My replies to the second question inline. Feel free to ask follow ups.
-Bryan
On Jun 26, 2008, at 5:24 PM, Andra Adams wrote:
Hi,
I've been looking through the HBase code and I was wondering if I
could get some clarification on two points.
1. Why doesn't HRegion's static merge method check that the two
regions specified are adjacent?
As far as I can tell, HRegion's merge method is called from the
Merge tool which gets its region names from command line
arguments. As far as I can see, merging non-adjacent regions would
break many of the assertions that HBase depends on, yet all calls
to HRegion's merge method result in a merged region. So how come
the caller of the Merge tool is being trusted to ensure the
adjacency of the regions it is specifying on the command line?
( Although admittedly, the adjacency check could be quite
computationally-expensive since it would involve a complete scan of
all regions in the "parent" META table (either .META. or -ROOT-) to
ensure that there are no regions in the "daughter" (either a user
table or .META.) table that have a start key between the end key
and start key of the regions being asked to merge).
2. Can I get an overview of the algorithm used to determine the
best candidate key in HStore's getRowKeyAtOrBefore (including
Memcache's internalGetRowKeyAtOrBefore, and HStore's
rowAtOrBeforeFromMapFile)?
I'm having trouble figuring out why HStore's getFull method looks
through the mc, snapshot and storefiles in reverse chronological
order (i.e. mc, then snapshot, then store files), while the
getRowKeyAtOrBefore looks through the storefiles, then the mc, then
the snapshot (in apparently no chronological order...?). Why does
getFull create a map of deletes (and older entries check this map
before inserting their values in the results map), while
getRowAtOrBefore opts to remove entries from the results map if a
delete is found at a later time?
Aside from the difference in style between getFull and
getRowAtOrBefore, I'm also wondering why the discovery of a deleted
value sometimes removes that key from the candidateKeys map, and
other times is simply ignored. (It could be that I'm missing some
of the concepts behind the algorithm).
The idea of getRowKeyAtOrBefore is to discover the row that comes
immediately before or right upon the search row. This is used
exclusively when trying to locate which region a key resides in. The
reasoning behind this is a little tricky. Regions in HBase are keyed
on their start row, which is inclusive. The end row is implied by the
presence of the next region. So, when you have an arbitrary key you'd
like to perform some operation on, you need to find the region which
contains it, which you can only know by scanning past it.
getRowKeyAtOrBefore is a specific, internal-only RPC method that does
this operation. In order to actually do the work, at the HStore
level, we have to decide amongst the possible keys that presented by
the memcache (including the snapshot) and all of the store files. The
order here is unimportant, because ultimately, we're going to have to
look at every one of those things unless we encounter a precise
match. Moreover, there could be deletes in any one of them, so we
have to carry the candidates along with us and apply the deletes
where they are required. The reasoning here is that if a row is
completely deleted, that is, all cells are suppressed by deletes,
even if it matches precisely, we don't want to return it as a
candidate key. Deletes are ignored when the don't apply to the data
we've already found, usually because there's a newer piece of data
than there is a delete (this is simply a memory optimization).
Likewise, getFull tries to find a whole row of information about a
key at a time. We need to follow deletes around here for the same
reason that we do it in regular get: we don't want to return deleted
data. We go in reverse chronological order here because that allows
the most recent data to easily take precedence.
Thanks,
Andra
[EMAIL PROTECTED]
