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From Benjamin Roth <benjamin.r...@jaumo.com>
Subject Efficient model for a sorting
Date Tue, 04 Oct 2016 11:27:54 GMT
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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