# migration-architecture.yaml
apiVersion: v1
kind: ConfigMap
metadata:
  name: database-migration-architecture
  namespace: database-ops
data:
  architecture: |
    ┌─────────────────────────────────────────────────────────────────┐
    │                 Zero-Downtime Migration Architecture             │
    ├─────────────────────────────────────────────────────────────────┤
    │                                                                  │
    │  ┌─────────────────────────────────────────────────────────┐   │
    │  │                    Application Layer                      │   │
    │  │  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐     │   │
    │  │  │   App v1    │  │   App v2    │  │   App v3    │     │   │
    │  │  │ (Previous)  │  │ (Current)   │  │   (Next)    │     │   │
    │  │  └──────┬──────┘  └──────┬──────┘  └──────┬──────┘     │   │
    │  │         └─────────────────┴─────────────────┘            │   │
    │  └───────────────────────────┬─────────────────────────────┘   │
    │                              │                                   │
    │  ┌───────────────────────────┴─────────────────────────────┐   │
    │  │                  Migration Controller                     │   │
    │  │  ┌────────────┐  ┌────────────┐  ┌────────────┐        │   │
    │  │  │  Version   │  │   Schema   │  │  Rollback  │        │   │
    │  │  │  Manager   │  │  Migrator  │  │  Handler   │        │   │
    │  │  └────────────┘  └────────────┘  └────────────┘        │   │
    │  └───────────────────────────┬─────────────────────────────┘   │
    │                              │                                   │
    │  ┌───────────────────────────┴─────────────────────────────┐   │
    │  │                    Database Layer                         │   │
    │  │  ┌─────────────┐  ┌─────────────┐  ┌─────────────┐     │   │
    │  │  │  Primary    │  │  Migration  │  │  Rollback   │     │   │
    │  │  │  Database   │  │  Staging    │  │  Snapshot   │     │   │
    │  │  └─────────────┘  └─────────────┘  └─────────────┘     │   │
    │  │                                                           │   │
    │  │  ┌─────────────────────────────────────────────────┐     │   │
    │  │  │            Change Data Capture (CDC)             │     │   │
    │  │  │  ┌───────────┐  ┌───────────┐  ┌───────────┐  │     │   │
    │  │  │  │  Binlog   │  │   WAL     │  │  OpLog    │  │     │   │
    │  │  │  │  (MySQL)  │  │(PostgreSQL)│  │ (MongoDB) │  │     │   │
    │  │  │  └───────────┘  └───────────┘  └───────────┘  │     │   │
    │  │  └─────────────────────────────────────────────────┘     │   │
    │  └───────────────────────────────────────────────────────────┘ │
    └─────────────────────────────────────────────────────────────────┘

// migration-controller.go
package migration

import (
    "context"
    "database/sql"
    "fmt"
    "log"
    "sync"
    "time"
)

// MigrationController orchestrates zero-downtime migrations
type MigrationController struct {
    primaryDB        *sql.DB
    stagingDB        *sql.DB
    migrationEngine  MigrationEngine
    versionManager   VersionManager
    healthChecker    HealthChecker
    rollbackHandler  RollbackHandler
    mu               sync.RWMutex
    state            MigrationState
}

// MigrationState tracks the current migration status
type MigrationState struct {
    CurrentVersion   string
    TargetVersion    string
    Phase            MigrationPhase
    StartTime        time.Time
    LastCheckpoint   string
    RollbackEnabled  bool
    Metrics          MigrationMetrics
}

// MigrationPhase represents the current phase of migration
type MigrationPhase int

const (
    PhaseIdle MigrationPhase = iota
    PhasePreCheck
    PhaseSchemaExpand
    PhaseDataMigration
    PhaseValidation
    PhaseSchemaContract
    PhaseCleanup
    PhaseComplete
    PhaseRollback
)

// ExecuteMigration performs a zero-downtime migration
func (mc *MigrationController) ExecuteMigration(ctx context.Context, targetVersion string) error {
    mc.mu.Lock()
    if mc.state.Phase != PhaseIdle {
        mc.mu.Unlock()
        return fmt.Errorf("migration already in progress: %v", mc.state.Phase)
    }
    
    mc.state = MigrationState{
        CurrentVersion:  mc.versionManager.GetCurrentVersion(),
        TargetVersion:   targetVersion,
        Phase:           PhasePreCheck,
        StartTime:       time.Now(),
        RollbackEnabled: true,
    }
    mc.mu.Unlock()

