[[high-availability-cluster-design]]
== Designing for resilience

Distributed systems like {es} are designed to keep working even if some of
their components have failed. As long as there are enough well-connected
nodes to take over their responsibilities, an {es} cluster can continue
operating normally if some of its nodes are unavailable or disconnected.

There is a limit to how small a resilient cluster can be. All {es} clusters
require:

- One <<modules-discovery-quorums,elected master node>> node
- At least one node for each <<modules-node,role>>.
- At least one copy of every <<scalability,shard>>.

A resilient cluster requires redundancy for every required cluster component.
This means a resilient cluster must have:

- At least three master-eligible nodes
- At least two nodes of each role
- At least two copies of each shard (one primary and one or more replicas,
  unless the index is a <<searchable-snapshots,searchable snapshot index>>)

A resilient cluster needs three master-eligible nodes so that if one of
them fails then the remaining two still form a majority and can hold a
successful election.

Similarly, redundancy of nodes of each role means that if a node for a
particular role fails, another node can take on its responsibilities.

Finally, a resilient cluster should have at least two copies of each shard. If
one copy fails then there should be another good copy to take over. {es}
automatically rebuilds any failed shard copies on the remaining nodes in order
to restore the cluster to full health after a failure.

Failures temporarily reduce the total capacity of your cluster. In addition,
after a failure the cluster must perform additional background activities to
restore itself to health. You should make sure that your cluster has the
capacity to handle your workload even if some nodes fail.

Depending on your needs and budget, an {es} cluster can consist of a single
node, hundreds of nodes, or any number in between. When designing a smaller
cluster, you should typically focus on making it resilient to single-node
failures. Designers of larger clusters must also consider cases where multiple
nodes fail at the same time. The following pages give some recommendations for
building resilient clusters of various sizes:

- <<high-availability-cluster-small-clusters>>
- <<high-availability-cluster-design-large-clusters>>

[[high-availability-cluster-small-clusters]]
=== Resilience in small clusters

In smaller clusters, it is most important to be resilient to single-node
failures. This section gives some guidance on making your cluster as resilient
as possible to the failure of an individual node.

[[high-availability-cluster-design-one-node]]
==== One-node clusters

If your cluster consists of one node, that single node must do everything.
To accommodate this, {es} assigns nodes every role by default.

A single node cluster is not resilient. If the node fails, the cluster will
stop working. Because there are no replicas in a one-node cluster, you cannot
store your data redundantly. However, by default at least one replica is
required for a <<cluster-health,`green` cluster health status>>. To ensure your
cluster can report a `green` status, override the default by setting
<<dynamic-index-settings,`index.number_of_replicas`>> to `0` on every index.

If the node fails, you may need to restore an older copy of any lost indices
from a <<modules-snapshots,snapshot>>.

Because they are not resilient to any failures, we do not recommend using
one-node clusters in production.

[[high-availability-cluster-design-two-nodes]]
==== Two-node clusters

If you have two nodes, we recommend they both be data nodes. You should also
ensure every shard is stored redundantly on both nodes by setting
<<dynamic-index-settings,`index.number_of_replicas`>> to `1` on every index
that is not a <<searchable-snapshots,searchable snapshot index>>. This is the
default behaviour but may be overridden by an <<index-templates,index
template>>. <<dynamic-index-settings,Auto-expand replicas>> can also achieve
the same thing, but it's not necessary to use this feature in such a small
cluster.

We recommend you set `node.master: false` on one of your two nodes so that it is
not <<master-node,master-eligible>>. This means you can be certain which of your
nodes is the elected master of the cluster. The cluster can tolerate the loss of
the other master-ineligible node. If you don't set `node.master: false` on one
node, both nodes are master-eligible. This means both nodes are required for a
master election. Since the election will fail if either node is unavailable,
your cluster cannot reliably tolerate the loss of either node.

By default, each node is assigned every role. We recommend you assign both nodes
all other roles except master eligibility. If one node fails, the other node can
handle its tasks.

