[Solr Wiki] Update of "SolrCloud2" by Mark Miller

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The "SolrCloud2" page has been changed by Mark Miller:
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== Getting Started ==
Check out and build the branch: https://svn.apache.org/repos/asf/lucene/dev/branches/solrcloud and build the example server with {{{cd solr; ant example}}}.

If you haven't yet, go through the simple [[http://lucene.apache.org/solr/tutorial.html|Solr Tutorial]] to familiarize yourself with Solr.

Solr embeds and uses Zookeeper as a repository for cluster configuration and coordination - think of it as a distributed filesystem that contains information about all of the Solr servers.

=== Example A: Simple two shard cluster ===
This example simply creates a cluster consisting of two solr servers representing two different shards of a collection.

Since we'll need two solr servers for this example, simply make a copy of the example directory for the second server.

{{{
cp -r example example2
}}}
This command starts up a Solr server and bootstraps a new solr cluster.

{{{
cd example
java -Dbootstrap_confdir=./solr/conf -Dcollection.configName=myconf -DzkRun -DnumShards=2 -jar start.jar
}}}
 * {{{-DzkRun}}} causes an embedded zookeeper server to be run as part of this Solr server.
 * {{{-Dbootstrap_confdir=./solr/conf}}} Since we don't yet have a config in zookeeper, this parameter causes the local configuration directory {{{./solr/conf}}} to be uploaded as the "myconf" config.  The name "myconf" is taken from the "collection.configName" param below.
 * {{{-Dcollection.configName=myconf}}} sets the config to use for the new collection. Omitting this param will cause the config name to default to "configuration1".
 * {{{-DnumShards=2}}} the number of logical partitions we plan on splitting the index into.

Browse to http://localhost:8983/solr/collection1/admin/zookeeper.jsp to see the state of the cluster (the zookeeper distributed filesystem).

You can see from the zookeeper browser that the Solr configuration files were uploaded under "myconf", and that a new document collection called "collection1" was created.  Under collection1 is a list of shards, the pieces that make up the complete collection.

Now we want to start up our second server - it will automatically be assigned to shard2 because we don't explicitly set the shard id.

Then start the second server, pointing it at the cluster:

{{{
java -Djetty.port=7574 -DzkHost=localhost:9983 -DnumShards=2 -jar start.jar
}}}
 * {{{-Djetty.port=7574}}}  is just one way to tell the Jetty servlet container to use a different port.
 * {{{-DzkHost=localhost:9983}}} points to the Zookeeper ensemble containing the cluster state.  In this example we're running a single Zookeeper server embedded in the first Solr server.  By default, an embedded Zookeeper server runs at the Solr port plus 1000, so 9983.
 * {{{-DnumShards=2}}} again, we need to specify the number of shards we want when a SolrCore is first started and registers in ZooKeeper.

If you refresh the zookeeper browser, you should now see both shard1 and shard2 in collection1.

Next, index some documents. If you want to whip up some Java you can use the CloudSolrServer solrj impl and simply init it with the address to ZooKeeper. Or simply randomly choose which instance to add documents too - they will be automatically forwarded to where they belong:

{{{
cd exampledocs
java -Durl=http://localhost:8983/solr/collection1/update -jar post.jar ipod_video.xml
java -Durl=http://localhost:8983/solr/collection1/update -jar post.jar monitor.xml
java -Durl=http://localhost:8983/solr/collection1/update -jar post.jar mem.xml
}}}
And now, a request to either server with "distrib=true" results in a distributed search that covers the entire collection:

http://localhost:8983/solr/collection1/select?distrib=true&q=*:*

If at any point you wish to start over fresh or experiment with different configurations, you can delete all of the cloud state contained within zookeeper by simply deleting the solr/zoo_data directory after shutting down the servers.

=== Example B: Simple two shard cluster with shard replicas ===
This example will simply build off of the previous example by creating another copy of shard1 and shard2.  Extra shard copies can be used for high availability and fault tolerance, or simply for increasing the query capacity of the cluster.


First, run through the previous example so we already have two shards and some documents indexed into each.  Then simply make a copy of those two servers:

{{{
cp -r example exampleB
cp -r example2 example2B
}}}
Then start the two new servers on different ports, each in its own window:

{{{
cd exampleB
java -Djetty.port=8900 -DzkHost=localhost:9983 -jar start.jar
}}}
{{{
cd example2B
java -Djetty.port=7500 -DzkHost=localhost:9983 -jar start.jar
}}}
Refresh the zookeeper browser page http://localhost:8983/solr/admin/zookeeper.jsp and verify that 4 solr nodes are up, and that each shard is present at 2 nodes.

Now send a query to any of the servers to query the cluster:

http://localhost:7500/solr/collection1/select?distrib=true&q=*:*

Send this query multiple times and observe the logs from the solr servers.  From your web browser, you may need to hold down CTRL while clicking on the browser refresh button to bypass the HTTP caching in your browser.  You should be able to observe Solr load balancing the requests (done via LBHttpSolrServer ?) across shard replicas, using different servers to satisfy each request.  There will be a log statement for the top-level request in the server the browser sends the request to, and then a log statement for each sub-request that are merged to produce the complete response.

