TimeUUIDs should be used for data that is time-based and requires uniqueness.
TimeUUID comparisons compare the time-based portion of the UUID. So no, you do not need to know the MAC addresses. In fact, for languages that cannot get to that low of a level to access a MAC address (like Java), the timeUUID tools generate random data for that part of the UUID. I don't understand your "user1"/"user2" scenario. The timeUUIDs in that scenario wouldn't even come in to question because the columns would be in two different rows since they pertain to two different users (unless they are referencing some other Column Family where those same TimeUUIDs are rows or columns in the same row). A good example of something that timeUUIDs would be great for would be friend requests. Regular time-based strings would not be sufficient in this case since it's possible that two requests can be sent from two different computers at the same time. Thus, you can store the requests as columns (or super columns) each named by a timeUUID. Later (assuming you've chosen to let Cassandra sort the columns or supercolumns by timeUUID), you can fetch all the requests for a given user in either most-recent, or least-recent order. From: Sameer Farooqui [mailto:cassandral...@gmail.com] Sent: Tuesday, June 14, 2011 8:16 PM To: user@cassandra.apache.org Subject: When does it make sense to use TimeUUID? I would like to store some timestamped user info in a Column Family with the usernames as the row key and different timestamps as column names. Each user might have a thousand timestamped data. I understand that the ver 1 UUIDs that Cassandra combines the MAC address of the computer generating the UUID with the number of 100-nanosecond intervals since the beginning of the Gregorian calendar. So, if user1 had data stored for an event at Jan 30, 2011/2:15pm and user2 had an event at the exact same time, the data could potentially be stored in different column names? So, I would have to know the MAC of the generating computer in order to do a column slice, right? When does it make sense to use TimeUUID vs just a time string like 20110130141500 and comparator type UTF8? - Sameer
