redis-patterns-expert

Use when designing caching strategies (cache-aside, write-through, write-behind), implementing distributed locks, building rate limiters, leaderboards, real-time streams (XADD/consumer groups), pub/sub, or tuning eviction policies. Triggers: thundering-herd on cache miss, dogpile on key expiry, Redlock vs SET-NX-PX choice, sliding-window rate limiter, hot-key on a single cluster slot, big-key blowup, MULTI/EXEC across slots, KEYS in production. NOT for Redis Cluster operations/admin (different domain), embedded KV (SQLite, leveldb), in-process LRU caches, or Memcached.

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Repository: curiositech/port-daddy
Last source activity: July 4, 2026 at 16:49
Detected SKILL.md language: English
Stars: 2
Forks: 0

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license	Apache-2.0
allowed-tools	Read,Write,Edit,Bash,Glob,Grep,WebSearch,WebFetch
name	redis-patterns-expert
description	Use when designing caching strategies (cache-aside, write-through, write-behind), implementing distributed locks, building rate limiters, leaderboards, real-time streams (XADD/consumer groups), pub/sub, or tuning eviction policies. Triggers: thundering-herd on cache miss, dogpile on key expiry, Redlock vs SET-NX-PX choice, sliding-window rate limiter, hot-key on a single cluster slot, big-key blowup, MULTI/EXEC across slots, KEYS in production. NOT for Redis Cluster operations/admin (different domain), embedded KV (SQLite, leveldb), in-process LRU caches, or Memcached.
metadata	{"category":"Backend & Infrastructure","tags":["redis","caching","rate-limiting","distributed-locks","streams","sorted-sets"],"provenance":{"kind":"first-party","owners":["port-daddy"]},"pairs-with":[{"skill":"background-job-orchestrator","reason":"Redis Streams + consumer groups are the durable-queue substrate this skill designs; job orchestration semantics (retries, dead-letter, scheduling) live there."},{"skill":"event-driven-architecture-expert","reason":"Choosing pub/sub vs Streams here is one leg of the broader event-delivery-guarantee design that skill owns."},{"skill":"websocket-streaming","reason":"Redis pub/sub is the standard fan-out layer behind multi-node WebSocket/SSE broadcast."}],"io-contract":{"kind":"deliverable","consumes":["[Truncated]","[Truncated]"],"produces":["[Truncated]","[Truncated]"]}}

Redis Patterns Expert

Redis is a data-structure server. The interesting part isn't the commands — it's the patterns. Most of the trouble comes from treating Redis as a database when you should treat it as a coordination primitive, or vice versa.

When to use

Adding a cache layer in front of a slow data source.
Need a distributed lock (single-leader election, mutual exclusion).
Rate limiting an API by user/IP/key.
Real-time leaderboards or sorted feeds.
Event streams (XADD/XREAD with consumer groups) for multi-consumer durable queues.
Pub/sub for notifications (best-effort, single delivery attempt).

Core capabilities

Caching strategies

Cache-aside (most common):

async function getUser(id: string) {
  const cached = await redis.get(`user:${id}`);
  if (cached) return JSON.parse(cached);
  const fresh = await db.users.findById(id);
  // SET with EX in one command — never SET then EXPIRE.
  await redis.set(`user:${id}`, JSON.stringify(fresh), 'EX', 300 + Math.floor(Math.random() * 60));
  return fresh;
}

The + random() jitter prevents the dogpile when many keys expire together. Without it, every cache key set during a deploy expires within the same minute 5 minutes later → thundering herd.

const SOFT = 300, HARD = 600;
const entry = await redis.get(key);
if (entry) {
  const { value, mtime } = JSON.parse(entry);
  if (Date.now() - mtime < SOFT * 1000) return value;
  // Stale but valid — refresh in background, return cached.
  void refresh(key);
  return value;
}
// No entry — block on origin.

