Feature Flag Targeting Guide

This guide covers advanced feature flag patterns with CEL targeting, rollout strategies, and per-instance evaluation.

CEL Expression Language

Kaiten uses CEL (Common Expression Language) for targeting rules. CEL is a safe, fast expression language designed for policy evaluation.

Basic Comparisons

tenantSlug == "sakura-tokyo"
orderCount > 100
region != "eu-west"

Logical Operators

tenantSlug == "sakura-tokyo" && orderCount > 10
region == "us-east" || region == "us-west"
!(tenantSlug == "demo-tenant")

List Membership

region in ["us-east", "us-west", "eu-west"]

String Functions

tenantSlug.startsWith("test-")
tenantSlug.endsWith("-production")
tenantSlug.contains("demo")

Rollout Strategies

Gradual Rollout

Use percentage-based rollout for the default variant to gradually enable a feature:

{
  "defaultVariant": {
    "type": "rolloutPercentage",
    "value": {
      "rolloutPercentage": {
        "distribution": {"on": 10, "off": 90}
      }
    }
  }
}

Increase the percentage over time: 10% → 25% → 50% → 100%.

Canary Deployment

Combine targeting + rollout:

1. Targeting rule: tenantSlug == "internal-test" → on (always on for your test tenant)
2. Default variant: 5% rollout to on (canary for everyone else)

Feature Kill Switch

Set default to off with no targeting rules. Enable for specific tenants as needed.

Per-Instance Evaluation

When evaluating flags, pass the instance tenantSlug as a context variable:

{
  "context": {
    "tenantSlug": "sakura-tokyo",
    "orderCount": 42
  }
}

This scopes the evaluation to that specific tenant, allowing per-customer feature control.

Rule Ordering

Targeting rules are evaluated in order — the first matching rule determines the variant. Order your rules from most specific to least specific:

1. tenantSlug == "vip-customer" → premium
2. region == "us-east" → beta
3. Default → off