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From tsuraan <tsur...@gmail.com>
Subject Re: Modelling assets and user permissions
Date Tue, 20 Apr 2010 16:47:42 GMT
> Suppose I have a CF that holds some sort of assets that some users of
> my program have access to, and that some do not.  In SQL-ish terms it
> would look something like this:
>
> TABLE Assets (
>  asset_id serial primary key,
>  ...
> );
>
> TABLE Users (
>  user_id serial primary key,
>  user_name text
> );
>
> TABLE Permissions (
>  asset_id integer references(Assets),
>  user_id integer references(Users)
> )
>
> Now, I can generate UUIDs for my asset keys without any trouble, so
> the serial that I have in my pseudo-SQL Assets table isn't a problem.
> My problem is that I can't see a good way to model the relationship
> between user ids and assets.  I see one way to do this, which has
> problems, and I think I sort of see a second way.
>
> The obvious way to do it is have the Assets CF have a SuperColumn that
> somehow enumerates the users allowed to see it, so when retrieving a
> specific Asset I can retrieve the users list and ensure that the user
> doing the request is allowed to see it.  This has quite a few
> problems.  The foremost is that Cassandra doesn't appear to have much
> for conflict resolution (at least I can't find any docs on it), so if
> two processes try to add permissions to the same Asset, it looks like
> one process will win and I have no idea what happens to the loser.
> Another problem is that Cassandra's SuperColumns don't appear to be
> ideal for storing lists of things; they store maps, which isn't a
> terrible problem, but it feels like a bit of a mismatch in my design.
> A SuperColumn mapping from user_ids to an empty byte array seems like
> it should work pretty efficiently for checking whether a user has
> permissions on an Asset, but it also seems pretty evil.
>
> The other idea that I have is a seperate CF for AssetPermissions that
> somehow stores pairs of asset_ids and user_names.  I don't know what
> I'd use for a key in that situation, so I haven't really gotten too
> far in seeing what else is broken with that idea.  I think it would
> get around the race condition, but I don't know how to do it, and I'm
> not sure how efficient it could be.
>
> What do people normally use in this situation?  I assume it's a pretty
> common problem, but I haven't see it in the various data modelling
> examples on the Wiki.

I'm wondering, is my question too vague, too specific, off topic for
this list, or answered in the docs somewhere that I missed?

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