Add UPGRADE-8.0 guide

.gitattributes

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 /.gitignore export-ignore
 /.php-cs-fixer.dist.php export-ignore
 /CHANGELOG.md merge=union
-/codecov.yml export-ignore
 /CONTRIBUTING.md export-ignore
+/UPGRADE-8.0.md export-ignore
+/codecov.yml export-ignore
 /docker export-ignore
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 /phive.xml export-ignore

UPGRADE-8.0.md

@@ -0,0 +1,110 @@
+UPGRADE FROM 7.3 to 8.0
+=======================
+
+Client configuration has been significantly reworked.
+Elastica rely on official elasticsearch PHP client parameters now.
+
+Client
+-------
+* `host` parameter has been renamed to `hosts` and it's type has been set to array
+  *Before*
+    ```php
+    new Elastica\Elastica([
+        'host' => 'http://es.host.com',
+    ])
+    ```
+
+  *After*
+    ```php
+    new Elastica\Elastica([
+        'hosts' => ['http://es.host.com'],
+    ])
+    ```
+
+* `port` parameter has been removed.
+  *Before*
+    ```php
+    new Elastica\Elastica([
+        'host' => 'http://es.host.com',
+        'port' => 9200,
+    ])
+    ```
+
+  *After*
+    ```php
+    new Elastica\Elastica([
+        'hosts' => ['http://es.host.com:9200'],
+    ])
+    ```
+
+* `url` no longer supported in favor of `hosts`
+    *Before*
+    ```php
+    new Elastica\Elastica([
+        'url' => 'http://es.host.com',
+    ])
+    ```
+
+    *After*
+    ```php
+    new Elastica\Elastica([
+        'hosts' => ['http://es.host.com'],
+    ])
+    ```
+
+* `roundRobin` parameter has been removed
+  *Before*
+    ```php
+    new Elastica\Elastica([
+        'url' => 'http://es.host.com',
+        'roundRobin' => true,
+    ])
+    ```
+
+  *After*
+    ```php
+    use Elastic\Transport\NodePool\Resurrect\ElasticsearchResurrect;
+    use Elastic\Transport\NodePool\Selector\RoundRobin;
+    use Elastic\Transport\NodePool\SimpleNodePool;
+
+    $nodePool = new SimpleNodePool(
+        new RoundRobin(),
+        new ElasticsearchResurrect()
+    );
+
+    new Client([
+        'hosts' => [
+            'https://es.host.com:9200',
+        ],
+        'transport_config' => [
+            'node_pool' => $nodePool,
+        ],
+    ]);
+    ```
+
+* `connections` parameter has been removed.
+  *Before*
+    ```php
+    new Elastica\Elastica([
+        'connections' => [
+            [
+                'host' => 'https://es.node1.com',
+                'port' => 9200,
+            ],
+            [
+                'host' => 'https://es.node2.com',
+                'port' => 9200,
+            ],
+        ],
+    ])
+    ```
+
+  *After*
+    ```php
+    new Elastica\Elastica([
+        'hosts' => [
+            'http://es.host.com',
+            'https://es.node2.com',
+        ],
+    ])
+    ```