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Dynamic Code Patterns: Extending Your Applications with Plugins Doug Hellmann – @doughellmann – PyCon 2013 1 Saturday, March 16, 13

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What is a “Plugin”? • Loaded Dynamically • Extends Core • Possibly Unknown Source 2 Saturday, March 16, 13

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Why Plugins? • Better API Abstraction • Reduce Core Dependencies • Strategy Pattern • Visitor Pattern • Indirect Code Contributions 3 Saturday, March 16, 13

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• OpenStack Metering • Measures Clouds • Varied Billing Requirements • Deployers Extend and Customize Ceilometer 4 Saturday, March 16, 13

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Ceilometer Design Compute Images Volumes Network Objects Notification Message Bus Event Listener Compute Pollsters Central Pollsters Collector Meter Data Message Bus 5 Saturday, March 16, 13

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Ceilometer Design Compute Images Volumes Network Objects Notification Message Bus Event Listener Compute Pollsters Central Pollsters Collector Meter Data Message Bus 5 Saturday, March 16, 13

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Ceilometer Design Compute Images Volumes Network Objects Notification Message Bus Event Listener Compute Pollsters Central Pollsters Collector Meter Data Message Bus 5 Saturday, March 16, 13

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Ceilometer Design Compute Images Volumes Network Objects Notification Message Bus Event Listener Compute Pollsters Central Pollsters Collector Meter Data Message Bus 5 Saturday, March 16, 13

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Ceilometer Design Compute Images Volumes Network Objects Notification Message Bus Event Listener Compute Pollsters Central Pollsters Collector Meter Data Message Bus 5 Saturday, March 16, 13

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Ceilometer Design Compute Images Volumes Network Objects Notification Message Bus Event Listener Compute Pollsters Central Pollsters Collector Meter Data Message Bus 5 Saturday, March 16, 13

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Ceilometer Design Compute Images Volumes Network Objects Notification Message Bus Event Listener Compute Pollsters Central Pollsters Collector Meter Data Message Bus 5 Saturday, March 16, 13

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Ceilometer Design Compute Images Volumes Network Objects Notification Message Bus Event Listener Compute Pollsters Central Pollsters Collector Meter Data Message Bus 5 Saturday, March 16, 13

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Ceilometer Design Compute Images Volumes Network Objects Notification Message Bus Event Listener Compute Pollsters Central Pollsters Collector Meter Data Message Bus 5 Saturday, March 16, 13

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Ceilometer Plugins • Message Bus • Receiving Notifications • Polling Compute • Polling Other • Storage 6 Saturday, March 16, 13

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Research • Blogofile • Sphinx • Mercurial • cliff • virtualenvwrapper • Nose • Trac • Django • Pyramid • SQLAlchemy • Diamond • Nova 7 Saturday, March 16, 13

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Discovery Explicit Scan File Import Reference Mercurial Diamond Blogofile Mercurial Django Pyramid Sphinx Nova Trac Nose cliff virtualenvwrapper 8 Saturday, March 16, 13

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Enabling Explicit Implicit Django Pyramid SQLAlchemy Blogofile Mercurial Trac Sphinx virtualenvwrapper cliff 9 Saturday, March 16, 13

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Importing Custom pkg_resources Django Pyramid Sphinx Diamond Nova Nose SQLAlchemy Blogofile Trac Nose SQLAlchemy Blogofile cliff virtualenvwrapper 10 Saturday, March 16, 13

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Integration Fine Coarse Prompt Inspect Nose virtualenvwrapper Sphinx Trac cliff Diamond Mercurial Blogofile Pyramid Django 11 Saturday, March 16, 13

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Integration Fine Coarse Prompt Inspect Nose virtualenvwrapper Sphinx Trac cliff Diamond Mercurial Blogofile Pyramid Django 12 Saturday, March 16, 13

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API Enforcement Convention Base Class / Interface Blogofile Mercurial Django Sphinx Pyramid virtualenvwrapper Nose (optional) Trac (interface) Diamond cliff (abc) 13 Saturday, March 16, 13

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Invocation Driver Dispatcher Iterator SQLAlchemy Nova Mercurial Django Pyramid cliff Nose Diamond virtualenvwrapper 14 Saturday, March 16, 13

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Discovery / Importing • Entry Points • Distribute and pkg_resources • Be Consistent 15 Saturday, March 16, 13

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Enabling • Explicit disabling • Automatic disabling 16 Saturday, March 16, 13

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Integration • Fine • Inspect • Application Owns Relationship 17 Saturday, March 16, 13

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API Enforcement • Abstract Base Classes • Duck Typing 18 Saturday, March 16, 13

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Invocation • Storage – Driver • Notifications – Dispatcher • Pollsters – Iterator 19 Saturday, March 16, 13

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stevedore • Implements Patterns • Wraps pkg_resources 20 Saturday, March 16, 13

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NamedExtensionManager • Multiple Plugins • Only Loads Named Plugins • map() 21 Saturday, March 16, 13

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EnabledExtensionManager • Multiple Plugins • Checks Each with Function on Load • map() 22 Saturday, March 16, 13

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DispatchExtensionManager • Multiple Plugins • Invokes Subset on map() 23 Saturday, March 16, 13

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DriverManager • Single Plugin • Direct Access 24 Saturday, March 16, 13

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Dynamic Code Patterns: Extending Your Applications with Plugins Doug Hellmann – @doughellmann – PyCon 2013 http://packages.python.org/distribute/pkg_resources.html https://github.com/dreamhost/stevedore https://launchpad.net/ceilometer http://doughellmann.com 25 Saturday, March 16, 13