You should avoid sending client requests to just one of your nodes. If you do
and this node fails, such requests will not receive responses even if the
remaining node is a healthy cluster on its own. Ideally, you should balance your
client requests across both nodes. A good way to do this is to specify the
addresses of both nodes when configuring the client to connect to your cluster.
Alternatively, you can use a resilient load balancer to balance client requests
across the nodes in your cluster.

Because it's not resilient to failures, we do not recommend deploying a two-node
cluster in production.

[[high-availability-cluster-design-two-nodes-plus]]
==== Two-node clusters with a tiebreaker

Because master elections are majority-based, the two-node cluster described
above is tolerant to the loss of one of its nodes but not the
other one. You cannot configure a two-node cluster so that it can tolerate
the loss of _either_ node because this is theoretically impossible. You might
expect that if either node fails then {es} can elect the remaining node as the
master, but it is impossible to tell the difference between the failure of a
remote node and a mere loss of connectivity between the nodes. If both nodes
were capable of running independent elections, a loss of connectivity would
lead to a {wikipedia}/Split-brain_(computing)[split-brain
problem] and therefore data loss. {es} avoids this and
protects your data by electing neither node as master until that node can be
sure that it has the latest cluster state and that there is no other master in
the cluster. This could result in the cluster having no master until
connectivity is restored.

You can solve this problem by adding a third node and making all three nodes
master-eligible. A <<modules-discovery-quorums,master election>> requires only
two of the three master-eligible nodes. This means the cluster can tolerate the
loss of any single node. This third node acts as a tiebreaker in cases where the
two original nodes are disconnected from each other. You can reduce the resource
requirements of this extra node by making it a <<voting-only-node,dedicated
voting-only master-eligible node>>, also known as a dedicated tiebreaker.
Because it has no other roles, a dedicated tiebreaker does not need to be as
powerful as the other two nodes. It will not perform any searches nor coordinate
any client requests and cannot be elected as the master of the cluster.

The two original nodes should not be voting-only master-eligible nodes since a
resilient cluster requires at least three master-eligible nodes, at least two
of which are not voting-only master-eligible nodes. If two of your three nodes
are voting-only master-eligible nodes then the elected master must be the third
node. This node then becomes a single point of failure.

We recommend assigning both non-tiebreaker nodes all other roles. This creates
redundancy by ensuring any task in the cluster can be handled by either node.

You should not send any client requests to the dedicated tiebreaker node.
You should also avoid sending client requests to just one of the other two
nodes. If you do, and this node fails, then any requests will not
receive responses, even if the remaining nodes form a healthy cluster. Ideally,
you should balance your client requests across both of the non-tiebreaker
nodes. You can do this by specifying the address of both nodes
when configuring your client to connect to your cluster. Alternatively, you can
use a resilient load balancer to balance client requests across the appropriate
nodes in your cluster. The {ess-trial}[Elastic Cloud] service
provides such a load balancer.

A two-node cluster with an additional tiebreaker node is the smallest possible
cluster that is suitable for production deployments.

[[high-availability-cluster-design-three-nodes]]
==== Three-node clusters

If you have three nodes, we recommend they all be <<data-node,data nodes>> and
every index that is not a <<searchable-snapshots,searchable snapshot index>>
should have at least one replica. Nodes are data nodes by default. You may
prefer for some indices to have two replicas so that each node has a copy of
each shard in those indices. You should also configure each node to be
<<master-node,master-eligible>> so that any two of them can hold a master
election without needing to communicate with the third node. Nodes are
master-eligible by default. This cluster will be resilient to the loss of any
single node.

You should avoid sending client requests to just one of your nodes. If you do,
and this node fails, then any requests will not receive responses even if the
remaining two nodes form a healthy cluster. Ideally, you should balance your
client requests across all three nodes. You can do this by specifying the
address of multiple nodes when configuring your client to connect to your
cluster. Alternatively you can use a resilient load balancer to balance client
requests across your cluster. The {ess-trial}[Elastic Cloud]
service provides such a load balancer.

