impala/docs/topics/impala_components.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--
Licensed to the Apache Software Foundation (ASF) under one
or more contributor license agreements.  See the NOTICE file
distributed with this work for additional information
regarding copyright ownership.  The ASF licenses this file
to you under the Apache License, Version 2.0 (the
"License"); you may not use this file except in compliance
with the License.  You may obtain a copy of the License at

  http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing,
software distributed under the License is distributed on an
"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
KIND, either express or implied.  See the License for the
specific language governing permissions and limitations
under the License.
-->
<!DOCTYPE concept PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd">
<concept id="intro_components">

  <title>Components of the Impala Server</title>
  <titlealts audience="PDF"><navtitle>Components</navtitle></titlealts>
  <prolog>
    <metadata>
      <data name="Category" value="Impala"/>
      <data name="Category" value="Concepts"/>
      <data name="Category" value="Administrators"/>
      <data name="Category" value="Developers"/>
      <data name="Category" value="Data Analysts"/>
    </metadata>
  </prolog>

  <conbody>

    <p>
      The Impala server is a distributed, massively parallel processing (MPP) database engine. It consists of
      different daemon processes that run on specific hosts within your CDH cluster.
    </p>

    <p outputclass="toc inpage"/>
  </conbody>

  <concept id="intro_impalad">

    <title>The Impala Daemon</title>

    <conbody>

      <p>
        The core Impala component is a daemon process that runs on each DataNode of the cluster, physically represented
        by the <codeph>impalad</codeph> process. It reads and writes to data files; accepts queries transmitted
        from the <codeph>impala-shell</codeph> 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.
      </p>

      <p>
        You can submit a query to the Impala daemon running on any DataNode, and that instance of the daemon serves as the
        <term>coordinator node</term> 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 <codeph>impala-shell</codeph> 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.
      </p>

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

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

      <p>
        <b>Related information:</b> <xref href="impala_config_options.xml#config_options"/>,
        <xref href="impala_processes.xml#processes"/>, <xref href="impala_timeouts.xml#impalad_timeout"/>,
        <xref href="impala_ports.xml#ports"/>, <xref href="impala_proxy.xml#proxy"/>
      </p>
    </conbody>
  </concept>

  <concept id="intro_statestore">

    <title>The Impala Statestore</title>

    <conbody>

      <p>
        The Impala component known as the <term>statestore</term> 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 <codeph>statestored</codeph>; 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.
      </p>

      <p>
        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.
      </p>

      <p conref="../shared/impala_common.xml#common/statestored_catalogd_ha_blurb"/>

      <p>
        <b>Related information:</b>
      </p>

      <p>
        <xref href="impala_scalability.xml#statestore_scalability"/>,
        <xref href="impala_config_options.xml#config_options"/>, <xref href="impala_processes.xml#processes"/>,
        <xref href="impala_timeouts.xml#statestore_timeout"/>, <xref href="impala_ports.xml#ports"/>
      </p>
    </conbody>
  </concept>

  <concept rev="1.2" id="intro_catalogd">

    <title>The Impala Catalog Service</title>

    <conbody>

      <p>
        The Impala component known as the <term>catalog service</term> relays the metadata changes from Impala SQL
        statements to all the DataNodes in a cluster. It is physically represented by a daemon process named
        <codeph>catalogd</codeph>; 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 <cmdname>statestored</cmdname> and
        <cmdname>catalogd</cmdname> services on the same host.
      </p>

      <p>
        The catalog service avoids the need to issue
        <codeph>REFRESH</codeph> and <codeph>INVALIDATE METADATA</codeph> 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 <codeph>REFRESH</codeph> or <codeph>INVALIDATE METADATA</codeph> on an Impala node
        before executing a query there.
      </p>

      <p>
        This feature touches a number of aspects of Impala:
      </p>

<!-- This was formerly a conref, but since the list of links also included a link
     to this same topic, materializing the list here and removing that
     circular link. (The conref is still used in Incompatible Changes.)

      <ul conref="../shared/impala_common.xml#common/catalogd_xrefs">
        <li/>
      </ul>
-->

      <ul id="catalogd_xrefs">
        <li>
          <p>
            See <xref href="impala_install.xml#install"/>, <xref href="impala_upgrading.xml#upgrading"/> and
            <xref href="impala_processes.xml#processes"/>, for usage information for the
            <cmdname>catalogd</cmdname> daemon.
          </p>
        </li>

        <li>
          <p>
            The <codeph>REFRESH</codeph> and <codeph>INVALIDATE METADATA</codeph> statements are not needed
            when the <codeph>CREATE TABLE</codeph>, <codeph>INSERT</codeph>, 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
            <xref href="impala_refresh.xml#refresh"/> and
            <xref href="impala_invalidate_metadata.xml#invalidate_metadata"/> for the latest usage information for
            those statements.
          </p>
        </li>
      </ul>

      <p conref="../shared/impala_common.xml#common/load_catalog_in_background"/>

      <p conref="../shared/impala_common.xml#common/statestored_catalogd_ha_blurb"/>

      <note>
        <p conref="../shared/impala_common.xml#common/catalog_server_124"/>
      </note>

      <p>
        <b>Related information:</b> <xref href="impala_config_options.xml#config_options"/>,
        <xref href="impala_processes.xml#processes"/>, <xref href="impala_ports.xml#ports"/>
      </p>
    </conbody>
  </concept>
</concept>