Redis Write Failures — Default Eviction Policy

# --- Step 1: Start the Redis server (run in one terminal) ---
# This launches Redis with default config on port 6379
redis-server

# --- Step 2: Connect with the Redis CLI (run in another terminal) ---
redis-cli

# --- Step 3: Ping the server to confirm it's alive ---
127.0.0.1:6379> PING
# Redis responds with PONG — the simplest health check you'll ever do

# --- Step 4: Store a string value (the most basic operation) ---
# SET key value
# We're caching a user's display name keyed by their user ID
127.0.0.1:6379> SET user:1001:display_name "Alice Nguyen"

# --- Step 5: Retrieve it ---
127.0.0.1:6379> GET user:1001:display_name

# --- Step 6: Store a value with an expiry (TTL = Time To Live) ---
# EX sets expiry in seconds — this key auto-deletes after 300 seconds (5 minutes)
# This is the pattern for caching: store it, let it expire, recompute if missing
127.0.0.1:6379> SET product:42:price "29.99" EX 300

# --- Step 7: Check how many seconds remain before expiry ---
127.0.0.1:6379> TTL product:42:price

# --- Step 8: Check if a key exists without fetching its value ---
127.0.0.1:6379> EXISTS user:1001:display_name

# =========================================================
# STRINGS — atomic counter for API rate limiting
# =========================================================

# Track how many API calls user 2055 has made this minute
# INCR is atomic — safe even with concurrent requests
127.0.0.1:6379> INCR api_calls:user:2055:minute:2024061514
127.0.0.1:6379> INCR api_calls:user:2055:minute:2024061514
127.0.0.1:6379> INCR api_calls:user:2055:minute:2024061514

# Set it to expire at the end of the minute (60 seconds)
127.0.0.1:6379> EXPIRE api_calls:user:2055:minute:2024061514 60

# =========================================================
# HASHES — store a user profile without one giant JSON blob
# =========================================================

# HSET sets one or more fields on a hash key
# Updating just the email later only rewrites that one field
127.0.0.1:6379> HSET user:2055 username "bob_the_dev" email "bob@example.com" plan "pro" login_count 0

# Retrieve a single field — no need to deserialize a full JSON object
127.0.0.1:6379> HGET user:2055 email

# Retrieve all fields at once
127.0.0.1:6379> HGETALL user:2055

# Atomically increment just the login counter
127.0.0.1:6379> HINCRBY user:2055 login_count 1

# =========================================================
# SORTED SETS — real-time game leaderboard
# =========================================================

# ZADD leaderboard_key score member
# Score is the player's points — Redis sorts automatically
127.0.0.1:6379> ZADD game:leaderboard 4200 "player:alice"
127.0.0.1:6379> ZADD game:leaderboard 8750 "player:bob"
127.0.0.1:6379> ZADD game:leaderboard 6100 "player:carol"

# Fetch top 3 players, highest score first (WITHSCORES shows the score)
# ZREVRANGE = reverse order = highest to lowest
127.0.0.1:6379> ZREVRANGE game:leaderboard 0 2 WITHSCORES

# Get a specific player's rank (0-indexed, 0 = top)
127.0.0.1:6379> ZREVRANK game:leaderboard "player:alice"

# =========================================================
# LISTS — lightweight task queue
# =========================================================

# LPUSH adds to the LEFT (head) of the list
# Workers will RPOP from the RIGHT (tail) — FIFO queue
127.0.0.1:6379> RPUSH email_queue "{\"to\":\"alice@example.com\",\"subject\":\"Welcome\"}"
127.0.0.1:6379> RPUSH email_queue "{\"to\":\"bob@example.com\",\"subject\":\"Reset\"}"

# BLPOP = blocking pop — worker waits up to 5 seconds for a job
# This is more efficient than polling in a loop
127.0.0.1:6379> BLPOP email_queue 5

import redis
import json
import time

# --- Connect to Redis ---
# decode_responses=True means Redis returns strings instead of bytes
redis_client = redis.Redis(
    host="localhost",
    port=6379,
    db=0,
    decode_responses=True
)

# --- Simulated database fetch (replace with your real DB query) ---
def fetch_product_from_database(product_id: int) -> dict:
    """
    Simulates a slow database query.
    In production this would be: cursor.execute('SELECT * FROM products WHERE id = %s', [product_id])
    """
    print(f"[DB] Querying database for product {product_id}...")
    time.sleep(0.05)  # simulate 50ms DB query latency
    return {
        "id": product_id,
        "name": "Mechanical Keyboard TKL",
        "price": 129.99,
        "stock": 42
    }

def get_product(product_id: int) -> dict:
    """
    Cache-Aside Pattern implementation.
    Always check Redis first. Fall back to DB on miss. Always set a TTL.
    """
    cache_key = f"product:{product_id}"  # namespaced key following the colon convention
    cache_ttl_seconds = 300              # cache is valid for 5 minutes

    # --- Step 1: Try the cache first ---
    cached_value = redis_client.get(cache_key)

    if cached_value is not None:
        # Cache HIT — data found in Redis, no DB query needed
        print(f"[CACHE] Hit for key '{cache_key}'")
        return json.loads(cached_value)  # deserialize from JSON string back to dict

