Zen Discoveryedit

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:

Pingedit

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).

Multicastedit

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:

Setting Description

group

The group address to use. Defaults to 224.2.2.4.

port

The port to use. Defaults to 54328.

ttl

The ttl of the multicast message. Defaults to 3.

address

The address to bind to, defaults to null which means it will bind to all available network interfaces.

enabled

Whether multicast ping discovery is enabled. Defaults to true.

Unicastedit

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:

Setting Description

hosts

Either an array setting or a comma delimited setting. Each value is either in the form of host:port, or in the form of host[port1-port2].

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

Master Electionedit

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.

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 win a master election. It 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

Fault Detectionedit

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:

Setting Description

ping_interval

How often a node gets pinged. Defaults to 1s.

ping_timeout

How long to wait for a ping response, defaults to 30s.

ping_retries

How many ping failures / timeouts cause a node to be considered failed. Defaults to 3.

External Multicastedit

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.

Cluster state updatesedit

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

No master blockedit

Note

Added in 1.4.0.Beta1.

For a node to be fully operational, it must have an active master. 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:

all

All operations on the node—i.e. both read & writes—will be rejected. This also applies for api cluster state read or write operations, like the get index settings, put mapping and cluster state api.

write

(default) Write operations will be rejected. Read operations will succeed, based on the last known cluster configuration. This may result in partial reads of stale data as this node may be isolated from the rest of the cluster.

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.