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Docs: Document the most important changes to zen discovery. #7746

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Docs: Document the most important changes to zen discovery. #7746

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Docs: Document the most important changes to zen discovery.
martijnvg Sep 15, 2014
3f9a48e
Applied Boaz's feedback.
martijnvg Sep 25, 2014
e8836cf
applied zack's feedback
martijnvg Sep 26, 2014
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7 changes: 7 additions & 0 deletions docs/reference/migration/migrate_1_x.asciidoc
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Expand Up @@ -68,4 +68,11 @@ curl -XGET 'http://localhost:9200/_all/_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 operations.
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maybe add some or all? i.e., "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 can end up being partial and possible also be stale.
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I think the read operations part should mention what happens. Something like "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>>
27 changes: 23 additions & 4 deletions docs/reference/modules/discovery/zen.asciidoc
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Expand Up @@ -68,7 +68,7 @@ perform the discovery.
[[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
`discovery.zen.ping_timeout` (which defaults to `3s`) allows to
configure the election to handle cases of slow or congested networks
Expand All @@ -77,15 +77,20 @@ will send a join request to the master (`discovery.zen.join_timeout`)
with a timeout defaulting at 20 times the ping timeout.
added[1.3.0,Previously defaulted to 10 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 protection against (partial) network failures where node may unjustly
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as a protection.

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
`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` allows to control the minimum
number of master eligible nodes a node should "see" in order to operate
within the cluster. Its recommended to set it to a higher value than 1
when running more than 2 nodes in the cluster.
number of master eligible nodes a node should "see" in order to win master election.
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win a master election..

Its recommended to set it to a higher value than 1 when running more than 2 nodes in the cluster.
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we recommend setting it to a quorum of your master eligible nodes.


[float]
[[fault-detection]]
Expand Down Expand Up @@ -163,3 +168,17 @@ 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,cluster update settings api>> added[1.1.0, The
setting existed before but wasn't dynamic].

[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.]
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extra ']'?


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.
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Does this include read-only cluster-metadata APIs like Node Stats, Cluster State, etc? Should probably add a note regardless of what the answer is, just to be explicit.

* `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.