async function getUserSingleflight(id: string) {
  const cacheKey = `user:${id}`;
  const lockKey  = `user:${id}:lock`;
  const cached = await redis.get(cacheKey);
  if (cached) return JSON.parse(cached);

  const lockAcquired = await redis.set(lockKey, '1', 'NX', 'EX', 5);
  if (!lockAcquired) {
    // Someone else is fetching. Sleep + retry the cache.
    await sleep(50);
    return getUserSingleflight(id);
  }
  try {
    const fresh = await db.users.findById(id);
    await redis.set(cacheKey, JSON.stringify(fresh), 'EX', 300);
    return fresh;
  } finally {
    await redis.del(lockKey);
  }
}

-- release.lua
if redis.call('GET', KEYS[1]) == ARGV[1] then
  return redis.call('DEL', KEYS[1])
else
  return 0
end

const token = crypto.randomUUID();
const acquired = await redis.set(lockKey, token, 'NX', 'PX', leaseMs);
if (!acquired) throw new Error('lock not acquired');
try {
  // ... critical section, must complete before leaseMs ...
} finally {
  await redis.eval(releaseScript, 1, lockKey, token);
}

-- sliding-window.lua
-- KEYS[1] = bucket key  ARGV[1] = window seconds  ARGV[2] = limit  ARGV[3] = now-ms
local now = tonumber(ARGV[3])
local window = tonumber(ARGV[1]) * 1000
redis.call('ZREMRANGEBYSCORE', KEYS[1], 0, now - window)
local count = redis.call('ZCARD', KEYS[1])
if count >= tonumber(ARGV[2]) then
  return 0
end
redis.call('ZADD', KEYS[1], now, now)
redis.call('PEXPIRE', KEYS[1], window)
return 1

// Top scorers
await redis.zincrby('game:scores', points, userId);
await redis.zrevrange('game:scores', 0, 9, 'WITHSCORES');

// Time-series with score = epoch
await redis.zadd('user:42:events', Date.now(), JSON.stringify(event));
await redis.zrangebyscore('user:42:events', Date.now() - 3600_000, '+inf');

// Cap to last 1000 events
await redis.zremrangebyrank('user:42:events', 0, -1001);

// Producer
await redis.xadd('jobs', '*', 'type', 'send_email', 'to', user.email);

// One-time setup
await redis.xgroup('CREATE', 'jobs', 'workers', '$', 'MKSTREAM');

// Consumer
const messages = await redis.xreadgroup(
  'GROUP', 'workers', `worker-${pid}`,
  'COUNT', 10, 'BLOCK', 5000,
  'STREAMS', 'jobs', '>',
);
for (const [stream, entries] of messages ?? []) {
  for (const [id, fields] of entries) {
    try {
      await handle(fields);
      await redis.xack('jobs', 'workers', id);
    } catch {
      // No XACK → message stays in the PEL (pending entries list).
    }
  }
}

// Reclaim work from a crashed peer
const pending = await redis.xpending('jobs', 'workers', 'IDLE', 60000, '-', '+', 100);
for (const { id } of pending) {
  await redis.xclaim('jobs', 'workers', `worker-${pid}`, 60000, id);
}

// Cap stream growth
await redis.xadd('jobs', 'MAXLEN', '~', 100000, '*', ...fields);

const sub = redis.duplicate(); // SUBSCRIBE blocks the connection
await sub.subscribe('user-events');
sub.on('message', (channel, payload) => handle(JSON.parse(payload)));

await redis.publish('user-events', JSON.stringify({ userId, kind: 'login' }));

maxmemory                100mb
maxmemory-policy         allkeys-lru

Policy	When to use
`noeviction`	Redis as durable store (you should be using a real DB).
`allkeys-lru`	Pure cache.
`allkeys-lfu`	Cache where some keys are hot for a long time.
`volatile-lru`	Mixed: some keys must persist (no TTL), evict only TTL'd ones.
`volatile-ttl`	Evict shortest-TTL first; useful for staged data.

node scripts/redis_patterns_audit.mjs --input examples/sample-input.json