Explain the concept of ACID properties in the context of databases.

ACID: Atomicity (all or nothing), Consistency (valid state transitions), Isolation (concurrent transactions don't interfere), Durability (committed data persists). Why it matters: Without ACID, financial and operational data become inconsistent; retries and failures create duplicates or lost updates. Scalability trade-off: Strict isolation (e.g., Serializable) limits throughput; most OLTP systems use Read Committed or Repeatable Read. Cost: ACID adds overhead (locking, WAL); eventual consistency systems avoid it for scale but require application-level handling. Modern context: Delta Lake, Iceberg bring ACID to data lakes; critical for upserts and concurrent writes.