To demonstrate fail over for high availability, go ahead and kill any one of the Solr servers (just press CTRL-C in the window running the server) and and send another query request to any of the remaining servers that are up.

=== Example C: Two shard cluster with shard replicas and zookeeper ensemble ===
The problem with example B is that while there are enough Solr servers to survive any one of them crashing, there is only one zookeeper server that contains the state of the cluster.  If that zookeeper server crashes, distributed queries will still work since the solr servers remember the state of the cluster last reported by zookeeper.  The problem is that no new servers or clients will be able to discover the cluster state, and no changes to the cluster state will be possible.

Running multiple zookeeper servers in concert (a zookeeper ensemble) allows for high availability of the zookeeper service.  Every zookeeper server needs to know about every other zookeeper server in the ensemble, and a majority of servers are needed to provide service.  For example, a zookeeper ensemble of 3 servers allows any one to fail with the remaining 2 constituting a majority to continue providing service.  5 zookeeper servers are needed to allow for the failure of up to 2 servers at a time.

For production, it's recommended that you run an external zookeeper ensemble rather than having Solr run embedded zookeeper servers.  For this example, we'll use the embedded servers for simplicity.

First, stop all 4 servers and then clean up the zookeeper data directories for a fresh start.

{{{
rm -r example*/solr/zoo_data
}}}
We will be running the servers again at ports 8983,7574,8900,7500.  The default is to run an embedded zookeeper server at hostPort+1000, so if we run an embedded zookeeper on the first three servers, the ensemble address will be {{{localhost:9983,localhost:8574,localhost:9900}}}.

As a convenience, we'll have the first server upload the solr config to the cluster.  You will notice it block until you have actually started the second server.  This is due to zookeeper needing a quorum before it can operate.

{{{
cd example
java -Dbootstrap_confdir=./solr/conf -Dcollection.configName=myconf -DzkRun -DzkHost=localhost:9983,localhost:8574,localhost:9900  -jar start.jar
}}}
{{{
cd example2
java -Djetty.port=7574 -DzkRun -DzkHost=localhost:9983,localhost:8574,localhost:9900 -jar start.jar
}}}
{{{
cd exampleB
java -Djetty.port=8900 -DzkRun -DzkHost=localhost:9983,localhost:8574,localhost:9900 -jar start.jar
}}}
{{{
cd example2B
java -Djetty.port=7500 -DzkHost=localhost:9983,localhost:8574,localhost:9900 -jar start.jar
}}}
Now since we are running three embedded zookeeper servers as an ensemble, everything can keep working even if a server is lost. To demonstrate this, kill the exampleB server by pressing CTRL+C in it's window and then browse to http://localhost:8983/solr/admin/zookeeper.jsp to verify that the zookeeper service still works.

== ZooKeeper ==
Multiple Zookeeper servers running together for fault tolerance and high availability is called an ensemble.  For production, it's recommended that you run an external zookeeper ensemble rather than having Solr run embedded servers.  See the [[http://zookeeper.apache.org/|Apache ZooKeeper]] site for more information on downloading and running a zookeeper ensemble.

When Solr runs an embedded zookeeper server, it defaults to using the solr port plus 1000 for the zookeeper client port.  In addition, it defaults to adding one to the client port for the zookeeper server port, and two for the zookeeper leader election port.  So in the first example with Solr running at 8983, the embedded zookeeper server used port 9983 for the client port and 9984,9985 for the server ports.

== Creating cores via CoreAdmin ==
New Solr cores may also be created and associated with a collection via CoreAdmin.

Additional cloud related parameters for the CREATE action:

 * '''collection''' - the name of the collection this core belongs to.  Default is the name of the core.
 * '''shard''' - the shard id this core represents
 * '''collection.<param>=<value>''' - causes a property of <param>=<value> to be set if a new collection is being created.
  * Use  collection.configName=<configname> to point to the config for a new collection.

Example:

{{{
curl 'http://localhost:8983/solr/admin/cores?action=CREATE&name=mycore&collection=collection1&shard=shard2'
}}}
== Distributed Requests ==
Explicitly specify the addresses of shards you want to query:

{{{
shards=localhost:8983/solr,localhost:7574/solr
}}}
Explicitly specify the addresses of shards you want to query, giving alternatives (delimited by `|`) used for load balancing and fail-over:

{{{
shards=localhost:8983/solr|localhost:8900/solr,localhost:7574/solr|localhost:7500/solr
}}}
Query all shards of a collection (the collection is implicit in the URL):

{{{
http://localhost:8983/solr/collection1/select?distrib=true
}}}
Query specific shard ids. In this example, the user has partitioned the index by date, creating a new shard every month.

{{{
http://localhost:8983/solr/collection1/select?shards=shard_200812,shard_200912,shard_201001&distrib=true
}}}
NOT IMPLEMENTED: Query all shards of a compatible collection, explicitly specified:

{{{
http://localhost:8983/solr/collection1/select?collection=collection1_recent
}}}
NOT IMPLEMENTED: Query all shards of multiple compatible collections, explicitly specified:

{{{
http://localhost:8983/solr/collection1/select?collection=collection1_NY,collection1_NJ,collection1_CT
}}}