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== Future Mode

The client offers a mode called "future" or "async" mode.  This allows batch processing of requests (sent in parallel
to the cluster), which can have a dramatic impact on performance and throughput.

PHP is fundamentally single-threaded, however libcurl provides functionality called the "multi interface".  This allows
languages like PHP to gain concurrency by providing a batch of requests to process.  The batch is executed in a parallel
fashion by the underlying multithreaded libcurl library, and the batch of responses is then returned to PHP.

In a single-threaded environment, the time to execute `n` requests is the sum of those `n` requests' latencies.  With
the multi interface, the time to execute `n` requests is the latency of the slowest request (assuming enough handles
are available to execute all requests in parallel).

Furthermore, the multi-interface allows requests to different hosts simultaneously, which means the Elasticsearch-PHP
client can more effectively utilize your full cluster.

=== Using Future Mode

Utilizing this feature is relatively straightforward, but it does introduce more responsibility into your code.  To enable
future mode, set the `future` flag in the client options to `'lazy'`:

[source,php]
----
$client = ClientBuilder::create()->build();

$params = [
    'index' => 'test',
    'type' => 'test',
    'id' => 1,
    'client' => [
        'future' => 'lazy'
    ]
];

$future = $client->get($params);
----

This will return a _future_, rather than the actual response.  A future represents a _future computation_ and acts like
a placeholder.  You can pass a future around your code like a regular object.  When you need the result values, you
can _resolve_ the future.  If the future has already resolved (due to some other activity), the values will be immediately
available.  If the future has not resolved yet, the resolution will block until those values have become available (e.g.
after the API call completes).

In practice, this means you can queue up a batch of requests by using `future: lazy` and they will pend until you resolve
the futures, at which time all requests will be sent in parallel to the cluster and return asynchronously to curl.

This sounds tricky, but it is actually very simple thanks to RingPHP's `FutureArray` interface, which makes the future
act like a simple associative array.  For example:

[source,php]
----
$client = ClientBuilder::create()->build();

$params = [
    'index' => 'test',
    'type' => 'test',
    'id' => 1,
    'client' => [
        'future' => 'lazy'
    ]
];

$future = $client->get($params);

$doc = $future['_source'];    // This call will block and force the future to resolve
----

Interacting with the future as an associative array, just like a normal response, will cause the future to resolve
that particular value (which in turn resolves all pending requests and values).  This allows patterns such as:

[source,php]
----
$client = ClientBuilder::create()->build();
$futures = [];

for ($i = 0; $i < 1000; $i++) {
    $params = [
        'index' => 'test',
        'type' => 'test',
        'id' => $i,
        'client' => [
            'future' => 'lazy'
        ]
    ];

    $futures[] = $client->get($params);     //queue up the request
}


foreach ($futures as $future) {
    // access future's values, causing resolution if necessary
    echo $future['_source'];
}
----

The queued requests will execute in parallel and populate their futures after execution.  Batch size defaults to
100 requests-per-batch.

If you wish to force future resolution, but don't actually need the values immediately, you can call `wait()` on the future
to force resolution too:

[source,php]
----
$client = ClientBuilder::create()->build();
$futures = [];

for ($i = 0; $i < 1000; $i++) {
    $params = [
        'index' => 'test',
        'type' => 'test',
        'id' => $i,
        'client' => [
            'future' => 'lazy'
        ]
    ];

    $futures[] = $client->get($params);     //queue up the request
}

//wait() forces future resolution and will execute the underlying curl batch
$futures[999]->wait();
----

=== Changing batch size

The default batch size is 100, meaning 100 requests will queue up before the client forces futures to begin resolving
(e.g. initiate a `curl_multi` call).  The batch size can be changed depending on your preferences.  The batch size
is controllable via the `max_handles` setting when configuring the handler:

[source,php]
----
$handlerParams = [
    'max_handles' => 500
];

$defaultHandler = ClientBuilder::defaultHandler($handlerParams);

$client = ClientBuilder::create()
            ->setHandler($defaultHandler)
            ->build();
----

This will change the behavior to wait on 500 queued requests before sending the batch.  Note, however, that forcing a
future to resolve will cause the underlying curl batch to execute, regardless of if the batch is "full" or not.  In this
example, only 499 requests are added to the queue...but the final future resolution will force the batch to flush
anyway:

[source,php]
----
$handlerParams = [
    'max_handles' => 500
];

$defaultHandler = ClientBuilder::defaultHandler($handlerParams);

$client = ClientBuilder::create()
            ->setHandler($defaultHandler)
            ->build();

$futures = [];

for ($i = 0; $i < 499; $i++) {
    $params = [
        'index' => 'test',
        'type' => 'test',
        'id' => $i,
        'client' => [
            'future' => 'lazy'
        ]
    ];

    $futures[] = $client->get($params);     //queue up the request
}

// resolve the future, and therefore the underlying batch
$body = $future[499]['body'];
----

=== Heterogeneous batches are OK

It is possible to queue up heterogeneous batches of requests.  For example, you can queue up several GETs, indexing requests
and a search:

[source,php]
----
$client = ClientBuilder::create()->build();
$futures = [];

$params = [
    'index' => 'test',
    'type' => 'test',
    'id' => 1,
    'client' => [
        'future' => 'lazy'
    ]
];

$futures['getRequest'] = $client->get($params);     // First request

$params = [
    'index' => 'test',
    'type' => 'test',
    'id' => 2,
    'body' => [
        'field' => 'value'
    ],
    'client' => [
        'future' => 'lazy'
    ]
];

$futures['indexRequest'] = $client->index($params);       // Second request

$params = [
    'index' => 'test',
    'type' => 'test',
    'body' => [
        'query' => [
            'match' => [
                'field' => 'value'
            ]
        ]
    ],
    'client' => [
        'future' => 'lazy'
    ]
];

$futures['searchRequest'] = $client->search($params);      // Third request

// Resolve futures...blocks until network call completes
$searchResults = $futures['searchRequest']['hits'];

// Should return immediately, since the previous future resolved the entire batch
$doc = $futures['getRequest']['_source'];
----

=== Caveats to Future mode

There are a few caveats to using future mode.  The biggest is also the most obvious: you need to deal with resolving the
future yourself.  This is usually trivial, but can sometimes introduce unexpected complications.

For example, if you resolve manually using `wait()`, you may need to call `wait()` several times if there were retries.
This is because each retry will introduce another layer of wrapped futures, and each needs to be resolved to get the
final result.

This is not needed if you access values via the ArrayInterface however (e.g. `$response['hits']['hits']`), since
FutureArrayInterface will automatically and fully resolve the future to provide values.

Another caveat is that certain APIs will lose their "helper" functionality.  For example, "exists" APIs (e.g.
`$client->exists()`, `$client->indices()->exists`, `$client->indices->templateExists()`, etc) typically return a true
or false under normal operation.

When operated in future mode, unwrapping of the future is left to your application,
which means the client can no longer inspect the response and return a simple true/false.  Instead, you'll see the raw
response from Elasticsearch and will have to take action appropriately.

This also applies to `ping()`.