The JgroupARM Dependable Computing Toolkit

1
The Jgroup/ARMDependable Computing
Toolkit Hein Meling Stavanger University College
NorwayDepartment of Electrical and Computer
Engineering Alberto Montresor University of
Bologna - ItalyDepartment of Computer Science
2
Context

• (Distributed) systems that require
• Reliable and high-availability operation
• Fault tolerance
• (Load balancing)
• Based on cheap hardware and software
• Commercial off the shelf, and not custom hardware
• Heterogenous software (OS) architectures
• Middleware architectures for distributed
  computing
• Middleware between the application and OS

3
Types of Failures

• Processor failures
• Crash failures
• Value failures (very expensive)
• Network failures
• Operating System hangs
• Memory leaks
• Software design errors(beyond state-of-the-art)

4
Overview

• Jgroup
• A toolkit aimed at supporting the development of
  reliable and highly-available applications.
• Autonomous Replication Management (ARM)
• A framework for server replica deployment and
  recovery without user intervention.
• History
• Formal specification (1996-97)
• Algorithm description and Jgroup implementation
• Integration with existing technologies (Java RMI
  / Jini)
• The ARM framework (2000-03)
• Development of Jgroup-based applications

5

• Summary
• Introduction
• Object Group Communication
• The ARM framework
• Integration with Java RMI / Jini
• Conclusions

6
The Problem

• Some environments supporting distributed
  computing
• CORBA (OMG)
• DCOM / .NET (Microsoft)
• Java RMI / Jini / EJB (Sun)
• Characteristics
• Object-oriented
• Based on client - server remote method
  invocations
• Promote modularity, reusability,
  interoperability, portability

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Java Remote Method Invocations

• Java RMI protocol
• enables objects residing in different JVMs to
  communicate through remote method invocations

JVM1
JVM2
8
Java Remote Method Invocations
method()
return x
JVM1
JVM2
9
The Problem

• Distributed computing environments did not
  provide adequate support for developing reliable
  and high-available applications
• Lack of reliable one-to-many interaction
  primitives
• From the clients point of view non-transparent
  access to replicated servers
• From the servers point of view no support for
  maintaining consistency

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The Solution The Object Group Paradigm

• Object group
• A dynamic collection of server objects that
  cooperate in order to deliver some service and
  maintain shared state
• Group method invocations
• The act of invoking a method on an object group
• The method is executed by a certain number of
  servers in the object group, depending on the
  invocation semantics

ObjectGroup
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The Solution The Object Group Paradigm

• From the clients point of view
• Groups must be transparent - like standard remote
  objects
• Clients need not be aware that they are
  interacting with an object group instead of a
  single server
• From the servers point of view
• Server implementation - as transparent as
  possible
• Servers forming a group
• must cooperate to maintain shared state and
• to appear as a single object

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Group Communication

• Group communication has been shown to be a
  powerful paradigm for supporting the development
  of dependable applications in distributed systems
• Management of dynamic groups(join/leave
  operations)
• Failure monitoring(crashes / partitionings)
• One-to-many communication
• Ordering of events (FIFO, Causal, Atomic)
• State synchronization tools

13
Other Object Group Systems

• CORBA
• Electra Cornell, Zurich
• Object Group Service (OGS) EPFL, Lausanne
• Eternal UC Santa Barbara, Eternal Systems
• Newtop Newcastle, UK
• Java RMI
• Filterfresh Bell Labs
• JavaGroups Cornell
• Aroma UC Santa Barbara
• DCOM
• Quintet Cornell

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Jgroup Yet Another Object Group Service?

• Support for partition-awareness
• Modern wide-area communication networks are often
  characterized as highly partitionable
• Jgroup supports the development of reliable and
  high-available applications in partitionable
  systems
• Moreover
• Is extends modern technologies like Java RMI and
  Jini
• Is completely written in Java (portability)
• Supports complex merging service
• Extensible deployment, recovery and upgrade
  facilities

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Autonomous Replication Management

• Support for transparent replica deployment
• Placing server replicas on machines in the
  network
• Selecting machines so that each application can
  tolerate both network and machine failures
• Support for replica recovery
• Jgroup detect and report failures
• ARM replace any crashed server replica with a new
  instance

16

• Summary
• Introduction
• Object Group Communication
• The ARM framework
• Integration with Java RMI / Jini
• Conclusions

17
Group Membership

• Group membership service tracks both voluntary
  and involuntary changes in the groups membership
• Variations are reported to group members through
  the installation of views
• Installed views
• Consist of a collection of members
• Correspond to the groups current membership as
  perceived by the members included in the view

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Group Membership A Simple Scenario
view
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Partition-awareness

• What kind of behavior can we expect from
  fault-tolerant applications in the presence of
  network partitioning?
• The primary-partition approach

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Support for partition-awareness

• Jgroup supports dependability in partitionable
  systems
• Development of applications aware of the
  existence of partitions (on the server-side)
• Partition-aware applications take advantage of
  their semantics in order to be more available
• Computations continue in all partitions of the
  system

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Group Membership A Partitioning Scenario
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Example Task Execution Service
Primary Partition
Partition-aware
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Comparison

• Primary-partition approach
• Easy to maintain a single, coherent shared
  state(strong consistency)
• Servers in non-primary partitions unable to serve
  requests (low availability)
• Partition-aware approach
• Servers in multiple partitions may be able to
  serve requests(high availability)
• Partitions evolve independently, possibly leading
  to inconsistent states (loose consistency)

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Comparison (Cont.)

