Status

Current state: Under Discussion ( Unable to render Jira issues macro, execution error. )

Motivation

So far, Solr only has a single type of node, one that is capable of assuming all kinds of tasks. There are usecases where one would like dedicated nodes for specific types of workloads. For example, a dedicated overseer node or a dedicated data node and query node or a node with no data hosted on it, one that can be used for administrative tasks or running plugins etc. Elasticsearch, Vespa etc. have first class support for node roles.

Proposal

Every node in Solr has one or more “roles”. The following roles are proposed:

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.
3. “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 Unable to render Jira issues macro, execution error. . This is very similar in concept to ElasticSearch's coordinating nodes. A coordinator node would be assumed to have no data hosted on it.
   
4. “zookeeper” 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

Notes:

1. If "-Dnode.roles" parameter is not passed, it is implicitly assumed to be "-Dnodes.role=data".
   

Public Interfaces

There will just one supported way to use the roles functionality:

Startup parameters

Examples:

1. Data node that can act as preferred overseer too:
       -Dnode.roles=overseer
       -Dnode.roles=overseer,data
2. Preferred overseer node with no data (dedicated overseer): -Dnode.roles=overseer
3. Preferred overseer with data: -Dnode.roles=overseer,data
4. Coordinator node: -Dnode.roles=coordinator

Cluster API

As of today, there is ADDROLE and REMOVEROLE APIs to add/remove roles at run time to nodes. It supports only OVERSEERROLE. We propose to deprecate this API, and recommend users to use startup params for achieving the same. Supporting both ways is tricky and will lead to a lot of confusion among users.

Example scenario

There's a Solr cluster with the following:

* Layer1: There are about 100 nodes, each node has many data replicas.
* Layer2: To manage such a large cluster reliably, they keep aside 4-5 dedicated overseer nodes.
* Layer3: Since query aggregations/coordination can potentially be expensive, they keep aside 5-10 query nodes.

Proposing the roles as:
* Layer1 nodes are the "data nodes" and hence get either no role defined for them or -Dnode.roles=data.
* Layer2 nodes are "overseer nodes" (though, only one of them can be an overseer at a time). They get -Dnode.roles=overseer
* Layer3 nodes are "coordinator nodes", no data must be hosted on these nodes and they are started with -Dnode.roles=coordinator

How to Retrieve Roles?

Public API

To Read the values use HTTP GET

GET /api/cluster/roles

{

     “node1”: [“overseer”],

     “node2”: [“overseer”, “data”],

     “node3”: [“data”]

}

Internal representation in ZK

This is where the roles are internally published for anyone who wishes to see this.

/roles.json

{

“overseer” : [“node1”, “node2],

“data” : [“node1”, “node2”]

}

Other notes

• Every time a node starts up with specified roles, the node assumes it is the correct role for that node and publishes those roles in ZK after successful startup.
• If a node is started with a -Dnode.roles parameter that doesn't have a data role, but it already has data hosting replicas on it, the startup fails with an error (and a hint indicating how to move replicas away from this replica).
• If a coordinator node is started with "data" role also, it fails to startup with a message indicating a node cannot both be coordinator and data node.

Compatibility, Deprecation, and Migration Plan

• Deprecate APIS ADDROLE, REMOVEROLE
• New V2 API for GET /api/cluster/roles to have nodes as key. 

Major Risks

None

Security considerations

None

Test Plan

Testing should mainly focus on how the nodes behave when roles are added to and removed from the nodes.

Rejected Alternatives

There is no proper alternative today. 

There are ways to achieve similar functionality:

• Use autoscaling to stop nodes from being placed on nodes. But, that framework itself has been re-written from Solr 8x to 9x, hence we don’t have a recommendation for users for a consistent way to achieve this.
• OVERSEER role is already available today.