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From Alexander Dejanovski <a...@thelastpickle.com>
Subject Re: Too many tombstones using TTL
Date Tue, 16 Jan 2018 09:32:19 GMT
Hi,

could you be more specific about the deletes you're planning to perform ?
This will end up moving your problem somewhere else as you'll be generating
new tombstones (and if you're planning on deleting rows, be aware that row
level tombstones aren't reported anywhere in the metrics, logs and query
traces).
Currently you can delete your data at the partition level, which will
create a single tombstone that will shadow all your expired (and non
expired) data and is very efficient. The read path is optimized for such
tombstones and the data won't be fully read from disk nor exchanged between
replicas. But that's of course if your use case allows to delete full
partitions.

We usually model so that we can restrict our reads to live data.
If you're creating time series, your clustering key should include a
timestamp, which you can use to avoid reading expired data. If your TTL is
set to 60 days, you can read only data that is strictly younger than that.
Then you can partition by time ranges, and access exclusively partitions
that have no chance to be expired yet.
Those techniques usually work better with TWCS, but the former could make
you hit a lot of SSTables if your partitions can spread over all time
buckets, so only use TWCS if you can restrict individual reads to up to 4
time windows.

Cheers,


On Tue, Jan 16, 2018 at 10:01 AM Python_Max <python.max@gmail.com> wrote:

> Hi.
>
> Thank you very much for detailed explanation.
> Seems that there is nothing I can do about it except delete records by key
> instead of expiring.
>
>
> On Fri, Jan 12, 2018 at 7:30 PM, Alexander Dejanovski <
> alex@thelastpickle.com> wrote:
>
>> Hi,
>>
>> As DuyHai said, different TTLs could theoretically be set for different
>> cells of the same row. And one TTLed cell could be shadowing another cell
>> that has no TTL (say you forgot to set a TTL and set one afterwards by
>> performing an update), or vice versa.
>> One cell could also be missing from a node without Cassandra knowing. So
>> turning an incomplete row that only has expired cells into a tombstone row
>> could lead to wrong results being returned at read time : the tombstone row
>> could potentially shadow a valid live cell from another replica.
>>
>> Cassandra needs to retain each TTLed cell and send it to replicas during
>> reads to cover all possible cases.
>>
>>
>> On Fri, Jan 12, 2018 at 5:28 PM Python_Max <python.max@gmail.com> wrote:
>>
>>> Thank you for response.
>>>
>>> I know about the option of setting TTL per column or even per item in
>>> collection. However in my example entire row has expired, shouldn't
>>> Cassandra be able to detect this situation and spawn a single tombstone for
>>> entire row instead of many?
>>> Is there any reason not doing this except that no one needs it? Is this
>>> suitable for feature request or improvement?
>>>
>>> Thanks.
>>>
>>> On Wed, Jan 10, 2018 at 4:52 PM, DuyHai Doan <doanduyhai@gmail.com>
>>> wrote:
>>>
>>>> "The question is why Cassandra creates a tombstone for every column
>>>> instead of single tombstone per row?"
>>>>
>>>> --> Simply because technically it is possible to set different TTL
>>>> value on each column of a CQL row
>>>>
>>>> On Wed, Jan 10, 2018 at 2:59 PM, Python_Max <python.max@gmail.com>
>>>> wrote:
>>>>
>>>>> Hello, C* users and experts.
>>>>>
>>>>> I have (one more) question about tombstones.
>>>>>
>>>>> Consider the following example:
>>>>> cqlsh> create keyspace test_ttl with replication = {'class':
>>>>> 'SimpleStrategy', 'replication_factor': '1'}; use test_ttl;
>>>>> cqlsh> create table items(a text, b text, c1 text, c2 text, c3 text,
>>>>> primary key (a, b));
>>>>> cqlsh> insert into items(a,b,c1,c2,c3) values('AAA', 'BBB', 'C111',
>>>>> 'C222', 'C333') using ttl 60;
>>>>> bash$ nodetool flush
>>>>> bash$ sleep 60
>>>>> bash$ nodetool compact test_ttl items
>>>>> bash$ sstabledump mc-2-big-Data.db
>>>>>
>>>>> [
>>>>>   {
>>>>>     "partition" : {
>>>>>       "key" : [ "AAA" ],
>>>>>       "position" : 0
>>>>>     },
>>>>>     "rows" : [
>>>>>       {
>>>>>         "type" : "row",
>>>>>         "position" : 58,
>>>>>         "clustering" : [ "BBB" ],
>>>>>         "liveness_info" : { "tstamp" : "2018-01-10T13:29:25.777Z",
>>>>> "ttl" : 60, "expires_at" : "2018-01-10T13:30:25Z", "expired" : true },
>>>>>         "cells" : [
>>>>>           { "name" : "c1", "deletion_info" : { "local_delete_time" :
>>>>> "2018-01-10T13:29:25Z" }
>>>>>           },
>>>>>           { "name" : "c2", "deletion_info" : { "local_delete_time" :
>>>>> "2018-01-10T13:29:25Z" }
>>>>>           },
>>>>>           { "name" : "c3", "deletion_info" : { "local_delete_time" :
>>>>> "2018-01-10T13:29:25Z" }
>>>>>           }
>>>>>         ]
>>>>>       }
>>>>>     ]
>>>>>   }
>>>>> ]
>>>>>
>>>>> The question is why Cassandra creates a tombstone for every column
>>>>> instead of single tombstone per row?
>>>>>
>>>>> In production environment I have a table with ~30 columns and It gives
>>>>> me a warning for 30k tombstones and 300 live rows. It is 30 times more
then
>>>>> it could be.
>>>>> Can this behavior be tuned in some way?
>>>>>
>>>>> Thanks.
>>>>>
>>>>> --
>>>>> Best regards,
>>>>> Python_Max.
>>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> Best regards,
>>> Python_Max.
>>>
>>
>>
>> --
>> -----------------
>> Alexander Dejanovski
>> France
>> @alexanderdeja
>>
>> Consultant
>> Apache Cassandra Consulting
>> http://www.thelastpickle.com
>>
>
>
>
> --
> Best regards,
> Python_Max.
>


-- 
-----------------
Alexander Dejanovski
France
@alexanderdeja

Consultant
Apache Cassandra Consulting
http://www.thelastpickle.com

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