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/**
 * Classify database exceptions to determine appropriate handling strategy
 */
public class TransactionErrorClassifier {
    
    public ErrorCategory classify(Exception e) {
        if (e instanceof SQLException sqlEx) {
            return classifySqlException(sqlEx);
        }
        if (e instanceof DataAccessException dae) {
            return classifySpringException(dae);
        }
        // Unknown errors default to permanent (fail fast)
        return ErrorCategory.PERMANENT;
    }
    
    private ErrorCategory classifySqlException(SQLException e) {
        String sqlState = e.getSQLState();
        
        if (sqlState == null) {
            return ErrorCategory.PERMANENT;
        }
        
        // Class 40: Transaction Rollback (deadlock, serialization failure)
        if (sqlState.startsWith("40")) {
            return ErrorCategory.TRANSIENT;
        }
        
        // Class 08: Connection Exception
        if (sqlState.startsWith("08")) {
            return ErrorCategory.TRANSIENT;
        }
        
        // Class 23: Integrity Constraint Violation
        if (sqlState.startsWith("23")) {
            return ErrorCategory.CONSTRAINT_VIOLATION;
        }
        
        // Class 53: Insufficient Resources
        if (sqlState.startsWith("53")) {
            return ErrorCategory.RESOURCE_EXHAUSTION;
        }
        
        // Class 57: Operator Intervention (admin shutdown, crash recovery)
        if (sqlState.startsWith("57")) {
            return ErrorCategory.PERMANENT;
        }
        
        // Default to permanent for unknown states
        return ErrorCategory.PERMANENT;
    }
    
    private ErrorCategory classifySpringException(DataAccessException e) {
        // Spring's DataAccessException hierarchy
        if (e instanceof DeadlockLoserDataAccessException) {
            return ErrorCategory.TRANSIENT;
        }
        if (e instanceof CannotAcquireLockException) {
            return ErrorCategory.TRANSIENT;
        }
        if (e instanceof OptimisticLockingFailureException) {
            return ErrorCategory.TRANSIENT;
        }
        if (e instanceof DataIntegrityViolationException) {
            return ErrorCategory.CONSTRAINT_VIOLATION;
        }
        if (e instanceof CannotGetJdbcConnectionException) {
            return ErrorCategory.TRANSIENT;  // Connection pool issues
        }
        return ErrorCategory.PERMANENT;
    }
}
 
public enum ErrorCategory {
    TRANSIENT,              // Retry will likely succeed
    CONSTRAINT_VIOLATION,   // Data is invalid, don't retry
    RESOURCE_EXHAUSTION,    // Back off, then maybe retry
    PERMANENT               // Fail fast, alert
}

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/**
 * Comprehensive transaction retry implementation
 */
public class TransactionRetryTemplate {
    
    private static final int MAX_RETRIES = 3;
    private static final long BASE_DELAY_MS = 100;
    private static final double JITTER_FACTOR = 0.3;
    
    private final TransactionErrorClassifier classifier = new TransactionErrorClassifier();
    private final Random random = new Random();
    
    /**
     * Execute operation with automatic retry for transient failures
     */
    public <T> T executeWithRetry(Supplier<T> operation, String operationName) {
        int attempt = 0;
        Exception lastException = null;
        
        while (attempt < MAX_RETRIES) {
            try {
                return operation.get();
            } catch (Exception e) {
                lastException = e;
                ErrorCategory category = classifier.classify(e);
                
                if (category != ErrorCategory.TRANSIENT) {
                    // Non-retriable error - fail immediately
                    throw wrapException(e, category, attempt);
                }
                
                attempt++;
                
                if (attempt >= MAX_RETRIES) {
                    logger.warn("Operation '{}' failed after {} attempts", 
                        operationName, attempt);
                    throw new RetryExhaustedException(operationName, lastException);
                }
                
                long delayMs = calculateBackoff(attempt);
                
                logger.info("Transient failure in '{}', attempt {}/{}, retrying in {}ms",
                    operationName, attempt, MAX_RETRIES, delayMs);
                
                sleep(delayMs);
            }
        }
        
        throw new RetryExhaustedException(operationName, lastException);
    }
    
    /**
     * Exponential backoff with jitter
     */
    private long calculateBackoff(int attempt) {
        // Base delay doubles each attempt: 100, 200, 400, 800...
        long exponentialDelay = BASE_DELAY_MS * (1L << (attempt - 1));
        
