Zen Discovery

The zen discovery is the built in discovery module for elasticsearch and the default. It provides unicast discovery, but can be extended to support cloud environments and other forms of discovery.

The zen discovery is integrated with other modules, for example, all communication between nodes is done using the transport module.

It is separated into several sub modules, which are explained below:

This is the process where a node uses the discovery mechanisms to find other nodes.

The unicast discovery requires a list of hosts to use that will act as gossip routers. It provides the following settings with the discovery.zen.ping.unicast prefix:

The unicast discovery uses the transport module to perform the discovery.

As part of the ping process a master of the cluster is either elected or joined to. This is done automatically. The discovery.zen.ping_timeout (which defaults to 3s) allows for the tweaking of election time to handle cases of slow or congested networks (higher values assure less chance of failure). Once a node joins, it will send a join request to the master (discovery.zen.join_timeout) with a timeout defaulting at 20 times the ping timeout.

When the master node stops or has encountered a problem, the cluster nodes start pinging again and will elect a new master. This pinging round also serves as a protection against (partial) network failures where node may unjustly think that the master has failed. In this case the node will simply hear from other nodes about the currently active master.

If discovery.zen.master_election.filter_client is true, pings from client nodes (nodes where node.client is true, or both node.data and node.master are false) are ignored during master election; the default value is true. If discovery.zen.master_election.filter_data is true, pings from non-master-eligible data nodes (nodes where node.data is true and node.master is false) are ignored during master election; the default value is false. Pings from master-eligible nodes are always observed during master election.

Nodes can be excluded from becoming a master by setting node.master to false. Note, once a node is a client node (node.client set to true), it will not be allowed to become a master (node.master is automatically set to false).

The discovery.zen.minimum_master_nodes sets the minimum number of master eligible nodes that need to join a newly elected master in order for an election to complete and for the elected node to accept it’s mastership. The same setting controls the minimum number of active master eligible nodes that should be a part of any active cluster. If this requirement is not met the active master node will step down and a new mastser election will be begin.

This setting must be set to a quorum of your master eligible nodes. It is recommended to avoid having only two master eligible nodes, since a quorum of two is two. Therefore, a loss of either master node will result in an inoperable cluster.

There are two fault detection processes running. The first is by the master, to ping all the other nodes in the cluster and verify that they are alive. And on the other end, each node pings to master to verify if its still alive or an election process needs to be initiated.

The following settings control the fault detection process using the discovery.zen.fd prefix:

The master node is the only node in a cluster that can make changes to the cluster state. The master node processes one cluster state update at a time, applies the required changes and publishes the updated cluster state to all the other nodes in the cluster. Each node receives the publish message, updates its own cluster state and replies to the master node, which waits for all nodes to respond, up to a timeout, before going ahead processing the next updates in the queue. The discovery.zen.publish_timeout is set by default to 30 seconds and can be changed dynamically through the cluster update settings api

For the cluster to be fully operational, it must have an active master and the number of running master eligible nodes must satisfy the discovery.zen.minimum_master_nodes setting if set. The discovery.zen.no_master_block settings controls what operations should be rejected when there is no active master.

The discovery.zen.no_master_block setting has two valid options:

The discovery.zen.no_master_block setting doesn’t apply to nodes based apis (for example cluster stats, node info and node stats apis) which will not be blocked and try to execute on any node possible.