1:53 p.m.
Lukas do you have any insight into how Titan supports transactions with Cassandra? I know
that Titan still uses the thrift API,which presumably rules out the light weight
transactions introduced in C* 2.0. These are not to be confused with ACID transactions. It
is more of an atomic update using a consensus protocol. Then you have atomic or logged
batches which are atomic in the sense that either all of the statements will succeed or
none will; however, they can succeed *eventually*. These have more overhead than unlogged
batches because the mutations are first written to a batch log. Applying multiple
mutations to the same partitions is an atomic operation. In other words, whether you
update 1 or 50 columns, if it is done within the same partition (and within the same
operation), it is atomic.
My point being that Cassandra is definitely not a transactional data store, so I am really
curious about what Titan is doing.
On Feb 23, 2016, at 1:48 PM, Lukas Krejci <lkrejci(a)redhat.com>
wrote:
On 02/23/2016 07:42 PM, Jay Shaughnessy wrote:
>
> Lukas,
>
> That's excellent news. Multiple backends is not something we really
> want to deal with. Also, it might be nice to see a short presentation
> on the "best practices" for Tx handling. But then again, that Tx stuff
> is handled at the Gremlin level? So, perhaps not relevant to direct C*
> consumers like Alerts.
>
You're right. Inventory uses Gremlin to handle transactions, so it
doesn't directly "see" what is Titan doing behind the scenes.
> On 2/23/2016 12:43 PM, Lukas Krejci wrote:
>> Hi all,
>>
>> lately I've become really dissatisfied with how Inventory performed and
>> semi-publicly blamed Titan for that (because that was what looked like
>> the cause of all world's problems in my then uneducated eyes ;) ).
>>
>> I decided to do some performance comparisons. Because we didn't want
>> Hawkular to ship with 2 different NoSQL backends (C* for metrics and
>> whatever else for Inventory), I chose an RDBMS as a good conservative
>> alternative (because people, IMHO, are still more comfortable dealing
>> with an RDBMS than with NoSQL databases).
>>
>> Currently, inventory is written against the graph DSL called Gremlin
>> (from Tinkerpop 2.6.0). Fortunately, there exists a "toy" SQL backend
>> for Tinkerpop 2 that we could try and see if it performed any good
>> (which would frankly be surprising, given the fact it stores the graph
>> data rather naively). With some luck, no code would have to be changed
>> on our side to see the results.
>>
>> We had no such luck.
>>
>> Making the inventory run with the SQL backend was literally a day worth
>> of work (if that) and the first preliminary tests showed that Inventory
>> with Postgres backend performed much much better that Titan with
>> embedded Cassandra. But the tests also uncovered some problems with the
>> way Inventory code handled transactions.
>>
>> Fast forward 3 weeks and see large parts of Hawkular inventory updated
>> to correctly handle transactions. Now a single call to Inventory really
>> results in at most 1 transaction in the backend.
>>
>> So, I went and re-ran the tests. Also, I refrained from using embedded
>> Cassandra and instead use a locally running 2-node cluster.
>>
>> The results caught me by surprise. Not so much that the naive SQL
>> backend didn't perform particularly well, but the difference between the
>> performance of Titan before and after the transaction handling fixes.
>>
>> To not keep you waiting any longer for the results: Titan + C* is the
>> winner.
>>
>> For nice charts that include comparison to the old misbehaving impl, see:
>> https://dashboards.ly/ua-tALzrY9rEoRBXvsLXbZJHT
>>
>> Cheers,
>>
>
>
>
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--
Lukas Krejci
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