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From: Bob Hutchison <hutch-lists@recursive.ca>
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Subject: How do you construct an index and use it, especially in Ruby
Date: Sun, 25 Apr 2010 14:14:39 -0400
Message-Id: <BE92AEA8-895B-4FDE-B11E-21B8B9FE3E92@recursive.ca>
Cc: Bob Hutchison <hutch-lists@recursive.ca>
To: user@cassandra.apache.org
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Hi,

I'm new to Cassandra and trying to work out how to do something that =
I've implemented any number of times (e.g. TokyoCabinet, Perst, even the =
filesystem using grep :-) I've managed to get some of this working in =
Cassandra but not all.

So here's the core of the situation.

I have this opaque chunk of data that I want to store in Cassandra and =
then find it again.

I can generate a key when the data is created very easily, and I've =
stored it in a straight forward manner: in a column with a key whose =
value is the data. And I can retrieve it when I know the key. No =
difficulties here at all, works fine.

Now I want to index this data taking what I imagine to be a pretty =
typical approach.

Lets say there's two many-to-one indexes: 'colour', and 'size'. Each =
colour value will have more than one chunk of data, same for size.

What I thought I'd do is make a super column and index the chunk of data =
kind of like: { 'colour' =3D> { 'blue' =3D> 1 }, 'size' =3D> { 'large' =
=3D> 1}} with the key equal to the key of the chunk of data. And =
Cassandra stores it without error like that. So using the Ruby gem, it'd =
be something along the lines of:

  cassandra.insert(:Indexes, key-of-the-chunk-of-data, { 'colour' =3D> { =
'blue' =3D> 1 }, 'size' =3D> { 'large' =3D> 1 } })

Q1: is this a reasonable approach? It *seems* to be what I've read is =
supposed to be done. The 1 is meaningless. Anyway, it executes without =
error in Ruby.

Q2: what is the syntax of the (Ruby) query to find the keys of all =
'blue' chunks of data? I'm assuming get_range is the correct method, but =
what are the parameters? The docs say: get_range(column_family, =
options=3D{}) but that seems to be missing a bit of detail, in =
particular the super column name.

Q2a: So I know there's a :start and :finish key supported in the options =
hash, inclusive, exclusive respectively. How do you define a range for =
equals with a UTF8 key? Surely not 'blue'.succ?? or by some kind of =
suffix??

Q2b: How do you specify the super column name 'colour'? Looking at the =
(Ruby) source of the get_range method and I'm unconvinced that this is =
implemented (seems to be a constant '' used where the super column name =
makes sense to be.)

Anyway I ended up hacking at the Ruby gem's source to use the column =
name where the '' was in the original, and didn't really get anywhere =
useful (I can find nothing, or everything, nothing in between).

Q3: If I am correct about what is supposed to be done, does the Ruby gem =
support it?

Q4: Does anyone know of some Ruby code that does and indexed lookup that =
they could point me at. (lots of code that indexes but nothing that =
searches by the index)

I'll try to take a look at some of the other Cassandra client =
implementations and see if I can get this model to work. Maybe just a =
Ruby problem?? With any luck, it'll be me messing up.

If it'd help I can post the source of what I have, but it'll need some =
cleanup. Let me know.

Thanks for taking the time to read this far :-)

Bob

----
Bob Hutchison
Recursive Design Inc.
http://www.recursive.ca/
weblog: http://xampl.com/so


----
Bob Hutchison
Recursive Design Inc.
http://www.recursive.ca/
weblog: http://xampl.com/so