Update services.rst

docs/guide/services.rst

@@ -1,21 +1,33 @@
 Services and dependencies (draft implementation)
 ================================================
 
+:Status: Draft implementation
+:Stability: Alpha
+:Last-Modified: 27 apr 2017
+
 Most probably, you'll want to use external systems within your transformations. Those systems may include databases,
 apis (using http, for example), filesystems, etc.
 
-For a start, including those services hardcoded in your transformations can do the job, but you'll pretty soon feel
+Hardocing those services in your code can do the job at first, but you'll pretty soon feel limited, for a few reasons:
-limited, for two main reasons:
 
-* Hardcoded and tightly linked dependencies make your transformation atoms hard to test.
+* Hardcoded and tightly linked dependencies make your transformations hard to test, and hard to reuse.
 * Processing data on your laptop is great, but being able to do it on different systems (or stages), in different
-  environments, is more realistic.
+  environments, is more realistic? You probably want to contigure a different database on a staging environment,
+  preprod environment or production system. Maybe you have silimar systems for different clients and want to select
+  the system at runtime. Etc.
 
 Service injection
 :::::::::::::::::
 
-To solve this problem, we introduce a light dependency injection system that basically allows you to define named
+To solve this problem, we introduce a light dependency injection system. It allows to define named dependencies in
-dependencies in your transformations, and provide an implementation at runtime.
+your transformations, and provide an implementation at runtime.
+
+Class-based transformations
+---------------------------
+
+To define a service dependency in a class-based transformation, use :class:`bonobo.config.Service`, a special
+descriptor (and subclass of :class:`bonobo.config.Option`) that will hold the service names and act as a marker
+for runtime resolution of service instances.
 
 Let's define such a transformation:
 
@@ -24,7 +36,7 @@ Let's define such a transformation:
     from bonobo.config import Configurable, Service
 
     class JoinDatabaseCategories(Configurable):
-        database = Service(default='primary_sql_database')
+        database = Service('primary_sql_database')
 
         def __call__(self, database, row):
             return {
@@ -35,6 +47,14 @@ Let's define such a transformation:
 This piece of code tells bonobo that your transformation expect a sercive called "primary_sql_database", that will be
 injected to your calls under the parameter name "database".
 
+Function-based transformations
+------------------------------
+
+No implementation yet, but expect something similar to CBT API, maybe using a `@Service(...)` decorator.
+
+Execution
+---------
+
 Let's see how to execute it:
 
 .. code-block:: python
@@ -50,6 +70,11 @@ Let's see how to execute it:
         }
     )
     
+A dictionary, or dictionary-like, "services" named argument can be passed to the :func:`bonobo.run` helper. The
+"dictionary-like" part is the real keyword here. Bonobo is not a DIC library, and won't become one. So the implementation
+provided is pretty basic, and feature-less. But you can use much more evolved libraries instead of the provided
+stub, and as long as it works the same (a.k.a implements a dictionary-like interface), the system will use it.
+
 Future
 ::::::
 
@@ -65,10 +90,10 @@ You can expect to see the following features pretty soon:
   database DSN or target filesystem on command line (or with shell environment).
 * Pool based locks that ensure that only one (or n) transformations are using a given service at the same time.
 
-This is under heavy development, let us know what you think (slack may be a good place for this).
+This is under development, let us know what you think (slack may be a good place for this).
 
 
 Read more
 :::::::::
 
-todo: example code.
+* See https://github.com/hartym/bonobo-sqlalchemy/blob/work-in-progress/bonobo_sqlalchemy/writers.py#L19 for example usage (work in progress).