[[high-availability-cluster-design-three-plus-nodes]]
==== Clusters with more than three nodes

Once your cluster grows to more than three nodes, you can start to specialise
these nodes according to their responsibilities, allowing you to scale their
resources independently as needed. You can have as many <<data-node,data
nodes>>, <<ingest,ingest nodes>>, <<ml-node,{ml} nodes>>, etc. as needed to
support your workload. As your cluster grows larger, we recommend using
dedicated nodes for each role. This lets you to independently scale resources
for each task.

However, it is good practice to limit the number of master-eligible nodes in
the cluster to three. Master nodes do not scale like other node types since
the cluster always elects just one of them as the master of the cluster. If
there are too many master-eligible nodes then master elections may take a
longer time to complete. In larger clusters, we recommend you
configure some of your nodes as dedicated master-eligible nodes and avoid
sending any client requests to these dedicated nodes. Your cluster may become
unstable if the master-eligible nodes are overwhelmed with unnecessary extra
work that could be handled by one of the other nodes.

You may configure one of your master-eligible nodes to be a
<<voting-only-node,voting-only node>> so that it can never be elected as the
master node. For instance, you may have two dedicated master nodes and a third
node that is both a data node and a voting-only master-eligible node. This
third voting-only node will act as a tiebreaker in master elections but will
never become the master itself.

[[high-availability-cluster-design-small-cluster-summary]]
==== Summary

The cluster will be resilient to the loss of any node as long as:

- The <<cluster-health,cluster health status>> is `green`.
- There are at least two data nodes. 
- Every index that is not a <<searchable-snapshots,searchable snapshot index>>
  has at least one replica of each shard, in addition to the primary.
- The cluster has at least three master-eligible nodes, as long as at least two
  of these nodes are not voting-only master-eligible nodes.
- Clients are configured to send their requests to more than one node or are
  configured to use a load balancer that balances the requests across an
  appropriate set of nodes. The {ess-trial}[Elastic Cloud] service provides such
  a load balancer.

[[high-availability-cluster-design-large-clusters]]
=== Resilience in larger clusters

It is not unusual for nodes to share some common infrastructure, such as a power
supply or network router. If so, you should plan for the failure of this
infrastructure and ensure that such a failure would not affect too many of your
nodes. It is common practice to group all the nodes sharing some infrastructure
into _zones_ and to plan for the failure of any whole zone at once.

Your cluster’s zones should all be contained within a single data centre. {es}
expects its node-to-node connections to be reliable and have low latency and
high bandwidth. Connections between data centres typically do not meet these
expectations. Although {es} will behave correctly on an unreliable or slow
network, it will not necessarily behave optimally. It may take a considerable
length of time for a cluster to fully recover from a network partition since it
must resynchronize any missing data and rebalance the cluster once the
partition heals. If you want your data to be available in multiple data centres,
deploy a separate cluster in each data centre and use
<<modules-cross-cluster-search,{ccs}>> or <<xpack-ccr,{ccr}>> to link the
clusters together. These features are designed to perform well even if the
cluster-to-cluster connections are less reliable or slower than the network
within each cluster.

After losing a whole zone's worth of nodes, a properly-designed cluster may be
functional but running with significantly reduced capacity. You may need
to provision extra nodes to restore acceptable performance in your
cluster when handling such a failure.

For resilience against whole-zone failures, it is important that there is a copy
of each shard in more than one zone, which can be achieved by placing data
nodes in multiple zones and configuring <<allocation-awareness,shard allocation
awareness>>. You should also ensure that client requests are sent to nodes in
more than one zone.

You should consider all node roles and ensure that each role is split
redundantly across two or more zones. For instance, if you are using
<<ingest,ingest pipelines>> or {ml}, you should have ingest or {ml} nodes in two
or more zones. However, the placement of master-eligible nodes requires a little
more care because a resilient cluster needs at least two of the three
master-eligible nodes in order to function. The following sections explore the
options for placing master-eligible nodes across multiple zones.