    // Create rollback point
    rollbackPoint, err := mc.rollbackHandler.CreateRollbackPoint()
    if err != nil {
        return fmt.Errorf("failed to create rollback point: %w", err)
    }

    // Execute migration phases
    phases := []struct {
        phase    MigrationPhase
        execute  func(context.Context) error
        rollback func(context.Context) error
    }{
        {
            phase:    PhasePreCheck,
            execute:  mc.executePreChecks,
            rollback: mc.rollbackPreChecks,
        },
        {
            phase:    PhaseSchemaExpand,
            execute:  mc.executeSchemaExpansion,
            rollback: mc.rollbackSchemaExpansion,
        },
        {
            phase:    PhaseDataMigration,
            execute:  mc.executeDataMigration,
            rollback: mc.rollbackDataMigration,
        },
        {
            phase:    PhaseValidation,
            execute:  mc.executeValidation,
            rollback: mc.rollbackValidation,
        },
        {
            phase:    PhaseSchemaContract,
            execute:  mc.executeSchemaContraction,
            rollback: mc.rollbackSchemaContraction,
        },
        {
            phase:    PhaseCleanup,
            execute:  mc.executeCleanup,
            rollback: nil, // Cleanup is optional
        },
    }

    for _, p := range phases {
        mc.updatePhase(p.phase)
        
        if err := p.execute(ctx); err != nil {
            log.Printf("Migration failed at phase %v: %v", p.phase, err)
            
            if mc.state.RollbackEnabled && p.rollback != nil {
                mc.updatePhase(PhaseRollback)
                if rollbackErr := mc.executeRollback(ctx, rollbackPoint); rollbackErr != nil {
                    return fmt.Errorf("migration failed and rollback failed: %v, rollback error: %w", err, rollbackErr)
                }
            }
            
            return err
        }
        
        // Create checkpoint after each successful phase
        if err := mc.createCheckpoint(p.phase); err != nil {
            log.Printf("Warning: failed to create checkpoint for phase %v: %v", p.phase, err)
        }
    }

    mc.updatePhase(PhaseComplete)
    return nil
}

// executePreChecks validates the migration can proceed
func (mc *MigrationController) executePreChecks(ctx context.Context) error {
    checks := []struct {
        name  string
        check func() error
    }{
        {"database_health", mc.checkDatabaseHealth},
        {"version_compatibility", mc.checkVersionCompatibility},
        {"disk_space", mc.checkDiskSpace},
        {"replication_lag", mc.checkReplicationLag},
        {"active_connections", mc.checkActiveConnections},
        {"long_running_queries", mc.checkLongRunningQueries},
    }

    for _, c := range checks {
        if err := c.check(); err != nil {
            return fmt.Errorf("pre-check '%s' failed: %w", c.name, err)
        }
    }

    return nil
}

// executeSchemaExpansion adds new schema elements
func (mc *MigrationController) executeSchemaExpansion(ctx context.Context) error {
    migrations := mc.migrationEngine.GetExpansionMigrations(
        mc.state.CurrentVersion,
        mc.state.TargetVersion,
    )

    for _, migration := range migrations {
        log.Printf("Executing expansion migration: %s", migration.ID)
        
        // Execute with retry logic
        err := mc.executeWithRetry(ctx, func() error {
            return mc.migrationEngine.ExecuteMigration(migration)
        }, 3, time.Second*5)
        
        if err != nil {
            return fmt.Errorf("expansion migration %s failed: %w", migration.ID, err)
        }
        
        // Verify migration success
        if err := mc.verifyMigration(migration); err != nil {
            return fmt.Errorf("verification failed for migration %s: %w", migration.ID, err)
        }
    }

    return nil
}

// executeDataMigration migrates data to new schema
func (mc *MigrationController) executeDataMigration(ctx context.Context) error {
    // Get data migration tasks
    tasks := mc.migrationEngine.GetDataMigrationTasks(
        mc.state.CurrentVersion,
        mc.state.TargetVersion,
    )

