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Consistency Models

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  1. Consistency Models

  2. A Replication Problem • Multiple copies may lead to consistency problems. • Whenever a copy is modified, that copy becomes different from the rest. • Modifications have to be carried out on all copies to ensure consistency. • The type of application has an impact on the consistency requirements needed and thus on the implementation.

  3. Consistency Model • A consistency model describes the rules to be used in updating replicated data • The rules define the order of operations. • Rules used depend on the application • Consistency defined within the context of read and write operations on shared data is data-centric • Strict • Sequential • Causal • FIFO

  4. Strict Consistency • Strict consistency is defined as follows: • Read is expected to return the value resulting from the most recent write operation • Assumes absolute global time • All writes are instantaneously visible to all • Suppose that process pi updates the value of x to 5 from 4 at time t1 and multicasts this value to all replicas • Process pj reads the value of x at t2 (t2 > t1). • Process pj should read x as 5 regardless of the size of the (t2-t1) interval.

  5. Strict Consistency • What if t2-t1 = 1 nsec and the optical fiber between the host machines with the two processes is 3 meters. • The update message would have to travel at 10 times the speed of light • Not allowed by Einsten’s special theory of relativity. • Can’t have strict consistency

  6. Sequential Consistency • Sequential Consistency: • The result of any execution is the same as if the (read and write) operations by all processes on the data were executed in some sequential order • Operations of each individual process appears in this sequence in the order specified by is programs. • We have seen this in the banking example • One implementation used Lamport’s clocks.

  7. Causal Consistency • Causal Consistency: That if one update, U1, causes another update, U2, to occur then U1 should be executed before U2 at each copy. • Application: Bulletin board • Possible Implementation: Using vector clocks

  8. FIFO Consistency • Writes done by a single process are seen by all other processes in the order in which they were issued • … but writes from different processes may be seen in a different order by different processes. • i.e., there are no guarantees about the order in which different processes see writes, except that two or more writes from a single source must arrive in order.

  9. FIFO Consistency • Caches in web browsers • All updates are updated by page owner. • No conflict between two writes • Note: If a web page is updated twice in a very short period of time then it is possible that the browser doesn’t see the first update. • Implementation: • Each process adds the following to an update message: (process id, sequence number) • Each other process applies the update messages in the order received from a single process.

  10. Consistency Models • Lots of them • We will discuss one more which will be related to the next paper you will review

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