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From "Sylvain Lebresne (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-5428) CQL3 don't validate that collections haven't more than 64K elements
Date Fri, 05 Apr 2013 10:56:18 GMT
Sylvain Lebresne created CASSANDRA-5428:
-------------------------------------------

             Summary: CQL3 don't validate that collections haven't more than 64K elements
                 Key: CASSANDRA-5428
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5428
             Project: Cassandra
          Issue Type: Bug
    Affects Versions: 1.2.0
            Reporter: Sylvain Lebresne
            Priority: Minor


This is somewhat similar to CASSANDRA-5355 but with a twist. When we serialize collections,
not only does the size of the elements is limited to 64K, but the number of elements is too
because it is also an unsigned short.

Now the same argument than in CASSANDRA-5355 that collections are "places to denormalize small
amounts of data" is true here too. So the fact that collections are limited to 64K elements
is something I could live with. However, we don't validate that no more than 64K elements
are inserted. And in fact, we can't validate it if the elements are added one by one.

So in practice, you can insert more than 64K elements, but if you try to read it, you will
only get back some subset of the collection. And the number of elements returned will correspond
to the 2 last bytes of the real size (so a collection of 65536 elements will be returned as
1 element). Imo, that's more problematic.

So since unfortunately we can't validate this at insertion, I suggest that as a first step
we:
# document that limitation (in http://cassandra.apache.org/doc/cql3/CQL.html typically)
# when we read a collection that has > 64K elements, we detect it and when serializing
that for the client, we:
** return as much as we can, i.e. the 64K first ones
** log a warning that something is wrong

On the longer term, for 2.0, maybe we should just change the serialization format and use
an int for the collection size, using an unsigned short was probably misguided. Of course
that changes said serialization format so we have to bump the native protocol version for
that (and thus can't do that in 1.2).

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