Prof. Orhan Gemikonakli Module Leader: Prof. Leonardo Mostarda Università di Camerino

1. Distributes Systems – Consistency (2) Prof. Orhan Gemikonakli Module Leader: Prof. Leonardo Mostarda Università di Camerino Prof. Orhan Gemikonakli - Camerino,

2. Last lecture State machine replication approach Paxos protocol Prof. Orhan Gemikonakli - Camerino,

3. Outline Replica management Permanent replicas Server initiated replicas Client initiated replicas Pull versus push protocols Consistency protocols Prof. Orhan Gemikonakli - Camerino,

4. Learning outcome • To understand the basic concepts related to replica management • Permanent replicas • Server initiated replicas • Client initiated replicas • To describe and discuss pull and push protocols • To understand various consistency protocols Prof. Orhan Gemikonakli - Camerino,

5. REPLICA MANAGEMENT • Replica management includes • placement • mechanisms to use for keeping the replicas consistent • The placement problem is split into two sub problems: • placing replica servers • placing content Prof. Orhan Gemikonakli - Camerino,

6. Replica Management • To decide • where • when and • by whom replicas should be placed. • Placement problem* • Placing servers • Placing content * Client aware placement / client unaware placement Prof. Orhan Gemikonakli - Camerino,

7. Replica Server Placement • FChoosing a proper cell size for server placement. Prof. Orhan Gemikonakli - Camerino,

8. Content Replication and Placement • The logical organization of different kinds of copies of a data store into three concentric rings. Prof. Orhan Gemikonakli - Camerino,

9. Content Replication and Placement • Permanent replicas can be considered as the initial set of replicas that constitute a distributed data store. • Web site distribution • files that constitute the Web are replicated in different servers, when a request comes in a round robin policy is applied • mirroring (the client choose which mirror to use) Prof. Orhan Gemikonakli - Camerino,

10. Content Replication and Placement • Server-initiated replicas are copies of a data store that are created to enhance performance. • Web server placed in New York. • it may happen that over a couple of days a sudden burst of requests come in from an unexpected location far from the server • it may be worthwhile to install a number of temporary replicas in regions where requests are coming from Prof. Orhan Gemikonakli - Camerino,

11. Server initiated replicas • Counting access requests from different clients. Prof. Orhan Gemikonakli - Camerino,

12. Content Replication and Placement • Client-initiated replicas are more commonly known as (client) caches • In principle, managing the cache is left entirely to the client. • However, there are many occasions in which the client can rely on participation from the data store • Data are generally kept in a cache for a limited amount of time • Whenever requested data can be fetched from the local cache, a cache hit is said to have occurred • What is the benefit of caching? Prof. Orhan Gemikonakli - Camerino,

13. Content Replication and Placement • Replica management also deals with propagation of (updated) content to the relevant replica servers. • Propagate only a notification of an update. • Transfer data from one copy to another. • Propagate the update operation to other copies. Prof. Orhan Gemikonakli - Camerino,

14. Content Replication and Placement • Propagating a notification is what invalidation protocols do. • The invalidation may specify which part of the data store has been updated • Advantages • little bandwidth is required to transfer the notification • work best when there are many update operations compared to read operations • For instance for a large data store 2 updates may happen without a read in the middle. Prof. Orhan Gemikonakli - Camerino,

15. Content Replication and Placement • Transferring the modified data among replicas is the second alternative • when the read-to-write ratio is relatively high • The third approach is not to transfer any data modifications at all, but to tell each replica which update operation it should perform (active replication) Prof. Orhan Gemikonakli - Camerino,

16. Pull versus Push Protocols • Another design issue is whether updates are pulled or pushed • In a push-based approach updates are propagated to other replicas without those replicas even asking for the updates. • Server-based protocols are applied when replicas need to be kept identical. • The read-to-update ratio at each replica is relatively high Prof. Orhan Gemikonakli - Camerino,

17. Push Technology Client registers interest for service, server pushes new content Prof. Orhan Gemikonakli - Camerino,

18. Pull versus Push Protocols • In a pull-based approach, a server or client requests another server to send it any updates it has at that moment. • are often used by client caches • is efficient when the read-to-update ratio is relatively low Prof. Orhan Gemikonakli - Camerino,

19. Browser cache Do you know how the browser cache works? The browser will first check its local cache to see whether the object requested from an origin server has already been stored locally on hard disk. If it has, and it is up-to-date, the object will be returned by the cache to the browser. HTTP supports the determination of whether the object is current via the If-Modified-Since header. Prof. Orhan Gemikonakli - Camerino,

20. CONSISTENCY PROTOCOLS • A consistency protocol describes an implementation of a specific consistency model. • primary-based protocol • all processes see all write operations in the same order Prof. Orhan Gemikonakli - Camerino,

21. Local-Write Protocols • Primary-backup protocol in which the primary migrates to the process wanting to perform an update. Prof. Orhan Gemikonakli - Camerino,

22. Summary Replica management Permanent replicas Server initiated replicas Client initiated replicas Pull versus push protocols Consistency protocols Prof. Orhan Gemikonakli - Camerino,

23. Next Lecture • Fault Tolerance • Basic concepts • Fault modelling • Failure masking • RPC Semantics in the Presence of Failures • Reliable multicast schemes • Virtual Synchrony • Two phase commit • |Three phase commit • Checkpointing and Message logging Prof. Orhan Gemikonakli - Camerino,