Deep Dive: How to Pass the Google Staff Engineer Bar in 2026 with System Design and Coding Interviews

package main

import (
    "context"
    "errors"
    "fmt"
    "net/http"
    "time"

    "github.com/go-redis/redis/v8" // https://github.com/go-redis/redis
    "github.com/prometheus/client_golang/prometheus" // https://github.com/prometheus/client_golang
    "github.com/prometheus/client_golang/prometheus/promhttp"
)

var (
    // rateLimitDecision is a Prometheus counter for rate limit decisions
    rateLimitDecision = prometheus.NewCounterVec(
        prometheus.CounterOpts{
            Name: "rate_limiter_decisions_total",
            Help: "Total number of rate limit decisions (allowed/denied)",
        },
        []string{"decision"},
    )
    ctx = context.Background()
)

func init() {
    prometheus.MustRegister(rateLimitDecision)
}

// TokenBucketRateLimiter implements a distributed token bucket rate limiter using Redis
type TokenBucketRateLimiter struct {
    redisClient *redis.Client
    capacity    int64       // Maximum number of tokens in the bucket
    refillRate  int64       // Tokens refilled per second
    keyPrefix   string      // Prefix for Redis keys to avoid collisions
}

// NewTokenBucketRateLimiter initializes a new rate limiter with validation
func NewTokenBucketRateLimiter(redisAddr string, capacity int64, refillRate int64) (*TokenBucketRateLimiter, error) {
    if capacity <= 0 {
        return nil, errors.New("capacity must be positive")
    }
    if refillRate <= 0 {
        return nil, errors.New("refill rate must be positive")
    }

    client := redis.NewClient(&redis.Options{
        Addr:     redisAddr,
        Password: "", // Set via environment variable in production
        DB:       0,
    })

    // Verify Redis connection during initialization
    if err := client.Ping(ctx).Err(); err != nil {
        return nil, fmt.Errorf("failed to connect to Redis: %w", err)
    }

    return &TokenBucketRateLimiter{
        redisClient: client,
        capacity:    capacity,
        refillRate:  refillRate,
        keyPrefix:   "rate_limiter:",
    }, nil
}

// Allow checks if a request from the given identifier is allowed under the rate limit
func (rl *TokenBucketRateLimiter) Allow(identifier string) (bool, error) {
    key := rl.keyPrefix + identifier
    now := time.Now().UnixNano()

    // Lua script to atomically update token bucket state in Redis
    luaScript := `
        local key = KEYS[1]
        local capacity = tonumber(ARGV[1])
        local refillRate = tonumber(ARGV[2])
        local now = tonumber(ARGV[3])
        local lastRefillTime = tonumber(redis.call('HGET', key, 'last_refill_time') or now)
        local tokens = tonumber(redis.call('HGET', key, 'tokens') or capacity)

        -- Calculate tokens to add since last refill
        local elapsedSeconds = (now - lastRefillTime) / 1e9
        local tokensToAdd = math.floor(elapsedSeconds * refillRate)
        tokens = math.min(capacity, tokens + tokensToAdd)

        if tokens >= 1 then
            tokens = tokens - 1
            redis.call('HSET', key, 'tokens', tokens, 'last_refill_time', now)
            redis.call('EXPIRE', key, 3600) -- Expire keys after 1 hour of inactivity
            return 1
        else
            redis.call('HSET', key, 'tokens', tokens, 'last_refill_time', now)
            redis.call('EXPIRE', key, 3600)
            return 0
        end
    `

    // Execute Lua script atomically to avoid race conditions
    result, err := rl.redisClient.Eval(ctx, luaScript, []string{key}, rl.capacity, rl.refillRate, now).Result()
    if err != nil {
        return false, fmt.Errorf("failed to execute rate limit script: %w", err)
    }

    allowed := result == int64(1)
    if allowed {
        rateLimitDecision.WithLabelValues("allowed").Inc()
    } else {
        rateLimitDecision.WithLabelValues("denied").Inc()
    }

    return allowed, nil
}

// Example HTTP handler using the rate limiter
func rateLimitedHandler(limiter *TokenBucketRateLimiter) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Use X-Forwarded-For or API key as identifier; simplified here for demo
        identifier := r.Header.Get("X-API-Key")
        if identifier == "" {
            identifier = r.RemoteAddr
        }

