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[[selectors]]
=== Selectors
The connection pool maintains the list of connections, and decides when nodes
should transition from alive to dead (and vice versa). It has no logic to choose
connections, however. That job belongs to the selector class.
The job of a selector is to return a single connection from a provided array of
connections. Like the connection pool, there are several implementations to
choose from.
[discrete]
==== RoundRobinSelector (Default)
This selector returns connections in a round-robin fashion. Node #1 is selected
on the first request, Node #2 on the second request, and so on. This ensures an
even load of traffic across your cluster. Round-robining happens on a
per-request basis (for example sequential requests go to different nodes).
The `RoundRobinSelector` is default but if you wish to explicitly configure it
you can do:
[source,php]
----
$client = ClientBuilder::create()
->setSelector('\Elasticsearch\ConnectionPool\Selectors\RoundRobinSelector')
->build();
----
Note that the implementation is specified via a namespace path to the class.
[discrete]
==== StickyRoundRobinSelector
This selector is "sticky", so that it prefers to reuse the same connection
repeatedly. For example, Node #1 is chosen on the first request. Node #1 will
continue to be re-used for each subsequent request until that node fails. Upon
failure, the selector will round-robin to the next available node, then "stick"
to that node.
This is an ideal strategy for many PHP scripts. Since PHP scripts are
shared-nothing and tend to exit quickly, creating new connections for each
request is often a sub-optimal strategy and introduces a lot of overhead.
Instead, it is better to "stick" to a single connection for the duration of the
script.
By default, this selector randomizes the hosts upon initialization which still
guarantees an even load distribution across the cluster. It changes the
round-robin dynamics from per-request to per-script.
If you are using <<future_mode>>, the "sticky" behavior of this selector is
non-ideal, since all parallel requests go to the same node instead of multiple
nodes in your cluster. When using future mode, the default `RoundRobinSelector`
should be preferred.
If you wish to use this selector, you may do so with:
[source,php]
----
$client = ClientBuilder::create()
->setSelector('\Elasticsearch\ConnectionPool\Selectors\StickyRoundRobinSelector')
->build();
----
Note that the implementation is specified via a namespace path to the class.
[discrete]
==== RandomSelector
This selector returns a random node, regardless of state. It is generally just
for testing.
If you wish to use this selector, you may do so with:
[source,php]
----
$client = ClientBuilder::create()
->setSelector('\Elasticsearch\ConnectionPool\Selectors\RandomSelector')
->build();
----
Note that the implementation is specified via a namespace path to the class.
[discrete]
=== Custom Selector
You can implement your own custom selector. Custom selectors must implement
`SelectorInterface`:
[source,php]
----
namespace MyProject\Selectors;
use Elasticsearch\Connections\ConnectionInterface;
use Elasticsearch\ConnectionPool\Selectors\SelectorInterface
class MyCustomSelector implements SelectorInterface
{
/**
* Selects the first connection
*
* @param array $connections Array of Connection objects
*
* @return ConnectionInterface
*/
public function select($connections)
{
// code here
}
}
----
{zwsp} +
You can then use your custom selector either via object injection or namespace
instantiation:
[source,php]
----
$mySelector = new MyCustomSelector();
$client = ClientBuilder::create()
->setSelector($mySelector) // object injection
->setSelector('\MyProject\Selectors\FirstSelector') // or namespace
->build();
----
[discrete]
==== Quick setup
Selectors can be changed via the `setSelector()` method:
[source,php]
----
$selector = '\Elasticsearch\ConnectionPool\Selectors\StickyRoundRobinSelector';
$client = ClientBuilder::create()
->setSelector($selector)
->build();
----