#Task boundary

1. Implement exactly the task as stated. Do not add abstractions, options, config, or files the task did not name.
2. Comments, identifiers, commit messages and log strings are English only.
3. Stop when the checklist at the end passes. Do not refactor or "improve" surrounding code.
4. Every checklist item below is backed by an assertion in a test or by pasted command output, never by a sentence.

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#Purpose

Rules for Redis. Redis is fast because it is in memory and single-threaded for command execution. Both facts drive every rule here: memory is finite and must be bounded, and one slow command blocks every other client.

Decide first what role Redis plays. A cache may lose data. A queue or session store may not. Configure differently for each, and never mix roles in one instance.

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#Keys and memory

Every key needs a TTL unless you can state why it must live forever.

arduino
namespace:entity:id:field        →  session:user:8fd2:data

A flat, prefixed namespace makes it possible to reason about, measure, and expire whole classes of key. Untracked keys with no TTL are how an instance reaches maxmemory at 3am.

bash
maxmemory 4gb
maxmemory-policy allkeys-lru        # cache role
maxmemory-policy noeviction         # queue / session role — fail writes, don't drop data

Never run a queue or session store on allkeys-lru. Redis will evict a job or a live session under memory pressure and report nothing.

Never run KEYS * against production. It is O(N) and blocks the single command thread for the duration. Use SCAN with a cursor.

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#Atomicity

Redis executes each command atomically, but a read-then-write in application code is not atomic.

js
// Race — two clients both read 5, both write 6
const n = await redis.get(k);
await redis.set(k, Number(n) + 1);

// Atomic
await redis.incr(k);

For anything multi-step, use a Lua script — it runs as a single atomic unit:

lua
-- Release a lock only if we still own it. Compare-and-delete must be atomic;
-- a GET-then-DEL can delete a lock that expired and was re-acquired by another
-- client in between.
if redis.call("GET", KEYS[1]) == ARGV[1] then
  return redis.call("DEL", KEYS[1])
end
return 0

MULTI/EXEC batches commands but permits no logic between them. Use WATCH for optimistic concurrency, or Lua when you need branching.

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#Distributed locks

js
// Acquire: atomic set-if-absent with an expiry and a unique owner token
const token = crypto.randomUUID();
const ok = await redis.set(key, token, { NX: true, PX: 30_000 });

Three requirements, all mandatory:

1. NX — set only if absent, in one command. A separate EXISTS check races.
2. PX — always an expiry. A lock without one survives a crashed holder forever.
3. A unique token — released via the compare-and-delete Lua script above.

Never release a lock with a bare DEL. If your work outran the TTL, the lock is now held by someone else and you have just released theirs.

A single-instance Redis lock is not safe across failover — the lock lives only in memory on the primary and is lost on election. For correctness-critical mutual exclusion, use the database (SELECT … FOR UPDATE, or a unique constraint), not Redis. Redis locks are appropriate for reducing duplicate work, not for preventing double-spend.

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#Caching patterns

Cache-aside is the default: read cache, miss → read source → write cache with TTL.

js
const hit = await redis.get(k);
if (hit) return JSON.parse(hit);
const fresh = await loadFromDb();
await redis.set(k, JSON.stringify(fresh), { EX: 300 });
return fresh;

Two failure modes worth designing against:

1. Stampede. A hot key expires and a thousand requests hit the database at once. Fix with a short lock around the recompute, or jittered TTLs (EX: 300 + random(60)).
2. Stale after write. Invalidate on write; do not rely on TTL alone for data the user just changed and expects to see.

Every cache needs a stated invalidation mechanism before it is added. → Performance/caching

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#Persistence

A cache needs no persistence. A queue needs AOF at minimum — and if losing a job is genuinely unacceptable, it belongs in a durable broker or a database table, not in Redis. Be explicit about which you have chosen.

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#Anti-patterns

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#Checklist

• The role of each instance (cache / queue / session) is explicit
• maxmemory and an appropriate maxmemory-policy are set for that role
• Durable roles run noeviction, never allkeys-lru
• Every cache key carries a TTL, with jitter on hot keys
• Keys follow a documented namespace convention
• KEYS is not used in application or operational code
• Read-modify-write sequences use atomic commands or Lua
• Locks use SET NX PX with a unique token
• Lock release is a compare-and-delete Lua script
• Correctness-critical exclusion uses the database, not Redis
• Persistence mode matches the durability the role requires
• Cache invalidation on write is implemented, not just TTL expiry