Please see below,
Stefan Podkowinski wrote:
As far as i understand the main problem is that you need to create
splits from streaming data with an unknown number of records and
offsets. Its just the same problem as with externally compressed data
(.gz). You need to go through the complete stream (or do a table scan)
to create logical splits. Afterwards each map task needs to seek to
the appropriate offset on a new stream over again. Very expansive. As
with compressed files, no wonder only one map task is started for each
.gz file and will consume the complete file.
I cannot see an easy way to split the JDBC stream and pass them to nodes.
IMHO the DBInputFormat
should follow this behavior and just create 1 split whatsoever.
Why would we want to limit to 1 splits, which effectively resolves to
sequential computation?
Maybe a future version of hadoop will allow to create splits/map tasks
on the fly dynamically?
It is obvious that input residing in one database is not optimal for
hadoop, and in any case(even with sharding)
DB I/O would be the bottleneck. I guess DBInput/Output formats should be
used when data is small but computation is costly.
Stefan
On Thu, Feb 5, 2009 at 3:28 PM, Fredrik Hedberg <[email protected]> wrote:
Indeed sir.
The implementation was designed like you describe for two reasons. First and
foremost to make is as simple as possible for the user to use a JDBC
database as input and output for Hadoop. Secondly because of the specific
requirements the MapReduce framework brings to the table (split
distribution, split reproducibility etc).
This design will, as you note, never handle the same amount of data as HBase
(or HDFS), and was never intended to. That being said, there are a couple of
ways that the current design could be augmented to perform better (and, as
in its current form, tweaked, depending on you data and computational
requirements). Shard awareness is one way, which would let each
database/tasktracker-node execute mappers on data where each split is a
single database server for example.
If you have any ideas on how the current design can be improved, please do
share.
Fredrik
On Feb 5, 2009, at 11:37 AM, Stefan Podkowinski wrote:
The 0.19 DBInputFormat class implementation is IMHO only suitable for
very simple queries working on only few datasets. Thats due to the
fact that it tries to create splits from the query by
1) getting a count of all rows using the specified count query (huge
performance impact on large tables)
2) creating splits by issuing an individual query for each split with
a "limit" and "offset" parameter appended to the input sql query
Effectively your input query "select * from orders" would become
"select * from orders limit <splitlength> offset <splitstart>" and
executed until count has been reached. I guess this is not working sql
syntax for oracle.
Stefan
2009/2/4 Amandeep Khurana <[email protected]>:
Adding a semicolon gives me the error "ORA-00911: Invalid character"
Amandeep
Amandeep Khurana
Computer Science Graduate Student
University of California, Santa Cruz
On Wed, Feb 4, 2009 at 6:46 AM, Rasit OZDAS <[email protected]> wrote:
Amandeep,
"SQL command not properly ended"
I get this error whenever I forget the semicolon at the end.
I know, it doesn't make sense, but I recommend giving it a try
Rasit
2009/2/4 Amandeep Khurana <[email protected]>:
The same query is working if I write a simple JDBC client and query the
database. So, I'm probably doing something wrong in the connection
settings.
But the error looks to be on the query side more than the connection
side.
Amandeep
Amandeep Khurana
Computer Science Graduate Student
University of California, Santa Cruz
On Tue, Feb 3, 2009 at 7:25 PM, Amandeep Khurana <[email protected]>
wrote:
Thanks Kevin
I couldnt get it work. Here's the error I get:
bin/hadoop jar ~/dbload.jar LoadTable1
09/02/03 19:21:17 INFO jvm.JvmMetrics: Initializing JVM Metrics with
processName=JobTracker, sessionId=
09/02/03 19:21:20 INFO mapred.JobClient: Running job: job_local_0001
09/02/03 19:21:21 INFO mapred.JobClient: map 0% reduce 0%
09/02/03 19:21:22 INFO mapred.MapTask: numReduceTasks: 0
09/02/03 19:21:24 WARN mapred.LocalJobRunner: job_local_0001
java.io.IOException: ORA-00933: SQL command not properly ended
at
org.apache.hadoop.mapred.lib.db.DBInputFormat.getRecordReader(DBInputFormat.java:289)
at org.apache.hadoop.mapred.MapTask.run(MapTask.java:321)
at
org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:138)
java.io.IOException: Job failed!
