The Twelve-Factor App

1. Simon Vocella
The Twelve-Factor App

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The application

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I. Codebase
One codebase per app

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II. Dependencies
Never relies on implicit existence of system-wide
packages

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III. Config
Store config in environment variables

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IV. Backing Services
The code should not make no distinction between local
and third party services.

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XI. Logs
• Logs are the stream of aggregated, time-ordered
events collected from the output streams of all
running process and backing services
• An app never routes or storage its output stream. It
will always write every event to the stdout

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V. Build, release, run
• The build stage is a transform which converts a
code repo into an executable bundle known as a
build.
• The release stage takes the build produced by the
build stage and combines it with the deploy’s
current config.
• The run stage runs the app in the execution
environment, by launching some set of the app’s
processes against a selected release.

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X. Dev/prod parity
App is designed for continuous deployment by keeping
the gap between development and production small

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The process
One codebase per app

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VI. Processes
• Execute the app as one or more stateless
processes
• Shared nothing architecture
• Stateful data must be stored in a stateful backing
service (e.g. database).

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VII. Port Binding
The app is completely self-contained and does not rely
on runtime injection of a webserver into the execution
environment to create a web-facing service.
e.g. the web app exports HTTP as a service by binding
to a port, and listening to requests coming in on that port.

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VIII. Unix Process
Model (ex Concurrency)
Using this model, the developer can architect their app
to handle diverse workloads by assigning each type of
work to a process type.
The app should never daemonize or write PID files.

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IX. Disposability
Maximize robustness with fast startup and graceful shutdown:
All jobs are reetrant
All the operations are idempotent
Process should be robust against crash

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XII. Admin Processes
• Developers will often wish to do one-off
administrative or maintenance tasks for the app,
such as:
• Running database migrations
• Running a console to run arbitrary code
• Running one-time scripts committed into the
app’s repo

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What we achieve:
• Setup automation
• Clean contract
• Suitable for deployment
• Minimize divergence between different
environments
• Can scale up without significant changes

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Bibliography
• The Twelve-Factor App site - http://12factor.net/
• Shared nothing architecture - http://en.wikipedia.org/wiki/
Shared_nothing_architecture
• Sticky session - http://en.wikipedia.org/wiki/Load_balancing_
%28computing%29#Persistence
• Running processes - http://dustin.sallings.org/2010/02/28/running-
processes.html
• Applying the Unix Process Model to Web Apps - http://
adam.herokuapp.com/past/2011/5/9/
applying_the_unix_process_model_to_web_apps/
• Crash-only design - http://lwn.net/Articles/191059/

18. Simon Vocella
Thank you.
Questions?