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From DuyHai Doan <doanduy...@gmail.com>
Subject Re: Efficient model for a sorting
Date Tue, 04 Oct 2016 12:09:40 GMT
MV is right now your best choice for this kind of sorting behavior.

Secondary index (whatever the impl, SASI or Lucene) has a cost of
scatter-gather if your cluster scale out. With MV you're at least
guaranteed to hit a single node everytime

On Tue, Oct 4, 2016 at 1:56 PM, Dorian Hoxha <dorian.hoxha@gmail.com> wrote:

> Can you use the lucene index https://github.com/Stratio/
> cassandra-lucene-index ?
>
> On Tue, Oct 4, 2016 at 1:27 PM, Benjamin Roth <benjamin.roth@jaumo.com>
> wrote:
>
>> Hi!
>>
>> I have a frequently used pattern which seems to be quite costly in CS.
>> The pattern is always the same: I have a unique key and a sorting by a
>> different field.
>>
>> To give an example, here a real life example from our model:
>> CREATE TABLE visits.visits_in (
>>     user_id int,
>>     user_id_visitor int,
>>     created timestamp,
>>     PRIMARY KEY (user_id, user_id_visitor)
>> ) WITH CLUSTERING ORDER BY (user_id_visitor ASC)
>>
>> CREATE MATERIALIZED VIEW visits.visits_in_sorted_mv AS
>>     SELECT user_id, created, user_id_visitor
>>     FROM visits.visits_in
>>     WHERE user_id IS NOT NULL AND created IS NOT NULL AND user_id_visitor
>> IS NOT NULL
>>     PRIMARY KEY (user_id, created, user_id_visitor)
>>     WITH CLUSTERING ORDER BY (created DESC, user_id_visitor DESC)
>>
>> This simply represents people, that visited my profile sorted by date
>> desc but only one entry per visitor.
>> Other examples with the same pattern could be a whats-app-like inbox
>> where the last message of each sender is shown by date desc. There are lots
>> of examples for that pattern.
>>
>> E.g. in redis I'd just use a sorted set, where the key could be like
>> "visits_${user_id}", set key would be user_id_visitor and score
>> the created timestamp.
>> In MySQL I'd create the table with PK on user_id + user_id_visitor and
>> create an index on user_id + created
>> In C* i use an MV.
>>
>> Is this the most efficient approach?
>> I also could have done this without an MV but then the situation in our
>> app would be far more complex.
>> I know that denormalization is a common pattern in C* and I don't
>> hesitate to use it but in this case, it is not as simple as it's not an
>> append-only case but updates have to be handled correctly.
>> If it is the first visit of a user, it's that simple, just 2 inserts in
>> base table + denormalized table. But on a 2nd or 3rd visit, the 1st or 2nd
>> visit has to be deleted from the denormalized table before. Otherwise the
>> visit would not be unique any more.
>> Handling this case without an MV requires a lot more effort, I guess even
>> more effort than just using an MV.
>> 1. You need kind of app-side locking to deal with race conditions
>> 2. Read before write is required to determine if an old record has to be
>> deleted
>> 3. At least CL_QUORUM is required to make sure that read before write is
>> always consistent
>> 4. Old record has to be deleted on update
>>
>> I guess, using an MV here is more efficient as there is less roundtrip
>> between C* and the app to do all that and the MV does not require strong
>> consistency as MV updates are always local and are eventual consistent when
>> the base table is. So there is also no need for distributed locks.
>>
>> I ask all this as we now use CS 3.x and have been advised that 3.x is
>> still not considered really production ready.
>>
>> I guess in a perfect world, this wouldn't even require an MV if SASI
>> indexes could be created over more than 1 column. E.g. in MySQL this case
>> is nothing else than a BTree. AFAIK SASI indices are also BTrees, filtering
>> by Partition Key (which should to be done anyway) and sorting by a field
>> would perfectly do the trick. But from the docs, this is not possible right
>> now.
>>
>> Does anyone see a better solution or are all my assumptions correct?
>>
>> --
>> 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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