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IBM Labs in Haifa © 2004 IBM Corporation
DCS – Distribution and Consistency Services
Eliezer Dekel ([email protected])
Towards Flexible Levels of Availability
IBM Labs in Haifa
© 2004 IBM Corporation
Motivation� Today, development & management of applications supporting QoS is complex
� Intertwined concerns� Platform specific� Fix level of QoS at development time and not deployment time
� This increases the cost of application development and administration� Problem: the personnel cost is gradually eclipsing other system costs
� According to a recent white paper by GartnerConsulting1: "While clustering shows great promise as a high-availability solution, its viability today is limited by the complexity of implementing and maintaining the cluster... "
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IBM Labs in Haifa
© 2004 IBM Corporation
Motivation (Continued)
� Dave Patterson's IPDPS 2001 Keynote1: "systems should require only minimal ongoing human administration, regardless of scale or complexity: Today, cost of maintenance = 10X cost of purchase“
� Goals:� Reduce cost
�of developing (potentially) distributed applications and �administrating applications on heterogeneous and dynamically
changing platforms�End-to-end High Availability
� The evolution of e-commerce make High Availability (HA) and High Responsiveness (HR) fundamental requirements
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IBM Labs in Haifa
© 2004 IBM Corporation
Scalability
The capability of a system to adapt readily to a greater or lesser intensity of use, volume, or demand while still meeting its business objectives (acceptable levels of performance, availability, manageability etc.)
Ideal - Gracefully degrade as load increases. Seldom happens
Bad situation - Think it's OK until load increases. Poor design
Utilization increases faster than the load - Typical
Utilization increase linearly with load - Good Situation
Resource Utilization
Load
IBM Labs in Haifa
© 2004 IBM Corporation
The Scaling Paradox
� Vertical and Horizontal Scaling� Stateful and stateless applications are two extremes� The Scaling Paradox for stateful applications
� Having multiple copies (cached or replicated), leads to inconsistencies:� Modifying one copy makes that copy different from the rest.
� Keeping copies always consistent � Requires global synchronization on each modification.
� Global synchronization precludes large-scale solutions.� If we can tolerate inconsistencies, we may reduce the need for global
synchronization.� Tolerating inconsistencies is application dependent.� Allow applications to work at the level of inconsistency they tolerate
IBM Labs in Haifa
© 2004 IBM Corporation
Free Recovery?
IBM Labs in Haifa
© 2004 IBM Corporation
WebSphere High Availability
� WebSphere Application Server releases 6.0 and XD introduce a new high availability architecture that is based on an internal component referred to as Distribution and Consistency Services (DCS)
� DCS provides a mechanism for communicating information (Distribution) among members with a given (Consistency and Synchronization) Quality of Service (QoS) for� Performance, Scalability and Availability
� An inherent part of DCS and an integral part of its services is failure detection mechanisms that support and allow guaranteed QoS
� DCS Services are provided through two main interfaces :� Membership and Synchrony Service – HAManager Component� Data Replication Service – DRS Component
IBM Labs in Haifa
© 2004 IBM Corporation
HAManager
CoreDCSstack
DataDCS stack
HA MgrSubscriber
DRS
WAS Component
DataDCS stack
HA MgrSubscriber
WAS Component
DCS in WebSphere 6.0
WAS Component
Transport
IBM Labs in Haifa
© 2004 IBM Corporation
WAS XD 5.1 and WAS 6.0 DCS Component (Data & Core)
Membership Layer
Virtual Synchrony
Application Layer
RMM
Transport layer
DCS stack
IBM Labs in Haifa
© 2004 IBM Corporation
BACKUP
IBM Labs in Haifa
© 2004 IBM Corporation
DB2
Navajo Architecture�Data Partitioning
Clients Placing Orders
WASPresentationTier
DB2
WASBusiness LogicTier
INTC
MSFT
…
……
…
……
……
IBM
CSCO
HA
Man
ager
/DC
S
IBM“BUY”
IBM“BUY”
IBM“BUY”
B
C
A
Affinity based WLM
A
B
C
DBCache
DBCache
DBCache
� Caching
IBM Labs in Haifa
© 2004 IBM Corporation
DB2
Clients Placing Orders
WASPresentationTier
DB2
INTC
MSFT
…
……
…
……
……
IBM
CSCO
XA
B
CA
B
C
A
IBM
IBM“BUY”
IBM“BUY”
IBM“BUY”
IBM
Navajo Architecture�Fast-failover
Affinity based WLM
WASBusiness LogicTier
HA
Man
ager
/DC
S
IBM Labs in Haifa
© 2004 IBM Corporation
DB2
Clients Placing Orders
WASPresentationTier
DB2
INTC
MSFT
…
……
…
……
……
IBM
CSCO
A
B
CA
B
C
A
IBM
IBM“BUY”
IBM“BUY”
IBM“BUY”
IBM
Navajo Architecture�Repartitioning
WASBusiness LogicTier
HA
Man
ager
/DC
S
Affinity based WLM
