Feature Flags That Cost Nothing at Scale

# Kill switch check in deployment script
FLAG=$(dig TXT get.payments-v2.flags.acme.v1.resolvedb.net +short 2>/dev/null)
if echo "$FLAG" | grep -q '"enabled":false'; then
  echo "payments-v2 is disabled, skipping deployment"
  exit 0
fi

# Maintenance mode in cron job
if dig TXT get.maintenance.flags.acme.v1.resolvedb.net +short | grep -q '"enabled":true'; then
  echo "Maintenance mode - skipping batch job"
  exit 0
fi

# Simple flag check (any language with DNS works)
dig TXT get.dark-mode.flags.myapp.v1.resolvedb.net +short
# "v=rdb1;s=ok;f=json;ttl=60;d={\"enabled\":true}"

import dns.resolver
import json

def get_flag(flag_name: str, namespace: str = "myapp") -> dict:
    """Get feature flag - no SDK required."""
    fqdn = f"get.{flag_name}.flags.{namespace}.v1.resolvedb.net"
    answers = dns.resolver.resolve(fqdn, 'TXT')
    response = str(answers[0]).strip('"')
    parts = dict(p.split('=', 1) for p in response.split(';') if '=' in p)
    return json.loads(parts.get('d', '{}'))

# Kill switch check
if not get_flag("payments-v2")["enabled"]:
    print("Feature disabled")
    sys.exit(0)

# With SDK for user targeting (optional)
from resolvedb import ResolveDB
db = ResolveDB(namespace="myapp", secret="your-secret")

# CTP auto-encrypts user context
flags = db.flags.get_for_user("new-checkout", user={
    "id": "user-123", "premium": True, "region": "na"
})
print(f"New checkout: {flags['enabled']}")

import dns from 'dns/promises';

// No SDK required - just DNS
async function getFlag(flagName: string): Promise<{enabled: boolean}> {
  const fqdn = `get.${flagName}.flags.myapp.v1.resolvedb.net`;
  const records = await dns.resolveTxt(fqdn);
  const response = records.flat().join('');
  const match = response.match(/d=(.+?)(?:;|$)/);
  return match ? JSON.parse(match[1]) : { enabled: false };
}

// Edge function (Cloudflare Workers)
async function handleRequest(request: Request) {
  // DoH - works in edge runtime
  const res = await fetch(
    'https://dns.google/resolve?name=get.maintenance.flags.myapp.v1.resolvedb.net&type=TXT'
  );
  const data = await res.json();
  if (data.Answer?.[0]?.data?.includes('"enabled":true')) {
    return new Response('Maintenance mode', { status: 503 });
  }
  // Continue with request...
}

package main

import (
    "encoding/json"
    "net"
    "strings"
)

func getFlag(flagName, namespace string) (map[string]any, error) {
    fqdn := fmt.Sprintf("get.%s.flags.%s.v1.resolvedb.net", flagName, namespace)
    records, err := net.LookupTXT(fqdn)
    if err != nil { return nil, err }

    response := strings.Join(records, "")
    for _, seg := range strings.Split(response, ";") {
        if strings.HasPrefix(seg, "d=") {
            var result map[string]any
            json.Unmarshal([]byte(seg[2:]), &result)
            return result, nil
        }
    }
    return nil, nil
}

// Kill switch check
flag, _ := getFlag("payments-v2", "myapp")
if enabled, ok := flag["enabled"].(bool); !ok || !enabled {
    log.Fatal("Feature disabled")
}