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From Benjamin Roth <benjamin.r...@jaumo.com>
Subject Re: implementing a 'sorted set' on top of cassandra
Date Sat, 14 Jan 2017 18:25:35 GMT
Mike mentioned "increment" in his initial post. That let me think of a case
with increments and fetching a top list by a counter like
https://redis.io/commands/zincrby
https://redis.io/commands/zrangebyscore

1. Cassandra is absolutely not made to sort by a counter (or a non-counter
numeric incrementing value) but it is made to store counters. In this case
a partition could be seen as a set.
2. I thought of CS for persistence and - depending on the app requirements
like real-time and set size - still use redis as a read cache

2017-01-14 18:45 GMT+01:00 Jonathan Haddad <jon@jonhaddad.com>:

> Sorted sets don't have a requirement of incrementing / decrementing.
> They're commonly used for thing like leaderboards where the values are
> arbitrary.
>
> In Redis they are implemented with 2 data structures for efficient lookups
> of either key or value. No getting around that as far as I know.
>
> In Cassandra they would require using the score as a clustering column in
> order to select top N scores (and paginate). That means a tombstone
> whenever the value for a key in the set changes. In sets with high rates of
> change that means a lot of tombstones and thus terrible performance.
> On Sat, Jan 14, 2017 at 9:40 AM DuyHai Doan <doanduyhai@gmail.com> wrote:
>
>> Sorting on an "incremented" numeric value has always been a nightmare to
>> be done properly in C*
>>
>> Either use Counter type but then no sorting is possible since counter
>> cannot be used as type for clustering column (which allows sort)
>>
>> Or use simple numeric type on clustering column but then to increment the
>> value *concurrently* and *safely* it's prohibitive (SELECT to fetch current
>> value + UPDATE ... IF value = <old_value>) + retry
>>
>>
>>
>> On Sat, Jan 14, 2017 at 8:54 AM, Benjamin Roth <benjamin.roth@jaumo.com>
>> wrote:
>>
>> If your proposed solution is crazy depends on your needs :)
>> It sounds like you can live with not-realtime data. So it is ok to cache
>> it. Why preproduce the results if you only need 5% of them? Why not use
>> redis as a cache with expiring sorted sets that are filled on demand from
>> cassandra partitions with counters?
>> So redis has much less to do and can scale much better. And you are not
>> limited on keeping all data in ram as cache data is volatile and can be
>> evicted on demand.
>> If this is effective also depends on the size of your sets. CS wont be
>> able to sort them by score for you, so you will have to load the complete
>> set to redis for caching and / or do sorting in your app on demand. This
>> certainly won't work out well with sets with millions of entries.
>>
>> 2017-01-13 23:14 GMT+01:00 Mike Torra <mtorra@demandware.com>:
>>
>> We currently use redis to store sorted sets that we increment many, many
>> times more than we read. For example, only about 5% of these sets are ever
>> read. We are getting to the point where redis is becoming difficult to
>> scale (currently at >20 nodes).
>>
>> We've started using cassandra for other things, and now we are
>> experimenting to see if having a similar 'sorted set' data structure is
>> feasible in cassandra. My approach so far is:
>>
>>    1. Use a counter CF to store the values I want to sort by
>>    2. Periodically read in all key/values in the counter CF and sort in
>>    the client application (~every five minutes or so)
>>    3. Write back to a different CF with the ordered keys I care about
>>
>> Does this seem crazy? Is there a simpler way to do this in cassandra?
>>
>>
>>
>>
>> --
>> Benjamin Roth
>> Prokurist
>>
>> Jaumo GmbH · www.jaumo.com
>> Wehrstraße 46 · 73035 Göppingen · Germany
>> Phone +49 7161 304880-6 <+49%207161%203048806> · Fax +49 7161 304880-1
>> <+49%207161%203048801>
>> AG Ulm · HRB 731058 · Managing Director: Jens Kammerer
>>
>>
>>


-- 
Benjamin Roth
Prokurist

Jaumo GmbH · www.jaumo.com
Wehrstraße 46 · 73035 Göppingen · Germany
Phone +49 7161 304880-6 · Fax +49 7161 304880-1
AG Ulm · HRB 731058 · Managing Director: Jens Kammerer

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