ACID Properties — Atomicity, Consistency, Isolation, Durability

Direct Answer

ACID describes four transaction properties: Atomicity means all of a transaction commits or none does; Consistency means a successful transaction preserves the rules the database and application enforce; Isolation controls what concurrent transactions can observe; and Durability means a successful commit survives a restart within the configured storage guarantees.

Code Example

import org.springframework.stereotype.Service;
import org.springframework.transaction.annotation.Transactional;
import java.math.BigDecimal;

@Service
public class BankTransferService {
    private final AccountRepository accountRepo;

    public BankTransferService(AccountRepository accountRepo) {
        this.accountRepo = accountRepo;
    }

    @Transactional
    public void transfer(Long fromId, Long toId, BigDecimal amount) {
        var from = accountRepo.findByIdForUpdate(fromId).orElseThrow();
        var to = accountRepo.findByIdForUpdate(toId).orElseThrow();

        from.debit(amount);
        to.credit(amount);
        // Both managed changes share one transaction. The explicit locking
        // policy addresses concurrent transfers; @Transactional alone does not.
    }
}

Common Mistakes & Pitfalls

• Confusing two meanings of consistency: ACID consistency is about preserving invariants; replica consistency is about what copies of data return.
• Treating isolation as serial execution: Weaker isolation levels intentionally allow some observations, so choose the level and concurrency control from the business rule.
• Treating @Transactional as magic: It declares a boundary. It does not add missing constraints, make remote calls atomic, or override the database's configured durability.