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From rouble <r.ou...@gmail.com>
Subject Re: Determining if data will be created on Cassandra Write Exceptions
Date Thu, 16 Feb 2017 15:33:41 GMT
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.guyomar@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 <edlinuxguru@gmail.com>
> wrote:
>
>>
>>
>> On Tue, Feb 14, 2017 at 2:30 PM, rouble <r.ouble@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.
>>
>
>

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