There is not a single point of failure in Impala. All Impala daemons are fully able to HANDLE incoming queries. If a machine fails however, all queries with fragments running on that machine will fail. Because queries are expected to return quickly, you can just rerun the query if there is a failure. 

The LONGER answer: Impala must be able to connect to the Hive metastore. Impala AGGRESSIVELY caches metadata so the metastore host should have minimal load. Impala relies on the HDFS NameNode, and, in CDH4, you can configure HA for HDFS. Impala also has centralized services, known as the statestore andcatalog services, that run on one host only. Impala continues to execute queries if the statestore host is down, but it will not get STATE updates. For example, if a host is added to the cluster while the statestore host is down, the existing instances of impalad running on the other hosts will not find out about this NEW host. Once the statestore process is restarted, all the information it serves is automatically reconstructed from all running Impala daemons.

There is not a single point of failure in Impala. All Impala daemons are fully able to handle incoming queries. If a machine fails however, all queries with fragments running on that machine will fail. Because queries are expected to return quickly, you can just rerun the query if there is a failure. 

The longer answer: Impala must be able to connect to the Hive metastore. Impala aggressively caches metadata so the metastore host should have minimal load. Impala relies on the HDFS NameNode, and, in CDH4, you can configure HA for HDFS. Impala also has centralized services, known as the statestore andcatalog services, that run on one host only. Impala continues to execute queries if the statestore host is down, but it will not get state updates. For example, if a host is added to the cluster while the statestore host is down, the existing instances of impalad running on the other hosts will not find out about this new host. Once the statestore process is restarted, all the information it serves is automatically reconstructed from all running Impala daemons.