• Primary-partition approach
• Development of fault-tolerant applications is
  simpler(active replication of existing non
  fault-tolerant servers)
• Developers cannot exploit application semantics
  in order to provide a more available service
• Partition-aware approach
• Applications adapt their behavior and remain
  available in many partitions (perhaps by reducing
  their quality of service)
• Development of fault-tolerant applications is
  more complex (case-by-case design is needed)

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The State Merging Problem

• During partitioning, the state of servers
  belonging to distinct partitions may become
  inconsistent
• When the partition disappears, an
  application-specific state merging protocol may
  be needed
• Servers participating in the protocol try to
  define a new shared state that reconciles (when
  possible) the divergences

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The State Merging Problem

• During partitioning, the state of servers
  belonging to distinct partitions may become
  inconsistent
• When the partition disappears, an
  application-specific state merging protocol may
  be needed
• Servers participating in the protocol try to
  define a new shared state that reconciles (when
  possible) the divergences

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The State Merging Problem

• State merging protocols are based on the exchange
  of information among servers that have been
  partitioned
• Jgroup provides a state merging service (SMS)
  that simplifies the development of state merging
  protocols
• NOTE
• Determining
• what information needs to be exchanged
• how to use it to construct a new consistent
  shared state
• is an application-dependent problem

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General Schema for State Merging Protocols

• In each of the merging partitions, a coordinator
  is selected
• SMS interrogates each coordinator to obtain
  information about its current state
• State information from a coordinator is passed to
  servers that used to be partitioned from it
• Each of the servers merge information from
  coordinators with their own state

S1
getState()
S2
putState()
S3
S4
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General Schema for State Merging Protocols

• In each of the merging partitions, a coordinator
  is selected
• SMS interrogates each coordinator to obtain
  information about its current state
• State information from a coordinator is passed to
  servers that used to be partitioned from it
• Each of the servers merge information from
  coordinators with their own state

S1
getState()
S2
putState()
S3
S4
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Full Object-Orientation

• Existing object group systems fail to provide a
  completely object-oriented environment for
  software developers

Remote methodinvocations
Messagemulticasting
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View Synchrony

• View synchrony (1)
• If a correct server S executes an invocation
  during a view, then
• all servers within the view will also execute the
  invocation,
• or S will install a new view
• View synchrony does not admit executions like
  this

S1
S2
S3
S4
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View Synchrony

• View Synchrony (2)
• All servers that survive from one view to the
  same next view execute the same set of
  invocations in the original view
• View synchrony does not admit executions like
  this

S1
S2
S3
S4
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Internal Group Method Invocations

• Synchronous invocations
• The method invocation terminates by returning a
  vector of return values, one from each server at
  which the method was executed
• Asynchronous invocations
• The method invocation terminates immediately
  replies (if any) are returned to a callback
  object
• Can be used to simulate message multicasting
  through void methods (one-way)

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Internal Invocations example
Synchronous invocation
S1
S2
S3
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Internal Invocations example
S1
S2
S3
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External Group Method Invocations

• Anycast invocations
• Are executed by at least one server in the object
  group (unless the client is partitioned from the
  group)
• Efficiency (same cost as standard RMI
  interactions)
• Useful for read methods on replicated databases
• Multicast invocations
• Are executed by all servers in a view, following
  the view synchrony semantics
• More costly (involve several servers)
• Useful for write methods on replicated databases

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External invocations example
C1
S1
S2
S3
C2
Multicast invocation registry.bind(name, obj)

Anycast invocation registry.lookup(name)

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• Summary
• Introduction
• Object Group Communication
• The ARM framework
• Integration with Java RMI / Jini
• Conclusions

39
Replication Management The Problem

• Object Group Systems support replication
  transparency
• Membership management
• Reliable multicast
• But does not support full failure transparency
• Application or manual support to distribute
  replicas
• Application support or manual intervention
  required to recover from replica failures
• Complicated tasks
• Application implementations prone to contain
  errors
• These tasks should not be left to the application
  developer

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Solution Autonomous Replication Management

• Support for creating object groups
• By placing individual members on distinct
  machines
• Each application may specify a replication policy
• For example, redundancy level 3
• Support for failure recovery
• Jgroup detects and reports failures to ARM
• ARM reacts by creating a replacement member for
  each failed member, perhaps on a different
  machine
• Each application may specify a recovery policy

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ARM Replica Distribution
item.ntnu.no
Management Client
Router
ux.his.no
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ARM Replica Distribution
item.ntnu.no
Management Client
Router
ux.his.no
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ARM Recovery from Crash Failure
item.ntnu.no
Group Leader
Management Client
Router
ux.his.no
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ARM Recovery from Crash Failure
item.ntnu.no
Group Leader
Management Client
Router
ux.his.no
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• Summary
• Introduction
• Object Group Communication
• The ARM framework
• Integration with Java RMI / Jini
• Conclusions

