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Feature Flags

EmailEngine supports feature flags that allow you to enable or disable specific features at startup. Feature flags are controlled via environment variables and are primarily used for experimental or optional functionality.

How Feature Flags Work​

Feature flags are set using environment variables with the prefix EENGINE_FEATURE_:

EENGINE_FEATURE_<NAME>=true

Enabling a feature:

Use one of the conventional values: y, yes, 1, true

EENGINE_FEATURE_EXAMPLE=true
EENGINE_FEATURE_EXAMPLE=1
EENGINE_FEATURE_EXAMPLE=yes

Note that the value check is loose - any value matching the regular expression /^y|1|t/i enables the flag. This means a value starting with "y", or containing "1" or "t" anywhere, counts as enabled (for example, EENGINE_FEATURE_EXAMPLE=later would enable the flag because it contains a "t"). Stick to the conventional values above to avoid surprises.

Disabling a feature:

Remove the environment variable entirely. This is the only safe way to disable a feature flag.

Do not set a "disable" value

Setting a feature flag environment variable to a non-enabling value such as false, 0, or no prevents EmailEngine from starting in current versions - the startup code throws an error when it encounters such a value. Always unset the variable instead.

Name Formatting​

Feature names are normalized internally -- hyphens, underscores, and spaces are treated as underscores:

# These all refer to the same feature:
EENGINE_FEATURE_MY_FEATURE=true
EENGINE_FEATURE_MY-FEATURE=true

Startup Logging​

When feature flags are enabled, EmailEngine logs them at startup:

{"msg":"Enabled feature flags","featureFlags":["my_feature"]}

Configuration Examples​

.env file:

EENGINE_FEATURE_NEW_UI=true

Docker Compose:

environment:
  - EENGINE_FEATURE_NEW_UI=true

Command line:

EENGINE_FEATURE_NEW_UI=true emailengine

Usage Notes​

Feature flags are used internally to gate experimental functionality. The available flags may change between EmailEngine versions. When a feature flag is not recognized by the current version, it is silently ignored.

Feature flags require an application restart to take effect -- they cannot be changed at runtime.

See Also​