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Proposal

Every node in Solr has to have one or more “roles”.

What is a role?

A role is a designation of a node that indicates that the node may perform a certain functionality that is governed by the role. A node that doesn't have a role may not perform the functionality associated with the role.

For example:
- Nodes with "data" role MAY host replicas (i.e. nodes without MAY NOT)
- Nodes with (UPCOMING ROLE) "zk" role MAY run zk (i.e. nodes without the role MAY NOT)
- Nodes with (IMAGINARY EXAMPLE) "worker" role MAY execute streaming map/reduce work
- Nodes with (IMAGINARY EXAMPLE) "ingest" role MAY run Tika parsing, OCR, data prepping etc

The following roles are proposed (based on existing functionality):

1. “data” role: A node with this role can host data hosting replicas. By default, this is the case for all nodes.
   
2. “overseer” role: A node with this role indicates that this node is a preferred overseer. When one or more such nodes are live, Solr guarantees that one of those nodes become the overseer.

Roles that might be introduced in future (specifics are outside the scope of this SIP, except for examples):

1. “coordinator” role [UPCOMING FEATURE]: This role can be associated with a node to where requests can be sent, and this node sends out other remote calls to data hosting nodes, aggregates the results and sends back to user. This will be useful for dealing with distributed query requests, bulk indexing & streaming expressions based queries. See

   Jira
   server ASF JIRA serverId 5aa69414-a9e9-3523-82ec-879b028fb15b key SOLR-15715

   . This is very similar in concept to ElasticSearch's coordinating nodes. A coordinator node would be assumed to have no data hosted on it.
   
2. “zookeeper” “zk” role [UPCOMING FEATURE]: This role can be associated with nodes that can have embedded ZK nodes. See: https://cwiki.apache.org/confluence/display/SOLR/SIP-14+Embedded+Zookeeper

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