        allowed, err := limiter.Allow(identifier)
        if err != nil {
            http.Error(w, "Internal server error", http.StatusInternalServerError)
            return
        }

        if !allowed {
            http.Error(w, "Rate limit exceeded", http.StatusTooManyRequests)
            return
        }

        fmt.Fprintln(w, "Request allowed")
    }
}

func main() {
    limiter, err := NewTokenBucketRateLimiter("localhost:6379", 100, 10) // 100 tokens capacity, 10 tokens/sec refill
    if err != nil {
        panic(fmt.Sprintf("Failed to initialize rate limiter: %v", err))
    }

    http.HandleFunc("/api/resource", rateLimitedHandler(limiter))
    http.Handle("/metrics", promhttp.Handler())

    fmt.Println("Server starting on :8080")
    if err := http.ListenAndServe(":8080", nil); err != nil {
        panic(fmt.Sprintf("Server failed: %v", err))
    }
}

package main

import (
    "context"
    "errors"
    "fmt"
    "hash/fnv"
    "log"
    "sync"
    "time"

    "github.com/gorilla/mux" // https://github.com/gorilla/mux
    "net/http"
)

// CacheNode represents a single node in the distributed cache cluster
type CacheNode struct {
    ID      string
    Address string
    Weight  int // Relative weight for load balancing
}

// ConsistentHashDistributedCache implements a distributed cache with consistent hashing
type ConsistentHashDistributedCache struct {
    nodes           []*CacheNode
    hashRing        map[uint32]*CacheNode
    sortedHashes    []uint32
    mu              sync.RWMutex
    virtualReplicas int // Number of virtual replicas per physical node
}

// NewConsistentHashDistributedCache initializes a new distributed cache
func NewConsistentHashDistributedCache(virtualReplicas int) (*ConsistentHashDistributedCache, error) {
    if virtualReplicas <= 0 {
        return nil, errors.New("virtual replicas must be positive")
    }

    return &ConsistentHashDistributedCache{
        nodes:           make([]*CacheNode, 0),
        hashRing:        make(map[uint32]*CacheNode),
        sortedHashes:    make([]uint32, 0),
        virtualReplicas: virtualReplicas,
    }, nil
}

// hash generates a 32-bit hash for a given key using FNV-1a
func hash(key string) uint32 {
    h := fnv.New32a()
    h.Write([]byte(key))
    return h.Sum32()
}

// AddNode adds a new node to the distributed cache cluster
func (dc *ConsistentHashDistributedCache) AddNode(node *CacheNode) {
    dc.mu.Lock()
    defer dc.mu.Unlock()

    // Add virtual replicas for the node to improve distribution
    for i := 0; i < dc.virtualReplicas; i++ {
        virtualKey := fmt.Sprintf("%s#%d", node.ID, i)
        virtualHash := hash(virtualKey)
        dc.hashRing[virtualHash] = node
        dc.sortedHashes = append(dc.sortedHashes, virtualHash)
    }

    // Sort hashes to enable binary search for key lookup
    for i := 0; i < len(dc.sortedHashes); i++ {
        for j := i + 1; j < len(dc.sortedHashes); j++ {
            if dc.sortedHashes[i] > dc.sortedHashes[j] {
                dc.sortedHashes[i], dc.sortedHashes[j] = dc.sortedHashes[j], dc.sortedHashes[i]
            }
        }
    }

    dc.nodes = append(dc.nodes, node)
    log.Printf("Added node %s (address: %s) with %d virtual replicas", node.ID, node.Address, dc.virtualReplicas)
}

// GetNode returns the cache node responsible for storing the given key
func (dc *ConsistentHashDistributedCache) GetNode(key string) (*CacheNode, error) {
    dc.mu.RLock()
    defer dc.mu.RUnlock()

    if len(dc.sortedHashes) == 0 {
        return nil, errors.New("no nodes available in cluster")
    }

    keyHash := hash(key)
    // Binary search for the first hash >= keyHash
    idx := -1
    for i, h := range dc.sortedHashes {
        if h >= keyHash {
            idx = i
            break
        }
    }