        // Add random jitter: ± 30%
        double jitter = 1 + (random.nextDouble() - 0.5) * 2 * JITTER_FACTOR;
        
        return (long) (exponentialDelay * jitter);
    }
    
    private void sleep(long ms) {
        try {
            Thread.sleep(ms);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            throw new RuntimeException("Interrupted during retry backoff", e);
        }
    }
}
 
/**
 * Spring-based implementation using @Retryable
 */
@Service
public class OrderService {
    
    /**
     * Declarative retry with Spring Retry
     */
    @Retryable(
        value = {DeadlockLoserDataAccessException.class, 
                 OptimisticLockingFailureException.class,
                 CannotAcquireLockException.class},
        maxAttempts = 3,
        backoff = @Backoff(delay = 100, multiplier = 2, maxDelay = 1000)
    )
    @Transactional
    public Order placeOrder(OrderRequest request) {
        // If this fails with a transient error, entire method is retried
        // Note: Transaction is completely new on each retry
        Order order = createOrder(request);
        reserveInventory(order);
        chargePayment(order);
        return order;
    }
    
    /**
     * Recovery method called when all retries exhausted
     */
    @Recover
    public Order placeOrderFallback(RuntimeException e, OrderRequest request) {
        logger.error("Order placement failed after all retries", e);
        metrics.incrementCounter("orders.failed.retry_exhausted");
        throw new OrderPlacementException(
            "Unable to place order. Please try again later.", e
        );
    }
}

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/**
 * Deadlock-resistant transaction implementation
 */
public class DeadlockAwareTransactionTemplate {
    
    private static final int MAX_DEADLOCK_RETRIES = 5;
    private static final long DEADLOCK_RETRY_DELAY_MS = 50;
    
    /**
     * Execute transaction with deadlock retry
     */
    @Transactional
    public <T> T executeWithDeadlockRetry(TransactionCallback<T> callback) {
        for (int attempt = 1; attempt <= MAX_DEADLOCK_RETRIES; attempt++) {
            try {
                return transactionTemplate.execute(callback);
            } catch (DeadlockLoserDataAccessException e) {
                if (attempt == MAX_DEADLOCK_RETRIES) {
                    logger.error("Deadlock persisted after {} attempts", attempt);
                    throw e;
                }
                
                logger.info("Deadlock detected (attempt {}), retrying...", attempt);
                metrics.incrementCounter("database.deadlocks");
                
                // Brief sleep to let the winner complete
                sleep(DEADLOCK_RETRY_DELAY_MS);
            }
        }
        throw new IllegalStateException("Should not reach here");
    }
    
    /**
     * Prevent deadlocks by locking in consistent order
     */
    @Transactional
    public void transferFunds(UUID fromAccount, UUID toAccount, BigDecimal amount) {
        // CRITICAL: Always lock accounts in a deterministic order (e.g., by ID)
        // This prevents the deadlock where T1 locks A→B while T2 locks B→A
        
        UUID firstLock = fromAccount.compareTo(toAccount) < 0 ? fromAccount : toAccount;
        UUID secondLock = fromAccount.compareTo(toAccount) < 0 ? toAccount : fromAccount;
        
        // Acquire locks in consistent order
        Account first = accountRepository.findByIdForUpdate(firstLock)
            .orElseThrow();
        Account second = accountRepository.findByIdForUpdate(secondLock)
            .orElseThrow();
        
        // Now perform the actual transfer
        Account source = fromAccount.equals(firstLock) ? first : second;
        Account destination = fromAccount.equals(firstLock) ? second : first;
        
        if (source.getBalance().compareTo(amount) < 0) {
            throw new InsufficientFundsException();
        }
        
        source.debit(amount);
        destination.credit(amount);
        
        accountRepository.save(source);
        accountRepository.save(destination);
    }
}
 
/**
 * Alternative: Use NOWAIT to detect potential deadlocks early
 */
@Repository
public class AccountRepository {
    
    @Query("SELECT a FROM Account a WHERE a.id = :id")
    @Lock(LockModeType.PESSIMISTIC_WRITE)
    @QueryHints(@QueryHint(name = "javax.persistence.lock.timeout", value = "0"))
    Optional<Account> findByIdWithNoWait(@Param("id") UUID id);
    
    // If lock cannot be acquired immediately, throws exception
    // Prevents waiting that could lead to deadlock
}

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/**
 * Sophisticated constraint violation handling
 */
@Service
public class UserRegistrationService {
    
    @Transactional
    public User registerUser(RegistrationRequest request) {
        try {
            User user = new User();
            user.setEmail(request.getEmail());
            user.setUsername(request.getUsername());
            user.setPasswordHash(passwordEncoder.encode(request.getPassword()));
            
            return userRepository.save(user);
            
