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. –Version x i [t] is the result of a series of write operations at L i that took place since initialization. This is represented by WS(x i [t]). –At time t, If operations in WS(x[t1]) have also been performed at local copy L j, we write it as WS(x i [t 1 ];x j [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?

Which one of these obeys this model?

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.

Which one of these obeys that?

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?

Which of these is read-your-writes?

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.

Which of these obeys writes-follow-reads?