Improve documentation for 0.4 release

components.rst

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+Components
+==========
+The library consists of three main parts.
+
+TaskScheduler
+-------------
+The ``TaskScheduler`` stores tasks and schedules executions. For each task
+there exists exactly one execution in the status ``planned`` or ``running``. If
+the task will only one time it only get one execution at all. For recurring
+tasks a new execution will be generated when the old one is ``completed`` or
+``failed``.
+
+Each task consists of:
+
+- ``uuid``: unique identifier
+- ``handlerClass``: class-name of the handler which will be executed for this
+  task
+- ``workload``: the workload will be passed to the handler
+- ``interval``: for recurring tasks this fields contains a cron-expression
+- ``firstExecution``: the earliest possible execution
+- ``lastExecution``: the latest possible execution
+
+TaskRunner
+----------
+The ``TaskRunner`` takes the ``planned`` executions and executes the ``Handler``
+with the workload of the task.
+
+Handler
+-------
+The ``Handler`` implements the domain-logic for a task. The library implements
+a Factory which uses Reflection to create a new instance for the
+``handlerClass`` of the task.
+
+.. note::
+
+    The :doc:`symfony` uses tagged-services to find available ``Handler``.