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.
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 <<HdfsDirectoryFactory Parameters,below>>), or passed to the `bin/solr` script at startup.
This example will start Solr in standalone mode, using the defined JVM properties (explained in more detail <<HdfsDirectoryFactory Parameters,below>>).
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`
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:
[source,bash]
----
# 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:
[source,bash]
----
-XX:MaxDirectMemorySize=20g
----
== HdfsDirectoryFactory Parameters
The `HdfsDirectoryFactory` has a number of settings that are defined as part of the `directoryFactory` configuration.
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`.
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:
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.
The Kerberos principal that Solr should use to authenticate to secure Hadoop; the format of a typical Kerberos V5 principal is: `primary/instance@realm`.
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`.
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`.
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*:
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):