    // Wrap around to first node if keyHash is larger than all hashes
    if idx == -1 {
        idx = 0
    }

    nodeHash := dc.sortedHashes[idx]
    return dc.hashRing[nodeHash], nil
}

// Example HTTP handler to get cache node for a key
func getCacheNodeHandler(cache *ConsistentHashDistributedCache) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        key := r.URL.Query().Get("key")
        if key == "" {
            http.Error(w, "key query parameter is required", http.StatusBadRequest)
            return
        }

        node, err := cache.GetNode(key)
        if err != nil {
            http.Error(w, fmt.Sprintf("Failed to get cache node: %v", err), http.StatusInternalServerError)
            return
        }

        fmt.Fprintf(w, "Key %s maps to node %s (address: %s)", key, node.ID, node.Address)
    }
}

func main() {
    cache, err := NewConsistentHashDistributedCache(3) // 3 virtual replicas per node
    if err != nil {
        log.Fatalf("Failed to initialize cache: %v", err)
    }

    // Add sample nodes (in production, these would be discovered via service registry)
    cache.AddNode(&CacheNode{ID: "node-1", Address: "10.0.0.1:11211", Weight: 1})
    cache.AddNode(&CacheNode{ID: "node-2", Address: "10.0.0.2:11211", Weight: 1})
    cache.AddNode(&CacheNode{ID: "node-3", Address: "10.0.0.3:11211", Weight: 2}) // Higher weight for more capacity

    r := mux.NewRouter()
    r.HandleFunc("/cache/node", getCacheNodeHandler(cache))

    log.Println("Cache service starting on :8081")
    if err := http.ListenAndServe(":8081", r); err != nil {
        log.Fatalf("Server failed: %v", err)
    }
}

package main

import (
    "context"
    "errors"
    "fmt"
    "log"
    "sync"
    "time"

    "github.com/samuel/go-zookeeper/zk" // https://github.com/samuel/go-zookeeper
)

const (
    zkServers          = "localhost:2181"
    leaderElectionPath = "/google-staff-leader-election"
    electionTimeout    = 10 * time.Second
)

// LeaderElectionService manages leader election using ZooKeeper
type LeaderElectionService struct {
    zkClient    *zk.Conn
    nodePath    string
    isLeader    bool
    leaderID    string
    mu          sync.RWMutex
    onElected   func() // Callback when this node is elected leader
    onDemoted   func() // Callback when this node is demoted from leader
}

// NewLeaderElectionService initializes a new leader election service
func NewLeaderElectionService(zkAddr string, nodeID string, onElected, onDemoted func()) (*LeaderElectionService, error) {
    if nodeID == "" {
        return nil, errors.New("node ID cannot be empty")
    }

    // Connect to ZooKeeper with timeout
    ctx, cancel := context.WithTimeout(context.Background(), electionTimeout)
    defer cancel()

    conn, _, err := zk.Connect([]string{zkAddr}, electionTimeout)
    if err != nil {
        return nil, fmt.Errorf("failed to connect to ZooKeeper: %w", err)
    }

    // Verify connection
    _, _, _, err = conn.ExistsW(ctx, leaderElectionPath)
    if err != nil {
        conn.Close()
        return nil, fmt.Errorf("failed to verify ZooKeeper path: %w", err)
    }

    // Create election path if it doesn't exist
    _, err = conn.Create(leaderElectionPath, []byte{}, 0, zk.WorldACL(zk.PermAll))
    if err != nil && err != zk.ErrNodeExists {
        conn.Close()
        return nil, fmt.Errorf("failed to create election path: %w", err)
    }

    return &LeaderElectionService{
        zkClient:  conn,
        nodePath:  fmt.Sprintf("%s/%s", leaderElectionPath, nodeID),
        isLeader:  false,
        leaderID:  "",
        onElected: onElected,
        onDemoted: onDemoted,
    }, nil
}

// Run starts the leader election loop
func (les *LeaderElectionService) Run(ctx context.Context) error {
    defer les.zkClient.Close()

    // Create ephemeral sequential node for this candidate
    path, err := les.zkClient.Create(les.nodePath, []byte(les.nodePath), zk.FlagEphemeral|zk.FlagSequence, zk.WorldACL(zk.PermAll))
    if err != nil {
        return fmt.Errorf("failed to create candidate node: %w", err)
    }
    log.Printf("Created candidate node: %s", path)

