How do you implement feature flags and canary deployments in a Go microservice?

Feature flags decouple deployment from release — code is deployed to all servers but activated for a subset of users or traffic. Canary deployments route a small percentage of traffic to a new version, monitoring for errors before full rollout.

// Feature flag implementation
type FeatureFlags struct {
    mu    sync.RWMutex
    flags map[string]FlagConfig
}

type FlagConfig struct {
    Enabled    bool
    Percentage int    // 0-100: % of users who see the feature
    Allowlist  []string // specific user IDs
}

func (f *FeatureFlags) IsEnabled(flagName, userID string) bool {
    f.mu.RLock()
    cfg, ok := f.flags[flagName]
    f.mu.RUnlock()
    if !ok || !cfg.Enabled { return false }

    // Always-on for allowlisted users (internal testing)
    for _, id := range cfg.Allowlist {
        if id == userID { return true }
    }

    // Percentage rollout: deterministic based on user ID
    // Same user always gets same experience
    h := fnv32(userID) % 100
    return int(h) < cfg.Percentage
}

// In handler
func userHandler(w http.ResponseWriter, r *http.Request) {
    claims, _ := claimsFromCtx(r.Context())
    if flags.IsEnabled("new-profile-ui", claims.UserID) {
        renderNewProfile(w, r)
        return
    }
    renderLegacyProfile(w, r)
}

// Kubernetes canary via Argo Rollouts
// spec.strategy.canary:
//   steps:
//   - setWeight: 10    # 10% traffic to new version
//   - pause: {duration: 5m}
//   - analysis:        # check error rate
//       templates: [{templateName: error-rate-analysis}]
//   - setWeight: 50    # 50% if analysis passed
//   - pause: {duration: 10m}
//   - setWeight: 100   # full rollout

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