# Combining Optimism and Pessimism in Managing Replicas in Distributed

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```					Combining Optimism and
Pessimism in Accessing
Replicas in Distributed
Systems
Joel M. Crichlow
INTRODUCTION
The system contains a network of computing
nodes.
The data that can be accessed can have any
level of replication across the nodes.
Replication is employed in order to increase
availability.
Replication increases the need for effective
control measures to preserve some level of
mutual consistency.
The scheme under discussion combines
Optimism and Pessimism.
Introduction
Front
End

x,y,z     x,y,z         x,y,z

x,y           x
Rationale
Simplicity – KIS (note only one S)
Increasingly more powerful computers
Faster networks
Wireless?
Objectives
Provide a high level of availability at a
known penalty to the application
Preserve data integrity
Build a system that is conceptually
simple
The Cost Bound
Maximum Numerical Error
Determines the extent to which a
replica can act independently
We are considering resources that can
be counted and are of the same type
The Cost Bound
A request for r resources can be
satisfied if the reachable pool is
controlled by a cost bound of C
resources and r  C.
We can split the cost bound C among n
replicas such that each replica i has
n

c
i 1
i   C
The Cost Bound
Application characteristics can permit
temporary over-allocation of resources
(like over-booking), hence, if we let R
be the no. of resources initially
available, then a cost bound ci can be
set at each replica i such that
n

c
i 1
i   C R
The Cost Bound
We propose a dynamic C value which
changes in two ways:
In response to pessimistic global processing,
and
In response to optimistic local processing.
Dynamic C
On pessimistic run

Cnew  (1  ) * CinitRA
R
Cnew is new cost bound;
Cinit is cost bound initially set;
RA is no. of resources allocated via requests;
R is no. of resources initially available.
Dynamic C
On pessimistic run

ci  wi C
n
where     w
i 1
i     1

Let the resources allocated at replica i after a
pessimistic cycle be RAi then
n
RA   RAi
i 1

Then let

wi  ( RAi  1) /( RA  n)
Dynamic C
On optimistic run
)x (  ic  ic
where
x  {allocatereq,deallocatereq}
Testing
There are published performance
results for a prototype first coded in C
and now coded in Java.
In each cycle we used wi = 1/n.
Due to the expert work of Prof. Steve
Hartley the system now runs on a Linux
four-node platform in Rob.308.
Thanks Steve!!!
Testing
Three replicas process transactions that
are generated by a transaction
generator located at a remote
computer.
Testing
Each transaction is expressed as a
parent and child transactions.
The parent remains at the owner while
the children are sent to all the replicas.
Parents are processed pessimistically.
Children are processed optimistically.
Testing
Conclusion
A system has been designed to
investigate the use of optimism and
pessimism in accessing replicas in a
distributed system. The viability of the
system has been established and a Java
test bed is being used to examine
alternative features.

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 views: 5 posted: 5/3/2012 language: English pages: 17