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Weaken language about "low-latency" networks (#89198)

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Today we say that voting-only nodes require a "low-latency" network.
This term has a specific meaning in some operating environments which is
different from our intended meaning. To avoid this confusion this commit
removes the absolute term "low-latency" in favour of describing the
requirements relative to the user's own performance goals.
This commit is contained in:
David Turner 2022-08-09 13:15:37 +01:00 • committed by GitHub
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75
docs/reference/modules/discovery/publishing.asciidoc
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@ -1,38 +1,40 @@
[[cluster-state-publishing]]
=== Publishing the cluster state
The master node is the only node in a cluster that can make changes to the
cluster state. The master node processes one batch of cluster state updates at
a time, computing the required changes and publishing the updated cluster state
to all the other nodes in the cluster. Each publication starts with the master
broadcasting the updated cluster state to all nodes in the cluster. Each node
responds with an acknowledgement but does not yet apply the newly-received
state. Once the master has collected acknowledgements from enough
master-eligible nodes, the new cluster state is said to be _committed_ and the
master broadcasts another message instructing nodes to apply the now-committed
state. Each node receives this message, applies the updated state, and then
sends a second acknowledgement back to the master.
The elected master node is the only node in a cluster that can make changes to
the cluster state. The elected master node processes one batch of cluster state
updates at a time, computing the required changes and publishing the updated
cluster state to all the other nodes in the cluster. Each publication starts
with the elected master broadcasting the updated cluster state to all nodes in
the cluster. Each node responds with an acknowledgement but does not yet apply
the newly-received state. Once the elected master has collected
acknowledgements from enough master-eligible nodes, the new cluster state is
said to be _committed_ and the master broadcasts another message instructing
nodes to apply the now-committed state. Each node receives this message,
applies the updated state, and then sends a second acknowledgement back to the
master.
The master allows a limited amount of time for each cluster state update to be
completely published to all nodes. It is defined by the
The elected master allows a limited amount of time for each cluster state
update to be completely published to all nodes. It is defined by the
`cluster.publish.timeout` setting, which defaults to `30s`, measured from the
time the publication started. If this time is reached before the new cluster
state is committed then the cluster state change is rejected and the master
considers itself to have failed. It stands down and starts trying to elect a
new master.
state is committed then the cluster state change is rejected and the elected
master considers itself to have failed. It stands down and starts trying to
elect a new master node.
If the new cluster state is committed before `cluster.publish.timeout` has
elapsed, the master node considers the change to have succeeded. It waits until
the timeout has elapsed or until it has received acknowledgements that each
node in the cluster has applied the updated state, and then starts processing
and publishing the next cluster state update. If some acknowledgements have not
been received (i.e. some nodes have not yet confirmed that they have applied
the current update), these nodes are said to be _lagging_ since their cluster
states have fallen behind the master's latest state. The master waits for the
lagging nodes to catch up for a further time, `cluster.follower_lag.timeout`,
which defaults to `90s`. If a node has still not successfully applied the
cluster state update within this time then it is considered to have failed and
is removed from the cluster.
elapsed, the elected master node considers the change to have succeeded. It
waits until the timeout has elapsed or until it has received acknowledgements
that each node in the cluster has applied the updated state, and then starts
processing and publishing the next cluster state update. If some
acknowledgements have not been received (i.e. some nodes have not yet confirmed
that they have applied the current update), these nodes are said to be
_lagging_ since their cluster states have fallen behind the elected master's
latest state. The elected master waits for the lagging nodes to catch up for a
further time, `cluster.follower_lag.timeout`, which defaults to `90s`. If a
node has still not successfully applied the cluster state update within this
time then it is considered to have failed and the elected master removes it
from the cluster.
Cluster state updates are typically published as diffs to the previous cluster
state, which reduces the time and network bandwidth needed to publish a cluster
@ -40,12 +42,19 @@ state update. For example, when updating the mappings for only a subset of the
indices in the cluster state, only the updates for those indices need to be
published to the nodes in the cluster, as long as those nodes have the previous
cluster state. If a node is missing the previous cluster state, for example
when rejoining a cluster, the master will publish the full cluster state to
that node so that it can receive future updates as diffs.
when rejoining a cluster, the elected master will publish the full cluster
state to that node so that it can receive future updates as diffs.
NOTE: {es} is a peer to peer based system, in which nodes communicate with one
another directly. The high-throughput APIs (index, delete, search) do not
normally interact with the master node. The responsibility of the master node
is to maintain the global cluster state and reassign shards when nodes join or
leave the cluster. Each time the cluster state is changed, the new state is
published to all nodes in the cluster as described above.
normally interact with the elected master node. The responsibility of the
elected master node is to maintain the global cluster state which includes
reassigning shards when nodes join or leave the cluster. Each time the cluster
state is changed, the new state is published to all nodes in the cluster as
described above.
The performance characteristics of cluster state updates are a function of the
speed of the storage on each master-eligible node, as well as the reliability
and latency of the network interconnections between all nodes in the cluster.
You must therefore ensure that the storage and networking available to the
nodes in your cluster are good enough to meet your performance goals.

21
docs/reference/modules/node.asciidoc
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@ -194,13 +194,6 @@ High availability (HA) clusters require at least three master-eligible nodes, at
least two of which are not voting-only nodes. Such a cluster will be able to
elect a master node even if one of the nodes fails.
Since voting-only nodes never act as the cluster's elected master, they may
require less heap and a less powerful CPU than the true master nodes.
However all master-eligible nodes, including voting-only nodes, require
reasonably fast persistent storage and a reliable and low-latency network
connection to the rest of the cluster, since they are on the critical path for
<<cluster-state-publishing,publishing cluster state updates>>.
Voting-only master-eligible nodes may also fill other roles in your cluster.
For instance, a node may be both a data node and a voting-only master-eligible
node. A _dedicated_ voting-only master-eligible nodes is a voting-only
@ -212,6 +205,20 @@ dedicated voting-only master-eligible node, set:
node.roles: [ master, voting_only ]
-------------------
Since dedicated voting-only nodes never act as the cluster's elected master,
they may require less heap and a less powerful CPU than the true master nodes.
However all master-eligible nodes, including voting-only nodes, are on the
critical path for <<cluster-state-publishing,publishing cluster state
updates>>. Cluster state updates are usually independent of
performance-critical workloads such as indexing or searches, but they are
involved in management activities such as index creation and rollover, mapping
updates, and recovery after a failure. The performance characteristics of these
activities are a function of the speed of the storage on each master-eligible
node, as well as the reliability and latency of the network interconnections
between the elected master node and the other nodes in the cluster. You must
therefore ensure that the storage and networking available to the nodes in your
cluster are good enough to meet your performance goals.
[[data-node]]
==== Data node