Adding documentation on link types

docs/source/link_types/index.rst

@@ -0,0 +1,71 @@
+Node classes
+------------
+There are two *Node* sub-classes, *Data* and *Calculation*. The *Code* nodes
+can be considered as sub-classes of *Data* nodes. *JobCalculations*,
+*InlineCalulations* and the *WorkCalculations* are subclasses of the
+*Calculation* class.
+
+AiIDA graph
+-----------
+The AiIDA graph is a `directed graph <https://en.wikipedia.org/wiki/Directed_graph>`_.
+The links of the graph are arrows connecting two nodes. An arrow (x, y) is
+considered to be directed from x to y; y is called the **head** and x is called
+the **tail** of the arrow; y is said to be a **direct successor** of x and x
+is said to be a **direct predecessor** of y. If a path leads from x to y, then
+y is said to be a **successor** of x and reachable from x, and x is said to be a
+**predecessor** of y. The arrow (y, x) is called the inverted arrow of (x, y).
+
+AiiDA links
+-----------
+The following table summarises the available AiiDA links and their properties.
+Each line of the table corresponds to a different link, it mentions the tail
+of the arrow (from column), the head of the arrow (to column) and finally
+the constraints that are related to the label of the link and the nodes involved.
+
+========= =================== ================= ==========================================
+Link type Tail of link (from) Head of link (to)	Unique condition (combinations that should
+                                                be unique in the link table)
+========= =================== ================= ==========================================
+INPUT     Data	              Calculation	    (head Calculation node, link label)
+CREATE	  Calculation	      Data	            (head Data node)
+RETURN    Calculation         Data	            (tail Calculation node, label)
+CALL	  Calculation	      Calculation	    (head Calculation node)
+========= =================== ================= ==========================================
+
+
+There are four different links available: **INPUT**, **CREATE**, **RETURN**
+and **CALL**.
+
+* The **INPUT** link is always from a *Data* node (tail of the link) to
+  a *Calculation* node (head of the link). The unique constraint means that
+  each *Calculation* node can have only one input *Data* node with given label
+  (i.e., a calculation cannot accept two inputs with the same label).
+
+* The **CREATE** link is always from a *Calculation* node (tail of the link)
+  to a *Data* node (head of the link). The unique constraint means that each
+  *Data* node can be created by one and only one *Calculation* node.
+
+* The **RETURN** link is always from a *Calculation* node (tail of the link)
+  to a *Data* node (head of the link). The unique constraint means that a
+  *Calculation* cannot return two or more *Data* nodes with the same label. Code
+  implementation detail: For the moment there is always and only a **CREATE**
+  link from a *JobCalculation* to the generated *Data*. A **RETURN** link is
+  implied with the conditions of a **RETURN** link (the implementation will be
+  corrected to comply shortly).
+
+* The **CALL** link is always from a *Calculation* to another *Calculation*
+  node. A given *Calculation* node cannot be called by more than one
+  *Calculation* node. In practice, the caller cannot be a *JobCalculation* but
+  it is always a *WorkCalculation*. Instead called calculations can be of any
+  subclass of *Calculation*.
+
+Graph navigation
+----------------
+The links can be followed in both possible directions (forward & reverse) using
+the QueryBuilder. This requires to define additional “names” for each direction
+of the link, and they are documented at the
+:doc:`QueryBuilder section <../querying/querybuilder/usage>`. For example,
+if there is an **INPUT** link from data D to calculation C, D is the
+“input_of” C, or equivalently D is the “output_of” C. Currently, in the
+QueryBuilder, input_of and output_of refer to any link type, where C is the
+head of the arrow and D is the tail.

docs/source/working_with_aiida/index.rst

@@ -128,6 +128,14 @@ Interfaces have been written for some of the most used schedulers.
 
     ../scheduler/index
 
+==========
+Link types
+==========
+
+.. toctree::
+    :maxdepth: 4
+
+    ../link_types/index.rst
 
 =============
 Querying data