Thanks for the links. I get that all queries need to be idempotent, and we should use retries for data consistency.
But, what happens when the retries fail? Then, the data *may* be there. To maintain consistency we need to rollback any created data, correct? tia, rouble On Wed, Feb 15, 2017 at 4:53 AM, Nicolas Guyomar <nicolas.guyo...@gmail.com> wrote: > Hi Rouble, > > I usually have to read javadoc in java driver to get my ideas straight > regarding exception handling. > > You can find informations reading : http://docs.datastax.com/en/ > drivers/java/3.1/com/datastax/driver/core/policies/RetryPolicy.html and > for instance http://docs.datastax.com/en/drivers/java/3.1/com/ > datastax/driver/core/policies/DowngradingConsistencyRetryPolicy.html > with the onWriteTimeout method which differentiate several case of error. > > As Edward stated, you can know how many replica acknowleged the write in > Cassandra response. > > Keep in mind that retrying usually mean your write query is idempotent or > you don't care having duplicate entries > > > On 14 February 2017 at 21:49, Edward Capriolo <edlinuxg...@gmail.com> > wrote: > >> >> >> On Tue, Feb 14, 2017 at 2:30 PM, rouble <r.ou...@gmail.com> wrote: >> >>> Cassandra Gurus, >>> >>> I have a question open on stackoverlow on how to determine if data is >>> actually created when a Write exception is thrown: >>> http://stackoverflow.com/questions/42231140/determin >>> ing-if-data-will-be-created-on-cassandra-write-exceptions >>> >>> From my discussion on the question, it seems that on *any* Cassandra >>> write, *any* exception, means the data may or may not be written. With the >>> exception of InvalidQueryException. >>> >>> I find this unsettling. Maybe I need time to adjust from my RDBMS >>> background, but how is Cassandra supposed to be used for systems that need >>> user feedback? or is it? >>> >>> Let me use the simple example of account creation. User tries to create >>> an account, and we need to indicate one way or the other whether the >>> account was created. Lets say a WriteTimeoutException is thrown while >>> trying to add the user. User may or may not be written, what do we tell the >>> user? Should we just rollback the change and tell the user that it failed. >>> This seems like the only thing we can do deterministically (and I notice >>> someone doing just that here: http://stackoverflow.com/a/34860495/215120 >>> ). >>> >>> How are people handling WriteTimeoutExceptions or UnavailableExceptions? >>> Rolling back in every case does not seem practical. >>> >>> tia, >>> Rouble >>> >> >> There is a difference between WriteTimeoutException and >> UnavailableException. >> >> UnavailableException indicates the write was never even attempted. >> >> WriteTimeoutException means the write was attempted. I believe you can >> interrogate the exception to determine if the operation was successful on >> any of the natural endpoints. >> >> The way to "cope" is idempotent writes and retries. If that model does >> not fit it is a square peg round hole discussion. >> > >
