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From Tomas Bartalos <tomas.barta...@gmail.com>
Subject Re: [EXTERNAL] Howto avoid tombstones when inserting NULL values
Date Fri, 04 Jan 2019 18:48:57 GMT
Hello Jon,

I thought having tombstones is much higher overhead than just overwriting
values. The compaction overhead can be l similar, but I think the read
performance is much worse.

Tombstones accumulate and hang for 10 days (by default) before they are
eligible for compaction.

Also we have tombstone warning and error thresholds. If cassandra scans
more than 10 000 tombstones, she will abort the query.

According to this article:
https://opencredo.com/blogs/cassandra-tombstones-common-issues/

"The cassandra.yaml comments explain in perfectly: *“When executing a scan,
within or across a partition, we need to keep the tombstones seen in memory
so we can return them to the coordinator, which will use them to make sure
other replicas also know about the deleted rows. With workloads that
generate a lot of tombstones, this can cause performance problems and even
exhaust the server heap. "*

Regards,
Tomas

On Fri, 4 Jan 2019, 7:06 pm Jonathan Haddad <jon@jonhaddad.com wrote:

> If you're overwriting values, it really doesn't matter much if it's a
> tombstone or any other value, they still need to be compacted and have the
> same overhead at read time.
>
> Tombstones are problematic when you try to use Cassandra as a queue (or
> something like a queue) and you need to scan over thousands of tombstones
> in order to get to the real data.  You're simply overwriting a row and
> trying to avoid a single tombstone.
>
> Maybe I'm missing something here.  Why do you think overwriting a single
> cell with a tombstone is any worse than overwriting a single cell with a
> value?
>
> Jon
>
>
> On Fri, Jan 4, 2019 at 9:57 AM Tomas Bartalos <tomas.bartalos@gmail.com>
> wrote:
>
>> Hello,
>>
>> I beleive your approach is the same as using spark with "
>> spark.cassandra.output.ignoreNulls=true"
>> This will not cover the situation when a value have to be overwriten with
>> null.
>>
>> I found one possible solution - change the schema to keep only primary
>> key fields and move all other fields to frozen UDT.
>> create table (year, month, day, id, frozen<Event>, primary key((year,
>> month, day), id) )
>> In this way anything that is null inside event doesn't create tombstone,
>> since event is serialized to BLOB.
>> The penalty is in need of deserializing the whole Event when selecting
>> only few columns.
>> Can anyone confirm if this is good solution performance wise?
>>
>> Thank you,
>>
>> On Fri, 4 Jan 2019, 2:20 pm DuyHai Doan <doanduyhai@gmail.com wrote:
>>
>>> "The problem is I can't know the combination of set/unset values" -->
>>> Just for this requirement, Achilles has a working solution for many years
>>> using INSERT_NOT_NULL_FIELDS strategy:
>>>
>>> https://github.com/doanduyhai/Achilles/wiki/Insert-Strategy
>>>
>>> Or you can use the Update API that by design only perform update on not
>>> null fields:
>>> https://github.com/doanduyhai/Achilles/wiki/Quick-Reference#updating-all-non-null-fields-for-an-entity
>>>
>>>
>>> Behind the scene, for each new combination of INSERT INTO table(x,y,z)
>>> statement, Achilles will check its prepared statement cache and if the
>>> statement does not exist yet, create a new prepared statement and put it
>>> into the cache for later re-use for you
>>>
>>> Disclaiment: I'm the creator of Achilles
>>>
>>>
>>>
>>> On Thu, Dec 27, 2018 at 10:21 PM Tomas Bartalos <
>>> tomas.bartalos@gmail.com> wrote:
>>>
>>>> Hello,
>>>>
>>>> The problem is I can't know the combination of set/unset values. From
>>>> my perspective every value should be set. The event from Kafka represents
>>>> the complete state of the happening at certain point in time. In my table
I
>>>> want to store the latest event so the most recent state of the happening
>>>> (in this table I don't care about the history). Actually I used wrong
>>>> expression since its just the opposite of "incremental update", every event
>>>> carries all data (state) for specific point of time.
>>>>
>>>> The event is represented with nested json structure. Top level elements
>>>> of the json are table fields with type like text, boolean, timestamp, list
>>>> and the nested elements are UDT fields.
>>>>
>>>> Simplified example:
>>>> There is a new purchase for the happening, event:
>>>> {total_amount: 50, items : [A, B, C, new_item], purchase_time :
>>>> '2018-12-27 13:30', specials: null, customer : {... }, fare_amount,...}
>>>> I don't know what actually happened for this event, maybe there is a
>>>> new item purchased, maybe some customer info have been changed, maybe the
