[[rolling-upgrades]]
== Rolling upgrades

A rolling upgrade allows an Elasticsearch cluster to be upgraded one node at
a time so upgrading does not interrupt service. Running multiple versions of
Elasticsearch in the same cluster beyond the duration of an upgrade is
not supported, as shards cannot be replicated from upgraded nodes to nodes
running the older version.

Rolling upgrades can be performed between minor versions. Elasticsearch
6.x supports rolling upgrades from *Elasticsearch 5.6*.
Upgrading from earlier 5.x versions requires a <<restart-upgrade,
full cluster restart>>. You must <<reindex-upgrade,reindex to upgrade>> from
versions prior to 5.x.

To perform a rolling upgrade:

. *Disable shard allocation*.
+
--
include::disable-shard-alloc.asciidoc[]
--

. *Stop non-essential indexing and perform a synced flush.* (Optional)
+
--
While you can continue indexing during the upgrade, shard recovery
is much faster if you temporarily stop non-essential indexing and perform a
<<indices-synced-flush, synced-flush>>.

include::synced-flush.asciidoc[]

--

. [[upgrade-node]] *Shut down a single node*.
+
--
include::shut-down-node.asciidoc[]
--

. *Upgrade the node you shut down.*
+
--
include::upgrade-node.asciidoc[]
include::set-paths-tip.asciidoc[]
--

. *Upgrade any plugins.*
+
Use the `elasticsearch-plugin` script to install the upgraded version of each
installed Elasticsearch plugin. All plugins must be upgraded when you upgrade
a node.

. *Start the upgraded node.*
+
--

Start the newly-upgraded node and confirm that it joins the cluster by checking
the log file or by submitting a `_cat/nodes` request:

[source,sh]
--------------------------------------------------
GET _cat/nodes
--------------------------------------------------
// CONSOLE
--

. *Reenable shard allocation.*
+
--

Once the node has joined the cluster, reenable shard allocation to start using
the node:

[source,js]
--------------------------------------------------
PUT _cluster/settings
{
  "transient": {
    "cluster.routing.allocation.enable": "all"
  }
}
--------------------------------------------------
// CONSOLE
--

. *Wait for the node to recover.*
+
--

Before upgrading the next node, wait for the cluster to finish shard allocation.
You can check progress by submitting a <<cat-health,`_cat/health`>> request:

[source,sh]
--------------------------------------------------
GET _cat/health
--------------------------------------------------
// CONSOLE

Wait for the `status` column to switch from `yellow` to `green`. Once the
node is `green`, all primary and replica shards have been allocated.

[IMPORTANT]
====================================================
During a rolling upgrade, primary shards assigned to a node running the new
version cannot have their replicas assigned to a node with the old
version. The new version might have a different data format that is
not understood by the old version.

If it is not possible to assign the replica shards to another node
(there is only one upgraded node in the cluster), the replica
shards remain unassigned and status stays `yellow`.

In this case, you can proceed once there are no initializing or relocating shards
(check the `init` and `relo` columns).

As soon as another node is upgraded, the replicas can be assigned and the
status will change to `green`.
====================================================

Shards that were not <<indices-synced-flush,sync-flushed>> might take longer to
recover.  You can monitor the recovery status of individual shards by
submitting a <<cat-recovery,`_cat/recovery`>> request:

[source,sh]
--------------------------------------------------
GET _cat/recovery
--------------------------------------------------
// CONSOLE

If you stopped indexing, it is safe to resume indexing as soon as
recovery completes.
--

. *Repeat*
+
--

When  the node has recovered and the cluster is stable, repeat these steps
for each node that needs to be updated.

--

[IMPORTANT]
====================================================

During a rolling upgrade, the cluster continues to operate normally. However,
any new functionality is disabled or operates in a backward compatible mode
until all nodes in the cluster are upgraded. New functionality
becomes operational once the upgrade is complete and all nodes are running the
new version. Once that has happened, there's no way to return to operating
in a backward compatible mode. Nodes running the previous major version will
not be allowed to join the fully-updated cluster.

In the unlikely case of a network malfunction during the upgrade process that
isolates all remaining old nodes from the cluster, you must take the
old nodes offline and upgrade them to enable them to join the cluster.

====================================================