Modelling the √N + ROWA Model Approach Inside the WS-ReplicationResource

1. Manuel Salvadores, Pilar Herrero, María S. Pérez, Alberto Sanchez Modelling the √N + ROWA Model Approach Inside the WS-ReplicationResource Grid Computing and its Application to Data Analysis (GADA'05) Facultad de Informática Universidad Politécnica de Madrid

2. 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.

3. 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.

4. 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

5. 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

6. 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:

7. A Decentralised Scenario: Use Case

8. 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.

9. 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) i.e. given four nodes (1,2,3,4):

10. Our Approach • Called √ N + ROWA • Association of two of these algorithms: • √N algorithm • The ROWA technique.

11. Our Approach 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.

12. 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.

13. Architecture

14. 1 . - Writing Request 2 6 . - Writing Request 2 . - Replication 1 3 4 . - Lock Quorum 7 . - Lock Quorum Only for Writing & & Replication 4 Operation 3 Replication Before performing Operation 1 operation 6 5.- Return Value 3 . - Reading Request S = { 1 , 2 } 1 S = { 2 , 3 } 2 S = { 1 , 3 , 4 } 3 S = { 2 , 4 } 4 How it works ?

15. 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

16. Scalability I • 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

17. Scalability II Taking into account that and

18. Average message exchange vs. Quorums’ length

19. Average message exchange for different quorums’ length

20. Future Work • Our currently effort have been focused in: • Definition of message level. • Draft of WSDL and Types schemas for WS-ReplicationResource • Definitions of the mechanism and related operations to group quorums. • Design of the WS-ReplicationResource Client Toolkit.

21. More information

22. Modelling the √N + ROWA Model Approach Inside the WS-ReplicationResource Thank you!