Blog · Architecture

Caching Strategies for Time-Sensitive Data

TTL-Based Cache Invalidation

Absolute instants beat relative English phrases in configuration. When a feed publisher tells you the next quote refresh is at epoch T, your edge should not guess "five minutes" — compute the remaining seconds at write time. Store cached_at alongside payload bodies for observability so support can answer "how old was this JSON?" from logs without replaying user traffic.

async function getCachedData(key, fetchFn, ttlSeconds = 300) {
  const raw = await redis.get(key);
  if (raw) {
    const { data, cached_at } = JSON.parse(raw);
    const age = Math.floor(Date.now() / 1000) - cached_at;
    console.log(`Cache hit — age: ${age}s`);
    return data;
  }
  const data = await fetchFn();
  await redis.setEx(
    key,
    ttlSeconds,
    JSON.stringify({ data, cached_at: Math.floor(Date.now() / 1000) })
  );
  return data;
}

Timestamp-Based ETags

A strong ETag derived from a monotonic updated_at lets browsers and intermediaries skip body download entirely. Use quoting rules from RFC 9110; compare with byte-for-byte equality on If-None-Match.

app.get('/api/rates', async (req, res) => {
  const rates = await getRates();
  const etag = `"${rates.last_updated}"`;
  if (req.headers['if-none-match'] === etag) {
    res.status(304).end();
    return;
  }
  res.setHeader('ETag', etag);
  res.setHeader('Cache-Control', 'public, max-age=60');
  res.json(rates);
});

CDN Cache with Surrogate Keys

Fastly, Cloudflare Workers, and Varnish support surrogate key headers listing logical entities (article-42, user-7). When an upstream mutation completes, purge every key containing that entity without flushing the whole domain. The purge API itself should be idempotent and authenticated — attackers love wide bans triggered through stolen dashboard cookies.

Cache Stampede Prevention with Timestamps

When a hot key expires, thundering herds recompute the same expensive query. Mitigations include single-flight locks, stale-while-revalidate with a probabilistic early refresh, and jitter added to TTL so synchronized clients do not line up on the same second. Encode lock owner + lease deadline as integers for easy sorting in Redis streams or DynamoDB conditional writes.

Best Practices Summary

Key takeaways

• Compute TTL from authoritative next-change time, not vibes.
• ETags from updated_at enable cheap revalidation.
• Surrogate keys connect business events to CDN invalidation.
• Add jitter and single-flight to prevent stampedes.
• Log cached_at for debugging stale UI screenshots.

Timestamp converter