added retry chapter (#7)

index.rst

@@ -31,6 +31,7 @@ Contents
 
     components
     locking
+    retry
     quick-example
     symfony
 

retry.rst

@@ -0,0 +1,41 @@
+Retry
+=====
+Some of the tasks which will be executed by a handler are risky and could fail
+(e.g. long running, i/o ...). To allow retry of this tasks the handler is able
+to implement the interface ``RetryTaskHandlerInterface`` and specify a maximum
+amount of attempts to pass the task.
+
+The retries will be scheduled as soon as possible and the following tasks will
+be scheduled after this retry later. This prevent the following tasks to fail
+because of bad starting conditions because of the previous task.
+
+Example
+*******
+
+.. code-block:: php
+
+    <?php
+
+    include __DIR__ . '/vendor/autoload.php';
+
+    class ImageResizeHandler implements Task\Handler\TaskHandlerInterface, Task\Executor\RetryTaskHandlerInterface
+    {
+        public function handle($workload)
+        {
+            try {
+                $this->doSomething();
+            } catch (SpecificException $exception) {
+                throw new FailedException($exception);
+            }
+
+            // other exceptions will be propagated to the runner
+            // the runner will retry the execution until the max-attempts are reached
+        }
+
+        public function getMaximumAttempts()
+        {
+            return 3;
+        }
+    }
+
+