KV

KV is a globally replicated key-value store optimized for high-read, low-write configuration, feature flags, JWT blocklists, and cache-like lookups from Workers. Values are eventually consistent — writes propagate globally with delay; do not use KV as a strongly consistent database.

					const theme = (await env.FLAGS.get(`user:${userId}:theme`)) ?? 'light';
await env.FLAGS.put('maintenance', 'true', { expirationTtl: 3600 });
				

| Pattern | Fit | |---------|-----| | Config blob | JSON settings read on every request | | Negative cache | Store 404 markers to protect origin | | Session hint | Non-authoritative prefs only | | Counter | Poor fit — use Durable Objects instead |

List operations are slow and paginated; design keys with predictable prefixes. TTL and expiration purge keys automatically.

On interviews: eventual consistency implications, KV vs Redis vs D1, key naming, write rate limits, and cache stampede mitigation.

Common pitfalls: storing authoritative balances or inventory; expecting immediate read-your-writes globally; huge values; frequent writes to the same hot key.

The trade-off is cheap global reads and simplicity versus weak consistency and unsuitability for transactional state.

Checklist:

• Use KV for read-heavy, tolerate-stale data.
• Namespace keys by tenant and feature.
• Set TTL on ephemeral flags and caches.
• Move counters and locks to Durable Objects.