Exceptable

Exceptable

Tuesday, October 20, 2009

Exception Propagation, in
PostgreSQL and Python
Aurynn Shaw, Commandprompt, Inc.

PostgreSQL Conference West, 2009


Background

• Large, enterprise Python + Pylons application
• Simpycity ORM utilizes signiﬁcant stored
procedures

• Strong procedural interfaces


Stored Procedures are Rare

• Most people don’t use Stored Procedures.


The Present System
Wherein

• A Database Query
• A Stored Procedure
• Or, A Database Query
• Exceptions Galore


A Database Query

• The App queries the Database


A Database Query

• Any variety of query


A Database Query
>>> c.Raw("SELECT * FROM auth_only")()
Traceback (most recent call last):
... <SNIP> ...
psycopg2.ProgrammingError: permission denied for
relation auth_only


A Database Query

• Any variety of query
• Or A Stored Procedure


Or a Stored Procedure
CREATE OR REPLACE FUNCTION except_test() RETURNS
VOID AS $$
BEGIN
RAISE EXCEPTION 'Test!';
END;
$$ LANGUAGE PLPGSQL;
>>> c.Function("except_test")()
Traceback (most recent call last):
... <SNIP> ...
psycopg2.InternalError: Test!


Beget an Exception

• Stored procedures raise InternalErrors
• Every procedural exception becomes this
• Permissions violations raise ProgrammingErrors
• I haven’t tested if this is all PG exceptions.


Seriously?

• This is what we have to work with?


Why this is Important
Wherein

• Why?
• Deﬁning your APIs
• Separation of Concerns
• Procedural Spaghetti


Why?

• Limited information to propagate upwards
• Stored Procedures are Harder to use
• Permissions violations are difﬁcult to detect


API Undeﬁned

• Exceptions are part of your API


API Undeﬁned

• Easily overlooked


API Undeﬁned

• Easily overlooked
• Delineations will make life easier


Separation of Concerns

• The Database is for Database Logic


Separation of Concerns

• The Database is for Database Logic
• Harder to write Data Logic


Spaghetti
Early version of our
app didn’t have
Exceptable - we
were left catching
InternalErrors and
guessing at what
• InternalErrors everywhere the error was, based
on timing.


Spaghetti

• InternalErrors everywhere
• Insufﬁciency of Information


Spaghetti

• InternalErrors everywhere
• Insufﬁciency of Information
• Considerable Repeated Code


A Saving Grace

• Violating Procedure Signatures ->
Python DataError


A Better Deal
Wherein

• Database API
• Easy Python implementation
• Universality
• Exceptable


Database API

• Easier to deﬁne an API


Database API

• The DB becomes part of that API


Database API

• Simple Stored Procedure interface


Database API

• Simple Stored Procedure interface
• Easily declare new Exceptions


Wonderfully Python

• A Simple Decorator


Simply Decorated
from exceptable.exceptable import Except

base = Except()

@base
def db_function():
pass


Wonderfully Python

• A Simple Decorator
• Catches and re-emits Exceptions
• The core of Exceptable
• Easy to Integrate - 2 lines, in Simpycity


Universality

• Exceptable Procedures never change
• DB logic doesn’t change
• Application support is Easy


Exceptable

• More Pythonic Database Access
• Better exceptions means better app ﬂow


Example Code
Wherein

• The DB Library
• The Application Implementation
• Catching Permissions Violations
• PostgreSQL 8.4


To Start,
CREATE TABLE exceptions (
name text primary key,
description text not null,
parent text references exceptions(name)
);
INSERT INTO exceptions VALUES ('Exception',
'Base exception',NULL);
INSERT INTO exceptions VALUES
('NotFoundException', 'Could not find
specified record', 'Exception');


Which leads to
CREATE OR REPLACE FUNCTION not_found (
in_reason TEXT
) RETURNS VOID as $body$

SELECT exceptaple.raise(
'NotFoundException',
$1
);
$body$ LANGUAGE SQL;


Application Level

• Easy to Query the Exception tables


Application Level

• Easy to Query the Exception tables
• Easy to set up a new library
• Python took 50 lines


Our Python Example

• The Exceptable decorator is easy to set up
• Designed for DB-API integration


In the Application
base = Except(InternalError, {
'Exception': Exception,
'NotFoundException': NotFoundError,
})


Our Python Example

• User-deﬁned


User Deﬁnitions
base = Except(InternalError, {
'PermissionError': PermissionError,
'UnknownUser': UnknownUserError,
'NotFoundException': NotFoundError,
})


Our Python Example

• User-deﬁned, and soon, table introspection


base is a decorator
@base
def db_api(query):
con = db.connect(conn_string)
cur = con.cursor()
return cur(query)


Which leads to
try:
rs = db_api(‘select * from test_api()’)
except NotFoundError, e:
# A hah! A usable error!
pass


As Opposed To
try:
rs = db_api(‘select * from test_api()’)
except InternalError, e:
if “NotFoundException” in str(e):
raise NotFoundError(str(e))
elif “PermissionsError” in str(e):
raise PermissionsError(str(e))


Our Python Example

• User-deﬁned, and soon, table introspection
• Existing decorators can be expanded easily


Grow, my Pretties!
class NotNullError(BaseException):
pass
class SCE(BaseException):
pass

base.add({
‘NotNullException’: NotNullError,
‘SufficientCoffeeException’: SCE
})


Permission Denied

• Exceptable.py also wraps the base permission
denied

• DB permissions violations work at the app level


Vertically Challenged
a = c.Function(‘test_auth’);
try:
result = a()
except PermissionDenied, e:
abort(403)
except NoSuchUser, e:
abort(401)


AND THUS
Questions?


Get It!

https://projects.commandprompt.com/
public/exceptable/


THANK YOU!


Exceptable

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