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Golang / GoLang System Architecture and Testing Interview Questions

What makes a Go microservice horizontally scalable and what patterns break scaling?

A horizontally scalable service can handle more load by adding replicas — each replica is identical and stateless. Go services are well-suited to horizontal scaling due to small memory footprint, but certain patterns break scalability.

Scalable vs Non-Scalable Patterns
PatternScalable?Problem / Fix
Global in-memory cache (map/slice)NOEach replica has its own cache — inconsistent reads. Fix: use Redis or Memcached
In-memory session storeNOSessions are lost on pod restart. Fix: Redis-backed sessions
Background goroutine per replicaCautionN replicas run N workers — duplicated work. Fix: use a job queue with exactly-one semantics
Stateless request handlerYESEach request fully handled without shared state
Externally stored state (DB, cache)YESState survives restarts; any replica can handle any request
Distributed locks (Redis SETNX)YESEnables exactly-one execution across replicas for cron jobs
Idempotent APIYESClients safely retry on any replica
// ANTI-PATTERN: in-memory state that breaks horizontal scaling
var sessionStore = map[string]Session{} // lost on pod restart!

// CORRECT: external session storage
type SessionStore struct{ redis *redis.Client }

func (s *SessionStore) Get(ctx context.Context, id string) (*Session, error) {
    data, err := s.redis.Get(ctx, "sess:"+id).Bytes()
    if err != nil { return nil, err }
    var sess Session
    return &sess, json.Unmarshal(data, &sess)
}

// Distributed cron: only one replica should run a job
func runIfLeader(ctx context.Context, rdb *redis.Client, jobFn func()) {
    key := "job:lock:daily-report"
    acquired, err := rdb.SetNX(ctx, key, "1", 5*time.Minute).Result()
    if err != nil || !acquired {
        return // another replica has the lock
    }
    defer rdb.Del(ctx, key) // release after job
    jobFn()
}

Why does storing session data in a Go service's in-memory map break horizontal scaling?
What pattern enables exactly-one execution of a cron job across multiple replicas?

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