Twelve-Factor App | Declare and Isolate

The second factor of the Twelve-Factor App methodology highlights the importance of explicitly declaring and isolating these dependencies.

Why Declare and Isolate Dependencies?

Declaring and isolating dependencies ensures that the application is self-contained and does not rely on the implicit existence of system-wide packages.

  • Reproducibility: Ensures that the application runs consistently across different environments.
  • Scalability: Facilitates easy scaling without conflicts between dependencies.
  • Security: Allows for targeted updates and monitoring of specific dependencies.

How to Declare and Isolate Dependencies

Use Dependency Management Tools

Modern programming languages offer package managers to handle dependencies, such as:

  • NPM for JavaScript
  • Maven for Java
  • Pip for Python
Example: Using Pip for Python
pip install -r requirements.txt

Where requirements.txt lists the dependencies:


Containers like Docker ensure that dependencies are isolated and consistent across different environments.

Example: Dockerfile
FROM python:3.8
COPY requirements.txt .
RUN pip install --no-cache-dir -r requirements.txt

Deployment Strategies with Dependencies

In a pipeline, dependencies must be handled at the build stage, ensuring the application has everything it needs before deployment.

  • Build Stage: Install dependencies using a package manager.
  • Test Stage: Run tests to verify that dependencies are functioning correctly.
  • Deployment Stage: Deploy the application along with its dependencies to the target environment.

Managing dependencies explicitly is essential for maintaining a robust, scalable, and secure application.