    // Execute migrations in parallel with controlled concurrency
    sem := make(chan struct{}, 4) // Max 4 concurrent migrations
    errChan := make(chan error, len(tasks))
    var wg sync.WaitGroup

    for _, task := range tasks {
        wg.Add(1)
        go func(t DataMigrationTask) {
            defer wg.Done()
            
            sem <- struct{}{} // Acquire semaphore
            defer func() { <-sem }() // Release semaphore
            
            if err := mc.executeDataMigrationTask(ctx, t); err != nil {
                errChan <- fmt.Errorf("task %s failed: %w", t.ID, err)
            }
        }(task)
    }

    wg.Wait()
    close(errChan)

    // Check for errors
    for err := range errChan {
        if err != nil {
            return err
        }
    }

    return nil
}

// executeDataMigrationTask executes a single data migration task
func (mc *MigrationController) executeDataMigrationTask(ctx context.Context, task DataMigrationTask) error {
    log.Printf("Starting data migration task: %s", task.ID)
    
    // Create progress tracker
    progress := NewProgressTracker(task.EstimatedRows)
    
    // Execute in batches
    batchSize := 1000
    offset := 0
    
    for {
        select {
        case <-ctx.Done():
            return ctx.Err()
        default:
        }
        
        // Process batch
        processed, err := mc.processBatch(task, offset, batchSize)
        if err != nil {
            return err
        }
        
        if processed == 0 {
            break // No more rows
        }
        
        offset += processed
        progress.Update(processed)
        
        // Log progress
        if progress.ShouldLog() {
            log.Printf("Task %s progress: %s", task.ID, progress.String())
        }
        
        // Throttle if needed
        if mc.shouldThrottle() {
            time.Sleep(100 * time.Millisecond)
        }
    }
    
    log.Printf("Completed data migration task: %s", task.ID)
    return nil
}

// Blue-Green Deployment Implementation
type BlueGreenController struct {
    blueDB     *sql.DB
    greenDB    *sql.DB
    loadBalancer LoadBalancer
    cdcEngine  CDCEngine
    validator  DataValidator
}

// ExecuteBlueGreenMigration performs blue-green deployment
func (bgc *BlueGreenController) ExecuteBlueGreenMigration(ctx context.Context, migration Migration) error {
    // Phase 1: Setup green environment
    log.Println("Phase 1: Setting up green environment")
    if err := bgc.setupGreenEnvironment(); err != nil {
        return fmt.Errorf("failed to setup green environment: %w", err)
    }

    // Phase 2: Start CDC replication
    log.Println("Phase 2: Starting CDC replication")
    replicationHandle, err := bgc.startCDCReplication()
    if err != nil {
        return fmt.Errorf("failed to start CDC: %w", err)
    }
    defer replicationHandle.Stop()

    // Phase 3: Apply schema changes to green
    log.Println("Phase 3: Applying schema changes")
    if err := bgc.applySchemaChanges(migration); err != nil {
        return fmt.Errorf("failed to apply schema changes: %w", err)
    }

    // Phase 4: Wait for replication to catch up
    log.Println("Phase 4: Waiting for replication sync")
    if err := bgc.waitForReplicationSync(ctx, replicationHandle); err != nil {
        return fmt.Errorf("replication sync failed: %w", err)
    }

    // Phase 5: Validate data consistency
    log.Println("Phase 5: Validating data consistency")
    if err := bgc.validateDataConsistency(); err != nil {
        return fmt.Errorf("data validation failed: %w", err)
    }

    // Phase 6: Switch traffic to green
    log.Println("Phase 6: Switching traffic")
    if err := bgc.switchTraffic(); err != nil {
        return fmt.Errorf("traffic switch failed: %w", err)
    }

    // Phase 7: Monitor and validate
    log.Println("Phase 7: Post-switch validation")
    if err := bgc.postSwitchValidation(ctx); err != nil {
        // Rollback if validation fails
        log.Println("Post-switch validation failed, rolling back")
        if rollbackErr := bgc.rollbackTrafficSwitch(); rollbackErr != nil {
            return fmt.Errorf("validation failed and rollback failed: %v, rollback error: %w", err, rollbackErr)
        }
        return err
    }

    return nil
}

// CDC Engine for real-time replication
type CDCEngine interface {
    StartReplication(source, target *sql.DB) (ReplicationHandle, error)
    GetReplicationLag() (time.Duration, error)
    ValidateConsistency() error
}

