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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
Either an array setting or a comma delimited setting. Each
value should be in the form of
The amount of time to wait for DNS lookups on each round of pinging. Specified as time units. Defaults to 5s.
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
Once a node decides to join an existing formed cluster, it
will send a join request to the master (
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.
true, pings from nodes that are not master
eligible (nodes where
false) are ignored during master election; the default value is
Nodes can be excluded from becoming a master by setting
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.
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
How often a node gets pinged. Defaults to
How long to wait for a ping response, defaults to
How many ping failures / timeouts cause a node to be
considered failed. Defaults to
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
discovery.zen.minimum_master_nodes nodes within a certain time (controlled by
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
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.
discovery.zen.no_master_block setting has two valid options:
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.
(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.
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.
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