    // Watch for changes to the election path to recheck leadership
    watchChan := make(chan zk.Event)
    _, _, watch, err := les.zkClient.ChildrenW(leaderElectionPath)
    if err != nil {
        return fmt.Errorf("failed to set watch on election path: %w", err)
    }

    for {
        select {
        case <-ctx.Done():
            log.Println("Leader election service stopped")
            return nil
        case event := <-watch:
            if event.Type == zk.EventNodeChildrenChanged {
                les.checkLeadership()
                // Reset watch
                _, _, newWatch, err := les.zkClient.ChildrenW(leaderElectionPath)
                if err != nil {
                    return fmt.Errorf("failed to reset watch: %w", err)
                }
                watch = newWatch
            }
        }
    }
}

// checkLeadership determines if this node is the leader
func (les *LeaderElectionService) checkLeadership() {
    les.mu.Lock()
    defer les.mu.Unlock()

    // Get all children of the election path, sorted by sequence number
    children, _, err := les.zkClient.Children(leaderElectionPath)
    if err != nil {
        log.Printf("Failed to get election children: %v", err)
        return
    }

    if len(children) == 0 {
        log.Println("No candidates in election")
        return
    }

    leaderNode := children[0] // First node in sorted list is leader
    currentNode := les.nodePath[len(leaderElectionPath)+1:] // Extract node ID from path

    if leaderNode == currentNode {
        if !les.isLeader {
            les.isLeader = true
            les.leaderID = currentNode
            log.Printf("Node %s elected leader", currentNode)
            if les.onElected != nil {
                les.onElected()
            }
        }
    } else {
        if les.isLeader {
            les.isLeader = false
            log.Printf("Node %s demoted from leader; new leader is %s", currentNode, leaderNode)
            if les.onDemoted != nil {
                les.onDemoted()
            }
        }
        les.leaderID = leaderNode
    }
}

// IsLeader returns true if this node is the current leader
func (les *LeaderElectionService) IsLeader() bool {
    les.mu.RLock()
    defer les.mu.RUnlock()
    return les.isLeader
}

// GetLeaderID returns the ID of the current leader
func (les *LeaderElectionService) GetLeaderID() string {
    les.mu.RLock()
    defer les.mu.RUnlock()
    return les.leaderID
}

func main() {
    ctx, cancel := context.WithCancel(context.Background())
    defer cancel()

    // Example callbacks
    onElected := func() {
        log.Println("Running leader-specific tasks (e.g., scheduling jobs)")
    }
    onDemoted := func() {
        log.Println("Stopping leader-specific tasks")
    }

    service, err := NewLeaderElectionService(zkServers, "staff-engineer-node-1", onElected, onDemoted)
    if err != nil {
        log.Fatalf("Failed to initialize leader election: %v", err)
    }

    go func() {
        if err := service.Run(ctx); err != nil {
            log.Fatalf("Leader election failed: %v", err)
        }
    }()

    // Simulate node running
    time.Sleep(5 * time.Minute)
    cancel()
}

// Short snippet to calculate latency vs cost tradeoff
func calculateTradeoff(cacheNodes int, redisCostPerNode float64, latencyReductionMs int) (float64, string) {
    totalCost := float64(cacheNodes) * redisCostPerNode
    roi := (float64(latencyReductionMs) * 100) / totalCost // ROI per ms reduction per dollar
    return roi, fmt.Sprintf("Adding %d cache nodes costs $%.2f/month, reduces latency by %dms, ROI: %.2f ms/$", cacheNodes, totalCost, latencyReductionMs, roi)
}

// Short snippet for proper error wrapping with context
func (s *Service) ProcessRequest(ctx context.Context, req *pb.Request) (*pb.Response, error) {
    // Validate request first
    if req.Id == "" {
        return nil, fmt.Errorf("process request: %w", errors.New("request ID cannot be empty"))
    }

    // Pass context to downstream calls
    resp, err := s.downstreamClient.Call(ctx, req)
    if err != nil {
        return nil, fmt.Errorf("process request %s: downstream call failed: %w", req.Id, err)
    }

    return resp, nil
}

// Short snippet to track cross-team impact (metric struct)
type ImpactMetric struct {
    TeamCount             int
    LatencyReductionMs    int
    EngineeringHoursSaved int
}

func (m *ImpactMetric) String() string {
    return fmt.Sprintf("Influenced %d teams, reduced latency by %dms, saved %d engineering hours", m.TeamCount, m.LatencyReductionMs, m.EngineeringHoursSaved)
}