HBASE: Table already exists
Last weekend while expermenting on Hbase component of Hadoop
ecosystem; and i came across an issue while creating table.
=> Hbase::Table - emp
hbase(main):003:0> create 'emp', 'personal data',
'professional data'
ERROR: Table already exists: emp!
Cool so table is already present; that’s ok; might be it
is already present; sounds easy peasy right but somehow it turned out to be
very interesting for me.
At first i checked its existence and i listed all the
table and unfortunately it’s not there.
hbase(main):004:0> list
TABLE
emp_v
random_test
temp_test
So to outsmart Hbase:) i tried deletion
hbase(main):005:0> drop 'emp'
ERROR: Table emp does not exist.
Excellent so in one context Hbase table is not present but
it is present sounds whacky right; but all in all its good there is something
for me to dig in and there is a saying "They Say Half Knowledge is a
Dangerous thing"; so why not add another half and make it full.
I went ahead and tried to see if there is any trace of a
file which might left behind and from that Hbase might be reading.
I checked that and I saw only one namespace i.e.
"default"
#hdfs dfs -ls /hbase/data
drwxr-xr-x - hbase hbase
0 2016-12-11 20:51 /hbase/data/default
you can see nowhere a single directory.
# hdfs dfs -ls /hbase/data/default
drwxr-xr-x - hbase hbase
0 2016-12-08 18:20 /hbase/data/default/emp_v
drwxr-xr-x - hbase hbase
0 2016-12-09 13:45 /hbase/data/default/random_test
drwxr-xr-x - hbase hbase
0 2016-12-09 00:08 /hbase/data/default/temp_test
So above didn’t helped either; but this is true that Hbase
is reading it from somewhere but where; i started from basics that how client coordinate
with Hbase and this proved to be fruitful step for me.
Way forward was "The Zookeeper".
A distributed HBase relies completely on Zookeeper (for
cluster configuration and management). In HBase, Zookeeper coordinates,
communicates, and shares state between the Masters and Region Servers.
Zookeeper is a client/server system for distributed
coordination that exposes an interface similar to a filesystem, where each
node (called a Znode) may contain data and a set of children.
Each Znode has a name and can be identified using a
filesystem-like path (for example, /root-znode/sub-znode/my-znode).
Znode which seems to be important for this context
Znode
|
Description
|
hbase/table
(zookeeper.znode.masterTableEnableDisable)
|
Used by the
master to track the table state during assignments (disabling/enabling
states, for example).
|
Zookeeper can be accessed via zkcli and i logged in to
zookeeper client via hbase zkcli or you
may use zookeeper-client command.
i did ls command and issue seems cracked; here is the
bright and shiny emp table.
ls /hbase/table
[ random_test, emp, hbase:meta,
hbase:namespace, temp_test, emp_v]
As I mentioned above as
part of Znode that HBase stores some data schema and state of each table in
zookeeper to be able to coordinate between all the region servers .
You might be wondering why
table metadata still present although dropped from Hbase??
What is the solution??
Since it seems an orphan
kind of table to me; so why not remove the metadata from zookeeper; Zookeeper
provides rmr command to remove a specified Znode.
So from same login I executed
below command which expected to remove a Znode i.e. <Metadata of emp table>
rmr /hbase/emp
and I checked ls command again that whether emp table is still present
or not; finally table is removed a big relief to me.
ls /hbase/table
[ random_test, hbase:meta, hbase:namespace,
temp_test, emp_v]
And I created table again and this time I succeeded J
hbase(main):001:0> create
'emp', 'personal data', 'professional data'
0 row(s) in 2.5890 seconds
=> Hbase::Table - emp
