[[modules-gateway]]
== Gateway

The gateway module allows one to store the state of the cluster meta
data across full cluster restarts. The cluster meta data mainly holds
all the indices created with their respective (index level) settings and
explicit type mappings.

Each time the cluster meta data changes (for example, when an index is
added or deleted), those changes will be persisted using the gateway.
When the cluster first starts up, the state will be read from the
gateway and applied.

The gateway set on the node level will automatically control the index
gateway that will be used. For example, if the `fs` gateway is used,
then automatically, each index created on the node will also use its own
respective index level `fs` gateway. In this case, if an index should
not persist its state, it should be explicitly set to `none` (which is
the only other value it can be set to).

The default gateway used is the
<<modules-gateway-local,local>> gateway.

[float]
=== Recovery After Nodes / Time

In many cases, the actual cluster meta data should only be recovered
after specific nodes have started in the cluster, or a timeout has
passed. This is handy when restarting the cluster, and each node local
index storage still exists to be reused and not recovered from the
gateway (which reduces the time it takes to recover from the gateway).

The `gateway.recover_after_nodes` setting (which accepts a number)
controls after how many data and master eligible nodes within the
cluster recovery will start. The `gateway.recover_after_data_nodes` and
`gateway.recover_after_master_nodes` setting work in a similar fashion,
except they consider only the number of data nodes and only the number
of master nodes respectively. The `gateway.recover_after_time` setting
(which accepts a time value) sets the time to wait till recovery happens
once all `gateway.recover_after...nodes` conditions are met.

The `gateway.expected_nodes` allows to set how many data and master
eligible nodes are expected to be in the cluster, and once met, the
`recover_after_time` is ignored and recovery starts. The
`gateway.expected_data_nodes` and `gateway.expected_master_nodes`
settings are also supported. For example setting:

[source,js]
--------------------------------------------------
gateway:
    recover_after_nodes: 1
    recover_after_time: 5m
    expected_nodes: 2
--------------------------------------------------

In an expected 2 nodes cluster will cause recovery to start 5 minutes
after the first node is up, but once there are 2 nodes in the cluster,
recovery will begin immediately (without waiting).

Note, once the meta data has been recovered from the gateway (which
indices to create, mappings and so on), then this setting is no longer
effective until the next full restart of the cluster.

Operations are blocked while the cluster meta data has not been
recovered in order not to mix with the actual cluster meta data that
will be recovered once the settings has been reached.

include::gateway/local.asciidoc[]

include::gateway/fs.asciidoc[]

include::gateway/hadoop.asciidoc[]

include::gateway/s3.asciidoc[]
