OpenSearch/docs/reference/upgrade/rolling_upgrade.asciidoc

[[rolling-upgrades]]
== Rolling upgrades

A rolling upgrade allows an {es} cluster to be upgraded one node at
a time so upgrading does not interrupt service. Running multiple versions of
{es} 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.

It is best to upgrade the master-eligible nodes in your cluster after all of
the other nodes. Once you have started to upgrade the master-eligible nodes
they may form a cluster that nodes of older versions cannot join. If you
upgrade the master-eligible nodes last then all the other nodes will not be
running an older version and so they will be able to join the cluster.

Rolling upgrades are supported:

* Between minor versions
* {stack-ref-68}/upgrading-elastic-stack.html[From 5.6 to 6.8]
* From 6.8 to {version}

Upgrading directly to {version} from 6.7 or earlier requires a
<<restart-upgrade, full cluster restart>>.

To perform a rolling upgrade from 6.8 to {version}:

. *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-api, synced-flush>>.

include::synced-flush.asciidoc[]

--

. *Temporarily stop the tasks associated with active {ml} jobs and {dfeeds}.* (Optional)
+
--
include::close-ml.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[]

[[rolling-upgrades-bootstrapping]]
NOTE: You should leave `cluster.initial_master_nodes` unset while performing a
rolling upgrade. Each upgraded node is joining an existing cluster so there is
no need for <<modules-discovery-bootstrap-cluster,cluster bootstrapping>>.
--

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

. If you use {es} {security-features} to define realms, verify that your realm
settings are up-to-date. The format of realm settings changed in version 7.0, in
particular, the placement of the realm type changed. See
<<realm-settings,Realm settings>>. 

. *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,console]
--------------------------------------------------
GET _cat/nodes
--------------------------------------------------
--

. *Reenable shard allocation.*
+
--

Once the node has joined the cluster, remove the `cluster.routing.allocation.enable`
setting to enable shard allocation and start using the node:

[source,console]
--------------------------------------------------
PUT _cluster/settings
{
  "persistent": {
    "cluster.routing.allocation.enable": null
  }
}
--------------------------------------------------
--

. *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,console]
--------------------------------------------------
GET _cat/health?v
--------------------------------------------------

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-api,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,console]
--------------------------------------------------
GET _cat/recovery
--------------------------------------------------

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.

--

. *Restart machine learning jobs.*
+
--
include::open-ml.asciidoc[]
--


[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.

If you stop half or more of the master-eligible nodes all at once during the
upgrade then the cluster will become unavailable, meaning that the upgrade is
no longer a _rolling_ upgrade. If this happens, you should upgrade and restart
all of the stopped master-eligible nodes to allow the cluster to form again, as
if performing a <<restart-upgrade,full-cluster restart upgrade>>. It may also
be necessary to upgrade all of the remaining old nodes before they can join the
cluster after it re-forms.

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