Merge branch 'master' into develop

docs/guide/_toc.rst

@@ -0,0 +1,11 @@
+.. toctree::
+    :maxdepth: 2
+
+    introduction
+    transformations
+    graphs
+    services
+    environment
+    purity
+    debugging
+    plugins

docs/guide/graphs.rst

@@ -5,6 +5,92 @@ Graphs are the glue that ties transformations together. They are the only data-s
 must be acyclic, and can contain as many nodes as your system can handle. However, although in theory the number of nodes can be rather high, practical use cases usually do not exceed more than a few hundred nodes and only then in extreme cases.
 
 
+Within a graph, each node are isolated and can only communicate using their
+input and output queues. For each input row, a given node will be called with
+the row passed as arguments. Each *return* or *yield* value will be put on the
+node's output queue, and the nodes connected in the graph will then be able to
+process it.
+
+|bonobo| is a line-by-line data stream processing solution.
+
+Handling the data-flow this way brings the following properties:
+
+- **First in, first out**: unless stated otherwise, each node will receeive the
+  rows from FIFO queues, and so, the order of rows will be preserved. That is
+  true for each single node, but please note that if you define "graph bubbles"
+  (where a graph diverge in different branches then converge again), the
+  convergence node will receive rows FIFO from each input queue, meaning that
+  the order existing at the divergence point wont stay true at the convergence
+  point.
+
+- **Parallelism**: each node run in parallel (by default, using independant
+  threads). This is useful as you don't have to worry about blocking calls.
+  If a thread waits for, let's say, a database, or a network service, the other
+  nodes will continue handling data, as long as they have input rows available.
+
+- **Independance**: the rows are independant from each other, making this way
+  of working with data flows good for line-by-line data processing, but
+  also not ideal for "grouped" computations (where an output depends on more
+  than one line of input data). You can overcome this with rolling windows if
+  the input required are adjacent rows, but if you need to work on the whole
+  dataset at once, you should consider other software.
+
+Graphs are defined using :class:`bonobo.Graph` instances, as seen in the
+previous tutorial step.
+
+What can be a node?
+:::::::::::::::::::
+
+**TL;DR**: … anything, as long as it’s callable().
+
+Functions
+---------
+
+.. code-block:: python
+
+    def get_item(id):
+        return id, items.get(id)
+
+
+
+Each node of a graph will be executed in isolation from the other nodes, and the data is passed from one node to the
+next using FIFO queues, managed by the framework. It's transparent to the end-user, though, and you'll only use
+function arguments (for inputs) and return/yield values (for outputs).
+
+Each input row of a node will cause one call to this node's callable. Each output is cast internally as a tuple-like
+data structure (or more precisely, a namedtuple-like data structure), and for one given node, each output row must
+have the same structure.
+
+If you return/yield something which is not a tuple, bonobo will create a tuple of one element.
+
+Properties
+----------
+
+|bonobo| assists you with defining the data-flow of your data engineering process, and then streams data through your
+callable graphs.
+
+* Each node call will process one row of data.
+* Queues that flows the data between node are first-in, first-out (FIFO) standard python :class:`queue.Queue`.
+* Each node will run in parallel
+* Default execution strategy use threading, and each node will run in a separate thread.
+
+Fault tolerance
+---------------
+
+Node execution is fault tolerant.
+
+If an exception is raised from a node call, then this node call will be aborted but bonobo will continue the execution
+with the next row (after outputing the stack trace and incrementing the "err" counter for the node context).
+
+It allows to have ETL jobs that ignore faulty data and try their best to process the valid rows of a dataset.
+
+Some errors are fatal, though.
+
+If you pass a 2 elements tuple to a node that takes 3 args, |bonobo| will raise an :class:`bonobo.errors.UnrecoverableTypeError`, and exit the
+current graph execution as fast as it can (finishing the other node executions that are in progress first, but not
+starting new ones if there are remaining input rows).
+
+
 Definitions
 :::::::::::
 

docs/guide/index.rst

@@ -3,13 +3,8 @@ Guides
 
 This section will guide you through your journey with Bonobo ETL.
 
-.. toctree::
-    :maxdepth: 2
 
-    introduction
-    transformations
-    graphs
-    services
-    environment
-    purity
+.. include:: _toc.rst
+
+