>>>> specials have been revoked and I have to reset them. I just need to store
>>>> the state as it artived from Kafka, there might already be an event for
>>>> this happening saved before, or maybe this is the first one.
>>>>
>>>> BR,
>>>> Tomas
>>>>
>>>>
>>>> On Thu, 27 Dec 2018, 9:36 pm Eric Stevens <mightye@gmail.com wrote:
>>>>
>>>>> Depending on the use case, creating separate prepared statements for
>>>>> each combination of set / unset values in large INSERT/UPDATE statements
>>>>> may be prohibitive.
>>>>>
>>>>> Instead, you can look into driver level support for UNSET values.
>>>>> Requires Cassandra 2.2 or later IIRC.
>>>>>
>>>>> See:
>>>>> Java Driver:
>>>>> https://docs.datastax.com/en/developer/java-driver/3.0/manual/statements/prepared/#parameters-and-binding
>>>>> Python Driver:
>>>>> https://www.datastax.com/dev/blog/python-driver-2-6-0-rc1-with-cassandra-2-2-features#distinguishing_between_null_and_unset_values
>>>>> Node Driver:
>>>>> https://docs.datastax.com/en/developer/nodejs-driver/3.5/features/datatypes/nulls/#unset
>>>>>
>>>>> On Thu, Dec 27, 2018 at 3:21 PM Durity, Sean R <
>>>>> SEAN_R_DURITY@homedepot.com> wrote:
>>>>>
>>>>>> You say the events are incremental updates. I am interpreting this
to
>>>>>> mean only some columns are updated. Others should keep their original
>>>>>> values.
>>>>>>
>>>>>> You are correct that inserting null creates a tombstone.
>>>>>>
>>>>>> Can you only insert the columns that actually have new values? Just
>>>>>> skip the columns with no information. (Make the insert generator
a bit
>>>>>> smarter.)
>>>>>>
>>>>>> Create table happening (id text primary key, event text, a text,
b
>>>>>> text, c text);
>>>>>> Insert into table happening (id, event, a, b, c) values
>>>>>> ("MainEvent","The most complete info we have right now","Priceless","10
>>>>>> pm","Grand Ballroom");
>>>>>> -- b changes
>>>>>> Insert into happening (id, b) values ("MainEvent","9:30 pm");
>>>>>>
>>>>>>
>>>>>> Sean Durity
>>>>>>
>>>>>>
>>>>>> -----Original Message-----
>>>>>> From: Tomas Bartalos <tomas.bartalos@gmail.com>
>>>>>> Sent: Thursday, December 27, 2018 9:27 AM
>>>>>> To: user@cassandra.apache.org
>>>>>> Subject: [EXTERNAL] Howto avoid tombstones when inserting NULL values
>>>>>>
>>>>>> Hello,
>>>>>>
>>>>>> I’d start with describing my use case and how I’d like to use
>>>>>> Cassandra to solve my storage needs.
>>>>>> We're processing a stream of events for various happenings. Every
>>>>>> event have a unique happening_id.
>>>>>> One happening may have many events, usually ~ 20-100 events. I’d
like
>>>>>> to store only the latest event for the same happening (Event is an
>>>>>> incremental update and it contains all up-to date data about happening).
>>>>>> Technically the events are streamed from Kafka, processed with Spark
>>>>>> an saved to Cassandra.
>>>>>> In Cassandra we use upserts (insert with same primary key).  So far
>>>>>> so good, however there comes the tombstone...
>>>>>>
>>>>>> When I’m inserting field with NULL value, Cassandra creates tombstone
>>>>>> for this field. As I understood this is due to space efficiency,
Cassandra
>>>>>> doesn’t have to remember there is a NULL value, she just deletes
the
>>>>>> respective column and a delete creates a ... tombstone.
>>>>>> I was hoping there could be an option to tell Cassandra not to be
so
>>>>>> space effective and store “unset" info without generating tombstones.
>>>>>> Something similar to inserting empty strings instead of null values:
>>>>>>
>>>>>> CREATE TABLE happening (id text PRIMARY KEY, event text); insert
into
>>>>>> happening (‘1’, ‘event1’); — tombstone is generated insert
into happening
>>>>>> (‘1’, null); — tombstone is not generated insert into happening
(‘1’, '’);
>>>>>>
>>>>>> Possible solutions:
>>>>>> 1. Disable tombstones with gc_grace_seconds = 0 or set to reasonable
>>>>>> low value (1 hour ?) . Not good, since phantom data may re-appear
2. ignore
>>>>>> NULLs on spark side with “spark.cassandra.output.ignoreNulls=true”.
Not
>>>>>> good since this will never overwrite previously inserted event field
with
>>>>>> “empty” one.
>>>>>> 3. On inserts with spark, find all NULL values and replace them with
>>>>>> “empty” equivalent (empty string for text, 0 for integer). Very
inefficient
>>>>>> and problematic to find “empty” equivalent for some data types.
>>>>>>
>>>>>> Until tombstones appeared Cassandra was the right fit for our use
>>>>>> case, however now I’m not sure if we’re heading the right direction.
>>>>>> Could you please give me some advice how to solve this problem ?
>>>>>>
>>>>>> Thank you,
>>>>>> Tomas
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>
> --
> Jon Haddad
> http://www.rustyrazorblade.com
> twitter: rustyrazorblade
>

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