Caching Strategy Advisor

Caching Strategy Advisor

What this skill does

This skill analyzes a performance bottleneck, a slow query, or an expensive operation and recommends the right caching strategy for it — which cache layer to use, what TTL to set, how to handle cache invalidation, and what to watch out for. It covers in-memory caches, Redis, HTTP caching headers, CDN caching, and database query caching, and explains the trade-offs of each recommendation.

Use this when you have a slow API, a database that's under heavy read load, a page that's slow to load, or any operation that's expensive and called frequently.

How to use

Claude Code / Cline

Copy this file to .agents/skills/caching-strategy-advisor/SKILL.md in your project root.

Then ask:

• "Use the Caching Strategy Advisor skill — our product listing API is slow and called thousands of times per minute."
• "We have a heavy database query that runs on every page load. Use the Caching Strategy Advisor skill to recommend a caching approach."

Provide:

• What operation is slow (SQL query, API call, computation, file read)
• How often it's called and by how many users
• How often the underlying data changes
• Your current tech stack (language, framework, existing infrastructure)
• What consistency guarantees you need (is stale data for a few seconds OK?)

Cursor

Add the instructions below to your .cursorrules or paste them into the Cursor AI pane with the context above.

Codex

Provide the operation description and context. Ask Codex to follow the instructions below to produce the caching recommendation.

The Prompt / Instructions for the Agent

When asked to advise on a caching strategy, follow these steps:

Step 1 — Understand the workload

Ask or infer:

• Read/write ratio: Is this data mostly read, or is it frequently updated?
• Call frequency: How many requests per second/minute hit this operation?
• Data freshness requirement: How stale can the data be before it causes a real problem? 1 second? 1 minute? 1 hour? Doesn't matter?
• Data size: How large is the response? (Affects cache storage cost and memory limits)
• User specificity: Is the data the same for all users (public), or different per user (personalized)?
• Consistency requirement: Is it OK to serve slightly stale data, or does every user need to see the absolute latest?

Step 2 — Identify the right cache layer

Choose the appropriate layer(s):

In-process / in-memory cache (e.g., a Map or LRU cache in the application itself)

• Best for: Frequently read, rarely changing data that's the same for all instances. Config values, lookup tables, small datasets.
• Trade-offs: Lost on restart, not shared across instances, can cause memory pressure if not bounded.
• Use when: Single-server deployments, data that almost never changes, sub-millisecond latency needed.

Redis / Memcached (distributed key-value cache)

• Best for: Data shared across multiple application instances. User sessions, computed results, rate limit counters, frequently read DB query results.
• Trade-offs: Additional infrastructure to manage, network hop adds ~1ms latency.
• Use when: Multi-instance deployments, data shared between requests from different users/servers.

HTTP response caching (Cache-Control headers)

• Best for: REST API responses that don't change per-user. Public endpoints.
• Trade-offs: Client controls freshness; requires proper ETag or Last-Modified for conditional requests.
• Use when: Public API responses, static-ish data served to browsers or API clients.

CDN caching (Cloudflare, CloudFront, Fastly)

• Best for: Static assets, public pages, API responses that are the same for all users globally.
• Trade-offs: Purging cached content requires a CDN API call; not suitable for personalized data.
• Use when: High global traffic, static or semi-static public content, reducing origin server load.

Database query cache / materialized views

• Best for: Expensive aggregation queries that are read-heavy.
• Trade-offs: Must be refreshed on a schedule or trigger; adds complexity to the data layer.
• Use when: The query result can't be moved to Redis easily, the query runs on a schedule anyway.

Step 3 — Recommend TTL and invalidation strategy

TTL (Time-to-live)

• For data that rarely changes: 1 hour to 24 hours
• For data that changes occasionally: 5–15 minutes
• For data that changes frequently but exact freshness isn't critical: 30–60 seconds
• For data where you need exact freshness: cache with event-based invalidation (not TTL)

Invalidation strategies:

• TTL-only (lazy expiry): Simplest. Cache expires after TTL. Good when stale data for a few minutes is acceptable.
• Write-through: Update the cache at the same time you write to the database. Good consistency, but write path is slower.
• Cache-aside (read-through): Check cache first; on miss, read from DB and populate cache. Most common pattern.
• Event-based invalidation: When data changes (on DB write, on event), explicitly delete or update the cache key. Needed for strong consistency.
• Stale-while-revalidate: Serve the stale value immediately while refreshing in the background. Great for reducing perceived latency.

