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From Jens Rantil <jens.ran...@tink.se>
Subject Re: Finding records that exist on Cassandra but not externally
Date Wed, 07 Sep 2016 20:47:44 GMT
Hi Chris,

Without fully knowing your usecase; You can't keep track of which keys have
changed in the external system somehow? Otherwise 2) sounds like the way to
go to me.

Cheers,
Jens

On Wed, Sep 7, 2016 at 9:47 AM <chris@cmartinit.co.uk> wrote:

> First off I hope this appropriate here- I couldn't decide whether this was
> a question for Cassandra users or spark users so if you think it's in the
> wiring place feel free to redirect me.
>
> I have a system that does a load of data manipulation using spark.  The
> output of this program is a effectively the new state that I want my
> Cassandra table to be in and the final step is to update Cassandra so that
> it matches this state.
>
> At present I'm currently inserting all rows in my generated state into
> Cassandra. This works for new rows and also for updating existing rows but
> doesn't of course delete any rows that were already in Cassandra but not in
> my new state.
>
> The problem I have now is how best to delete these missing rows. Options I
> have considered are:
>
> 1. Setting a ttl on inserts which is roughly the same as my data refresh
> period. This would probably be pretty performant but I really don't want to
> do this because it would mean that all data in my database would disappear
> if I had issues running my refresh task!
>
> 2. Every time I refresh the data I would first have to fetch all primary
> keys from Cassandra and, compare them to primary keys locally to create a
> list of pks to delete before the insert. This seems the most logicaly
> correct option but is going to result in reading vast amounts of data from
> Cassandra.
>
> 3. Truncating the entire table before refreshing Cassandra. This has the
> benefit of being pretty simple in code but I'm not sure of the performance
> implications of this and what will happen if I truncate while a node is
> offline.
>
> For reference the table is on the order of 10s of millions of rows and for
> any data refresh only a very small fraction (<.1%) will actually need
> deleting. 99% of the time I'll just be overwriting existing keys.
>
> I'd be grateful if anyone could shed some advice on the best solution here
> or whether there's some better way I haven't thought of.
>
> Thanks,
>
> Chris
>
-- 

Jens Rantil
Backend Developer @ Tink

Tink AB, Wallingatan 5, 111 60 Stockholm, Sweden
For urgent matters you can reach me at +46-708-84 18 32.

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