    # --- Step 2: Cache MISS — go to the database ---
    print(f"[CACHE] Miss for key '{cache_key}'")
    product_data = fetch_product_from_database(product_id)

    # --- Step 3: Populate the cache for next time ---
    # json.dumps serializes the dict to a JSON string for storage
    # ex=cache_ttl_seconds ensures the key auto-expires — NEVER skip this
    redis_client.set(
        cache_key,
        json.dumps(product_data),
        ex=cache_ttl_seconds
    )
    print(f"[CACHE] Stored '{cache_key}' with TTL={cache_ttl_seconds}s")

    return product_data

def invalidate_product_cache(product_id: int):
    """
    Call this whenever a product is updated in the database.
    Removing the key forces the next request to re-fetch fresh data.
    """
    cache_key = f"product:{product_id}"
    deleted_count = redis_client.delete(cache_key)
    if deleted_count > 0:
        print(f"[CACHE] Invalidated key '{cache_key}'")
    else:
        print(f"[CACHE] Key '{cache_key}' wasn't in cache — nothing to invalidate")

# --- Demo ---
if __name__ == "__main__":
    print("=== First request — cold cache ===")
    product = get_product(product_id=7)
    print(f"Result: {product}\n")

    print("=== Second request — warm cache ===")
    product = get_product(product_id=7)
    print(f"Result: {product}\n")

    print("=== Simulating a product update ===")
    invalidate_product_cache(product_id=7)

    print("\n=== Third request — cache was invalidated ===")
    product = get_product(product_id=7)
    print(f"Result: {product}")

# =========================================================
# CONFIGURING EVICTION POLICY (redis.conf or at runtime)
# =========================================================

# Set max memory to 256MB — Redis will start evicting when this is reached
127.0.0.1:6379> CONFIG SET maxmemory 268435456

# allkeys-lru = evict least-recently-used key from the ENTIRE keyspace
# Best default for a pure cache where all keys have roughly equal value
127.0.0.1:6379> CONFIG SET maxmemory-policy allkeys-lru

# Confirm the config was applied
127.0.0.1:6379> CONFIG GET maxmemory-policy

# =========================================================
# TTL COMMANDS — managing key lifetime
# =========================================================

# SET with expiry in seconds
127.0.0.1:6379> SET session:user:8821 "eyJhbGciOiJIUzI1NiJ9" EX 3600

# SET with expiry in milliseconds (for sub-second precision)
127.0.0.1:6379> SET rate_check:ip:192.168.1.1 "1" PX 60000

# Check TTL in seconds (-1 = no expiry, -2 = key doesn't exist)
127.0.0.1:6379> TTL session:user:8821

# Check TTL in milliseconds (more precise)
127.0.0.1:6379> PTTL rate_check:ip:192.168.1.1

# PERSIST removes the TTL — key lives forever (use with caution)
127.0.0.1:6379> PERSIST session:user:8821
127.0.0.1:6379> TTL session:user:8821

# =========================================================
# MUTEX PATTERN — prevent cache stampedes
# NX = only set if key does NOT exist (atomic check-and-set)
# This is a distributed lock: only the first caller wins
# =========================================================

# First request tries to acquire the rebuild lock (TTL=10s to auto-release)
127.0.0.1:6379> SET lock:product:7:rebuild "1" NX EX 10

# Second concurrent request tries the same — gets nil (lock is taken)
127.0.0.1:6379> SET lock:product:7:rebuild "1" NX EX 10

# After the first request rebuilds the cache and releases the lock:
127.0.0.1:6379> DEL lock:product:7:rebuild

# =========================================================
# CHECK MEMORY USAGE
# =========================================================

# See overall memory stats
127.0.0.1:6379> INFO memory

# See exactly how much RAM one key is using (in bytes)
127.0.0.1:6379> MEMORY USAGE session:user:8821

# =========================================================
# CONFIGURING PERSISTENCE (redis.conf)
# =========================================================

# --- RDB Snapshotting ---
# Format: save <seconds> <changes>
# Trigger: at least 1 change in 900s (15 min) OR 10 in 300s (5 min) OR 10000 in 60s
save 900 1
save 300 10
save 60 10000

# RDB file location and name
dbfilename dump.rdb
dir /var/lib/redis/

# Compress the RDB file (yes/no)
rdbcompression yes

# --- AOF Persistence ---
# Enable AOF (yes/no)
appendonly yes

# AOF file name
appendfilename "appendonly.aof"

# Sync policy:
# always = sync every write (safest, slowest)
# everysec = sync once per second (default, best trade-off)
# no = let OS decide (fastest, least safe)
appendfsync everysec

# --- AOF Rewrite ---
# Auto-rewrite AOF when it grows 100% in size (auto-aof-rewrite-percentage 100)
# and reaches at least 64MB (auto-aof-rewrite-min-size 64mb)
auto-aof-rewrite-percentage 100
auto-aof-rewrite-min-size 64mb

# --- Check current config ---
# From redis-cli
127.0.0.1:6379> CONFIG GET save
127.0.0.1:6379> CONFIG GET appendonly
127.0.0.1:6379> CONFIG GET appendfsync

# --- Force an RDB save immediately ---
127.0.0.1:6379> SAVE

# --- Force an AOF rewrite ---
127.0.0.1:6379> BGREWRITEAOF

# --- Check persistence stats ---
127.0.0.1:6379> INFO persistence