Backend Developer interview prep for Salesforce.

Deadlocks occur when two or more transactions simultaneously hold locks that the other transactions need to proceed. To remediate: 1) Enforce a deterministic global lock ordering (e.g., always acquire row locks sorted by entity primary key); 2) Keep transactions as short as possible—perform all external network calls and heavy business logic before opening the transactional database boundary; 3) Choose appropriate transaction isolation levels (e.g., Read Committed with optimistic locking using version columns instead of aggressive Pessimistic Serializable locking); 4) Analyze query execution plans, database lock graphs, and slow query logs; 5) Implement exponential backoff and randomized retry loops in application code for transient serialization failures.

Employ the Expand and Contract (Parallel Run) migration pattern: 1) Phase 1 (Expand): Add the new database column or table as nullable without modifying existing queries. Deploy code that writes dual-writes to both old and new representations while reading exclusively from the old; 2) Phase 2 (Backfill): Run asynchronous background migration jobs to backfill historical records in batches; 3) Phase 3: Switch application reads to the new schema representation while maintaining dual writes; 4) Phase 4 (Contract): Remove the dual-write logic and safely deprecate/drop the old column or table after monitoring error logs and verifying data parity.