// PostgreSQL CDC Implementation
type PostgreSQLCDC struct {
    config CDCConfig
    decoder LogicalDecoder
}

func (p *PostgreSQLCDC) StartReplication(source, target *sql.DB) (ReplicationHandle, error) {
    // Create replication slot
    slotName := fmt.Sprintf("migration_%s", time.Now().Format("20060102150405"))
    
    _, err := source.Exec(`
        SELECT pg_create_logical_replication_slot($1, 'pgoutput')
    `, slotName)
    if err != nil {
        return nil, fmt.Errorf("failed to create replication slot: %w", err)
    }

    // Start replication connection
    replConn, err := p.createReplicationConnection()
    if err != nil {
        return nil, err
    }

    // Create replication handle
    handle := &postgresReplicationHandle{
        slotName:  slotName,
        conn:      replConn,
        target:    target,
        decoder:   p.decoder,
        stopChan:  make(chan struct{}),
        errorChan: make(chan error, 1),
    }

    // Start replication worker
    go handle.startReplication()

    return handle, nil
}

// expand-contract.go
package migration

import (
    "context"
    "database/sql"
    "fmt"
    "time"
)

// ExpandContractMigration implements the expand-contract pattern
type ExpandContractMigration struct {
    db              *sql.DB
    compatibilityMgr CompatibilityManager
    featureFlags    FeatureFlagService
}

// Column addition with backward compatibility
func (ecm *ExpandContractMigration) AddColumnWithCompatibility(
    ctx context.Context,
    table string,
    column ColumnDefinition,
) error {
    // Phase 1: Add column as nullable
    log.Printf("Phase 1: Adding column %s.%s as nullable", table, column.Name)
    
    addColumnSQL := fmt.Sprintf(`
        ALTER TABLE %s 
        ADD COLUMN IF NOT EXISTS %s %s NULL
    `, table, column.Name, column.Type)
    
    if _, err := ecm.db.ExecContext(ctx, addColumnSQL); err != nil {
        return fmt.Errorf("failed to add column: %w", err)
    }

    // Phase 2: Add database trigger for backward compatibility
    if column.DefaultValue != "" {
        triggerSQL := ecm.createCompatibilityTrigger(table, column)
        if _, err := ecm.db.ExecContext(ctx, triggerSQL); err != nil {
            return fmt.Errorf("failed to create compatibility trigger: %w", err)
        }
    }

    // Phase 3: Backfill existing data
    if column.BackfillStrategy != nil {
        if err := ecm.backfillData(ctx, table, column); err != nil {
            return fmt.Errorf("failed to backfill data: %w", err)
        }
    }

    // Phase 4: Enable feature flag for new column usage
    if err := ecm.featureFlags.Enable(fmt.Sprintf("use_%s_%s", table, column.Name)); err != nil {
        return fmt.Errorf("failed to enable feature flag: %w", err)
    }

    return nil
}

// createCompatibilityTrigger creates triggers for backward compatibility
func (ecm *ExpandContractMigration) createCompatibilityTrigger(
    table string,
    column ColumnDefinition,
) string {
    triggerName := fmt.Sprintf("compat_%s_%s", table, column.Name)
    
    return fmt.Sprintf(`
        CREATE OR REPLACE FUNCTION %s_func()
        RETURNS TRIGGER AS $$
        BEGIN
            -- Ensure new column has value
            IF NEW.%s IS NULL THEN
                NEW.%s = %s;
            END IF;
            
            -- Sync with old column if exists
            IF TG_OP = 'UPDATE' AND OLD.%s IS DISTINCT FROM NEW.%s THEN
                -- Update related old columns
                %s
            END IF;
            
            RETURN NEW;
        END;
        $$ LANGUAGE plpgsql;

