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From stack <st...@duboce.net>
Subject Re: Low-cost and fast HTable.exists(...)?
Date Sun, 09 Nov 2008 21:50:16 GMT
Lars George wrote:
> Hi,
>
> I was wondering if there is a low cost (as in memory) and fast way to 
> check if a certain cell already exists? I need to insert a cell, but 
> based on if it was there before or not increase a counter (as in total 
> number of entries in a table). 

Does the count of elements have to be up-to-date?  Why not just scan the 
table every hour or so to get a count?  (Scans are fast in 0.19.0. Seven 
times faster than they were in 0.17.x and probably 100 times faster than 
what they are in 0.1.3 -- smile).

> I see that HTable.get(...) returns the byte array, means there are 
> memory, reading and network streaming involved. 

Yes.

> So if I do a
>
> if (table.get(row, col) == null) { incr(counter); }
> table.put(...);
>
> this seems like a waste of resources and may not be as fast as a true
>
> if (!table.exists(row, col))  { incr(counter); }
> table.put(...)
>
Its tough.  Ideal would be a bloom filter on the column.  You'd check 
for presence of a Cell in bloom filter.  It'd come back yes/no.  Would 
be an in-memory test but would involve a network trip (Maybe have a 
client-side bloomfilter too?  So, if exists, would save the network trip?).

The hard part about bloom filter though is that you would have specify 
exact coordinates as in exact row/column/timestamp.  The row/column part 
is easy but the timestamp less-so.  When you insert, you probably do not 
specify a timestamp letting the system set the timestamp to now.  If you 
then want to test existence in a bloomfilter, how you going to do it if 
you don't have the exact timestamp.  So, you end up using the hbase 
get(row, column) because it will return the latest insert if no 
timestamp specified.

Otherwise, looks like you would be happy with a bloomfilter that just 
recorded the row and column and not timestamp.  That'd work.   I think 
this is how bloomfilters work now in latest hbase.  We need to check.  
They used to be row/column/timestamp (They are broken till we release 
0.19.0 though -- in about a month).
> It looks like this is easily doable since get() also delegates to the 
> region servers.
>

> Am I missing something? Assuming HTable is sort of a Set 
> implementation I am confused as to way this check is missing.
>
Well, its not that straight-forward.  The only place to check presence 
of a column is by actually asking hbase and letting it check its 
memcache and then all of its storefiles.  This is only way to see if a 
row/column combination exists.  There is no short-circuit, say, a Set 
that holds all row/column combinations because it could be massive if a 
row had millions of columns (nothing to prevent this happening).

Is the lookup taking too long?  In 0.19.0, the speeds are all up.  There 
is a cache of file blocks maintained in the server.  If you can hit the 
cache, then you can see lookup rates double and even quadruple.

Would a bloom filter in your client work help?

St.Ack

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