[[client]]
== Client

You can use the *Java client* in multiple ways:

* Perform standard <<java-docs-index,index>>, <<java-docs-get,get>>,
  <<java-docs-delete,delete>> and <<java-search,search>> operations on an
  existing cluster
* Perform administrative tasks on a running cluster

Obtaining an elasticsearch `Client` is simple. The most common way to
get a client is by creating a <<transport-client,`TransportClient`>>
that connects to a cluster.

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

The client must have the same major version (e.g. `2.x`, or `5.x`) as the
nodes in the cluster. Clients may connect to clusters which have a different
minor version (e.g. `2.3.x`) but it is possible that new functionality may not
be supported.  Ideally, the client should have the same version as the
cluster.

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


[[transport-client]]
=== Transport Client

The `TransportClient` connects remotely to an Elasticsearch cluster
using the transport module. It does not join the cluster, but simply
gets one or more initial transport addresses and communicates with them
in round robin fashion on each action (though most actions will probably
be "two hop" operations).

[source,java]
--------------------------------------------------
// on startup

TransportClient client = new PreBuiltTransportClient(Settings.EMPTY)
        .addTransportAddress(new TransportAddress(InetAddress.getByName("host1"), 9300))
        .addTransportAddress(new TransportAddress(InetAddress.getByName("host2"), 9300));

// on shutdown

client.close();
--------------------------------------------------

Note that you have to set the cluster name if you use one different than
"elasticsearch":

[source,java]
--------------------------------------------------
Settings settings = Settings.builder()
        .put("cluster.name", "myClusterName").build();
TransportClient client = new PreBuiltTransportClient(settings);
//Add transport addresses and do something with the client...
--------------------------------------------------

The Transport client comes with a cluster sniffing feature which
allows it to dynamically add new hosts and remove old ones.
When sniffing is enabled, the transport client will connect to the nodes in its
internal node list, which is built via calls to `addTransportAddress`.
After this, the client will call the internal cluster state API on those nodes
to discover available data nodes. The internal node list of the client will
be replaced with those data nodes only. This list is refreshed every five seconds by default.
Note that the IP addresses the sniffer connects to are the ones declared as the 'publish'
address in those node's elasticsearch config.

Keep in mind that the list might possibly not include the original node it connected to
if that node is not a data node. If, for instance, you initially connect to a
master node, after sniffing, no further requests will go to that master node,
but rather to any data nodes instead. The reason the transport client excludes non-data
nodes is to avoid sending search traffic to master only nodes.

In order to enable sniffing, set `client.transport.sniff` to `true`:

[source,java]
--------------------------------------------------
Settings settings = Settings.builder()
        .put("client.transport.sniff", true).build();
TransportClient client = new PreBuiltTransportClient(settings);
--------------------------------------------------

Other transport client level settings include:

[cols="<,<",options="header",]
|=======================================================================
|Parameter |Description
|`client.transport.ignore_cluster_name` |Set to `true` to ignore cluster
name validation of connected nodes. (since 0.19.4)

|`client.transport.ping_timeout` |The time to wait for a ping response
from a node. Defaults to `5s`.

|`client.transport.nodes_sampler_interval` |How often to sample / ping
the nodes listed and connected. Defaults to `5s`.
|=======================================================================


[[client-connected-to-client-node]]
=== Connecting a Client to a Coordinating Only Node

You can start locally a {ref}/modules-node.html#coordinating-only-node[Coordinating Only Node]
and then simply create a <<transport-client,`TransportClient`>> in your
application which connects to this Coordinating Only Node.

This way, the coordinating only node will be able to load whatever plugin you
need (think about discovery plugins for example).
