Add Autogate

Adding an Autogate

Autogates enable gradual rollout of risky code changes independent of binary releases. Unlike compatibility flags (which are permanent, date-based behavioral changes), autogates are temporary gates that can be toggled on/off via internal tooling during rollout, then removed once the change is stable.

When to use an autogate vs a compat flag

| Use an autogate when... | Use a compat flag when... |

| --------------------------------------------- | ------------------------------------------ |

| Rolling out a risky internal change gradually | Changing user-visible behavior permanently |

| You need a kill switch during rollout | The change is tied to a compatibility date |

| The gate will be removed once stable | Users need to opt in or out explicitly |

Autogates and compat flags are separate mechanisms — an autogate does not become a compat flag.

Step 1: Add the enum value

Edit src/workerd/util/autogate.h. Add a new entry to the AutogateKey enum before NumOfKeys:

enum class AutogateKey {
  TEST_WORKERD,
  // ... existing gates ...
  // Brief description of what this gate controls.
  MY_NEW_FEATURE,
  NumOfKeys  // Reserved for iteration.
};

Naming convention: SCREAMING_SNAKE_CASE for the enum value.

Step 2: Add the string mapping

Edit src/workerd/util/autogate.c++. Add a case to the KJ_STRINGIFY switch before the NumOfKeys case:

kj::StringPtr KJ_STRINGIFY(AutogateKey key) {
  switch (key) {
    // ... existing cases ...
    case AutogateKey::MY_NEW_FEATURE:
      return "my-new-feature"_kj;
    case AutogateKey::NumOfKeys:
      KJ_FAIL_ASSERT("NumOfKeys should not be used in getName");
  }
}

Naming convention: kebab-case for the string name. This string is what appears in runtime configuration (prefixed with workerd-autogate-). The enum name and string name should match to avoid confusion.

Step 3: Guard your code

Use Autogate::isEnabled() to conditionally execute the new code path:

#include <workerd/util/autogate.h>

// At the point where behavior should change:
if (util::Autogate::isEnabled(util::AutogateKey::MY_NEW_FEATURE)) {
  // New code path
} else {
  // Old code path (keep until gate is removed)
}

Step 4: Test

Three ways to test autogated code:

A. The @all-autogates test variant (automatic):

Every wd_test() and kj_test() generates a @all-autogates variant that enables all gates. If your feature is tested by existing tests, they'll automatically run with the gate enabled:

just stream-test //src/workerd/api/tests:my-test@all-autogates

B. Targeted C++ test setup:

In a C++ test file, enable specific gates:

#include <workerd/util/autogate.h>

// In test setup:
util::Autogate::initAutogateNamesForTest({"my-new-feature"_kj});

// In test teardown:
util::Autogate::deinitAutogate();

C. Environment variable:

Set WORKERD_ALL_AUTOGATES=1 to enable all gates when no explicit config is provided.

Step 5: Build and verify

just build
just stream-test //path/to:my-test@               # Old behavior (gate off)
just stream-test //path/to:my-test@all-autogates   # New behavior (gate on)

Step 6: Remove the gate (after rollout)

Once the human user explicitly confirms that the feature is stable and fully rolled out:

• Remove the AutogateKey enum value from autogate.h
• Remove the case from KJ_STRINGIFY in autogate.c++
• Remove all Autogate::isEnabled() checks, keeping only the new code path

Checklist

• [ ] Enum value added to AutogateKey in autogate.h (before NumOfKeys)
• [ ] Comment describes what the gate controls
• [ ] String mapping added to KJ_STRINGIFY in autogate.c++
• [ ] Code guarded with Autogate::isEnabled()
• [ ] Old code path preserved (for rollback)
• [ ] @all-autogates test variant passes
• [ ] Tests cover both gated and ungated paths

Files touched

| File | What to do |

| ------------------------------- | --------------------------------------- |

| src/workerd/util/autogate.h | Add enum value with comment |

| src/workerd/util/autogate.c++ | Add case to KJ_STRINGIFY |

| Your feature file(s) | Guard code with Autogate::isEnabled() |