超大数据分页之Twitter的cursor方式进行Web数据分页

URL:

http://twitter.com/followers/ids.format

Returns an of numeric IDs for every user following the specified user.

Parameters:
* cursor. Required. Breaks the results into pages. Provide a value of -1 to begin paging. Provide values as returned to in the response body’s next_cursor and previous_cursor attributes to page back and forth in the list.
o Example: http://twitter.com/followers/ids/barackobama.xml?cursor=-1
o Example: http://twitter.com/followers/ids/barackobama.xml?cursor=-1300794057949944903

A cursor is an opaque deletion-tolerant index into a Btree keyed by source
userid and modification time. It brings you to a point in time in the
reverse chron sorted list. So, since you can’t change the past, other than
erasing it, it’s effectively stable. (Modifications bubble to the top.) But
you have to deal with additions at the list head and also block shrinkage
due to deletions, so your blocks begin to overlap quite a bit as the data
ages. (If you cache cursors and read much later, you’ll see the first few
rows of cursor[n+1]’s block as duplicates of the last rows of cursor[n]’s
block. The intersection cardinality is equal to the number of deletions in
cursor[n]’s block). Still, there may be value in caching these cursors and
then heuristically rebalancing them when the overlap proportion crosses some
threshold.

The page based approach does not scale with large sets. We can no
longer support this kind of API without throwing a painful number of
503s.

Working with row-counts forces the data store to recount rows in an O
(n^2) manner. Cursors avoid this issue by allowing practically
constant time access to the next block. The cost becomes O(n/
block_size) which, yes, is O(n), but a graceful one given n <10^7 and
a block_size of 5000. The cursor approach provides a more complete and
consistent result set.

Proportionally, very few users require multiple page fetches with a
page size of 5,000.

Also, scraping the social graph repeatedly at high speed is could
often be considered a low-value, borderline abusive use of the social
graph API.