get_columns_since
On Fri, Jul 3, 2009 at 7:21 PM, Evan Weaver<[email protected]> wrote: > 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 >