        } catch (DataIntegrityViolationException e) {
            // Parse the underlying cause to identify which constraint failed
            String message = extractConstraintName(e);
            
            if (message.contains("users_email_key")) {
                throw new RegistrationException(
                    "Email already registered. Please use a different email or log in.",
                    RegistrationErrorCode.EMAIL_TAKEN
                );
            }
            
            if (message.contains("users_username_key")) {
                throw new RegistrationException(
                    "Username already taken. Please choose a different username.",
                    RegistrationErrorCode.USERNAME_TAKEN
                );
            }
            
            // Unknown constraint - log for debugging, return generic message
            logger.error("Unexpected constraint violation during registration", e);
            throw new RegistrationException(
                "Registration failed. Please try again.",
                RegistrationErrorCode.UNKNOWN
            );
        }
    }
    
    private String extractConstraintName(DataIntegrityViolationException e) {
        Throwable cause = e.getCause();
        if (cause instanceof ConstraintViolationException cve) {
            return cve.getConstraintName() != null 
                ? cve.getConstraintName() 
                : cause.getMessage();
        }
        return e.getMessage();
    }
    
    /**
     * Alternative: Check before insert (has race condition but better UX)
     */
    @Transactional
    public ValidationResult validateRegistration(RegistrationRequest request) {
        List<String> errors = new ArrayList<>();
        
        // Pre-check (may have false negatives due to race, but improves UX)
        if (userRepository.existsByEmail(request.getEmail())) {
            errors.add("Email already registered");
        }
        
        if (userRepository.existsByUsername(request.getUsername())) {
            errors.add("Username already taken");
        }
        
        if (!errors.isEmpty()) {
            return ValidationResult.failed(errors);
        }
        
        // Note: Even if pre-checks pass, the actual insert might still fail
        // due to race condition. Handle constraint violation as fallback.
        return ValidationResult.passed();
    }
}
 
/**
 * Handle unique constraint as upsert (insert or update)
 */
@Service
public class SettingsService {
    
    @Transactional
    public UserSettings saveSettings(UUID userId, Map<String, String> settings) {
        for (Map.Entry<String, String> entry : settings.entrySet()) {
            // Use database's upsert capability
            settingsRepository.upsert(userId, entry.getKey(), entry.getValue());
        }
        return settingsRepository.findByUserId(userId);
    }
}
 
@Repository
public interface SettingsRepository extends JpaRepository<Setting, UUID> {
    
    // PostgreSQL upsert using native query
    @Modifying
    @Query(value = 
        "INSERT INTO user_settings (user_id, key, value) " +
        "VALUES (:userId, :key, :value) " +
        "ON CONFLICT (user_id, key) DO UPDATE SET value = :value",
        nativeQuery = true)
    void upsert(
        @Param("userId") UUID userId, 
        @Param("key") String key, 
        @Param("value") String value
    );
}

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/**
 * Robust connection and timeout handling
 */
@Configuration
public class DataSourceConfiguration {
    
    @Bean
    public DataSource dataSource() {
        HikariConfig config = new HikariConfig();
        
        // Connection pool settings
        config.setMaximumPoolSize(20);
        config.setMinimumIdle(5);
        
        // Connection validation
        config.setConnectionTestQuery("SELECT 1");
        config.setValidationTimeout(5000);  // 5 seconds to validate
        
        // Acquire timeout - fail fast if pool exhausted
        config.setConnectionTimeout(10000);  // 10s to acquire connection
        
        // Idle connections closed after this time
        config.setIdleTimeout(300000);  // 5 minutes
        
        // Connections closed after this time regardless of activity
        config.setMaxLifetime(1800000);  // 30 minutes
        
        // Leak detection
        config.setLeakDetectionThreshold(60000);  // Warn if connection held > 60s
        
        return new HikariDataSource(config);
    }
}
 
@Service
public class RobustDataAccessService {
    
    /**
     * Handle connection acquisition failures
     */
    public Order placeOrderWithConnectionRetry(OrderRequest request) {
        int attempts = 0;
        while (attempts < 3) {
            try {
                return orderService.placeOrder(request);
            } catch (CannotGetJdbcConnectionException e) {
                attempts++;
                logger.warn("Connection pool exhausted, attempt {}/3", attempts);
                
                if (attempts >= 3) {
                    throw new ServiceUnavailableException(
                        "Database temporarily unavailable. Please try again."
                    );
                }
                