46
Introduction to Jini

• Jini is an API built on top of the Java 2
  platform
• enables spontaneous networks of devices/software
  services to assemble into federations of objects
• addresses the distribution problems in these
  federations through a set of simple interfaces
  and protocols

Jini Network
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Jini Architecture

• The components of the Jini architecture may be
  divided in three categories
• Infrastructure i.e. the components that enables
  building a federated Jini system
• Model that supports and encourages the
  production of reliable distributed services
• Services that can be made part of a federated
  Jini system and which offer functionality to any
  other member of the federation
• Javaspaces

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Jini Infrastructure

• The infrastructure is composed of
• Java RMI protocolenables objects residing in
  different JVMs to communicate through remote
  method invocations

JVM1
JVM2
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Jini Infrastructure

• The infrastructure is composed of
• Lookup Service defines how services may become
  part of a Jini system and clients retrieve
  services by their types and attributes.

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The Jini Programming Model

• The programming model is based on three distinct
  paradigms for distributed computing
• Leases extend the Java programming model by
  adding the time to the notion of holding a
  reference to a resource
• Transactionsallow a set of operations on one or
  more remote participants to be grouped in such a
  way that either all succeed or all fail
• Eventsenable objects to register interest in
  changes of the abstract state of remote objects

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Jini and Fault Tolerance

• Jini fault tolerance is based on leases and
  transactions
• leases enable the detection of service failures
• transactions provide consistency by guaranteeing
  all-or-nothing semantics
• Unfortunately, no support for high-availability
  is present in Jini
• No support for replication
• Failure of transaction manager ? clients and
  participants must wait for the recovery of the
  manager before serving further requests

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Enhancing Jini with Fault-Tolerance

• Extending Jini with the Object Group Paradigm
• Infrastructure
• Extending Java RMI for Group Method Invocations
• Extending the Lookup Service for dealing with
  Group Proxies
• Programming Model
• Object Group Paradigm as alternative programming
  model
• Integration between transactional and object
  group model
• Services
• Replicated JavaSpaces

53
Extending Java RMI

• RMI group at Javasoft designed Java RMI in order
  to be extensible
• The RemoteRef interface enables programmers to
  write their own references to remote objects on
  the client-side
• Unfortunately, RemoteRefs are not sufficient
• There is no possibility to modify the behavior of
  RMI on the server side

RemoteRef
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The Jgroup Approach (Current Version)
Statically or dynamically generated
implementsthe remote interface
Method dispatchers
Fixed stub for server proxy
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Designing a New Java RMI API

• We have cooperated with Sun Microsystems to
  design a new RMI API
• Fully customizable, on both the client-side and
  the server-side
• Based on Dynamic Proxy Classes (JDK 1.3)(No need
  for static stub generators)
• Two different versions
• One-to-one (remote method invocations)
• Voted down in JSR-078
• Being included in the "Davis" release of Jini
• One-to-many (group method invocations)

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Jgroup with 1-to-1 Customizable RMI
Statically or dynamically generated
implementsthe remote interface
Method dispatchers
57
Jgroup with 1-to-Many Customizable RMI
Customizableobjects
Multicast RMI
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Extending the Lookup Service

• Jini enables the registration of customized
  proxies for services
• this feature can be used to register group
  proxies using any implementation of the lookup
  service
• Group proxies, however, differ from standard
  proxies as their contents may be dynamic
• server registration ? server reference added to
  group proxy
• server removal, lease expired ? server reference
  removed from group proxy
• We have developed an alternative implementation
  of the lookup specification capable to deal with
  group proxies

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The Jgroup Lookup Service
Lookup
Invocation
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Extending the Jini Programming Model

• Jgroup Jini programming model for
  fault-tolerance
• Leases transactions
• Object group communication
• Problem
• transactions and group communication considered
  as separate aspects of fault-tolerance
• their composition does not result in any
  meaningful combination of their respective
  strengths
• We need the possibility of using replication in
  transactions
• Transaction managers
• Participants
• Clients

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• Summary
• Introduction
• Object Group Communication
• The ARM framework
• Integration with Java RMI / Jini
• Conclusions

62
Applications (Research)

• Jgroup/ARM is being used for
• A distributed auction system
• Partitionable auctions
• Panzieri, Amoroso et al., University of Bologna,
  2002
• An online-upgrade service for active replication
• Solarski, GMD Fokus
• A replication management framework
• Application-specific replication and recovery
  strategies
• Meling, HiS
• Dependable naming service
• Support for extensible group proxies (JERI)
• Meling et al., HiS

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Applications (Education)

• Jgroup is being used at the
• Stavanger University College in the Advanced
  Programming course
• University of Bologna in the Distributed System
  course
• Norwegian University of Science and Technology in
  the Dependable Systems course
• Source for several projects and thesis
• Low-level communication protocols (Bologna)
• Replication services (Bologna)
• Wide-area distributed services (Padova)
• Management and deployment issues (HiS)

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• Thank You!
• http//jgroup.sourceforge.net/