[[high-availability-cluster-design-two-zones]]
==== Two-zone clusters

If you have two zones, you should have a different number of
master-eligible nodes in each zone so that the zone with more nodes will
contain a majority of them and will be able to survive the loss of the other
zone. For instance, if you have three master-eligible nodes then you may put
all of them in one zone or you may put two in one zone and the third in the
other zone. You should not place an equal number of master-eligible nodes in
each zone. If you place the same number of master-eligible nodes in each zone,
neither zone has a majority of its own. Therefore, the cluster may not survive
the loss of either zone.

[[high-availability-cluster-design-two-zones-plus]]
==== Two-zone clusters with a tiebreaker

The two-zone deployment described above is tolerant to the loss of one of its
zones but not to the loss of the other one because master elections are
majority-based. You cannot configure a two-zone cluster so that it can tolerate
the loss of _either_ zone because this is theoretically impossible. You might
expect that if either zone fails then {es} can elect a node from the remaining
zone as the master but it is impossible to tell the difference between the
failure of a remote zone and a mere loss of connectivity between the zones. If
both zones were capable of running independent elections then a loss of
connectivity would lead to a
{wikipedia}/Split-brain_(computing)[split-brain problem] and
therefore data loss. {es} avoids this and protects your data by not electing
a node from either zone as master until that node can be sure that it has the
latest cluster state and that there is no other master in the cluster. This may
mean there is no master at all until connectivity is restored.

You can solve this by placing one master-eligible node in each of your two
zones and adding a single extra master-eligible node in an independent third
zone. The extra master-eligible node acts as a tiebreaker in cases
where the two original zones are disconnected from each other. The extra
tiebreaker node should be a <<voting-only-node,dedicated voting-only
master-eligible node>>, also known as a dedicated tiebreaker. A dedicated
tiebreaker need not be as powerful as the other two nodes since it has no other
roles and will not perform any searches nor coordinate any client requests nor
be elected as the master of the cluster.

You should use <<allocation-awareness,shard allocation awareness>> to ensure
that there is a copy of each shard in each zone. This means either zone remains
fully available if the other zone fails.

All master-eligible nodes, including voting-only nodes, are on the critical path
for publishing cluster state updates. Because of this, these nodes require
reasonably fast persistent storage and a reliable, low-latency network
connection to the rest of the cluster. If you add a tiebreaker node in a third
independent zone then you must make sure it has adequate resources and good
connectivity to the rest of the cluster.

[[high-availability-cluster-design-three-zones]]
==== Clusters with three or more zones

If you have three zones then you should have one master-eligible node in each
zone. If you have more than three zones then you should choose three of the
zones and put a master-eligible node in each of these three zones. This will
mean that the cluster can still elect a master even if one of the zones fails.

As always, your indices should have at least one replica in case a node fails,
unless they are <<searchable-snapshots,searchable snapshot indices>>. You
should also use <<allocation-awareness,shard allocation awareness>> to limit
the number of copies of each shard in each zone. For instance, if you have an
index with one or two replicas configured then allocation awareness will ensure
that the replicas of the shard are in a different zone from the primary. This
means that a copy of every shard will still be available if one zone fails. The
availability of this shard will not be affected by such a failure.

[[high-availability-cluster-design-large-cluster-summary]]
==== Summary

The cluster will be resilient to the loss of any zone as long as:

- The <<cluster-health,cluster health status>> is `green`.
- There are at least two zones containing data nodes.
- Every index that is not a <<searchable-snapshots,searchable snapshot index>>
  has at least one replica of each shard, in addition to the primary.
- Shard allocation awareness is configured to avoid concentrating all copies of
  a shard within a single zone.
- The cluster has at least three master-eligible nodes. At least two of these 
  nodes are not voting-only master-eligible nodes, and they are spread evenly
  across at least three zones.
- Clients are configured to send their requests to nodes in more than one zone
  or are configured to use a load balancer that balances the requests across an
  appropriate set of nodes. The {ess-trial}[Elastic Cloud] service provides such
  a load balancer.
