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## PowerPoint Slideshow about ' CIS 720' - nariko

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### CIS 720

Lecture 16

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]).

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?

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.

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?

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.

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