.. -*- coding: utf-8 -*-

.. _hooks:

Hooks and Operations

====================

Generalities

------------

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 subclasses of the Hook class in

`server/hook.py`, implementing their own `call` method, and selected

over a set of pre-defined `events` (and possibly more conditions,

hooks being selectable AppObjects like views and components).

There are two families of events: data events and server events. In a

typical application, most of the Hooks are defined over data

events.

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 definition 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 value ranges and

  cardinalities, etc.)

* implement computed attributes

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, for instance a validation check which needs that all relations be

already set on an entity. 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.

Operations are subclasses of the Operation class in `server/hook.py`,

implementing `precommit_event` and other standard methods (wholly

described later in this chapter).

Events

------

Hooks are mostly defined and used to handle `dataflow`_ operations. It

means as data gets in (entities added, updated, relations set or

unset), 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_delete_relation

* before_add_relation

* before_delete_relation

This is an occasion to remind us that relations support the add/delete

operation, but no update.

Non data events also exist. These are called SYSTEM HOOKS.

* server_startup

* server_shutdown

* server_maintenance

* server_backup

* server_restore

* session_open

* session_close

Using dataflow Hooks

--------------------

XXX blabla

Validation Errors

~~~~~~~~~~~~~~~~~

When a condition is not met in a Hook/Operation, it must raise a

`ValidationError`. Raising anything but a (subclass of)

ValidationError is a programming error. Raising a ValidationError

entails aborting the current transaction.

The ValidationError exception is used to convey enough information up

to the user interface. Hence its constructor is different from the

default Exception constructor. It accepts, positionally:

* an entity eid,

* a dict whose keys represent attribute (or relation) names and values

  an end-user facing message (hence properly translated) relating the

  problem.

We will use a very simple example to show hooks usage. Let us start

with the following schema.

.. sourcecode:: python

   class Person(EntityType):

       age = Int(required=True)

An entity hook

~~~~~~~~~~~~~~

We would like to add a range constraint over a person's age. Let's

write an hook. It shall be placed into mycube/hooks.py. If this file

were to grow too much, we can easily have a mycube/hooks/... package

containing hooks in various modules.

.. sourcecode:: python

   from cubicweb import ValidationError

   from cubicweb.selectors import implements

   from cubicweb.server.hook import Hook

   class PersonAgeRange(Hook):

        __regid__ = 'person_age_range'

        events = ('before_add_entity', 'before_update_entity')

        __select__ = Hook.__select__ & implements('Person')

        def __call__(self):

            if 0 >= self.entity.age <= 120:

               return

            msg = self._cw._('age must be between 0 and 120')

            raise ValidationError(self.entity.eid, {'age': msg})

Hooks being AppObjects like views, they have a __regid__ and a

__select__ class attribute. The base __select__ is augmented with an

`implements` selector matching the desired entity type. The `events`

tuple is used by the Hook.__select__ base selector to dispatch the

hook on the right events. In an entity hook, it is possible to

dispatch on any entity event (e.g. 'before_add_entity',

'before_update_entity') at once if needed.

Like all appobjects, hooks have the `self._cw` attribute which

represents the current session. In entity hooks, a `self.entity`

attribute is also present.

A relation hook

~~~~~~~~~~~~~~~

Let us add another entity type with a relation to person (in

mycube/schema.py).

.. sourcecode:: python

   class Company(EntityType):

        name = String(required=True)

        boss = SubjectRelation('Person', cardinality='1*')

We would like to constrain the company's bosses to have a minimum

(legal) age. Let's write an hook for this, which will be fired when

the `boss` relation is established.

.. sourcecode:: python

   class CompanyBossLegalAge(Hook):

        __regid__ = 'company_boss_legal_age'

        events = ('before_add_relation',)

        __select__ = Hook.__select__ & match_rtype('boss')

        def __call__(self):

            boss = self._cw.entity_from_eid(self.eidto)

            if boss.age < 18:

                msg = self._cw._('the minimum age for a boss is 18')

                raise ValidationError(self.eidfrom, {'boss': msg})

We use the `match_rtype` selector to select the proper relation type.

The essential difference with respect to an entity hook is that there

is no self.entity, but `self.eidfrom` and `self.eidto` hook attributes

which represent the subject and object eid of the relation.

Using Operations

----------------

Let's augment our example with a new `subsidiary_of` relation on Company.

.. sourcecode:: python

   class Company(EntityType):

        name = String(required=True)

        boss = SubjectRelation('Person', cardinality='1*')

        subsidiary_of = SubjectRelation('Company', cardinality='*?')

Base example

~~~~~~~~~~~~

We would like to check that there is no cycle by the `subsidiary_of`

relation. This is best achieved in an Operation since all relations

are likely to be set at commit time.

