Zen Discoveryedit

Zen discovery is the built-in, default, discovery module for Elasticsearch. It provides unicast and file-based discovery, and can be extended to support cloud environments and other forms of discovery via plugins.

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.

Seed nodesedit

Zen discovery uses a list of seed nodes in order to start off the discovery process. At startup, or when electing a new master, Elasticsearch tries to connect to each seed node in its list, and holds a gossip-like conversation with them to find other nodes and to build a complete picture of the cluster. By default there are two methods for configuring the list of seed nodes: unicast and file-based. It is recommended that the list of seed nodes comprises the list of master-eligible nodes in the cluster.

Unicastedit

Unicast discovery configures a static list of hosts for use as seed nodes. 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.

The list of hosts is set using the discovery.zen.ping.unicast.hosts static setting. This is either an array of hosts or a comma-delimited string. Each value should be in the form of host:port or host (where port defaults to the setting transport.profiles.default.port falling back to transport.tcp.port if not set). Note that IPv6 hosts must be bracketed. The default for this setting is 127.0.0.1, [::1]

Additionally, the discovery.zen.ping.unicast.resolve_timeout configures the amount of time to wait for DNS lookups on each round of pinging. This is specified as a time unit and defaults to 5s.

Unicast discovery uses the transport module to perform the discovery.

File-basededit

In addition to hosts provided by the static discovery.zen.ping.unicast.hosts setting, it is possible to provide a list of hosts via an external file. Elasticsearch reloads this file when it changes, so that the list of seed nodes can change dynamically without needing to restart each node. For example, this gives a convenient mechanism for an Elasticsearch instance that is run in a Docker container to be dynamically supplied with a list of IP addresses to connect to for Zen discovery when those IP addresses may not be known at node startup.

To enable file-based discovery, configure the file hosts provider as follows:

discovery.zen.hosts_provider: file

Then create a file at $ES_PATH_CONF/unicast_hosts.txt in the format described below. Any time a change is made to the unicast_hosts.txt file the new changes will be picked up by Elasticsearch and the new hosts list will be used.

Note that the file-based discovery plugin augments the unicast hosts list in elasticsearch.yml: if there are valid unicast host entries in discovery.zen.ping.unicast.hosts then they will be used in addition to those supplied in unicast_hosts.txt.

The discovery.zen.ping.unicast.resolve_timeout setting also applies to DNS lookups for nodes specified by address via file-based discovery. This is specified as a time unit and defaults to 5s.

The format of the file is to specify one node entry per line. Each node entry consists of the host (host name or IP address) and an optional transport port number. If the port number is specified, is must come immediately after the host (on the same line) separated by a :. If the port number is not specified, a default value of 9300 is used.

For example, this is an example of unicast_hosts.txt for a cluster with four nodes that participate in unicast discovery, some of which are not running on the default port:

10.10.10.5
10.10.10.6:9305
10.10.10.5:10005
# an IPv6 address
[2001:0db8:85a3:0000:0000:8a2e:0370:7334]:9301

Host names are allowed instead of IP addresses (similar to discovery.zen.ping.unicast.hosts), and IPv6 addresses must be specified in brackets with the port coming after the brackets.

It is also possible to add comments to this file. All comments must appear on their lines starting with # (i.e. comments cannot start in the middle of a line).

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) 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 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.

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.

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, 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

No master blockedit

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:

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). Requests to these apis will not be blocked and can run on any available node.