Okay, but what benefit do you think you get from having the partitions on the same node – since they would be separate partitions anyway? I mean, what exactly do you think you’re going to do with them, that wouldn’t be a whole lot more performant by being able to process data in parallel from separate nodes? I mean, the whole point of Cassandra is scalability and distributed processing, right?
-- Jack Krupansky From: Drew Kutcharian Sent: Friday, August 29, 2014 7:31 PM To: user@cassandra.apache.org Subject: Re: Data partitioning and composite partition key Hi Jack, I think you missed the point of my email which was trying to avoid the problem of having very wide rows :) In the notation of sensorId-datatime, the datatime is a datetime bucket, say a day. The CQL rows would still be keyed by the actual time of the event. So you’d end up having SesonId->Datetime Bucket (day/week/month)->actual event. What I wanted to be able to do was to colocate all the events related to a sensor id on a single node (token). See "High Throughput Timelines” at http://www.datastax.com/dev/blog/advanced-time-series-with-cassandra - Drew On Aug 29, 2014, at 3:58 PM, Jack Krupansky <j...@basetechnology.com> wrote: With CQL3, you, the developer, get to decide whether to place a primary key column in the partition key or as a clustering column. So, make sensorID the partition key and datetime as a clustering column. -- Jack Krupansky From: Drew Kutcharian Sent: Friday, August 29, 2014 6:48 PM To: user@cassandra.apache.org Subject: Data partitioning and composite partition key Hey Guys, AFAIK, currently Cassandra partitions (thrift) rows using the row key, basically uses the hash(row_key) to decide what node that row needs to be stored on. Now there are times when there is a need to shard a wide row, say storing events per sensor, so you’d have sensorId-datetime row key so you don’t end up with very large rows. Is there a way to have the partitioner use only the “sensorId” part of the row key for the hash? This way we would be able to store all the data relating to a sensor in one node. Another use case of this would be multi-tenancy: Say we have accounts and accounts have users. So we would have the following tables: CREATE TABLE account ( id timeuuid PRIMARY KEY, company text //timezone ); CREATE TABLE user ( id timeuuid PRIMARY KEY, accountId timeuuid, email text, password text ); // Get users by account CREATE TABLE user_account_index ( accountId timeuuid, userId timeuuid, PRIMARY KEY(acid, id) ); Say I want to get all the users that belong to an account. I would first have to get the results from user_account_index and then use a multi-get (WHERE IN) to get the records from user table. Now this multi-get part could potentially query a lot of different nodes in the cluster. It’d be great if there was a way to limit storage of users of an account to a single node so that way multi-get would only need to query a single node. Note that the problem cannot be simply fixed by using (accountId, id) as the primary key for the user table since that would create a problem of having a very large number of (thrift) rows in the users table. I did look thru the code and JIRA and I couldn’t really find a solution. The closest I got was to have a custom partitioner, but then you can’t have a partitioner per keyspace and that’s not even something that’d be implemented in future based on the following JIRA: https://issues.apache.org/jira/browse/CASSANDRA-295 Any ideas are much appreciated. Best, Drew
