202 lines
8.1 KiB
Plaintext
202 lines
8.1 KiB
Plaintext
[[modules-discovery-zen]]
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=== Zen Discovery
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The zen discovery is the built in discovery module for elasticsearch and
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the default. It provides both multicast and unicast discovery as well
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being easily extended to support cloud environments.
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The zen discovery is integrated with other modules, for example, all
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communication between nodes is done using the
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<<modules-transport,transport>> module.
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It is separated into several sub modules, which are explained below:
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[float]
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[[ping]]
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==== Ping
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This is the process where a node uses the discovery mechanisms to find
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other nodes. There is support for both multicast and unicast based
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discovery (these mechanisms can be used in conjunction as well).
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[float]
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[[multicast]]
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===== Multicast
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Multicast ping discovery of other nodes is done by sending one or more
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multicast requests which existing nodes will receive and
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respond to. It provides the following settings with the
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`discovery.zen.ping.multicast` prefix:
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[cols="<,<",options="header",]
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|=======================================================================
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|Setting |Description
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|`group` |The group address to use. Defaults to `224.2.2.4`.
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|`port` |The port to use. Defaults to `54328`.
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|`ttl` |The ttl of the multicast message. Defaults to `3`.
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|`address` |The address to bind to, defaults to `null` which means it
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will bind `network.bind_host`
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|`enabled` |Whether multicast ping discovery is enabled. Defaults to `true`.
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|=======================================================================
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[float]
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[[unicast]]
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===== Unicast
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The unicast discovery allows for discovery when multicast is
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not enabled. It basically requires a list of hosts to use that will act
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as gossip routers. It provides the following settings with the
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`discovery.zen.ping.unicast` prefix:
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[cols="<,<",options="header",]
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|=======================================================================
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|Setting |Description
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|`hosts` |Either an array setting or a comma delimited setting. Each
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value is either in the form of `host:port`, or in the form of
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`host[port1-port2]`.
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|=======================================================================
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The unicast discovery uses the
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<<modules-transport,transport>> module to
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perform the discovery.
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[float]
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[[master-election]]
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==== Master Election
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As part of the ping process a master of the cluster is either
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elected or joined to. This is done automatically. The
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`discovery.zen.ping_timeout` (which defaults to `3s`) allows for the
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tweaking of election time to handle cases of slow or congested networks
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(higher values assure less chance of failure). Once a node joins, it
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will send a join request to the master (`discovery.zen.join_timeout`)
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with a timeout defaulting at 20 times the ping timeout.
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When the master node stops or has encountered a problem, the cluster nodes
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start pinging again and will elect a new master. This pinging round also
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serves as a protection against (partial) network failures where node may unjustly
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think that the master has failed. In this case the node will simply hear from
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other nodes about the currently active master.
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If `discovery.zen.master_election.filter_client` is `true`, pings from client nodes (nodes where `node.client` is
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`true`, or both `node.data` and `node.master` are `false`) are ignored during master election; the default value is
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`true`. If `discovery.zen.master_election.filter_data` is `true`, pings from non-master-eligible data nodes (nodes
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where `node.data` is `true` and `node.master` is `false`) are ignored during master election; the default value is
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`false`. Pings from master-eligible nodes are always observed during master election.
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Nodes can be excluded from becoming a master by setting `node.master` to
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`false`. Note, once a node is a client node (`node.client` set to
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`true`), it will not be allowed to become a master (`node.master` is
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automatically set to `false`).
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The `discovery.zen.minimum_master_nodes` sets the minimum
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number of master eligible nodes that need to join a newly elected master in order for an election to
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complete and for the elected node to accept it's mastership. The same setting controls the minimum number of
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active master eligible nodes that should be a part of any active cluster. If this requirement is not met the
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active master node will step down and a new mastser election will be begin.
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This setting must be set to a quorum of your master eligible nodes. It is recommended to avoid
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having only two master eligible nodes, since a quorum of two is two. Therefore, a loss
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of either master node will result in an inoperable cluster.
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[float]
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[[fault-detection]]
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==== Fault Detection
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There are two fault detection processes running. The first is by the
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master, to ping all the other nodes in the cluster and verify that they
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are alive. And on the other end, each node pings to master to verify if
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its still alive or an election process needs to be initiated.
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The following settings control the fault detection process using the
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`discovery.zen.fd` prefix:
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[cols="<,<",options="header",]
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|=======================================================================
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|Setting |Description
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|`ping_interval` |How often a node gets pinged. Defaults to `1s`.
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|`ping_timeout` |How long to wait for a ping response, defaults to
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`30s`.
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|`ping_retries` |How many ping failures / timeouts cause a node to be
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considered failed. Defaults to `3`.
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|=======================================================================
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[float]
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==== External Multicast
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The multicast discovery also supports external multicast requests to
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discover nodes. The external client can send a request to the multicast
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IP/group and port, in the form of:
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[source,js]
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--------------------------------------------------
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{
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"request" : {
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"cluster_name": "test_cluster"
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}
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}
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--------------------------------------------------
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And the response will be similar to node info response (with node level
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information only, including transport/http addresses, and node
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attributes):
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[source,js]
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--------------------------------------------------
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{
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"response" : {
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"cluster_name" : "test_cluster",
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"transport_address" : "...",
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"http_address" : "...",
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"attributes" : {
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"..."
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}
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}
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}
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--------------------------------------------------
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Note, it can still be enabled, with disabled internal multicast
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discovery, but still have external discovery working by keeping
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`discovery.zen.ping.multicast.enabled` set to `true` (the default), but,
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setting `discovery.zen.ping.multicast.ping.enabled` to `false`.
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[float]
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==== Cluster state updates
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The master node is the only node in a cluster that can make changes to the
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cluster state. The master node processes one cluster state update at a time,
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applies the required changes and publishes the updated cluster state to all
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the other nodes in the cluster. Each node receives the publish message,
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updates its own cluster state and replies to the master node, which waits for
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all nodes to respond, up to a timeout, before going ahead processing the next
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updates in the queue. The `discovery.zen.publish_timeout` is set by default
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to 30 seconds and can be changed dynamically through the
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<<cluster-update-settings,cluster update settings api>>
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[float]
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[[no-master-block]]
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==== No master block
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For the cluster to be fully operational, it must have an active master and the
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number of running master eligible nodes must satisfy the
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`discovery.zen.minimum_master_nodes` setting if set. The
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`discovery.zen.no_master_block` settings controls what operations should be
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rejected when there is no active master.
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The `discovery.zen.no_master_block` setting has two valid options:
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[horizontal]
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`all`:: All operations on the node--i.e. both read & writes--will be rejected. This also applies for api cluster state
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read or write operations, like the get index settings, put mapping and cluster state api.
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`write`:: (default) Write operations will be rejected. Read operations will succeed, based on the last known cluster configuration.
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This may result in partial reads of stale data as this node may be isolated from the rest of the cluster.
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The `discovery.zen.no_master_block` setting doesn't apply to nodes based apis (for example cluster stats, node info and
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node stats apis) which will not be blocked and try to execute on any node possible.
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