.. sourcecode:: python

    def check_cycle(self, session, eid, rtype, role='subject'):

        parents = set([eid])

        parent = session.entity_from_eid(eid)

        while parent.related(rtype, role):

            parent = parent.related(rtype, role)[0]

            if parent.eid in parents:

                msg = session._('detected %s cycle' % rtype)

                raise ValidationError(eid, {rtype: msg})

            parents.add(parent.eid)

    class CheckSubsidiaryCycleOp(Operation):

        def precommit_event(self):

            check_cycle(self.session, self.eidto, 'subsidiary_of')

    class CheckSubsidiaryCycleHook(Hook):

        __regid__ = 'check_no_subsidiary_cycle'

        events = ('after_add_relation',)

        __select__ = Hook.__select__ & match_rtype('subsidiary_of')

        def __call__(self):

            CheckSubsidiaryCycleOp(self._cw, eidto=self.eidto)

The operation is instantiated in the Hook.__call__ method.

An operation always takes a session object as first argument

(accessible as `.session` from the operation instance), and optionally

all keyword arguments needed by the operation. These keyword arguments

will be accessible as attributes from the operation instance.

Like in Hooks, ValidationError can be raised in Operations. Other

exceptions are programming errors.

Notice how our hook will instantiate an operation each time the Hook

is called, i.e. each time the `subsidiary_of` relation is set.

Using set_operation

~~~~~~~~~~~~~~~~~~~

There is an alternative method to schedule an Operation from a Hook,

using the `set_operation` function.

.. sourcecode:: python

   class CheckSubsidiaryCycleHook(Hook):

       __regid__ = 'check_no_subsidiary_cycle'

       events = ('after_add_relation',)

       __select__ = Hook.__select__ & match_rtype('subsidiary_of')

       def __call__(self):

           set_operation(self._cw, 'subsidiary_cycle_detection', self.eidto,

                         CheckCycleOp, rtype=self.rtype)

   class CheckSubsidiaryCycleOp(Operation):

       def precommit_event(self):

           for eid in self._cw.transaction_data['subsidiary_cycle_detection']:

               check_cycle(self.session, eid, self.rtype)

Here, we call set_operation with a session object, a specially forged

key, a value that is the actual payload of an individual operation (in

our case, the object of the subsidiary_of relation) , the class of the

Operation, and more optional parameters to give to the operation (here

the rtype which do not vary accross operations).

The body of the operation must then iterate over the values that have

been mapped in the transaction_data dictionary to the forged key.

This mechanism is especially useful on two occasions (not shown in our

example):

* massive data import (reduced memory consumption within a large

  transaction)

* when several hooks need to instantiate the same operation (e.g. an

  entity and a relation hook).

Operation: a small API overview

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. autoclass:: cubicweb.server.hook.Operation

.. autoclass:: cubicweb.server.hook.LateOperation

.. autofunction:: cubicweb.server.hook.set_operation

Hooks writing rules

-------------------

Remainder

~~~~~~~~~

Never, ever use the `entity.foo = 42` notation to update an entity. It

will not work.

How to choose between a before and an after event ?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Before hooks give you access to the old attribute (or relation)

values. By definition the database is not yet updated in a before

hook.

To access old and new values in an before_update_entity hook, one can

use the `server.hook.entity_oldnewvalue` function which returns a

tuple of the old and new values. This function takes an entity and an

attribute name as parameters.

In a 'before_add|update_entity' hook the self.entity contains the new

values. One is allowed to further modify them before database

operations, using the dictionary notation.

.. sourcecode:: python

   self.entity['age'] = 42

This is because using self.entity.set_attributes(age=42) will

immediately update the database (which does not make sense in a

pre-database hook), and will trigger any existing

before_add|update_entity hook, thus leading to infinite hook loops or

such awkward situations.

Beyond these specific cases, updating an entity attribute or relation

must *always* be done using `set_attributes` and `set_relations`

methods.

(Of course, ValidationError will always abort the current transaction,

whetever the event).

Peculiarities of inlined relations

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some relations are defined in the schema as `inlined` (see

:ref:`RelationType` for details). In this case, they are inserted in

the database at the same time as entity attributes.

Hence in the case of before_add_relation, such relations already exist

in the database.

Edited attributes

~~~~~~~~~~~~~~~~~

On udpates, it is possible to ask the `entity.edited_attributes`

variable whether one attribute has been updated.

.. sourcecode:: python

  if 'age' not in entity.edited_attribute:

      return

Deleted in transaction

~~~~~~~~~~~~~~~~~~~~~~

The session object has a deleted_in_transaction method, which can help

writing deletion Hooks.

.. sourcecode:: python

   if self._cw.deleted_in_transaction(self.eidto):

      return

Given this predicate, we can avoid scheduling an operation.

Disabling hooks

~~~~~~~~~~~~~~~

It is sometimes convenient to disable some hooks. For instance to

avoid infinite Hook loops. One uses the `hooks_control` context

manager.

This can be controlled more finely through the `category` Hook class

attribute.

.. sourcecode:: python

   with hooks_control(self.session, self.session.HOOKS_ALLOW_ALL, <category>):

       # ... do stuff

.. autoclass:: cubicweb.server.session.hooks_control