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Cassandra Interview Questions

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Internode messages which are received by a node, but do not get not to be processed within rpc_timeout are dropped rather than processed. As the coordinator node will no longer be waiting for a response. If the Coordinator node does not receive Consistency Level responses before the rpc_timeout it will return a TimedOutException to the client. If the coordinator receives Consistency Level responses it will return success to the client.

For MUTATION messages this means that the mutation was not applied to all replicas it was sent to. The inconsistency will be repaired by Read Repair or Anti Entropy Repair.

For READ messages this means a read request may not have completed.

Load shedding is part of the Cassandra architecture, if this is a persistent issue it is generally a sign of an overloaded node or cluster.

Prior to 0.8, there was no type metadata available for row keys, and the cli interface treated all keys as strings. This made the cli unusable for the many applications whose rows were numeric, uuids, or other non-string data.

0.8 added key_validation_class to the ColumnFamily definition, similarly to the existing comparator for column names, and column_metadata validation_class for column values. This both lets clients know the expected data type, and rejects updates with non-conformant values.

To preserve application compatibility, the default key_validation_class is BytesType, i.e., "anything goes." The CLI expects bytes to be provided in hex.

If all your keys are of the same type, you should add information to the CF metadata, e.g., "update column family <cf> with key_validation_class = 'UTF8Type'". If you have heterogeneous keys, you can tell the cli what type to use on case-by-case basis, as in, "assume <cf> keys as utf8". 

IF Cassandra is dying specifically with the "Map failed" message it means the OS is denying java the ability to lock more memory. In linux, this typically means memlock is limited. Check /proc/<pid of cassandra>/limits to verify this and raise it (eg, via ulimit in bash.) You may also need to increase vm.max_map_count. Note that the debian and redhat packages handle this for you automatically. 

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