Designing for PaaS -> Next Generation Application Design Principle

12 Factor Principle for Cloud Apps

Code is Version Controlled

• Always tracked in version control system
• 1:1 relationship between codebase and app
• Many deploys of a given app
• Codebase same across deploys, versions may differ

Dependencies are Declared and Isolated

• Never assume system-wide packages
• Dependency declaration manifest
• Isolated so no dependency “leak” from system
• Helps new developers

Configuration is Stored in the Environment

• Should store in environment variables
• Should not be “constants” in code
• Ideally not in configuration files
• Avoid grouping as “environments”

Backing Services as Attached Resources

• Services consumed over the network
• No distinction between local or 3rd party services
• Keep dependencies de-coupled
• Attach and detach at will

Build and Run Stages are Separated

• Impossible to change code at runtime
• Releases should have IDs
• Build may be complex, started by devs
• Run is simple and completely unattended

Application Executed as Stateless Processes

• Share nothing
• Persisted data in statefulbacking store
• Memory and file system is cache only
• Avoid sticky sessions

Services are Exported via Port Binding

• Self contained
• Embedded servers
• Listen on a specific port
• Very specific and idealistic

Application Scaled Out via Process Model

• Processes are first-class citizens
• Work assigned to a process type
• Applications have processes that span servers
• Use OS process managers, not daemons

Processes are Disposable

• Can be started or stopped at any time
• Minimal startup time, graceful shutdown
• Worker processes return work to the queue
• Robust against sudden death

Parity Between Application Environments

• Avoid time/personnel/tool gaps
• Design for continuous deployment
• Very important for backing services
• Containers and configmgmt. makes this easier!

Logs Treated as Event Streams

• Logs are a stream of time-ordered events
• App is never concerned with storing own logfiles
• Execution environment captures and stores logs
• May be routed to file, watched, sent to external service

Management Tasks Run as One-Off Processes

• Run in identical environment
• Separate out as scripts that are source controlled
• Don’t run from local terminal
• Don’t run directly against the database