at
org.apache.hadoop.mapred.JobClient.runJob(JobClient.java:1217)
at LoadTable1.run(LoadTable1.java:130)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:79)
at LoadTable1.main(LoadTable1.java:107)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown
Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at org.apache.hadoop.util.RunJar.main(RunJar.java:165)
at org.apache.hadoop.mapred.JobShell.run(JobShell.java:54)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:65)
at org.apache.hadoop.util.ToolRunner.run(ToolRunner.java:79)
at org.apache.hadoop.mapred.JobShell.main(JobShell.java:68)
Exception closing file
/user/amkhuran/contract_table/_temporary/_attempt_local_0001_m_000000_0/part-00000
java.io.IOException: Filesystem closed
at
org.apache.hadoop.hdfs.DFSClient.checkOpen(DFSClient.java:198)
at
org.apache.hadoop.hdfs.DFSClient.access$600(DFSClient.java:65)
at
org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.closeInternal(DFSClient.java:3084)
at
org.apache.hadoop.hdfs.DFSClient$DFSOutputStream.close(DFSClient.java:3053)
at
org.apache.hadoop.hdfs.DFSClient$LeaseChecker.close(DFSClient.java:942)
at org.apache.hadoop.hdfs.DFSClient.close(DFSClient.java:210)
at
org.apache.hadoop.hdfs.DistributedFileSystem.close(DistributedFileSystem.java:243)
at
org.apache.hadoop.fs.FileSystem$Cache.closeAll(FileSystem.java:1413)
at org.apache.hadoop.fs.FileSystem.closeAll(FileSystem.java:236)
at
org.apache.hadoop.fs.FileSystem$ClientFinalizer.run(FileSystem.java:221)
Here's my code:
public class LoadTable1 extends Configured implements Tool {
// data destination on hdfs
private static final String CONTRACT_OUTPUT_PATH =
"contract_table";
// The JDBC connection URL and driver implementation class
private static final String CONNECT_URL = "jdbc:oracle:thin:@dbhost
:1521:PSEDEV";
private static final String DB_USER = "user";
private static final String DB_PWD = "pass";
private static final String DATABASE_DRIVER_CLASS =
"oracle.jdbc.driver.OracleDriver";
private static final String CONTRACT_INPUT_TABLE =
"OSE_EPR_CONTRACT";
private static final String [] CONTRACT_INPUT_TABLE_FIELDS = {
"PORTFOLIO_NUMBER", "CONTRACT_NUMBER"};
private static final String ORDER_CONTRACT_BY_COL =
"CONTRACT_NUMBER";
static class ose_epr_contract implements Writable, DBWritable {
String CONTRACT_NUMBER;
public void readFields(DataInput in) throws IOException {
this.CONTRACT_NUMBER = Text.readString(in);
}
public void write(DataOutput out) throws IOException {
Text.writeString(out, this.CONTRACT_NUMBER);
}
public void readFields(ResultSet in_set) throws SQLException {
this.CONTRACT_NUMBER = in_set.getString(1);
}
@Override
public void write(PreparedStatement prep_st) throws SQLException
{
// TODO Auto-generated method stub
}
}
public static class LoadMapper extends MapReduceBase
implements Mapper<LongWritable,
ose_epr_contract, Text, NullWritable> {
private static final char FIELD_SEPARATOR = 1;
public void map(LongWritable arg0, ose_epr_contract arg1,
OutputCollector<Text, NullWritable> arg2, Reporter arg3)
throws IOException {
StringBuilder sb = new StringBuilder();
;
sb.append(arg1.CONTRACT_NUMBER);
arg2.collect(new Text (sb.toString()), NullWritable.get());
}
}
public static void main(String[] args) throws Exception {
Class.forName("oracle.jdbc.driver.OracleDriver");
int exit = ToolRunner.run(new LoadTable1(), args);
}
public int run(String[] arg0) throws Exception {
JobConf conf = new JobConf(getConf(), LoadTable1.class);
conf.setInputFormat(DBInputFormat.class);
DBConfiguration.configureDB(conf, DATABASE_DRIVER_CLASS,
CONNECT_URL, DB_USER, DB_PWD);
DBInputFormat.setInput(conf, ose_epr_contract.class,
"select CONTRACT_NUMBER from OSE_EPR_CONTRACT",
"select COUNT(CONTRACT_NUMBER) from OSE_EPR_CONTRACT");
FileOutputFormat.setOutputPath(conf, new
Path(CONTRACT_OUTPUT_PATH));
conf.setMapperClass(LoadMapper.class);
conf.setNumReduceTasks(0);
conf.setOutputKeyClass(Text.class);
conf.setOutputValueClass(NullWritable.class);
JobClient.runJob(conf);
return 0;
}
}
-Amandeep
Amandeep Khurana
Computer Science Graduate Student
University of California, Santa Cruz
On Tue, Feb 3, 2009 at 6:51 PM, Kevin Peterson <[email protected]
wrote:
On Tue, Feb 3, 2009 at 5:49 PM, Amandeep Khurana <[email protected]>
wrote:
In the setInput(...) function in DBInputFormat, there are two sets
of
arguments that one can use.
1. public static void *setInput*(JobConf
a) In this, do we necessarily have to give all the fieldNames (which
are
the
column names right?) that the table has, or do we need to specify
only
the
ones that we want to extract?
You may specify only those columns that you are interested in.
b) Do we have to have a orderBy or not necessarily? Does this relate
to
the
primary key in the table in any ways?
Conditions and order by are not necessary.
a) Is there any restriction on the kind of queries that this function
can take in the inputQuery string?
I don't think so, but I don't use this method -- I just use the
fieldNames
and tableName method.
I am facing issues in getting this to work with an Oracle database
and
have no idea of how to debug it (an email sent earlier).
Can anyone give me some inputs on this please?
Create a new table that has one column, put about five entries into
that
table, then try to get a map job working that outputs the values to a
text
file. If that doesn't work, post your code and errors.
--
M. Raşit ÖZDAŞ
