This helps a lot. However, I can't find any API method that actually lets me do a slice query on a time-sorted column, as necessary for the second blog example. I get the following error on r789419:
InvalidRequestException: get_slice_from requires CF indexed by name Evan On Tue, May 19, 2009 at 8:00 PM, Jonathan Ellis<[email protected]> wrote: > Mail storage, man, I think pretty much anything I could come up with > would look pretty simplistic compared to what "real" systems do in > that domain. :) > > But blogs, I think I can handle those. Let's make it ours multiuser > or there isn't enough scale to make it interesting. :) > > The interesting thing here is we want to be able to query two things > efficiently: > - the most recent posts belonging to a given blog, in reverse > chronological order > - a single post and its comments, in chronological order > > At first glance you might think we can again reasonably do this with a > single CF, this time a super CF: > > <ColumnFamily ColumnType="Super" ColumnSort="Time" Name="Post"/> > > The key is the blog name, the supercolumns are posts and the > subcolumns are comments. This would be reasonable BUT supercolumns > are just containers, they have no data or timestamp associated with > them directly (only through their subcolumns). So you cannot sort a > super CF by time. > > So instead what I would do would be to use two CFs: > > <ColumnFamily ColumnSort="Time" Name="Post"/> > <ColumnFamily ColumnSort="Time" Name="Comment"/> > > For the first, the keys used would be blog names, and the columns > would be the post titles and body. So to get a list of most recent > posts you just do a slice query. Even though Cassandra currently > handles large groups of columns sub-optimally, even with a blog > updated several times a day you'd be safe taking this approach (i.e. > we'll have that problem fixed before you start seeing it :). > > For the second, the keys are blog name<delimiter><post title>. The > columns are the comment data. You can serialize these a number of > ways; I would probably use title as the column name and have the value > be the author + body (e.g. as a json dict). Again we use the slice > call to get the comments in order. (We will have to manually reverse > what slice gives us since time sort is always reverse chronological > atm, but the overhead of doing this in memory will be negligible.) > > Does this help? > > -Jonathan > > On Tue, May 19, 2009 at 11:49 AM, Evan Weaver <[email protected]> wrote: >> Even if it's not actually in real-life use, some examples for common >> domains would really help clarify things. >> >> * blog >> * email storage >> * search index >> >> etc. >> >> Evan >> >> On Mon, May 18, 2009 at 8:19 PM, Jonathan Ellis <[email protected]> wrote: >>> Does anyone have a simple app schema they can share? >>> >>> I can't share the one for our main app. But we do need an example >>> here. A real one would be nice if we can find one. >>> >>> I checked App Engine. They don't have a whole lot of examples either. >>> They do have a really simple one: >>> http://code.google.com/appengine/docs/python/gettingstarted/usingdatastore.html >>> >>> The most important thing in Cassandra modeling is choosing a good key, >>> since that is what most of your lookups will be by. Keys are also how >>> Cassandra scales -- Cassandra can handle effectively infinite keys >>> (given enough nodes obviously) but only thousands to millions of >>> columns per key/CF (depending on what API calls you use -- Jun is >>> adding one now that does not deseriailze everything in the whole CF >>> into memory. The rest will need to follow this model eventually too). >>> >>> For this guestbook I think the choice is obvious: use the name as the >>> key, and have a single simple CF for the messages. Each column will >>> be a message (you can even use the mandatory timestamp field as part >>> of your user-visible data. win!). You get the list (or page) of >>> users with get_key_range and then their messages with get_slice. >>> >>> <ColumnFamily ColumnSort="Name" Name="Message"/> >>> >>> Anyone got another one for pedagogical purposes? >>> >>> -Jonathan >>> >> >> >> >> -- >> Evan Weaver >> > -- Evan Weaver
