Manage Feature Flags

Migrate and delete old feature flags

Codegen’s GraphSitter library provides powerful APIs for managing feature flag usage in code. This guide will walk you through the basics of how to detect and clean up obsolete feature flags.

Removing rolled out flags

Once a feature has been fully rolled out behind a flag and is stable on production, the next step is to remove old fallback code paths.

Common syntax for feature flags include:

Typsecript

feature_flag = codebase.get_symbol("ROLLED_OUT_FLAG_TO_DELETE")
feature_flag_name = feature_flag.name

# Iterate over all usages of the feature flag import
for usage_symbol in feature_flag.get_all_symbol_usages():
if isinstance(usage_symbol, Function):
    # Check statements within the function
    for statement in usage_symbol.code_block.get_statements():
        if isinstance(statement, IfBlockStatement) and feature_flag_name in statement.condition.source:
            # Simplify the condition of the if statement
            statement.reduce_condition(bool_condition=feature_flag_name == statement.condition.source)
elif isinstance(usage_symbol, Import):
    # Remove the import of the feature flag
    usage_symbol.remove()

Python

flag_name = "CODEMOD_RUN_SYNTAX_PREHIGHLIGHT"

# Get the feature flag variable
feature_flag_file = codebase.get_file("codegen-backend/app/utils/posthog/feature_flags.py")
flag_class = feature_flag_file.get_class("FeatureFlag")

# Check if the flag_var is None before proceeding
flag_var = flag_class.get_attribute(flag_name)
if not flag_var:
raise Exception(f'No such flag: {flag_name}')

# Remove the feature flag from the class
flag_var.remove()

# Remove all usages of the feature flag
for usage in flag_var.usages():
# If the usage is in an if statement, remove the entire if statement
if isinstance(usage.parent, IfBlockStatement):
  usage.parent.remove()
# If the usage is in a with statement, unwrap the with statement
elif isinstance(usage.parent, WithStatement):
  usage.parent.code_block.unwrap()
# Otherwise, just remove the usage
else:
  usage.remove()

Deleting unused feature flags

Detecting unused feature flag is a specific use case of deleting dead code, isolated to a specific search space.

Delete unused flag

print = codebase.log

feature_flag_usage = {}
feature_flag_class = codebase.get_class('FeatureFlag')

if feature_flag_class:
    # Initialize usage count for all attributes
    for attr in feature_flag_class.attributes:
        feature_flag_usage[attr.name] = 0

    # Get all usages of the FeatureFlag class
    for usage in feature_flag_class.usages():
        usage_source = usage.usage_symbol.source if hasattr(usage, 'usage_symbol') else str(usage)
        for flag_name in feature_flag_usage.keys():
            if f"FeatureFlag.{flag_name}" in usage_source:
                feature_flag_usage[flag_name] += 1

    sorted_flags = sorted(feature_flag_usage.items(), key=lambda x: x[1], reverse=True)

    print("Feature Flag Usage Table:")
    print("-------------------------")
    print(f"{'Feature Flag':<30} | {'Usage Count':<12}")
    print("-" * 45)
    for flag, count in sorted_flags:
        print(f"{flag:<30} | {count:<12}")

    print(f"\nTotal feature flags: {len(sorted_flags)}")
else:
    print("❗ FeatureFlag enum not found in the codebase")

Recommended Next Steps

Once rolled out feature flags are removed, new dead code paths may have been created. Run a delete dead code codemod to ensure complete clean-up.

Best Practices

1. Use Caution: Ensure that flags have been fully rolled out before deleting.

2. Fix Tests: Check for additional ways feature flags are used in the codebase (e.g. string arguments in test patches).

Related Skills

• reduce if statements
• unwrap with statements