Running Solr on HDFS | Apache Solr Reference Guide 7.0

Solr has support for writing and reading its index and transaction log files to the HDFS distributed filesystem.

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 Hadoop MapReduce to process Solr data, see the MapReduceIndexerTool in the Solr contrib area.

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

Standalone Solr Instances

For standalone Solr instances, there are a few parameters you should be sure to 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 dir of the form hdfs://host:port/path
You need to specify an UpdateLog location of 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 in standalone mode, 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 in SolrCloud mode, 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 is meant to replace the standard file system cache that Solr utilizes so much. 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 follow 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 that are defined as part of the directoryFactory configuration.

Solr HDFS Settings

solr.hdfs.home: 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: Enable the blockcache. The default is true.
solr.hdfs.blockcache.read.enabled: Enable the read cache. The default is true.
solr.hdfs.blockcache.direct.memory.allocation: Enable direct memory allocation. If this is false, heap is used. The default is true.
solr.hdfs.blockcache.slab.count: Number of memory slabs to allocate. Each slab is 128 MB in size. The default is 1.
solr.hdfs.blockcache.global: Enable/Disable using one global cache for all SolrCores. The settings used will be from the first HdfsDirectoryFactory created. The default is true.

NRTCachingDirectory Settings

solr.hdfs.nrtcachingdirectory.enable: true | Enable the use of NRTCachingDirectory. The default is true.
solr.hdfs.nrtcachingdirectory.maxmergesizemb: NRTCachingDirectory max segment size for merges. The default is 16.
solr.hdfs.nrtcachingdirectory.maxcachedmb: NRTCachingDirectory max cache size. The default is 192.

HDFS Client Configuration Settings

solr.hdfs.confdir: 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: Set to true to enable Kerberos authentication. The default is false.
solr.hdfs.security.kerberos.keytabfile: 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: The Kerberos principal that Solr should use to authenticate to secure Hadoop; the format of a typical Kerberos V5 principal is: primary/instance@realm.

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>

Automatically Add Replicas in SolrCloud

One benefit to running Solr in HDFS is the ability to automatically add new replicas when the Overseer notices that a shard has gone down. Because the "gone" index shards are stored in HDFS, the a new core will be created and the new core will point to the existing indexes in HDFS.

Collections created using autoAddReplicas=true on a shared file system have automatic addition of replicas enabled. The following settings can be used to override the defaults in the <solrcloud> section of solr.xml.

autoReplicaFailoverWorkLoopDelay: The time (in ms) between clusterstate inspections by the Overseer to detect and possibly act on creation of a replacement replica. The default is 10000.
autoReplicaFailoverWaitAfterExpiration: The minimum time (in ms) to wait for initiating replacement of a replica after first noticing it not being live. This is important to prevent false positives while stoping or starting the cluster. The default is 30000.
autoReplicaFailoverBadNodeExpiration: The delay (in ms) after which a replica marked as down would be unmarked. The default is 60000.

Temporarily Disable autoAddReplicas for the Entire Cluster

When doing offline maintenance on the cluster and for various other use cases where an admin would like to temporarily disable auto addition of replicas, the following APIs will disable and re-enable autoAddReplicas for all collections in the cluster:

Disable auto addition of replicas cluster wide by setting the cluster property autoAddReplicas to false:

http://localhost:8983/solr/admin/collections?action=CLUSTERPROP&name=autoAddReplicas&val=false

Re-enable auto addition of replicas (for those collections created with autoAddReplica=true) by unsetting the autoAddReplicas cluster property (when no val param is provided, the cluster property is unset):

http://localhost:8983/solr/admin/collections?action=CLUSTERPROP&name=autoAddReplicas

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