Howdah - An Application using Pylons, PostgreSQL, Simpycity and Exceptable

Aurynn Shaw

This mini-tutorial covers building a small application on Howdah, an open source, Python based web development framework by Commandprompt, Inc. We will cover the full process of designing a vertically coherent application on Howdah, integrating DB-level stored procedures, DB exception propagation through Exceptable, DB access through Simpycity, authentication through repoze.who, permissions through VerticallyChallenged, and application views through Pylons. By the end of the talk, we will have covered a full application built on The Stack, and how to cover common pitfalls in using Howdah components.

1. HOWDAH
A tutorial
Tuesday, October 20, 2009

2. Why?
Howdah is for people who really “get” databases
Howdah treats your database as code. Procedures,
and hand-written SQL.
Would you automate python generation? Then why
SQL?
Just because it’s not code you’re comfortable with,
doesn’t make it any less code.
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3. What shall we build?
Today, we’ll be talking about designing and
implementing a Wiki.
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4. Why a Wiki?
Canonical example
Simple enough to define in an afternoon
Complex enough to require in-depth exploration
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5. Why a Wiki?
Public and Private permissions/users
Read/write collaborative model
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6. Design vs. Code
Speaking more on Design than on Code
Why is more important than How.
How is still important
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7. STEP 1:
DESIGN
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8. Database
What do we need?
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9. Application
What do we need?
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10. Application
What do we need?
Anything else? Did we miss anything?
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11. STEP 2:
REVISIT
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12. Database
Based on our Application design, what expansions do
we need?
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13. Database
Based on our Application design, what expansions do
we need?
Why do we need them?
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14. Application
With the new DB features, what changes?
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15. Application
With the new DB features, what changes?
What new ideas are evident?
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16. Application
With the new DB features, what changes?
What new ideas are evident?
Do the changes make things easier?
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17. STEP 3:
API CONTRACTS
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18. Database
Defining our API
What stored procedures do we need?
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19. Database
Defining our API
What stored procedures do we need?
What should they do?
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20. Database
Defining our API
What exceptions do we need?
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21. Database
Defining our API
What exceptions do we need?
Null data
Bad data
No such record
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22. Application
Defining our API
What models do we need?
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23. Application
Defining our API
What exceptions do we need?
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24. Application
Defining our API
What exceptions do we need?
What do DB exceptions become?
What HTTP responses should the exceptions
raise?
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25. Application
Defining our API
What views do we need?
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26. Application
Defining our API
What views do we need?
What views are read-only? Read-write? Write-only?
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27. STEP 4:
FIRST EXPANSION
USERS
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28. Database
Users
User system!
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29. Database
Users
User system!
VerticallyChallenged for users
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30. Database
Users
User system!
VerticallyChallenged for users
How to set up VC
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31. Database
Users
Stored Procedures - How do we adapt them?
How does this affect our API contract?
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32. Application
Users
Using @needs to define permissions
How should views be protected?
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33. Application
Users
Using @needs to define permissions
How should views be protected?
Should anonymous users have write permission?
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34. Application
Users
Permissions violations
What should no user return?
What should a bad user return?
What should insufficient permissions return?
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35. Application
Users
Why - Are there better mechanisms?
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36. STEP 5:
ADMINISTRATIVE
USERS
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37. Database
Administrators
What delineates an admin?
What special things can an admin do?
Should admins be otherwise normal users?
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38. Database
Administrators
Root-level permissions:
Should the database superuser ever be allowed to log
in from the web app?
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39. Database
Administrators
Root-level permissions:
Should the database superuser ever be allowed to log
in from the web app?
Why?
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40. Application
Administrators
What delineates an Admin?
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41. Application
Administrators
Design
Are there special admin-only views?
How do we protect admin privileges?
Are there user-specific views? Do admins have
permission to access those?
Is anything changed by the DB layer?
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42. Application
Administrators
Should administrators be able to view everything?
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43. Application
Administrators
Should administrators be able to view everything?
What about privileged information?
HIPAA, lawyer confidentiality
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44. Application
Administrators
Should administrators be able to view everything?
What about privileged information?
HIPAA, lawyer confidentiality
How can we protect privileged information like this?
Can we ever guarantee protection? How?
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45. STEP 6:
A NEW FEATURE
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46. A New Feature
Let’s add a user profile page
Specifically list the pages that a user has edited
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47. Database
A New Feature
Design first!
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48. Database
A New Feature
Design first!
Do we need new stored procedures?
What are they?
Who has access to them?
Does this require write access?
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49. Application
A New Feature
What does the app need to support this?
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50. Application
A New Feature
What does the app need to support this?
What views do we need?
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51. Application
A New Feature
What does the app need to support this?
What views do we need?
Who has access to the views?
Logged-in users only?
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52. Application
A New Feature
What about security and data confidentiality?
What security issues could be present?
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53. Application
A New Feature
What about security and data confidentiality?
What security issues could be present?
Do we list entries that a user may not have read
access to?
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Tuesday, October 20, 2009

54. Application
A New Feature
What about security and data confidentiality?
What security issues could be present?
Do we list entries that a user may not have read
access to?
Should we list nothing, instead?
Why do it like this? Are there better solutions?
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