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.

Unicast discovery requires a list of hosts to use that will act as gossip routers. These hosts can be specified as hostnames or IP addresses; hosts specified as hostnames are resolved to IP addresses during each round of pinging. Note that if you are in an environment where DNS resolutions vary with time, you might need to adjust your JVM security settings.

It is recommended that the unicast hosts list be maintained as the list of master-eligible nodes in the cluster.

Unicast discovery 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) determines how long the node will wait before deciding on starting an election or joining an existing cluster. Three pings will be sent over this timeout interval. In case where no decision can be reached after the timeout, the pinging process restarts. In slow or congested networks, three seconds might not be enough for a node to become aware of the other nodes in its environment before making an election decision. Increasing the timeout should be done with care in that case, as it will slow down the election process. Once a node decides to join an existing formed cluster, 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 a 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.ignore_non_master_pings is true, pings from nodes that are not master eligible (nodes where node.master is false) are ignored during master election; the default value is false.

Nodes can be excluded from becoming a master by setting node.master 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 its 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 master 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 eligible 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, acknowledges it, but does not yet apply it. If the master does not receive acknowledgement from at least discovery.zen.minimum_master_nodes nodes within a certain time (controlled by the discovery.zen.commit_timeout setting and defaults to 30 seconds) the cluster state change is rejected.

Once enough nodes have responded, the cluster state is committed and a message will be sent to all the nodes. The nodes then proceed to apply the new cluster state to their internal state. The master node 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 is measured from the moment the publishing started. Both timeout settings 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). Requests to these apis will not be blocked and can run on any available node.