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Zen Discovery
editZen Discovery
editThe 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:
Ping
editThis is the process where a node uses the discovery mechanisms to find other nodes.
Unicast
editThe unicast discovery 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 |
---|---|
|
Either an array setting or a comma delimited setting. Each
value should be in the form of |
The unicast discovery uses the transport module to perform the discovery.
Master Election
editAs 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 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.filter_client
is true
, pings from client nodes (nodes where node.client
is
true
, or both node.data
and node.master
are false
) are ignored during master election; the default value is
true
. If discovery.zen.master_election.filter_data
is true
, pings from non-master-eligible data nodes (nodes
where node.data
is true
and node.master
is false
) are ignored during master election; the default value is
false
. Pings from master-eligible nodes are always observed during master election.
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 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 node will result in an inoperable cluster.
Fault Detection
editThere 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 |
---|---|
|
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 updates
editThe 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 block
editFor 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 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. |
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.