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    <p>Tim, you really should have posted your question separately, rather than as an answer to the OP, which it wasn't.</p> <p>But to answer, first, go read Ben Black's slides at <a href="http://www.slideshare.net/benjaminblack/introduction-to-cassandra-replication-and-consistency" rel="nofollow">http://www.slideshare.net/benjaminblack/introduction-to-cassandra-replication-and-consistency</a>.</p> <p>Done? Okay, now for the specific questions:</p> <p>"How would differences in [replica] data-state be reconciled on a subsequent read?"</p> <p>The highest timestamp wins.</p> <p>"Do all zones work off the same system clock?"</p> <p>Timestamps are provided by clients (i.e., your app server). They should be synchronized with e.g. ntpd (which is good practice anyway), but high precision is not required because if ordering matters you should be avoiding conflict either by using unique column names or by using external locking.</p> <p>For example: if you have a list of users following you in a Twitter clone, you should give each follower its own column and there will be no way to lose data no matter how out of sync the clocks are.</p> <p>If you have an admin tool for your website and two admins upload a new favicon "simultaneously," one update is going to win and it doesn't really matter which. Here, you do want your clocks synchronized but "within a few ms" is close enough.</p> <p>If you are managing user registration and you want to allow creating account "jbellis" only if it doesn't already exist, you need a lock manager no matter how closely synchronzied your clocks are.</p> <p>"Would stale data get returned?"</p> <p>A node (a better unit to think about than a "zone") will not have data it missed during its downtime until it is sent that data by read repair, hinted handoff, or anti-entropy repair. In the meantime, it will reply to read requests with stale data; if you use a high enough consistencylevel read requests will wait for enough other replies to make sure you always see the most recent version anyway, which may mean not being able to fulfil requests if enough other replicas are down.</p> <p>Otherwise, a low consistencylevel (e.g. ONE) implicitly means "I understand that the higher availability and lower latency I get with this lower consistencylevel means I'm okay with seeing stale data temporarily after downtime."</p>
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    1. POwhich NOSQL database tool is better to choose for my application?
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    1. COThanks for the clarification on the concurrency model. P.S. If comments could be longer on this site, and formatted, my reply would have been a comment, because it was 80% comment and only 20% answer. Sometimes one must violate the rules of this website in order to participate cogently in the conversation.
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