Fault tolerance and high availability in SolrCloud

Whenever SolrCloud is restarted, election happens again. If a particular shard that was a replica earlier comes up before the shard that was the leader, the replica shard becomes the leader and the leader shard becomes the replica. Whenever we restart the Tomcat and ZooKeeper servers for starting SolrCloud, we can expect the leaders and replicas to switch.

Let us check the availability of the cluster. We will bring down a leader node and a replica node to check whether the cluster is able to serve all the documents that we have indexed. First check the number of documents in mycollection:

http://solr5:8080/solr/mycollection/select/?q=*:*

We can see that there are nine documents in the collection. Similarly, run the following query on catcollection:

http://solr5:8080/solr/catcollection/select/?q=*:*

The catcollection contains 11 documents.

Now let us bring down Tomcat on solr2 and solr3, or simply turn off the machines. Check the SolrCloud graph...