The core Impala component is a daemon process that runs on each DataNode of the cluster, physically represented by the impalad process. It reads and writes to data files; accepts queries transmitted from the impala-shell command, Hue, JDBC, or ODBC; parallelizes the queries and distributes work across the cluster; and transmits intermediate query results back to the central coordinator node.

You can submit a query to the Impala daemon running on any DataNode, and that instance of the daemon serves as the coordinator node for that query. The other nodes transmit partial results back to the coordinator, which constructs the final result set for a query. When running experiments with functionality through the impala-shell command, you might always connect to the same Impala daemon for convenience. For clusters running production workloads, you might load-balance by submitting each query to a different Impala daemon in round-robin style, using the JDBC or ODBC interfaces.

The Impala daemons are in constant communication with the statestore, to confirm which nodes are healthy and can accept new work.

They also receive broadcast messages from the catalogd daemon (introduced in Impala 1.2) whenever any Impala node in the cluster creates, alters, or drops any type of object, or when an INSERT or LOAD DATA statement is processed through Impala. This background communication minimizes the need for REFRESH or INVALIDATE METADATA statements that were needed to coordinate metadata across nodes prior to Impala 1.2.

Related information: Modifying Impala Startup Options, Starting Impala, Setting the Idle Query and Idle Session Timeouts for impalad, Ports Used by Impala, Using Impala through a Proxy for High Availability

The Impala component known as the statestore checks on the health of Impala daemons on all the DataNodes in a cluster, and continuously relays its findings to each of those daemons. It is physically represented by a daemon process named statestored; you only need such a process on one host in the cluster. If an Impala daemon goes offline due to hardware failure, network error, software issue, or other reason, the statestore informs all the other Impala daemons so that future queries can avoid making requests to the unreachable node.

Because the statestore's purpose is to help when things go wrong, it is not critical to the normal operation of an Impala cluster. If the statestore is not running or becomes unreachable, the Impala daemons continue running and distributing work among themselves as usual; the cluster just becomes less robust if other Impala daemons fail while the statestore is offline. When the statestore comes back online, it re-establishes communication with the Impala daemons and resumes its monitoring function.

Most considerations for load balancing and high availability apply to the impalad daemon. The statestored and catalogd daemons do not have special requirements for high availability, because problems with those daemons do not result in data loss. If those daemons become unavailable due to an outage on a particular host, you can stop the Impala service, delete the Impala StateStore and Impala Catalog Server roles, add the roles on a different host, and restart the Impala service.

Related information:

Scalability Considerations for the Impala Statestore, Modifying Impala Startup Options, Starting Impala, Increasing the Statestore Timeout, Ports Used by Impala

The Impala component known as the catalog service relays the metadata changes from Impala SQL statements to all the DataNodes in a cluster. It is physically represented by a daemon process named catalogd; you only need such a process on one host in the cluster. Because the requests are passed through the statestore daemon, it makes sense to run the statestored and catalogd services on the same host.

The catalog service avoids the need to issue REFRESH and INVALIDATE METADATA statements when the metadata changes are performed by statements issued through Impala. When you create a table, load data, and so on through Hive, you do need to issue REFRESH or INVALIDATE METADATA on an Impala node before executing a query there.

This feature touches a number of aspects of Impala:

• See Impala Installation, Upgrading Impala and Starting Impala, for usage information for the catalogd daemon.

• The REFRESH and INVALIDATE METADATA statements are not needed when the CREATE TABLE, INSERT, or other table-changing or data-changing operation is performed through Impala. These statements are still needed if such operations are done through Hive or by manipulating data files directly in HDFS, but in those cases the statements only need to be issued on one Impala node rather than on all nodes. See REFRESH Statement and INVALIDATE METADATA Statement for the latest usage information for those statements.

By default, the metadata loading and caching on startup happens asynchronously, so Impala can begin accepting requests promptly. To enable the original behavior, where Impala waited until all metadata was loaded before accepting any requests, set the catalogd configuration option --load_catalog_in_background=false.

Most considerations for load balancing and high availability apply to the impalad daemon. The statestored and catalogd daemons do not have special requirements for high availability, because problems with those daemons do not result in data loss. If those daemons become unavailable due to an outage on a particular host, you can stop the Impala service, delete the Impala StateStore and Impala Catalog Server roles, add the roles on a different host, and restart the Impala service.

Related information: Modifying Impala Startup Options, Starting Impala, Ports Used by Impala