                // Back-pressure: wait before retry
                sleep(500 * attempts);
            }
        }
        throw new IllegalStateException("Should not reach here");
    }
    
    /**
     * Handle query timeouts with chunking
     */
    @Transactional(timeout = 30)  // 30 second timeout
    public void processLargeBatch(List<Record> records) {
        // If this times out, break into smaller chunks
        for (Record record : records) {
            processRecord(record);
        }
    }
    
    /**
     * Alternative: Process in smaller transactions
     */
    public BatchResult processInChunks(List<Record> records) {
        List<List<Record>> chunks = Lists.partition(records, 100);
        
        int processed = 0;
        List<String> errors = new ArrayList<>();
        
        for (List<Record> chunk : chunks) {
            try {
                // Each chunk is a separate transaction
                processChunk(chunk);  // @Transactional with shorter timeout
                processed += chunk.size();
            } catch (TransactionTimedOutException e) {
                logger.error("Chunk timed out at offset {}", processed);
                errors.add("Timeout at record " + processed);
                // Continue with next chunk or break depending on requirements
            }
        }
        
        return new BatchResult(processed, records.size(), errors);
    }
    
    @Transactional(timeout = 10)  // Smaller timeout per chunk
    public void processChunk(List<Record> records) {
        records.forEach(this::processRecord);
    }
}

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/**
 * Database circuit breaker implementation
 */
@Service
public class DatabaseCircuitBreaker {
    
    private final AtomicInteger failureCount = new AtomicInteger(0);
    private final AtomicReference<State> state = new AtomicReference<>(State.CLOSED);
    private volatile long lastFailureTime = 0;
    
    private static final int FAILURE_THRESHOLD = 5;
    private static final long OPEN_TIMEOUT_MS = 30000;  // 30 seconds
    
    public <T> T execute(Supplier<T> operation, Supplier<T> fallback) {
        if (state.get() == State.OPEN) {
            if (shouldAttemptReset()) {
                return attemptHalfOpen(operation, fallback);
            }
            return fallback.get();
        }
        
        try {
            T result = operation.get();
            onSuccess();
            return result;
        } catch (Exception e) {
            if (isCircuitBreakerException(e)) {
                onFailure();
            }
            throw e;
        }
    }
    
    private boolean shouldAttemptReset() {
        return System.currentTimeMillis() - lastFailureTime > OPEN_TIMEOUT_MS;
    }
    
    private <T> T attemptHalfOpen(Supplier<T> operation, Supplier<T> fallback) {
        state.set(State.HALF_OPEN);
        try {
            T result = operation.get();
            reset();  // Success! Close the circuit
            return result;
        } catch (Exception e) {
            tripBreaker();  // Still failing, reopen
            return fallback.get();
        }
    }
    
    private void onSuccess() {
        failureCount.set(0);
    }
    
    private void onFailure() {
        int failures = failureCount.incrementAndGet();
        lastFailureTime = System.currentTimeMillis();
        
        if (failures >= FAILURE_THRESHOLD) {
            tripBreaker();
        }
    }
    
    private void tripBreaker() {
        state.set(State.OPEN);
        logger.warn("Circuit breaker OPENED - database failures exceeded threshold");
        alertOps("Database circuit breaker opened");
    }
    
    private void reset() {
        state.set(State.CLOSED);
        failureCount.set(0);
        logger.info("Circuit breaker CLOSED - database recovered");
    }
    
    private boolean isCircuitBreakerException(Exception e) {
        // Only trip for infrastructure failures, not business errors
        return e instanceof CannotGetJdbcConnectionException
            || e instanceof QueryTimeoutException
            || e instanceof DataAccessResourceFailureException;
    }
    
    enum State { CLOSED, OPEN, HALF_OPEN }
}
 
/**
 * Using Resilience4j for production circuit breaker
 */
@Configuration
public class CircuitBreakerConfig {
    
    @Bean
    public CircuitBreakerRegistry circuitBreakerRegistry() {
        CircuitBreakerConfig config = CircuitBreakerConfig.custom()
            .failureRateThreshold(50)  // Open when 50% of calls fail
            .waitDurationInOpenState(Duration.ofSeconds(30))
            .slidingWindowSize(10)  // Evaluate last 10 calls
            .permittedNumberOfCallsInHalfOpenState(3)  // Test with 3 calls
            .recordExceptions(
                CannotGetJdbcConnectionException.class,
                DataAccessResourceFailureException.class
            )
            .build();
        
        return CircuitBreakerRegistry.of(config);
    }
}
 
@Service
public class OrderServiceWithCircuitBreaker {
    
    private final CircuitBreaker circuitBreaker;
    private final CachedOrderService cachedOrderService;
    
