Hi Wangda , I am not sure making overwrite=false , will solve the problem. As per java doc by making overwrite=false , it will throw an exception if the file already exists. So, for all the remaining mappers it will throw an exception.
Also I am very new to ZK and have very basic knowledge of it , I am not sure if it can solve the problem and if yes how. I am still going through available sources on the ZK. Can you please refer to me some source or link on ZK , that can help me in solving the problem. Best Saurabh On Tue, Aug 12, 2014 at 3:08 AM, Wangda Tan <wheele...@gmail.com> wrote: > Hi Saurabh, > It's an interesting topic, > > >> So , here is the question , is it possible to make sure that when one of > the mapper tasks is writing to a file , other should wait until the first > one is finished. ? I read that all the mappers task don't interact with > each other > > A simple way to do this is using HDFS namespace: > Create file using "public FSDataOutputStream create(Path f, boolean > overwrite)", overwrite=false. Only one mapper can successfully create file. > > After write completed, the mapper will create a flag file like "completed" > in the same folder. Other mappers can wait for the "completed" file > created. > > >> Is there any way to have synchronization between two independent map > reduce jobs? > I think ZK can do some complex synchronization here. Like mutex, master > election, etc. > > Hope this helps, > > Wangda Tan > > > > > On Tue, Aug 12, 2014 at 10:43 AM, saurabh jain <sauravma...@gmail.com> > wrote: > > > Hi Folks , > > > > I have been writing a map-reduce application where I am having an input > > file containing records and every field in the record is separated by > some > > delimiter. > > > > In addition to this user will also provide a list of columns that he > wants > > to lookup in a master properties file (stored in HDFS). If this columns > > (lets say it a key) is present in master properties file then get the > > corresponding value and update the key with this value and if the key is > > not present it in the master properties file then it will create a new > > value for this key and will write to this property file and will also > > update in the record. > > > > I have written this application , tested it and everything worked fine > > till now. > > > > *e.g :* *I/P Record :* This | is | the | test | record > > > > *Columns :* 2,4 (that means code will look up only field *"is" and > "test"* in > > the master properties file.) > > > > Here , I have a question. > > > > *Q 1:* In the case when my input file is huge and it is splitted across > > the multiple mappers , I was getting the below mentioned exception where > > all the other mappers tasks were failing. *Also initially when I started > > the job my master properties file is empty.* In my code I have a check if > > this file (master properties) doesn't exist create a new empty file > before > > submitting the job itself. > > > > e.g : If i have 4 splits of data , then 3 map tasks are failing. But > after > > this all the failed map tasks restarts and finally the job become > > successful. > > > > So , *here is the question , is it possible to make sure that when one of > > the mapper tasks is writing to a file , other should wait until the first > > one is finished. ?* I read that all the mappers task don't interact with > > each other. > > > > Also what will happen in the scenario when I start multiple parallel > > map-reduce jobs and all of them working on the same properties files. *Is > > there any way to have synchronization between two independent map reduce > > jobs*? > > > > I have also read that ZooKeeper can be used in such scenarios , Is that > > correct ? > > > > > > Error: > com.techidiocy.hadoop.filesystem.api.exceptions.HDFSFileSystemException: > IOException - failed while appending data to the file ->Failed to create > file [/user/cloudera/lob/master/bank.properties] for > [DFSClient_attempt_1407778869492_0032_m_000002_0_1618418105_1] on client > [10.X.X.17], because this file is already being created by > > [DFSClient_attempt_1407778869492_0032_m_000005_0_-949968337_1] on > [10.X.X.17] > > at > org.apache.hadoop.hdfs.server.namenode.FSNamesystem.recoverLeaseInternal(FSNamesystem.java:2548) > > at > org.apache.hadoop.hdfs.server.namenode.FSNamesystem.appendFileInternal(FSNamesystem.java:2377) > > at > org.apache.hadoop.hdfs.server.namenode.FSNamesystem.appendFileInt(FSNamesystem.java:2612) > > at > org.apache.hadoop.hdfs.server.namenode.FSNamesystem.appendFile(FSNamesystem.java:2575) > > at > org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.append(NameNodeRpcServer.java:522) > > at > org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.append(ClientNamenodeProtocolServerSideTranslatorPB.java:373) > > at > org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java) > > at > org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:585) > > at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:1026) > > at > org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:1986) > > at > org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:1982) > > at java.security.AccessController.doPrivileged(Native > Method) > > at javax.security.auth.Subject.doAs(Subject.java:415) > > at > org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1554) > > at > org.apache.hadoop.ipc.Server$Handler.run(Server.java:1980) > > > > >