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Zen Discovery
editZen Discovery
editThe 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:
Ping
editThis 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
editMulticast 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 |
---|---|
|
The group address to use. Defaults to |
|
The port to use. Defaults to |
|
The ttl of the multicast message. Defaults to |
|
The address to bind to, defaults to |
|
Whether multicast ping discovery is enabled. Defaults to |
Unicast
editThe 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 |
---|---|
|
Either an array setting or a comma delimited setting. Each
value is either in the form of |
The unicast discovery uses the transport module to perform the discovery.
Master Election
editAs 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).
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 |
External Multicast
editThe 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 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
[1.1.0]
Added in 1.1.0. The setting existed before but wasn’t dynamic
.