HBASE-493 was created, and seems similar. It's a write-if-not-
modified-since.
I would guess that you probably don't want to use HBase to maintain a
distributed auto-increment. You need to think of some other approach
the produces unique ids across concurrent access, like hash or GUID
or something like that.
-Bryan
On Apr 16, 2008, at 2:18 PM, Jim Kellerman wrote:
Row locks do not apply to reads, only updates. They prevent two
applications from updating the same row simultaneously. There is no
other locking mechanism in HBase. (It follows Bigtable in this
regard. See http://labs.google.com/papers/bigtable.html )
There has been some discussion about adding a conditional write
(i.e. only completes successfully if the current value of the cell
being updated has value x), but noone has thought it important
enough to enter an enhancement request on the HBase Jira: https://
issues.apache.org/jira/browse/HBASE
By the way, you will get a more timely response to HBase questions
if you address them to the hbase mailing list: hbase-
[EMAIL PROTECTED]
---
Jim Kellerman, Senior Engineer; Powerset
-----Original Message-----
From: Thomas Thevis [mailto:[EMAIL PROTECTED]
Sent: Wednesday, April 16, 2008 4:22 AM
To: core-user@hadoop.apache.org
Subject: ID Service with HBase?
Hello list readers,
I'd like to perform mass data operations resulting in several
output files with cross-references between lines in different
files. For this purpose, I want to use a kind of ID service
and I wonder whether I could use HBase for this task.
However, until now I was not able to use the HBase locking
mechanism in a way that newly created IDs are unique.
The setup:
- each Mapper has its own instance of an IDSevice implementation
- each IDService instance has its own reference to the ID
table in the HBase
The code snippet which is used to return and update IDs:
[code]
final String columnName = this.config.get(ID_COLUMN_ID);
final Text column = new Text(columnName); final String
tableName = this.config.get(ID_SERVICE_TABLE_ID);
final HTable table = new HTable(this.config, new
Text(tableName)); final Text rowName = new Text(namespace);
final long startValue;
final long lockid = table.startUpdate(rowName); final byte[]
bytes = table.get(rowName, column); if (bytes == null) {
startValue = 0;
} else {
final ByteArrayInputStream byteArrayInputStream
= new ByteArrayInputStream(bytes);
final LongWritable longWritable = new LongWritable();
longWritable.readFields(new
DataInputStream(byteArrayInputStream));
startValue = longWritable.get();
}
final long stopValue = startValue + size; table.put(lockid,
column, new LongWritable(stopValue)); table.commit(lockid); [/code]
As stated above, resulting IDs are not unique, about a
quarter of all created IDs appears several times.
Now my question: Do I use the locking mechanism the wrong way
or is my approach to use HBase locking and synchronizing for
this task completely wrong?
Thanks,
Thomas
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