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Feature-Flags.md

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Feature Flags

Feature flags allows to hide not-yet-ready feature from the user, making it possible to merge not fully completed features to Master and deploy to PROD.

Feature flags align with twelve-factor-app principles (as with any other settings they could be overriden via environment variables on Stage)

Here's the idea:

  1. You start to implement a feature
  2. You implement some very first steps, and according to trunk-driven-development want to merge it to master
  3. You need master to be deployable to PROD, but your feature isn't ready to be shown to the user yet (since you just started)
  4. You create a feature-flag and turn it ON in DEV or Canary environment, and turn it OFF on Prod (via environment variables on docker container)
  5. You add a code to visually hide / disable the feature if feature-flag is off.

Technical details

Backend

For backend you should add a feature-flag as a boolean setting in appsettings.json. You then read it from appsettings as a usual configuration. You could easily override anything specified in appsettings.json using env. variables. E.g. Email__Host=smtp.sendpulse.com.

Frontend

Check out env-variables.ts for a reference implementation (isMiniProfilerEnabled feature flag). Steps to add:

  1. Add variable to .env file
  2. Add variable with type STRING to env.d.ts
  3. Use normal ways to access env. variables (provided by vite), e.g. import.meta.env.REACT_APP_SENTRY_DSN
  4. Env. variables should have a prefix REACT_APP
  5. Set variables when running Docker image on certain stage (e.g. in Configuration section of App Service in Azure Portal)
  6. We use import-meta-env plugin so you could override these variables at runtime as well.
  7. On hosting machine, when Docker image starts run npx @import-meta-env/cli --example .env which will use env. variables to override previously defined values. Steps 6 and 7 are already implemented in a template, so no need to do them for each feature flag.