        CREATE TRIGGER %s
        BEFORE INSERT OR UPDATE ON %s
        FOR EACH ROW
        EXECUTE FUNCTION %s_func();
    `, triggerName, column.Name, column.Name, column.DefaultValue,
       column.Name, column.Name, column.SyncLogic,
       triggerName, table, triggerName)
}

// backfillData implements various backfill strategies
func (ecm *ExpandContractMigration) backfillData(
    ctx context.Context,
    table string,
    column ColumnDefinition,
) error {
    switch column.BackfillStrategy.Type {
    case BackfillBatch:
        return ecm.batchBackfill(ctx, table, column)
    case BackfillOnline:
        return ecm.onlineBackfill(ctx, table, column)
    case BackfillLazy:
        return ecm.lazyBackfill(ctx, table, column)
    default:
        return fmt.Errorf("unknown backfill strategy: %v", column.BackfillStrategy.Type)
    }
}

// batchBackfill performs batched data backfill
func (ecm *ExpandContractMigration) batchBackfill(
    ctx context.Context,
    table string,
    column ColumnDefinition,
) error {
    batchSize := 1000
    if column.BackfillStrategy.BatchSize > 0 {
        batchSize = column.BackfillStrategy.BatchSize
    }

    var lastID int64
    totalRows := 0

    for {
        // Update batch of rows
        query := fmt.Sprintf(`
            UPDATE %s 
            SET %s = %s
            WHERE id > $1 
            AND %s IS NULL
            ORDER BY id
            LIMIT $2
            RETURNING id
        `, table, column.Name, column.BackfillStrategy.ValueExpression,
           column.Name)

        rows, err := ecm.db.QueryContext(ctx, query, lastID, batchSize)
        if err != nil {
            return err
        }

        updatedCount := 0
        for rows.Next() {
            if err := rows.Scan(&lastID); err != nil {
                rows.Close()
                return err
            }
            updatedCount++
        }
        rows.Close()

        if updatedCount == 0 {
            break // No more rows to update
        }

        totalRows += updatedCount
        log.Printf("Backfilled %d rows (total: %d)", updatedCount, totalRows)

        // Throttle to avoid overload
        time.Sleep(100 * time.Millisecond)
    }

    return nil
}

// Table renaming with zero downtime
func (ecm *ExpandContractMigration) RenameTableZeroDowntime(
    ctx context.Context,
    oldName, newName string,
) error {
    // Step 1: Create updatable view with old name
    viewSQL := fmt.Sprintf(`
        CREATE OR REPLACE VIEW %s AS
        SELECT * FROM %s
    `, oldName, newName)
    
    if _, err := ecm.db.ExecContext(ctx, viewSQL); err != nil {
        return fmt.Errorf("failed to create compatibility view: %w", err)
    }

    // Step 2: Create INSTEAD OF triggers for the view
    triggers := []string{
        ecm.createInsertTrigger(oldName, newName),
        ecm.createUpdateTrigger(oldName, newName),
        ecm.createDeleteTrigger(oldName, newName),
    }

    for _, trigger := range triggers {
        if _, err := ecm.db.ExecContext(ctx, trigger); err != nil {
            return fmt.Errorf("failed to create trigger: %w", err)
        }
    }

    // Step 3: Update feature flag
    if err := ecm.featureFlags.Enable(fmt.Sprintf("use_table_%s", newName)); err != nil {
        return fmt.Errorf("failed to enable feature flag: %w", err)
    }

    return nil
}

// Complex schema transformation
type SchemaTransformation struct {
    Type        TransformationType
    Source      SchemaElement
    Target      SchemaElement
    Mapping     DataMapping
    Validation  ValidationRules
}

func (ecm *ExpandContractMigration) ExecuteTransformation(
    ctx context.Context,
    transformation SchemaTransformation,
) error {
    // Create transformation plan
    plan := ecm.createTransformationPlan(transformation)
    
    // Execute plan phases
    for i, phase := range plan.Phases {
        log.Printf("Executing transformation phase %d/%d: %s", 
            i+1, len(plan.Phases), phase.Description)
        
        // Create savepoint for phase
        savepoint := fmt.Sprintf("phase_%d", i)
        if _, err := ecm.db.ExecContext(ctx, "SAVEPOINT "+savepoint); err != nil {
            return err
        }
        
