# HG changeset patch
# User Aurelien Campeas <aurelien.campeas@logilab.fr>
# Date 1270748196 -7200
# Node ID 6d182c7d4392467138608f747b25b4e2cb3f18a7
# Parent  73bdc50d6af1ffb27631b30af8efbf076183eb1a
[doc/book] begin chapter on Hooks/Operations

diff -r 73bdc50d6af1 -r 6d182c7d4392 doc/book/en/development/devrepo/hooks.rst
--- a/doc/book/en/development/devrepo/hooks.rst	Thu Apr 08 18:05:41 2010 +0200
+++ b/doc/book/en/development/devrepo/hooks.rst	Thu Apr 08 19:36:36 2010 +0200
@@ -2,31 +2,103 @@
 
 .. _hooks:
 
-Hooks
-=====
+Hooks and Operations
+====================
+
+Principles
+----------
 
-XXX FILLME
+Paraphrasing the `emacs`_ documentation, let us say that hooks are an
+important mechanism for customizing an application. A hook is
+basically a list of functions to be called on some well-defined
+occasion (This is called `running the hook`).
+
+.. _`emacs`: http://www.gnu.org/software/emacs/manual/html_node/emacs/Hooks.html
+
+In CubicWeb, hooks are classes subclassing the Hook class in
+`server/hook.py`, implementing their own `call` method, and defined
+over pre-defined `events`.
+
+There are two families of events: data events and server events. In a
+typical application, most of the Hooks are defined over data
+events. There can be a lot of them.
 
-*Hooks* are executed before or after updating an entity or a relation in the
-repository.
+The purpose of data hooks is to complement the data model as defined
+in the schema.py, which is static by nature, with dynamic or value
+driven behaviours. It is functionally equivalent to a `database
+trigger`_, except that database triggers definitions languages are not
+standardized, hence not portable (for instance, PL/SQL works with
+Oracle and PostgreSQL but not SqlServer nor Sqlite).
+
+.. _`database trigger`: http://en.wikipedia.org/wiki/Database_trigger
+
+Data hooks can serve the following purposes:
+
+* enforcing constraints that the static schema cannot express
+  (spanning several entities/relations, exotic cardinalities, etc.)
 
-Their prototypes are as follows:
+* implement computed attributes (an example could be the maintenance
+  of a relation representing the transitive closure of another relation)
+
+Operations are Hook-like objects that are created by Hooks and
+scheduled to happen just before (or after) the `commit` event. Hooks
+being fired immediately on data operations, it is sometime necessary
+to delay the actual work down to a time where all other Hooks have run
+and the application state converges towards consistency. Also while
+the order of execution of Hooks is data dependant (and thus hard to
+predict), it is possible to force an order on Operations.
+
+Events
+------
 
-    * after_add_entity     (session, entity)
-    * after_update_entity  (session, entity)
-    * after_delete_entity  (session, eid)
-    * before_add_entity    (session, entity)
-    * before_update_entity (session, entity)
-    * before_delete_entity (session, eid)
+Hooks are mostly defined and used to handle `dataflow`_ operations. It
+means as data gets in (mostly), specific events are issued and the
+Hooks matching these events are called.
+
+.. _`dataflow`: http://en.wikipedia.org/wiki/Dataflow
+
+Below comes a list of the dataflow events related to entities operations:
+
+* before_add_entity
+
+* before_update_entity
+
+* before_delete_entity
+
+* after_add_entity
+
+* after_update_entity
+
+* after_delete_entity
+
+These define ENTTIES HOOKS. RELATIONS HOOKS are defined
+over the following events:
+
+* after_add_relation
 
-    * after_add_relation     (session, fromeid, rtype, toeid)
-    * after_delete_relation  (session, fromeid, rtype, toeid)
-    * before_add_relation    (session, fromeid, rtype, toeid)
-    * before_delete_relation (session, fromeid, rtype, toeid)
+* after_delete_relation
+
+* before_add_relation
+
+* before_delete_relation
+
+This is an occasion to remind us that relations support the add/delete
+operation, but no delete.
+
+Non data events also exist. These are called SYSTEM HOOKS.
+
+* server_startup
 
-    * server_startup
-    * server_shutdown
+* server_shutdown
+
+* server_maintenance
+
+* server_backup
 
-    * session_open
-    * session_close
+* server_restore
+
+* session_open
 
+* session_close
+
+
diff -r 73bdc50d6af1 -r 6d182c7d4392 server/hook.py
--- a/server/hook.py	Thu Apr 08 18:05:41 2010 +0200
+++ b/server/hook.py	Thu Apr 08 19:36:36 2010 +0200
@@ -30,7 +30,6 @@
 
 Session hooks (eg session_open, session_close) have no special attribute.
 
-
 :organization: Logilab
 :copyright: 2001-2010 LOGILAB S.A. (Paris, FRANCE), license is LGPL v2.
 :contact: http://www.logilab.fr/ -- mailto:contact@logilab.fr