MAXIME LEMAITRE – 18/6/2015

Scenario Driven Design… Yet another Blabla Driven Design …

Agenda

• Inspirations• The dashboard problem• Introducing Scenario Driven design• Examples

– VOD– Twitter– Office for Ipad– “Betclic”

• Conclusion

Inspirations

Scenario Driven API DesignOr: how to make more usable APIsBy Ivo Jansch (APIDays, Paris)

… a long time ago …

Our page is so slow to loadcan you fix the performance problem ?

5

• The design– /orders– /orders/:id– /orders/:id/details– /products– /products/:id– …

The basic design

“A Home page showing the user’s open orders, the order status, one product for each order plus the thumb, and a list of the last 5 sent

orders” … What about performance ?

26 API Calls !!“But our API is so RESTfull !”

What’s the problem ?

What’s the problem ?

What is a REST resource ? Is you raw data model ?

Where is the Business logic ? Where it should be ?

What if you need to release a new client ? (WebApp, Mobile, …)

What’s the problem² ?

• This is not a REST problem– REST is a great way to expose data

• But…– REST is not an excuse to expose your raw data model– REST is not an excuse to stop thinking about business logic

“Smart Data & dumb code works a lot better than the other way around“

The Cathedral & The Bazaar, Eric S. Raymond Smart APIs, dump clients

Note : Batch requests could help, but business logic is still on client side...

Introducing ‘Scenario Driven API Design’

Introducing Scenario Driven Design

1. Client analysis– Identify client needs– Usage scenarios

2. Service design– Optimal ways to service the client

scenarios3. Data modelling

– Create data model to support the scenario’s

– Low level and high level concepts4. Interface design

– REST/HAL etc. interface– Versioning, etc.

Reminders :• Data Driven Design

from Data to generic clients

• Client Driven Design from Specific clients to data

Typical VOD features

Scenario Driven Design Approach

• Identify all relevant scenarios– Order a movie– Watch a movie– See list of my movies– Promote content on the home screen– See what’s available– Like/unlike movies

Interactions are platform-specific, Scenarios usually aren’t.

Promote content on the home page

• All clients should present, on their home screen, after login:– The user’s most recently

rented movies– User specific

recommendations– Movie top 20– Featured movies

Scenario Driven Design

(1) ‘Home’ screen

(4) /promoslots/home

(2) Promote content to the home screen

(3) Movies, collections, banners,

Interface Design

• Exemples– /recommendations• or /users/@me/recommendations

– /promoslots/:screen– /users/@me/movies– /users/@me/movies/liked – /movies

• Consider scenario’s at the highest level– Business Logic entirely API side– Important: find balance between generic and specific

Framework Design

Framework Design Guidelines: Conventions, Idioms, and Patterns for Reusable .NET Librariesby Krzysztof Cwalina & Brad Abrams

” Frameworks often contain a very large set of APIs. This enables advanced scenarios that require power and expressiveness. However, most development revolves around a small set of common scenarios that use a relatively small subset of the full framework. To optimize the overall productivity of the developers using a framework, it is crucial to invest heavily in the design of APIs that are used in the most common scenarios.”

Do & Don’thttp://blogs.msdn.com/b/kcwalina/archive/2005/05/05/scenariodrivendesign.aspx

• Do make sure that the API design specification is the central part of the design of any feature that includes a publicly accessible API.

• Do define top usage scenarios for each major feature area.• Do ensure that the scenarios correspond to appropriate abstraction

level. They should roughly correspond to the end user use cases.• Do design APIs by first writing code samples for the main scenarios

and then defining the object model to support the code samples.• Do not rely solely on standard design methodologies (for example

UML) when designing the public APIs layer of a framework.• Do not start API design with object model diagrams.• Do organize usability studies to test APIs in main scenarios.

Examples

The example of Twitter API

• The Twitter API exposes many methods for tweets, favorites, retweets, followers, search, …

• What is the most common feature of twitter ? => Timelines • The home timeline is central to how most users interact with the Twitter service.

Do no try to reinvent the wheel Dumb clients, Smart APIs

Office for Ipad“designed for iPad” as opposed to “ported to iPad

• Several key mobile scenarios– Packing up for the ride home– Starting from scratch– Last minute scramble– Buttoning things up– Collaborating with coworkers– Taking notes

• “bringing over hundreds of Office commands, toolbars, and dialogs to the iPad would be, simply put, a mistake.”

Conclusion

- Better Usability- Better Performance- Better maintainability- Less duplicates

Questions

References

• http://blogs.msdn.com/b/kcwalina/archive/2005/05/05/scenariodrivendesign.aspx

• https://speakerdeck.com/ijansch/scenario-driven-api-design-apidays-paris• https://speakerdeck.com/ijansch/scenario-driven-api-design• https://github.com/dotnet/corefx/wiki/Framework-Design-Guidelines-Digest • http://www.slideshare.net/clintedmonson/advanced-oop-laws-principles-idioms-p

resentation?qid=63cb2104-5ac7-48e9-b3eb-aebf85cf7459&v=qf1&b=&from_search=1

• http://www.streetinsider.com/Insiders+Blog/Microsoft+%28MSFT%29+Shows+How+Office+for+iPad+was+Designed/9519656.html