Write-behind caching writes to cache first, then asynchronously persists to database. Fast, but risky.
Risks:
- Cache failure before DB write = data loss
- Ordering issues if writes processed out of order
- Debugging is harder (cache may not match DB)
Mitigations:
- Replicated cache (Redis Cluster) for durability
- Write-ahead log before cache write
- Careful ordering guarantees
When to use:
- High write volume where some loss is acceptable
- Analytics, counters, non-critical data
- Never for financial transactions
In interviews, mention write-behind for performance but acknowledge the risks.