Solr on HDFS

The Solr HDFS module has support for writing and reading its index and transaction log files to the HDFS distributed filesystem. This plugin must first be installed before using. This module needs to be installed as lib, it is planned to evolve to a Solr package in a later release.

This does not use Hadoop MapReduce to process Solr data, rather it only uses the HDFS filesystem for index and transaction log file storage.

To use HDFS rather than a local filesystem, you must be using Hadoop 2.x and you will need to instruct Solr to use the HdfsDirectoryFactory. There are also several additional parameters to define. These can be set in one of three ways:

  • Pass JVM arguments to the bin/solr script. These would need to be passed every time you start Solr with bin/solr.

  • Modify solr.in.sh (or solr.in.cmd on Windows) to pass the JVM arguments automatically when using bin/solr without having to set them manually.

  • Define the properties in solrconfig.xml. These configuration changes would need to be repeated for every collection, so is a good option if you only want some of your collections stored in HDFS.

Starting Solr on HDFS

User-Managed Cluters and Single-Node Installations

For user-managed clusters or single-node Solr installations, there are a few parameters you should modify before starting Solr. These can be set in solrconfig.xml (more on that below), or passed to the bin/solr script at startup.

  • You need to use an HdfsDirectoryFactory and a data directory in the form hdfs://host:port/path

  • You need to specify an updateLog location in the form hdfs://host:port/path

  • You should specify a lock factory type of 'hdfs' or none.

If you do not modify solrconfig.xml, you can instead start Solr on HDFS with the following command:

bin/solr start -Dsolr.directoryFactory=HdfsDirectoryFactory
     -Dsolr.lock.type=hdfs
     -Dsolr.data.dir=hdfs://host:port/path
     -Dsolr.updatelog=hdfs://host:port/path

This example will start Solr using the defined JVM properties (explained in more detail below).

SolrCloud Instances

In SolrCloud mode, it’s best to leave the data and update log directories as the defaults Solr comes with and simply specify the solr.hdfs.home. All dynamically created collections will create the appropriate directories automatically under the solr.hdfs.home root directory.

  • Set solr.hdfs.home in the form hdfs://host:port/path

  • You should specify a lock factory type of 'hdfs' or none.

bin/solr start -c -Dsolr.directoryFactory=HdfsDirectoryFactory
     -Dsolr.lock.type=hdfs
     -Dsolr.hdfs.home=hdfs://host:port/path

This command starts Solr using the defined JVM properties.

Modifying solr.in.sh (*nix) or solr.in.cmd (Windows)

The examples above assume you will pass JVM arguments as part of the start command every time you use bin/solr to start Solr. However, bin/solr looks for an include file named solr.in.sh (solr.in.cmd on Windows) to set environment variables. By default, this file is found in the bin directory, and you can modify it to permanently add the HdfsDirectoryFactory settings and ensure they are used every time Solr is started.

For example, to set JVM arguments to always use HDFS when running in SolrCloud mode (as shown above), you would add a section such as this:

# Set HDFS DirectoryFactory & Settings
-Dsolr.directoryFactory=HdfsDirectoryFactory \
-Dsolr.lock.type=hdfs \
-Dsolr.hdfs.home=hdfs://host:port/path \

The Block Cache

For performance, the HdfsDirectoryFactory uses a Directory that will cache HDFS blocks. This caching mechanism replaces the standard file system cache that Solr utilizes. By default, this cache is allocated off-heap. This cache will often need to be quite large and you may need to raise the off-heap memory limit for the specific JVM you are running Solr in. For the Oracle/OpenJDK JVMs, the following is an example command-line parameter that you can use to raise the limit when starting Solr:

-XX:MaxDirectMemorySize=20g

HdfsDirectoryFactory Parameters

The HdfsDirectoryFactory has a number of settings defined as part of the directoryFactory configuration.

Solr HDFS Settings

solr.hdfs.home

Required

Default: none

A root location in HDFS for Solr to write collection data to. Rather than specifying an HDFS location for the data directory or update log directory, use this to specify one root location and have everything automatically created within this HDFS location. The structure of this parameter is hdfs://host:port/path/solr.

