approach

• The study was done on the impact of incorporating
  fault tolerance tactics into system architecture
  that use architectural patterns.
• Primarily for Legacy systems and already existing
  systems but can be used for green systems.
• Fault Tolerance tactics employed detection,
  recovery( preparation, repair, reintroduction and
  prevention
• Ten architectural patterns including
  Client-Server, Broker, Layers, Model View, Pipes
  and Filters were used.
• An impact scale was mapped from easy, neutral,
  trivial and difficult to show which of these
  patterns are easily changed to include fault
  tolerance into the system architecture.
• Useful in giving a cost analysis of the
  comparative amount of work required to add fault
  tolerance to existing systems

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RobustBPEL Approach

• This study looked at a language-based approach
  to address reliability in the business layer.
• RobustBPEL is a part of the transparent shaping
  programming model
• A composite web service defined as a BPEL process
  is instrumented automatically to monitor its
  partner web services at runtime.
• Events such as faults and timeouts are monitored
  from within the adapted process.
• This adapted process is augmented with a proxy
  that dynamically replaces failed services.
• They assert that this will improve fault
  tolerance and performance of BPEL processes by
  transparently adapting their behavior.
• Transparency is achieved by using a dynamic
  proxy.
• No change is made to the BPEL engine.

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Autonomous functionality in Web Services Approach

• This study is primarily geared at high
  availability of long running web services by
  extending the Web Services Architecture. The
  following new components were proposed
• Component Health Monitoring (CHM) module
  represents a new service used to track the health
  of individual Web Services components. basically
  a failure detection service.
• Consistent and Reliable Messaging (CRM) a simple
  optimized group communication layer, limited to
  small groups that use virtual synchrony for
  replication.
• Data Dissemination (DDS) module provides
  reliable multicast-style data streaming from
  the Web Services platform to a potentially large
  number of clients that must link directly to the
  DDS protocol. the DDS framework standardizes such
  notions as joining a group, sending a message,
  and delivering a message, but offers plug-in
  flexibility with respect to the actual properties
  of the protocol.

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Autonomous functionality in Web Services Approach

• Monitoring, Distributed Control (MDC) component
  responds to the need for mechanisms capable of
  monitoring and managing the entire system, by
  tracking performance metrics and other state
  variables and reporting them out Bir04.
• Event Notification (EVN) is last major component
  proposed and is still at an early design stage.
  EVN service focuses on urgent, small, one-time
  events. They have a notion of using some form of
  distributed query processing, in which the
  components of a Web Services system are treated
  as small Databases that can be queried Bir04.
• These five new components along with the use of
  the WS-Reliability, WS-ReliableMessaging and
  WS-Transaction standards were proposed.

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Challenges

• Difficult to address reliability and fault
  tolerance in collaborating webs services in
  multiple application scenarios.
• Multiple services developed and maintained on
  different environments introduce new levels of
  complexity.
• Unreliable communication channels
• Long running composite web service applications
• If the architecture of system already exist, thus
  it is difficult to make substantial changes
• Cost of incorporating fault tolerance into a
  system
• Given the architecture used in a system, which
  patterns are easier to maintain and adjust when
  necessary.
• May Become too complex if one extends the Web
  Services architecture to include autonomous
  functionality.
• In extending the SOAn by adding new layers and
  frameworks, may introduce additional
  vulnerabilities to the architecture.
• May affect the robustness of the architecture.

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Conclusion

• In sum a lot of work is being done to achieve
  reliability and fault tolerance in web services,
  however analysis of some these approaches show
  that they can introduce undesirable consequences
  such as increase in overall cost, complexity and
  maintenance.
• The use of patterns can be useful in adding
  reliability and fault tolerance to the Service
  Oriented Architecture.
• Reliability Patterns - WS-ReliableMessaging and
  WS-reliability
• Fault Tolerance Patterns Acknowledgement and
  Active Replication
• Look into the feasibility of extending the SOA
  framework to make it more dependable.
• Its not enough to have a dependable SOA but also
  to design secure and dependable web services
  applications.
• Systematic methodology that can be used to aid
  designers in building dependable web services.

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Future Work

• Look into designing a methodology to implement
  fault tolerance, reliability and autonomous
  functionality into the framework of the Web
  Services Architecture.
• Autonomous Webs Services supports reliability,
  as such we can develop patterns that describes
  the big picture general properties which
  include self-monitoring, self-diagnosis of
  faults, self-contained, discoverable,
  self-adapting and self-repair properties that is
  being sought currently.

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Recommendations

• Suggestions/feedback

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References

• Bir04 Ken Birman, Robbert van Renesse and
  Werner Vogels. Adding High Availability and
  Autonomic Behavior to Web Services. Proceedings
  of 26th International Conference on Software
  Engineering (ICSE04).2004.
• Eze08 Onyeka Ezenwoye and S. Masoud Sadjadi. A
  language-based approach to addressing reliability
  in composite web services. In Proceedings of the
  20th International Conference on Software
  Engineering and Knowledge Engineering
  (SEKE'2008), pages 649-654, San Francisco Bay,
  USA, July 2008.
• Har08 Neil B. Harrison and Paris Avgeriou.
  Incorporating Fault Tolerance Tactics in Software
  Architecture Patterns. Proceedings of ACM. SERENE
  2008. NewCastle, UK, November 17-19,2008.
• W3c04 David Booth, et al. Web Services
  Architecture. http//www.w3.org/TR/2004/NOTE-ws-ar
  ch-20040211/wsa.pdf, February 2004.