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From <>
Subject Re: Re: Cassandra DataModeling recommendations
Date Mon, 05 Dec 2011 09:36:22 GMT
 Thanks for the answer, as I read the book on Cassandra, I was not aware  at that time on
Composite Key which I recently discovered.
 You mentioned a TTL and let the database remove the date for me. I  never read about that.
Is it possible without an external batch ?
 I will try to rephrase in any case my goal:
 - I would like to store for a user (identified by its id) several carts (BLOB).
 - Associated to these carts, I would like to attach metadata like expiration date and possibly
 - I would like to be able to retrieve all the carts of a given userId.
 - I would like to have a mean to remove expired carts.
 I think that this use case is not very complicated but I was wondering if there was any kind
of modelling recommendation.
 Thanks for your answer.
 Best Regards


 Not sure I understand your use case, but I think you could use a composite column instead
of composite key.

For example, 


This way, you could do a slice query on the time if you do not need all the carts, and you
could also get all the carts in one query.

 For expired carts, maybe you could attach TTL to each column that has time constraint, let
the database remove the data for you.

 Hi all,
In order to evaluate NoSQL solutions and to gain knowledge, I am currently working on a kind
of prototype. 
 Here is a brief overview of the scope:

I would like to manage user carts. Lets keep things simple:
A user can have up to n (lets say 3 for example) carts. Each cart will contain metadata and
among them an expiration date and a blob containing stuff (xml in fact but I really don't
care of the content).
A user can save, retrieve or delete his carts. Additionally, a dedicated batch process would
remove carts who are expired.

Basically I was thinking of two ways to model the data:
1- A ColumnFamily with the userid as a key and having several SuperColumns each one describing
a Cart and its content.
 This has the advantage that I can get all the Carts in a single get or can do some slice
queries to get only some Carts. The problem is that I cannot if I am right create a secondary
index on the expired date column inside each Cart.
 2- A ColumnFamily with a composite key like userid::cartId containing the expiration date
column and the blob. I can in that case create an index to perform a query on the expiration
timestamp. The drawback is that if I want to get all the Carts I need to create either a secondary
ColumnFamily listing the carts associated to a userid or use a kind of OrderPreservingPartitionner
if I want to perform a Key-Range Query.
I made some tests and I had some problems
First I was unable to perform queries in the case 2 like:
get Carts where timestamp < xxxxxxx; The (ugly, really!) workaround was to create a fake
column always set to true and the query that worked was:
 get Carts where dummy=true and timestamp < xxxxxxx; But I really dislike this solution
and I am almost sure this is not the right way to go.

I tried something different like creating a dedicated timestamp columnfamily associating a
key based on a timestamp and columns related to user and carts. In that case if I want outdated
entries I could perform a range query on keys of this columnfamily. But again in that case
I need an OrderPreservingPartionner and I fear that using a timestamp as a key would lead
to a bad repartition scheme among the nodes. If I fit to the second proposal (with Standard
Columns), columns could be directly the key like userId::cartId and there is no logic in the
removal process. If I fit to first solution solution, I need to have some logic to analyze
the column key or value to get userid + cartid.
 Another point, if I use this column family I have to manage "updates". If for example I replace
Cart2 of user1, I need to remove the corresponding entry and add a new one. This is honestly
probably not the hardest part.
I have the feeling that having a userId based ColumnFamily with SuperColumns inside and a
dedicated timestamp table is the best choice. In fact I think that basically my requests will
- Give me all the carts of a userId
 - Remove all the expired carts: which is probably in fact 2 requests: Find all carts whose
expiry date is before a given date. Delete the found stuff.

I am fairly new to NoSQL and especially to Cassandra so I would like to get any advice on:
 1- Is Cassandra suited to this kind of storage ? I would say yes
2- What is the right way to model the data and the related constraints.

If my description is unclear or anyone does need more details, do not hesitate to ask
 Thanks in advance for any help or advice




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