Thanks Jonathan & Michael,
No, I do not need incrementing columns. All I need is to increment a
counter of how many documents (cells) I have in a HBase table. The
initial count comes from a MR job counting them all and setting up the
counters in a memcachedb instance. Then when someone inserts a new
document - as opposed to replacing an existing one - I want to increase
that counter.
Overall it seems like I have to go with the get() == null method since
everything else seems kind of way too much work for what I need. I just
thought it would be easy to save some resources. Even doing the normal
get() just not returning anything from the server but the boolean flag
would have seemed a little bit better.
Thanks again,
Lars
Jonathan Gray wrote:
Unfortunately, presence in a bloom filter does not guarantee its existence. So
if you need to be absolutely certain, the bloom filter can only tell you
whether it definitely does not exist (or that it probably exists, with the
false positive error rate based on the size and parameters of the bloom). If
it is found in the bloom, you would still need to hit the actual record to be
sure. Bloom filters make things faster when you miss or if you can tolerate
false-positives.
You can of course save the expense of carrying the data through the network
back to the client, as an optimization.
Perhaps we need to get more at the real use case you're talking about. You
want to insert each new column in a row/family with an incrementing column
name? Do you just need the ordering or do you need the indexes as well? With
your current design, do you start at 0 and keep checking until you miss? If
so, you cannot tolerate false-positives and expect more hits than misses; bloom
filters will not help.
I have done some work with incrementing ids here:
https://issues.apache.org/jira/browse/HBASE-803.
Take a look at the code for incrementFamilyColumn(), should not be too difficult to modify it to get the functionality you want. This implementation would make the work transparent to your client and much faster as things will be processed in the local regionserver to the row, your latency/network cost would then only be for the put.
JG
-----Original Message-----
From: stack [mailto:[EMAIL PROTECTED]
Sent: Sunday, November 09, 2008 1:50 PM
To: [email protected]
Subject: Re: Low-cost and fast HTable.exists(...)?
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
