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CIS 720

CIS 720. Lecture 16. Client-Centric Consistency. Intended to address the issues in eventual consistency for mobile clients. Consistent for a single client. Notation x i [ t ] is the version of x at local copy L i at time t .

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CIS 720

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  1. CIS 720 Lecture 16

  2. Client-Centric Consistency • Intended to address the issues in eventual consistency for mobile clients. • Consistent for a single client. • Notation • xi[t] is the version of x at local copy Li at time t. • Version xi[t] is the result of a series of write operations at Li that took place since initialization. This is represented by WS(xi[t]). • At time t, If operations in WS(x[t1]) have also been performed at local copy Lj, we write it as WS(xi[t1];xj[t]).

  3. Read-after-read • A data store is said to provide read-read consistency if the following condition holds: • If a process reads the value of a data item x then any successive read operation on x by that process will always return that same value or a more recent value. • In other words, if a process has seen a value of x at time t, it will never see an older version of x at a later time. • Example: Suppose a user opens his mailbox in San Francisco, then flies to New York. Should he see an earlier version of his mailbox?

  4. Which one of these obeys this model?

  5. Write-after-write • In a writer-after-write consistent store, the following condition holds: • A write operation by a process on a data item x is completed before any successive write operation on x by the same process. • In other words, a write operation must wait for all preceding write operations.

  6. Which one of these obeys that?

  7. Read Your Writes • A data store is said to provide read-your-writes consistency, if the following condition holds: • The effect of a write operation by a process on data item x will always be seen by a successive read operation on x by the same process. • In other words a write operation is always completed before a successive read operation by the same process, no matter where the read operation takes place. • Suppose your web browser has a cache. • You update your web page on the server. • You refresh your browser. • Do you have read-your-writes consistency?

  8. Which of these is read-your-writes?

  9. Writes Follow Reads • A data store is said to provide writes-follow-reads consistency, if the following holds: • A write operation by a process on a data item x following a previous read operation on x by the same process is guaranteed to take place on the same or a more recent value of x that was read. • In other words, any successive write operation by a process on a data item x is guaranteed to take place on a copy of x that is up to date with the value most recently read. • Example: Suppose we are replicating a database for a blog. Performing a write amounts to posting a response. If we do not use writes-follow-reads, then it would be possible for a user to read a response without the original.

  10. Which of these obeys writes-follow-reads?

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