Block Cache Settings

solr.hdfs.blockcache.enabled

Optional

Default: true

Enable the blockcache.

solr.hdfs.blockcache.read.enabled

Optional

Default: true

Enable the read cache.

solr.hdfs.blockcache.direct.memory.allocation

Optional

Default: true

Enable direct memory allocation. If this is false, heap is used.

solr.hdfs.blockcache.slab.count

Optional

Default: 1

Number of memory slabs to allocate. Each slab is 128 MB in size.

solr.hdfs.blockcache.global

Optional

Default: true

Enable/Disable using one global cache for all SolrCores. The settings used will be from the first HdfsDirectoryFactory created.

NRTCachingDirectory Settings

solr.hdfs.nrtcachingdirectory.enable

Optional

Default: true

Enable the use of NRTCachingDirectory.

solr.hdfs.nrtcachingdirectory.maxmergesizemb

Optional

Default: 16

NRTCachingDirectory max segment size for merges.

solr.hdfs.nrtcachingdirectory.maxcachedmb

Optional

Default: 192

NRTCachingDirectory max cache size.

HDFS Client Configuration Settings

solr.hdfs.confdir

Optional

Default: none

Pass the location of HDFS client configuration files - needed for HDFS HA for example.

Kerberos Authentication Settings

Hadoop can be configured to use the Kerberos protocol to verify user identity when trying to access core services like HDFS. If your HDFS directories are protected using Kerberos, then you need to configure Solr’s HdfsDirectoryFactory to authenticate using Kerberos in order to read and write to HDFS. To enable Kerberos authentication from Solr, you need to set the following parameters:

solr.hdfs.security.kerberos.enabled

Optional

Default: false

Set to true to enable Kerberos authentication.

solr.hdfs.security.kerberos.keytabfile

Required

Default: none

A keytab file contains pairs of Kerberos principals and encrypted keys which allows for password-less authentication when Solr attempts to authenticate with secure Hadoop.

This file will need to be present on all Solr servers at the same path provided in this parameter.

solr.hdfs.security.kerberos.principal

Required

Default: none

The Kerberos principal that Solr should use to authenticate to secure Hadoop; the format of a typical Kerberos V5 principal is: primary/instance@realm.

Update Log settings

When using HDFS to store Solr indexes, it is recommended to also store the transaction logs on HDFS. This can be done by using the solr.HdfsUpdateLog update log hander class. The solrconfig.xml is often used to define an update log handler class name either using a variable reference or direct specification, for example:

<updateLog class="${solr.ulog:solr.UpdateLog}">

When specifying a class like this, it needs to be ensured that the correct class name is specified. When no class name is specified, Solr automatically picks the correct update log handler class solr.HdfsUpdateLog for collections which are configured to use the HdfsDirectory Factory.

Example solrconfig.xml for HDFS

Here is a sample solrconfig.xml configuration for storing Solr indexes on HDFS:

<directoryFactory name="DirectoryFactory" class="solr.HdfsDirectoryFactory">
  <str name="solr.hdfs.home">hdfs://host:port/solr</str>
  <bool name="solr.hdfs.blockcache.enabled">true</bool>
  <int name="solr.hdfs.blockcache.slab.count">1</int>
  <bool name="solr.hdfs.blockcache.direct.memory.allocation">true</bool>
  <int name="solr.hdfs.blockcache.blocksperbank">16384</int>
  <bool name="solr.hdfs.blockcache.read.enabled">true</bool>
  <bool name="solr.hdfs.nrtcachingdirectory.enable">true</bool>
  <int name="solr.hdfs.nrtcachingdirectory.maxmergesizemb">16</int>
  <int name="solr.hdfs.nrtcachingdirectory.maxcachedmb">192</int>
</directoryFactory>

If using Kerberos, you will need to add the three Kerberos related properties to the <directoryFactory> element in solrconfig.xml, such as:

<directoryFactory name="DirectoryFactory" class="solr.HdfsDirectoryFactory">
   ...
  <bool name="solr.hdfs.security.kerberos.enabled">true</bool>
  <str name="solr.hdfs.security.kerberos.keytabfile">/etc/krb5.keytab</str>
  <str name="solr.hdfs.security.kerberos.principal">solr/admin@KERBEROS.COM</str>
</directoryFactory>