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From Robert Wille <>
Subject Re: Schema questions for data structures with recently-modified access patterns
Date Fri, 24 Jul 2015 16:23:36 GMT
When performing an update, the following needs to happen:

1. Read document.last_modified
2. Get the current timestamp
3. Update document with last_modified=current timestamp
4. Insert into doc_by_last_modified with last_modified=current timestamp
5. Delete from doc_by_last_modified with last_modified=the timestamp from step 1

If two parties do the above at roughly the same time, such that in step 1 they both read the
same last_modified timestamp, then when they do step 5, they’ll both delete the same old
record from doc_by_last_modified, and you’ll get two records for the same document in doc_by_last_modified.

Would it work to put steps 3-5 into an atomic batch and use a lightweight transaction for
step 3? (e.g. UPDATE document SET doc = :doc, last_modified = :cur_ts WHERE docid = :docid
IF last_modified = :prev_ts) If a lightweight transaction is batched with other statements
on other tables, will the other statements get cancelled if the lightweight transaction is


On Jul 23, 2015, at 9:49 PM, Jack Krupansky <<>>

Concurrent update should not be problematic. Duplicate entries should not be created. If it
appears to be, explain your apparent issue so we can see whether it is a real issue.

But at least from all of the details you have disclosed so far, there does not appear to be
any indication that this type of time series would be anything other than a good fit for Cassandra.

Besides, the new materialized view feature of Cassandra 3.0 would make it an even easier fit.

-- Jack Krupansky

On Thu, Jul 23, 2015 at 6:30 PM, Robert Wille <<>>
I obviously worded my original email poorly. I guess that’s what happens when you post at
the end of the day just before quitting.

I want to get a list of documents, ordered from most-recently modified to least-recently modified,
with each document appearing exactly once.

Jack, your schema does exactly that, and is essentially the same as mine (with exception of
my missing the DESC clause, and I have a partitioning column and you only have clustering

The problem I have with my schema (or Jack’s) is that it is very easy for a document to
get in the list multiple times. Concurrent updates to the document, for example. Also, a consistency
issue could cause the document to appear in the list more than once.

I think that Alec Collier’s comment is probably accurate, that this kind of a pattern just
isn’t a good fit for Cassandra.

On Jul 23, 2015, at 1:54 PM, Jack Krupansky <<>>

Maybe you could explain in more detail what you mean by recently modified documents, since
that is precisely what I thought I suggested with descending ordering.

-- Jack Krupansky

On Thu, Jul 23, 2015 at 3:40 PM, Robert Wille <<>>
Carlos’ suggestion (nor yours) didn’t didn’t provide a way to query recently-modified

His updated suggestion provides a way to get recently-modified documents, but not ordered.

On Jul 22, 2015, at 4:19 PM, Jack Krupansky <<>>

"No way to query recently-modified documents."

I don't follow why you say that. I mean, that was the point of the data model suggestion I
proposed. Maybe you could clarify.

I also wanted to mention that the new materialized view feature of Cassandra 3.0 might handle
this use case, including taking care of the delete, automatically.

-- Jack Krupansky

On Tue, Jul 21, 2015 at 12:37 PM, Robert Wille <<>>
The time series doesn’t provide the access pattern I’m looking for. No way to query recently-modified

On Jul 21, 2015, at 9:13 AM, Carlos Alonso <<>>

Hi Robert,

What about modelling it as a time serie?

CREATE TABLE document (
  docId UUID,
  doc TEXT,
  last_modified TIMESTAMP
  PRIMARY KEY(docId, last_modified)

This way, you the lastest modification will always be the first record in the row, therefore
accessing it should be as easy as:

SELECT * FROM document WHERE docId == <the docId> LIMIT 1;

And, if you experience diskspace issues due to very long rows, then you can always expire
old ones using TTL or on a batch job. Tombstones will never be a problem in this case as,
due to the specified clustering order, the latest modification will always be first record
in the row.

Hope it helps.

Carlos Alonso | Software Engineer | @calonso<>

On 21 July 2015 at 05:59, Robert Wille <<>>
Data structures that have a recently-modified access pattern seem to be a poor fit for Cassandra.
I’m wondering if any of you smart guys can provide suggestions.

For the sake of discussion, lets assume I have the following tables:

CREATE TABLE document (
        docId UUID,
        doc TEXT,
        last_modified TIMEUUID,
        PRIMARY KEY ((docid))

CREATE TABLE doc_by_last_modified (
        date TEXT,
        last_modified TIMEUUID,
        docId UUID,
        PRIMARY KEY ((date), last_modified)

When I update a document, I retrieve its last_modified time, delete the current record from
doc_by_last_modified, and add a new one. Unfortunately, if you’d like each document to appear
at most once in the doc_by_last_modified table, then this doesn’t work so well.

Documents can get into the doc_by_last_modified table multiple times if there is concurrent
access, or if there is a consistency issue.

Any thoughts out there on how to efficiently provide recently-modified access to a table?
This problem exists for many types of data structures, not just recently-modified. Any ordered
data structure that can be dynamically reordered suffers from the same problems. As I’ve
been doing schema design, this pattern keeps recurring. A nice way to address this problem
has lots of applications.

Thanks in advance for your thoughts


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