Step 4 — Flag cache-specific risks

Always check for:

• Cache stampede: Many requests hit the cache at the same time after expiry, all flooding the DB. Mitigation: probabilistic early expiry, mutex/lock on cache refresh.
• Cache poisoning: Attackers inject bad data into the cache. Mitigation: validate data before caching, don't cache error responses.
• Thundering herd on cold start: Cache is empty on startup; all traffic hits DB. Mitigation: warm the cache on startup, use staggered TTLs.
• Memory limits: Unbounded caches grow until the process crashes. Always set a max size and eviction policy.
• Sensitive data in cache: PII or secrets in cache keys or values. Ensure cache is not exposed and keys don't leak user data.

Step 5 — Format the recommendation

## Caching Strategy Recommendation

### Problem Summary
[1–2 sentences describing the bottleneck and the key workload characteristics]

### Recommended Strategy
[Which cache layer(s) to use and why]

### Implementation Plan

#### Cache Layer: [Layer name]
- **What to cache**: [The exact query result, API response, computed value]
- **Cache key structure**: `[prefix]:[identifier]` (e.g., `product:list:category:electronics:page:1`)
- **TTL**: [Value and reasoning]
- **Invalidation**: [How and when to invalidate]
- **Storage estimate**: [Approximate size per entry × expected number of entries]

### Code Pattern
[Pseudocode or real code example showing the cache-aside or write-through pattern]

### Risks and Mitigations
| Risk | Mitigation |
|------|-----------|
| [Risk] | [How to address it] |

### Expected Impact
[Estimated reduction in DB load, latency improvement, or throughput increase]

### What NOT to cache
[Any related data that should NOT be cached, and why]

Example

Input to Agent:

"Use the Caching Strategy Advisor skill. Our GET /api/products endpoint queries the database for a product catalog (500 products, with category and price info). It's called 2,000 times per minute. Products are updated by staff roughly once per hour. Our stack is Node.js + PostgreSQL + Redis (already available). Stale data for up to 5 minutes is acceptable."

Output from Agent:

Caching Strategy Recommendation

Problem Summary

The product catalog is read-heavy (2,000 req/min), changes infrequently (~hourly), and can tolerate up to 5 minutes of staleness. This is an ideal candidate for a Redis cache with a TTL-based expiry.

Recommended Strategy

Cache the query result in Redis with a 5-minute TTL. Use a cache-aside pattern: on a cache miss, query PostgreSQL and populate the cache. No event-based invalidation needed given the acceptable staleness window.

Implementation Plan

Cache Layer: Redis

• What to cache: Full serialized response of GET /api/products (the array of 500 products with category and price)
• Cache key: products:catalog:all (if no filtering); products:catalog:category:[slug] (if filtered by category)
• TTL: 5 minutes (300 seconds)
• Invalidation: TTL-only. Optionally: explicitly delete the key after a staff product update for immediate freshness.
• Storage estimate: ~500 products × ~500 bytes = ~250KB per cache key — well within Redis limits

Code Pattern

async function getProducts(category?: string): Promise<Product[]> {
  const cacheKey = category ? `products:catalog:category:${category}` : 'products:catalog:all';

  const cached = await redis.get(cacheKey);
  if (cached) return JSON.parse(cached);

  const products = await db.query('SELECT * FROM products WHERE ...', [category]);
  await redis.setex(cacheKey, 300, JSON.stringify(products)); // TTL: 300s
  return products;
}

Risks and Mitigations

Risk	Mitigation
Cache stampede (all cache keys expire at once)	Add small random jitter to TTL: 300 + Math.floor(Math.random() * 30)
Cold start on deploy	Pre-warm cache on server startup by calling getProducts()

Expected Impact

DB read load for this endpoint drops by ~98% (cache miss rate will be very low at 2,000 req/min with 5-minute TTL).

Notes

• Don't cache what you can't invalidate. If you're not sure how to invalidate a cache entry when the data changes, use a shorter TTL until you have a proper invalidation strategy.
• Cache the output of expensive operations, not the input. Cache the rendered HTML or the query result, not intermediate variables.
• Measure before and after — add cache hit/miss metrics so you can verify the cache is actually being used.