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From "Tyler Hobbs (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-8005) Server-side DESCRIBE
Date Wed, 01 Oct 2014 19:22:34 GMT


Tyler Hobbs commented on CASSANDRA-8005:

bq. What I'm not convinced of is the usefulness of exposing such methods to clients through
the addition of DESCRIBE to CQL, because drivers already provide it and I generally prefer
not having 2 ways to do the same thing.

I agree, but I would prefer for server-side DESCRIBE to be the (eventual) only way to do it.

bq. The reason drivers provides a sort of DESCRIBE is just that generating it from those APIs
is a small amount of trivial code and there is thus no reason for drivers not to provide it
out of the box.

It is neither small nor trivial, especially when multiple versions of Cassandra have to be

For example, see the methods to {{DESCRIBE TABLE}} in the python driver:
 There have been multiple subtle bugs in this.

There are also other tricky details, like listing {{CREATE TYPE}} statements in an order that
handles cross-type dependencies.  (Even cqlsh got this wrong at first.)

bq. Once OPSC moves to the python-driver, it will also no longer have to reinvent this logic,
and use the driver's. And for CASSANDRA-7190 we don't really need a DESCRIBE statement in

There is a lot of value in having Cassandra create the schema file itself.  Other processes
can easily have permissions problems (I know that OpsCenter certainly had a lot of them).
 This doesn't mean that {{DESCRIBE}} has to be in CQL, but we need that same logic anyway.

> Server-side DESCRIBE
> --------------------
>                 Key: CASSANDRA-8005
>                 URL:
>             Project: Cassandra
>          Issue Type: New Feature
>          Components: API
>            Reporter: Tyler Hobbs
>            Priority: Minor
>              Labels: cql3
>             Fix For: 3.0
> The various {{DESCRIBE}} commands are currently implemented by cqlsh, and nearly identical
implementations exist in many drivers.  There are several motivations for making {{DESCRIBE}}
part of the CQL language:
> * Eliminate the (fairly complex) duplicate implementations across drivers and cqlsh
> * Get closer to allowing drivers to not have to fetch the schema tables. (Minor changes
to prepared statements are also needed.)
> * Have instantaneous support for new schema features in cqlsh.  (You currently have to
update the bundled python driver.)
> * Support writing out schemas where it makes sense.  One good example of this is backups.
 You need to restore the schema before restoring data in the case of total loss, so it makes
sense to write out the schema alongside snapshots.

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