Cassandra makes no attempt to provide cross-row isolation or atomicity. (Within a row, it technically provides atomicity but not isolation.)
On Fri, Sep 9, 2011 at 9:33 PM, <alta...@ceid.upatras.gr> wrote: > I a thinking about a scenario that goes like this: a node is reading a > secondary index to reply to a select query. While in the middle of this, > two rows are mutated, one that has already been read and considered for > the select result, and one that is yet to be processed. Say both rows > where changed in a way that causes them to be included to the result. The > result however will contain only the second one, and will not represent > the correct select result, neither before or after said mutation. > > This is extreme, I know, but given a cluster with enough activity, I don't > believe it's impossible. So I guess the answer is, Cassandra doesn't care, > the result is not guaranteed to represent a valid snapshot of the > database, but is very likely to? > > Alexander > >> Consider this scenario in a SQL database: >> >> UPDATE foo SET x = 1 WHERE key = 'asdf'; >> >> Now, "simultaneously," two clients run >> >> UPDATE foo SET x = 2 WHERE key = 'asdf'; >> and >> SELECT * FROM foo WHERE x = 1; >> >> Either you get back row asdf, or you don't. Either is valid. Same >> thing happens with Cassandra indexes. >> >> On Fri, Sep 9, 2011 at 10:41 AM, <alta...@ceid.upatras.gr> wrote: >>> I see that Cassandra updates secondary indices as soon as a value of the >>> indexed column is updated. This can happen, for example, during a select >>> query with a condition on a secondary index. Does Cassandra perform no >>> checking or locking? Will the result of this select, with old and new >>> values, be returned as is? Am I missing some reason why this isn't a >>> problem? >>> >>> Alexander >>> >> >> >> >> -- >> Jonathan Ellis >> Project Chair, Apache Cassandra >> co-founder of DataStax, the source for professional Cassandra support >> http://www.datastax.com >> >> > > -- Jonathan Ellis Project Chair, Apache Cassandra co-founder of DataStax, the source for professional Cassandra support http://www.datastax.com
