Ian -- Thanks for the summary -- I think this concurs with the kind of questions I had. It does seem like we would still save *some* space with a paginated implementation (user123 is repeated fewer times, even if everything else is stored the same number of times), but I agree that the savings may not be that large. I was just trying to see if there was any strong experiences either way. It's true that we can start simple, and then optimize. My main concern is that reloading everything if we make a bad choice is somewhat expensive as well. Thanks for the excellent summary of the issue. I'll need to go ahead and talk with our other engineers about reverting to a simpler implementation.
--Derek On Tue, Apr 3, 2012 at 12:59 PM, Ian Varley <[email protected]> wrote: > Hi Derek, > > If I understand you correctly, you're ultimately trying to store triples > in the form "user, valueid, value", right? E.g., something like: > > "user123, firstname, Paul", > "user234, lastname, Smith" > > (But the usernames are fixed width, and the valueids are fixed width). > > And, your access pattern is along the lines of: "for user X, list the next > 30 values, starting with valueid Y". Is that right? And these values should > be returned sorted by valueid? > > The tl;dr version is that you should probably go with one row per > user+value, and not build a complicated intra-row pagination scheme on your > own unless you're really sure it is needed. > > Your two options mirror a common question people have when designing HBase > schemas: should I go "tall" or "wide"? Your first schema is "tall": each > row represents one value for one user, and so there are many rows in the > table for each user; the row key is user + valueid, and there would be > (presumably) a single column qualifier that means "the value". This is > great if you want to scan over rows in sorted order by row key (thus my > question above, about whether these ids are sorted correctly). You can > start a scan at any user+valueid, read the next 30, and be done. What > you're giving up is the ability to have transactional guarantees around all > the rows for one user, but it doesn't sound like you need that. Doing it > this way is generally recommended (see here< > http://hbase.apache.org/book.html#schema.smackdown>). > > Your second option is "wide": you store a bunch of values in one row, > using different qualifiers (where the qualifier is the valueid). The simple > way to do that would be to just store ALL values for one user in a single > row. I'm guessing you jumped to the "paginated" version because you're > assuming that storing millions of columns in a single row would be bad for > performance, which may or may not be true; as long as you're not trying to > do too much in a single request, or do things like scanning over and > returning all of the cells in the row, it shouldn't be fundamentally worse. > The client has methods that allow you to get specific slices of columns. > > Note that neither case fundamentally uses more disk space than the other; > you're just "shifting" part of the identifying information for a value > either to the left (into the row key, in option one) or to the right (into > the column qualifiers in option 2). Under the covers, every key/value still > stores the whole row key, and column family name. (If this is a bit > confusing, take an hour and watch Lars George's excellent video about > understanding HBase schema design: > http://www.youtube.com/watch?v=_HLoH_PgrLk). > > A manually paginated version has lots more complexities, as you note, like > having to keep track of how many things are in each page, re-shuffling if > new values are inserted, etc. That seems significantly more complex. It > might have some slight speed advantages (or disadvantages!) at extremely > high throughput, and the only way to really know that would be to try it > out. If you don't have time to build it both ways and compare, my advice > would be to start with the simplest option (one row per user+value). Start > simple & iterate! :) > > (Let me know if I've misunderstood your situation.) > > Ian > > On Apr 2, 2012, at 3:10 PM, Derek Wollenstein wrote: > > We're looking at how to store a large amount of (per-user) list data in > hbase, and we were trying to figure out what kind of access pattern made > the most sense. One option is store the majority of the data in a key, so > we could have something like > <FixedWidthUserName><FixedWidthValueId1>:"" (no value) > <FixedWidthUserName><FixedWidthValueId2>:"" (no value) > <FixedWidthUserName><FixedWidthValueId3>:"" (no value) > > The other option we hade was to do this entirely using > > <FixedWidthUserName><FixedWidthPageNum0>:<FixedWidthLength><FixedIdNextPageNum><ValueId1><ValueId2><ValueId3>... > > <FixedWidthUserName><FixedWidthPageNum1>:<FixedWidthLength><FixedIdNextPageNum><ValueId1><ValueId2><ValueId3>... > > where each row would contain multiple values. > So in one case reading the first thirty values would be > scan { STARTROW => 'FixedWidthUsername' LIMIT => 30} > And in the second case it would be > get 'FixedWidthUserName\x00\x00\x00\x00' > > The general usage pattern would be to read only the first 30 values of > these lists, with infrequent access reading deeper into the lists. Some > users would have <= 30 total values in these lists, and some users would > have millions (i.e. power-law distribution) > > The single-value format seems like it would take up more space on hbase, > but would offer some improved retrieval / pagination flexibility. Would > there be any significant performance advantages to be able to paginate via > gets vs paginating with scans? > My initial understanding was that doing a scan should be faster if our > paging size is unknown (and caching is set appropriately), but that gets > should be faster if we'll always need the same page size. I've ended up > hearing different people tell me opposite things about performance. I > assume the page sizes would be relatively consistent, so for most use cases > we could guarantee that we only wanted one page of data in the > fixed-page-length case. I would also assume that we would have infrequent > updates, but may have inserts into the middle of these lists (meaning we'd > need to update all subsequent rows). > > Thanks for help / suggestions / followup questions > > --Derek > >
