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From DuyHai Doan <doanduy...@gmail.com>
Subject Re: Lightweight transaction inside a batch : request rejected
Date Mon, 24 Oct 2016 08:21:21 GMT
As far as I remember, there is an optimization in Cassandra to manage Paxos
ballot per table. So asking a Paxos operation to span multiple tables (even
if same partition key) would require a lot of changes in the current impl.

The question has already been raised, you may want to convince the
committers by adding some comments here:
https://issues.apache.org/jira/browse/CASSANDRA-10085

On Mon, Oct 24, 2016 at 9:58 AM, Mickael Delanoƫ <delanoemic@gmail.com>
wrote:

> Hi,
>
> I would like to use lightweight transaction inside a batch but the request
> is rejected by cassandra, however I think this is a use case than could be
> handled without problem.
> Below is what I wanted to do.
>
> I am using cassandra 3.7.
>
> CREATE KEYSPACE test_ksp WITH replication = {'class': 'SimpleStrategy',
> 'replication_factor': '1'};
>
> CREATE TABLE test_ksp.item (
>     user_id bigint,
>     item_id text,
>     item_value text,
>     item_key1 text,
>     item_key2 text,
> PRIMARY KEY ((user_id), item_id));
>
> CREATE TABLE test_ksp.item_id_by_key (
>     user_id bigint,
>     item_key text,
>     item_id text,
> PRIMARY KEY ((user_id), item_key));
>
> USE test_ksp;
>
> BEGIN BATCH
> INSERT INTO item (user_id, item_id, item_value, item_key1, item_key2)
> values (1,'i11','item-C', 'key-XYZ-123', 'key-ABC-789') IF NOT EXISTS;
> INSERT INTO item_id_by_key (user_id, item_key, item_id) VALUES (1,
> 'key-XYZ-123', 'i11');
> INSERT INTO item_id_by_key (user_id, item_key, item_id) VALUES (1,
> 'key-ABC-789', 'i11');
> APPLY BATCH;
>
>
> So as you can see this is a batch that targets 2 tables but with the same
> partition key (i.e the same target nodes). Moreover It uses only ONE
> condition on one table only.
> I don't understand why cassandra returns an error "Batch with conditions
> cannot span multiple tables" in that case.
>
> I understand that if I had used several conditions on different tables it
> could be a problem, but in my case there is only one condition and moreover
> I have always the same partition key for every table inside the batch.
> As there is only one condition, I expected the paxos protocol just act on
> this condition and as the partition keys are all the same, the paxos
> protocol has only to work with the same replica nodes (not span across
> multiple partition).
> In my point of view this is as if the LWT was in a single statement,
> except that after the LWT is accepted a complete batch has to be executed.
>
> Is there someone that could explain why this use case need to be rejected
> by cassandra? And do you think this is something that cassandra could
> handle in a future version ?
>
> Regards,
> Mickaƫl
>
>

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