Update

Rewritten-docs.md

@@ -487,14 +487,51 @@ described as well.
 
 ### Evaluation unit executed on backend
 
-@todo: describe possibilities of "piece of work" which can backend execute, how they can look like, describe our job and its tasks
-
-@todo: why is there division to internal and external tasks and why it is needed
+One of the bigger requests for the new system is to support complex
+configuration of execution pipeline. The idea comes from lecturers of Compiler
+principles class who want to migrate their semi-manual evaluation process to
+CodEx. Unfortunately, CodEx is not capable of such compilicated exercise setup.
+None of evaluation systems we found is capable of such task, so design from
+scratch is needed.
+
+There are two main approaches to design a complex execution configuration. It
+can be composed of small amount of relatively big components or much more small
+tasks. Big components are easy to write and whole configuration is reasonably
+small. The components are designed for current problems, so it is not scalable
+enough for pleasant future usage. This can be solved by introducing small set of
+single-purposed tasks which can be composed together. The whole configuration is
+then quite bigger, but with great adaptation ability for new conditions and also
+less amount of work programming them. For better user experience, configuration
+generators for some common cases can be introduced.
+
+ReCodEx target is to be continuously developed and used for many years, so the
+smaller tasks are the right choice. Observation of CodEx system shows that
+only a few tasks are needed. In extreme case, only one task is enough -- execute
+a binary. However, for better portability of configurations along different
+systems it is better to implement reasonable subset of operations directly
+without calling system provided binaries. These operations are copy file, create
+new directory, extract archive and so on, altogether called internal tasks.
+Another benefit from custom implementation of these tasks is guarantied safety,
+so no sandbox needs to be used as in exernal tasks case.
+
+For a job evaluation, the tasks needs to be executed sequentialy in a specified
+order. The idea of running independent tasks in parallel is bad because exact
+time measurement needs controled environment on target computer with
+minimalization of interrupts by other processes. It seems that connecting tasks
+into directed acyclic graph (DAG) can handle all possible problem cases. None of
+the authors, supervisors and involved faculty staff can think of a problem that
+cannot be decomposed into tasks connected in a DAG. The goal of evaluation is
+to satisfy as many tasks as possible. During execution there are sometimes
+multiple choices of next task. To control that, each task can have a priority,
+which is used as a secondary ordering criterium. For better understanding, here
+is a small example.
+
+![Task serialization](https://github.com/ReCodEx/wiki/raw/master/images/Assignment_overview.png)
+
+@todo: why we need priority and exact order of tasks?
 
 @todo: division to initiation, execution and evaluation why and what for
 
-@todo: in what order should tasks be executed, how to sort them
-
 @todo: how to solve problem with specific worker environment, mention internal job variables