    @Autowired
    public OrderServiceWithCircuitBreaker(
            CircuitBreakerRegistry registry,
            CachedOrderService cachedOrderService) {
        this.circuitBreaker = registry.circuitBreaker("database");
        this.cachedOrderService = cachedOrderService;
    }
    
    public Order getOrder(UUID orderId) {
        return circuitBreaker.executeSupplier(() -> 
            orderRepository.findById(orderId).orElseThrow()
        );
    }
    
    // With fallback
    public List<Order> getRecentOrders(UUID customerId) {
        try {
            return circuitBreaker.executeSupplier(() ->
                orderRepository.findRecentByCustomerId(customerId)
            );
        } catch (CallNotPermittedException e) {
            // Circuit is open - use fallback
            return cachedOrderService.getCachedOrders(customerId);
        }
    }
}

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/**
 * Idempotency key pattern for safe retries
 */
@Service
public class IdempotentOrderService {
    
    private final IdempotencyKeyRepository idempotencyRepo;
    private final OrderRepository orderRepository;
    
    /**
     * Client generates a unique key per logical operation.
     * If key exists, return cached result instead of re-executing.
     */
    @Transactional
    public OrderResult placeOrder(String idempotencyKey, OrderRequest request) {
        // Check if we've already processed this request
        Optional<IdempotencyRecord> existing = 
            idempotencyRepo.findByKey(idempotencyKey);
        
        if (existing.isPresent()) {
            IdempotencyRecord record = existing.get();
            
            if (record.isCompleted()) {
                // Already completed - return cached result
                logger.info("Returning cached result for idempotency key: {}", 
                    idempotencyKey);
                return record.getCachedResult();
            }
            
            if (record.isInProgress()) {
                // Another request is processing - tell client to wait
                throw new ConflictException(
                    "Request in progress. Please retry in a few seconds."
                );
            }
        }
        
        // Mark this key as in-progress
        IdempotencyRecord record = new IdempotencyRecord(
            idempotencyKey, 
            IdempotencyStatus.IN_PROGRESS
        );
        idempotencyRepo.save(record);
        
        try {
            // Execute the actual operation
            Order order = createOrder(request);
            reserveInventory(order);
            
            // Cache the result
            OrderResult result = new OrderResult(order);
            record.setStatus(IdempotencyStatus.COMPLETED);
            record.setCachedResult(result);
            idempotencyRepo.save(record);
            
            return result;
            
        } catch (Exception e) {
            // Mark failed - could retry or leave for investigation
            record.setStatus(IdempotencyStatus.FAILED);
            record.setError(e.getMessage());
            idempotencyRepo.save(record);
            throw e;
        }
    }
    
    /**
     * Alternative: Use natural keys for implicit idempotency
     * Works when operations can be uniquely identified by content
     */
    @Transactional
    public void recordPayment(PaymentNotification notification) {
        String paymentId = notification.getPaymentProviderTransactionId();
        
        // Check if payment already recorded (using unique constraint)
        if (paymentRepository.existsByExternalId(paymentId)) {
            logger.info("Payment {} already recorded, skipping", paymentId);
            return;  // Idempotent - no error, no duplicate
        }
        
        Payment payment = new Payment();
        payment.setExternalId(paymentId);  // Has unique constraint
        payment.setAmount(notification.getAmount());
        payment.setOrderId(notification.getOrderId());
        
        try {
            paymentRepository.save(payment);
        } catch (DataIntegrityViolationException e) {
            // Race condition: another thread saved first
            // This is fine - payment is recorded
            if (e.getMessage().contains("payments_external_id_key")) {
                logger.info("Payment {} recorded by concurrent request", paymentId);
                return;
            }
            throw e;
        }
    }
}
 
/**
 * IdempotencyRecord entity
 */
@Entity
@Table(name = "idempotency_keys")
public class IdempotencyRecord {
    @Id
    @Column(name = "idempotency_key")
    private String key;
    
    @Enumerated(EnumType.STRING)
    private IdempotencyStatus status;
    
    @Column(columnDefinition = "jsonb")
    @Type(JsonType.class)
    private OrderResult cachedResult;
    
    private String error;
    
    @CreationTimestamp
    private Instant createdAt;
    
    // Expire old keys
    @Column(name = "expires_at")
    private Instant expiresAt;
}