NoHostAvailableException Found at the Time of data reading from Cassandra - cassandra

Question

I am using Cassandra 2.1.5, and Cassandra-java-driver 2.0.10. I am facing below Exception when i am fetching Data from Cassandra Table.
com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (tried: /127.0.0.1:9042 (com.datastax.driver.core.TransportException: [/127.0.0.1:9042] Connection has been closed))
at com.datastax.driver.core.exceptions.NoHostAvailableException.copy(NoHostAvailableException.java:84)
at com.datastax.driver.core.DefaultResultSetFuture.extractCauseFromExecutionException(DefaultResultSetFuture.java:265)
at com.datastax.driver.core.DefaultResultSetFuture.getUninterruptibly(DefaultResultSetFuture.java:179)
at com.datastax.driver.core.AbstractSession.execute(AbstractSession.java:52)
at com.datastax.driver.core.AbstractSession.execute(AbstractSession.java:36)
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 com.datastax.spark.connector.cql.SessionProxy.invoke(SessionProxy.scala:33)
at com.sun.proxy.$Proxy8.execute(Unknown Source)
at com.exportstagging.SparkTest.ProductDataLoader.dbQuery(ProductDataLoader.java:418)
at com.exportstagging.SparkTest.ProductDataLoader.main(ProductDataLoader.java:442)
Caused by: com.datastax.driver.core.exceptions.NoHostAvailableException: All host(s) tried for query failed (tried: /127.0.0.1:9042 (com.datastax.driver.core.TransportException: [/127.0.0.1:9042] Connection has been closed))
at com.datastax.driver.core.RequestHandler.reportNoMoreHosts(RequestHandler.java:216)
at com.datastax.driver.core.RequestHandler.access$900(RequestHandler.java:45)
at com.datastax.driver.core.RequestHandler$SpeculativeExecution.sendRequest(RequestHandler.java:276)
at com.datastax.driver.core.RequestHandler$SpeculativeExecution$1.run(RequestHandler.java:374)
at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)
In my cassandra Table there is 50000 columns and 380000 rows. when i am fire this query i am getting above error.
Select * from mykeyspace.productdata Where id In (1,...,6000).
I have used token to create batching for fetching data from Cassandra.

Answer 1

Trying to fectch 6000 partitions at once, on 380000 rows with a table with 50000 columns seems totally overkill.
Multi partition select should be done using asynchronous queries, using one query per partition.
Having 50000 columns in a table looks like a data modeling problem. What is your use case ?
Fetching 380000 rows at once shouldn't be necessary (and will take a very long time anyway), and it looks like an analytical query which should be way better handled through Spark.
Trying to get so much data at once, you are most likely getting an OOM on your cassandra node, which could explain why you're getting the message "Connection has been closed"
My advice is to review your model, and try to split the load the way you should (async queries) and use proper paging.
If you want to crunch so much data at once, you have to do it with Spark through batch processing, output the result in another cassandra table, and access that result table through smaller, faster interactive queries.