Zen Discovery

The zen discovery is the built in discovery module for elasticsearch and the default. It provides both multicast and unicast discovery as well being easily extended to support cloud environments.

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. There is support for both multicast and unicast based discovery (these mechanisms can be used in conjunction as well).

Multicast ping discovery of other nodes is done by sending one or more multicast requests which existing nodes will receive and respond to. It provides the following settings with the discovery.zen.ping.multicast prefix:

The unicast discovery allows for discovery when multicast is not enabled. It basically 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 initial 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. [1.3.0] Added in 1.3.0. Previously defaulted to 10 times the ping timeout .

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 a node should "see" in order to operate within the cluster. Its recommended to set it to (N/2)+1, rounding down to the nearest integer, when running more than 2 nodes in the cluster (where N is the number of master eligible nodes in the 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 multicast discovery also supports external multicast requests to discover nodes. The external client can send a request to the multicast IP/group and port, in the form of:

{
    "request" : {
        "cluster_name": "test_cluster"
    }
}

And the response will be similar to node info response (with node level information only, including transport/http addresses, and node attributes):

{
    "response" : {
        "cluster_name" : "test_cluster",
        "transport_address" : "...",
        "http_address" : "...",
        "attributes" : {
            "..."
        }
    }
}

Note, it can still be enabled, with disabled internal multicast discovery, but still have external discovery working by keeping discovery.zen.ping.multicast.enabled set to true (the default), but, setting discovery.zen.ping.multicast.ping.enabled to false.

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 [1.1.0] Added in 1.1.0. The setting existed before but wasn’t dynamic .