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Docs: Document the most important changes to zen discovery. 

Closes #7746
This commit is contained in:
Martijn van Groningen 2014-09-15 10:00:28 +02:00
parent c86fdecd25
commit fca406415d
2 changed files with 38 additions and 4 deletions
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8
docs/reference/migration/migrate_1_x.asciidoc
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@ -71,4 +71,12 @@ curl -XGET 'http://localhost:9200/_all/_mappings'
curl -XGET 'http://localhost:9200/_mappings' curl -XGET 'http://localhost:9200/_mappings'
-------------------------------------------------- --------------------------------------------------
==== Zen discovery
Each cluster must have an elected master node in order to be fully operational. Once a node loses its elected master
node it will reject some or all operations.
On versions before `1.4.0.Beta1` all operations are rejected when a node loses its elected master. From `1.4.0.Beta1`
only write operations will be rejected by default. Read operations will still be served based on the information available
to the node, which may result in being partial and possibly also stale. If the default is undesired then the
pre `1.4.0.Beta1` behaviour can be enabled, see: <<modules-discovery-zen,no-master-block>>

34
docs/reference/modules/discovery/zen.asciidoc
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@ -68,22 +68,30 @@ perform the discovery.
[[master-election]] [[master-election]]
==== Master Election ==== Master Election
As part of the initial ping process a master of the cluster is either As part of the ping process a master of the cluster is either
elected or joined to. This is done automatically. The elected or joined to. This is done automatically. The
`discovery.zen.ping_timeout` (which defaults to `3s`) allows for the `discovery.zen.ping_timeout` (which defaults to `3s`) allows for the
tweaking of election time to handle cases of slow or congested networks tweaking of election time to handle cases of slow or congested networks
(higher values assure less chance of failure). Once a node joins, it (higher values assure less chance of failure). Once a node joins, it
will send a join request to the master (`discovery.zen.join_timeout`) will send a join request to the master (`discovery.zen.join_timeout`)
with a timeout defaulting at 20 times the ping 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 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.
Nodes can be excluded from becoming a master by setting `node.master` to 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 `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 `true`), it will not be allowed to become a master (`node.master` is
automatically set to `false`). automatically set to `false`).
The `discovery.zen.minimum_master_nodes` sets the minimum The `discovery.zen.minimum_master_nodes` sets the minimum
number of master eligible nodes a node should "see" in order to operate number of master eligible nodes a node should "see" in order to win a master election.
within the cluster. Its recommended to set it to a higher value than 1 It must be set to a quorum of your master eligible nodes. It is recommended to avoid
when running more than 2 nodes in the cluster. 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
[float] [float]
[[fault-detection]] [[fault-detection]]
@ -160,3 +168,21 @@ 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 updates in the queue. The `discovery.zen.publish_timeout` is set by default
to 30 seconds and can be changed dynamically through the to 30 seconds and can be changed dynamically through the
<<cluster-update-settings,cluster update settings api>> <<cluster-update-settings,cluster update settings api>>
[float]
[[no-master-block]]
==== No master block
coming[1.4.0.Beta1]
For a node to be fully operational, it must have an active master. 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` - Write operations will be rejected. Read operations will succeed, based on the last known cluster configuration.
This may result in partial reads or stale data as this node may be isolated from the rest of the cluster. This is the default.
The `discovery.zen.no_master_block` setting doesn't apply for nodes based apis (for example cluster stats, node info and
node stats apis) which will not be blocked and try to execute on any node possible.