ws-replicationresource modelling the √n + rowa model approach inside the ws-replicationresource...
Post on 21-Dec-2015
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Modelling the √N + ROWA Model Approach Inside the WS-ReplicationResource
Manuel Salvadores, Pilar Herrero, María S. Pérez, Alberto Sanchez
Facultad de InformáticaUniversidad Politécnica de Madrid
Grid Computing and its Application to Data Analysis (GADA'05)
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Introduction
● Open Grid Service Architecture enumerate those characteristics that Grid systems have to possess.
● High availability plays an important role among all these characteristics.
● The replication concept is close related to the availability concept, being one of the techniques more employed for failure recovery.
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How could a state be defined?
A resource could be defined as a● Web Service● With a set of properties, defined by the WS-
ResourceProperties.● Being its state the combination of all the values
associated to all these properties at a given moment.
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WS-ResourceProperties
Four operations are defined in the WS-ResourceProperties specification to access to the resource’s properties:
1. GetResourceProperty
2. GetMultipleResourceProperty
3. SetResourceProperties
4. QueryResourceProperties
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A Decentralised Scenario
● Each of the nodes could be the queries’ or updates receptor over a replicated resource
● Having an idea about:– Which of the resource’s properties could be accessed
at a given moment
– Making all this possible in an autonomous way
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A Decentralised Scenario: Use Case
• An initial scenario:– i nodes N={N1,N2,N3, …, Ni}– Each of these nodes could also be the receptor of reading or
writing requests. – In order to ensure the fairness of the actions to be carried out:
• each of the actions represented as a tuple (a, t), where ‘a’ represents the action to be carried out in the moment ‘t’.
If (N=4):
The casual order constraint:
A={ (a1,t1), (a2,t2), (a3,t3) ,(a4,t4) }
],1[11 Niaatt iiii
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A Decentralised Scenario: Use Case
NiN1
N2N3
...
ReplicationResource
ReplicationResource Replication
Resource
ReplicationResource
Process1
Process2
a1=SetProperties(“Counter”,5)t1
a2=SetProperties(“Counter”,10)t2
t1 < t2 < t3 < t4
Process3a3=GetProperty(“Counter”)
t3
Process 3 obtains counter = 10
Process4
a4=SetProperties(“Counter”,15)t4
WS-ReplicationResource
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Related Work I
Scalable Model: Quorum
“Let S = {S1, S2, …} be a set of sites. A quorum system Q is a set of subsets of S with pair-wise non-null intersection. Each element of Q is called a quorum”
if we have four sites:S1, S2, S3, S4.
A possible quorum system : {S1, S2, S3}, {S2, S3, S4}, {S1, S4}
ROWA(Read One write All)
• Read Operations: read from any site. If a site is down, try another site.
• Write operations: write to all sites. If any site rejects the write, abort the transaction.
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Related Work II
√N Quorums The √N algorithm, being N the number of nodes, is based on the association of nodes in N minimum subsets with no null intersection (between each two of them)
, 1 , ,i j i j N S Si j
S1 = {1,2} S2={2,3}
S3={1,3,4} S4={2,4}
i.e. given four nodes (1,2,3,4):
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Our Approach
– Called √ N + ROWA– Association of two of these algorithms:
● √N algorithm ● The ROWA technique.
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e Our approach will also consider two key factors while replicating the data through the nodes:
●The impact that the “lazy propagation” technique ●The scalability of the system
An i-node wish to carry out an writing operation:– it requires the votes of the quorum Si
A writing/reading operation over Sj being i≠j and – the node Nz will have to send Sj the updated modifications over
the synchronised element before giving Sj its vote.
Our Approach
i j zS S N
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Architecture
We have identified two components to be introduced inside each and every node
1. A mutex property deployed in the nodes as a WS-ResourceProperty.
2. The √N + ROWA engine that interacts with the mutex to take decisions and implements the read and write operations.
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Architecture
WS-ReplicationResource
WS-ResourcePropertyMutex
N √ + ROWA Engine
Reliable Applications
Read Write
(Operations)Data
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How it works ?
1
3
2
4
S1 = {1,2}S2 = {2,3}S3 = {1,3,4}S4 = {2,4}
1.- Writing Request
2.- Replication
3.- Reading Request
4.- Lock Quorum Only for Writing & Replication Operation 1
5.- Return Value
6.- Writing Request
7.- Lock Quorum & Replication Operation 3Before performing operation 6
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Scalability
•First work hypothesis:
There are not possible collisions in the system
•Second work hypothesis:
The time to process one operation is much lower
that the number of transactions per unit of time
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Scalability I
( 1) ( 1) ( )( 1) ( _ ) ( )( 1)m k k p w k p change q p w k
The average of messages to be sent depends on:
and it will depend on:
• the operation request as (k-1)• answers as (k-1) • the replication as (k-1) (only in a writing request) • if the last operation was carried out in another Quorum an additional factor has to be taken into account
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Scalability II
2 1N k k
1( _ ) Np change qN
Taking into account that
and
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eAverage message exchange vs. Quorums’ length
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eAverage message exchange for different quorums’ length
Number Nodes (N) Quorum Length (K) Messages Exchange
381 20 42
1561 40 86
3541 60 130
6321 80 174
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Future Work
● Our currently effort have been focused in:Definition of message level.Draft of WSDL and Types schemas for WS-
ReplicationResourceDefinitions of the mechanism and related
operations to group quorums.Design of the WS-ReplicationResource Client
Toolkit.
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eModelling the √N + ROWA Model Approach
Inside the WS-ReplicationResource