1. 7.1. CONSISTENCY AND REPLICATION INTRODUCTION
DINESH INAVOLU

2. Consistency
In Advanced Computing Systems, Consistency  refers to the requirement that any change to the system made by a transaction must be such that it keeps the system valid according to all system’s defined rules. 

3. Replication
In advanced Computing systems, Replication is about keeping resources redundant or copying resources , and maintaining consistency among them.
It should be transparent to an external user. Replication uses distributed technology to share data between multiple sites.

4. Reasons for Replication
Reliability
If one replica is unavailable or crashes, use another
Protect against corrupted data
Performance
Scale with size of the distributed system (replicated web servers)
Scale in geographically distributed systems (web proxies)

5. What may be replicated?
Data can be replicated by storing the same data on multiple storage devices. It is called Data Replication.
Computing tasks may be replicated, it is called computation replication  and it’s when the same computing task is executed either repeatedly on a single device( replication in time) or the same is executed on separated devices (replication in space)

6. Passive Replication
Where there is only one server (the primary server ) which processes client requests. After processing a request, it updates the state on the other servers, copy / replica and sends back the response to the client. If the primary server fails, one of the replica servers takes its place.

7. Passive Replication

8. Active Replication
Here each client request is sent to and processed by all the servers. For all the servers to receive the same sequence of operations, an atomic broadcast protocol must be used to guarantee that either all the servers receive a message or none, plus that they all receive messages in the same order.

9. Active Replication

10. Drawbacks of Replication
Increased overhead on update: When an update is required, a database system must ensure that all replicas / copy are updated.
Require more disk space, so increase resources usage: Storing replicas of same data at different sites consumes more disk space.
Expensive: Concurrency control and recovery techniques are more advanced and hence more expensive.

11. References
Distributed Systems, Principles and Paradigms, Second Edition, Andrew s. Tanenbaum Maarten Van Steen
Replication ,
Consistency and Replication: Distributed OS, University of Massachusetts.

12. THANK YOU