        // Execute phase
        if err := phase.Execute(ctx, ecm.db); err != nil {
            // Rollback to savepoint
            if _, rbErr := ecm.db.ExecContext(ctx, "ROLLBACK TO SAVEPOINT "+savepoint); rbErr != nil {
                return fmt.Errorf("phase failed and rollback failed: %v, rollback error: %w", err, rbErr)
            }
            return fmt.Errorf("transformation phase %d failed: %w", i+1, err)
        }
        
        // Validate phase results
        if err := phase.Validate(ctx, ecm.db); err != nil {
            return fmt.Errorf("validation failed for phase %d: %w", i+1, err)
        }
    }
    
    return nil
}

-- postgresql-expand-contract.sql
-- Advanced expand-contract patterns for PostgreSQL

-- Pattern 1: Adding NOT NULL column with zero downtime
-- Phase 1: Add nullable column
ALTER TABLE orders ADD COLUMN status varchar(50);

-- Phase 2: Add check constraint (not enforced yet)
ALTER TABLE orders 
ADD CONSTRAINT orders_status_not_null 
CHECK (status IS NOT NULL) NOT VALID;

-- Phase 3: Backfill in batches
DO $$
DECLARE
    batch_size INTEGER := 10000;
    updated INTEGER;
BEGIN
    LOOP
        UPDATE orders 
        SET status = 'pending'
        WHERE status IS NULL
        AND id IN (
            SELECT id FROM orders 
            WHERE status IS NULL 
            LIMIT batch_size
            FOR UPDATE SKIP LOCKED
        );
        
        GET DIAGNOSTICS updated = ROW_COUNT;
        
        IF updated = 0 THEN
            EXIT;
        END IF;
        
        -- Prevent long-running transaction
        COMMIT;
        
        -- Brief pause to reduce load
        PERFORM pg_sleep(0.1);
    END LOOP;
END $$;

-- Phase 4: Validate constraint
ALTER TABLE orders VALIDATE CONSTRAINT orders_status_not_null;

-- Phase 5: Convert to NOT NULL
ALTER TABLE orders ALTER COLUMN status SET NOT NULL;
ALTER TABLE orders DROP CONSTRAINT orders_status_not_null;

-- Pattern 2: Splitting tables with zero downtime
-- Original table: users (id, email, profile_data, settings_data)
-- Target: users (id, email), user_profiles (user_id, ...), user_settings (user_id, ...)

-- Phase 1: Create new tables
CREATE TABLE user_profiles (
    user_id INTEGER PRIMARY KEY REFERENCES users(id),
    bio TEXT,
    avatar_url VARCHAR(500),
    created_at TIMESTAMPTZ DEFAULT NOW(),
    updated_at TIMESTAMPTZ DEFAULT NOW()
);

CREATE TABLE user_settings (
    user_id INTEGER PRIMARY KEY REFERENCES users(id),
    theme VARCHAR(50) DEFAULT 'light',
    notifications_enabled BOOLEAN DEFAULT true,
    language VARCHAR(10) DEFAULT 'en',
    created_at TIMESTAMPTZ DEFAULT NOW(),
    updated_at TIMESTAMPTZ DEFAULT NOW()
);

-- Phase 2: Create triggers to keep data in sync
CREATE OR REPLACE FUNCTION sync_user_split_tables()
RETURNS TRIGGER AS $$
BEGIN
    IF TG_OP = 'INSERT' THEN
        -- Extract and insert profile data
        INSERT INTO user_profiles (user_id, bio, avatar_url)
        VALUES (
            NEW.id,
            (NEW.profile_data->>'bio')::TEXT,
            (NEW.profile_data->>'avatar_url')::VARCHAR(500)
        ) ON CONFLICT (user_id) DO UPDATE SET
            bio = EXCLUDED.bio,
            avatar_url = EXCLUDED.avatar_url,
            updated_at = NOW();
        
        -- Extract and insert settings data
        INSERT INTO user_settings (user_id, theme, notifications_enabled, language)
        VALUES (
            NEW.id,
            COALESCE((NEW.settings_data->>'theme')::VARCHAR(50), 'light'),
            COALESCE((NEW.settings_data->>'notifications_enabled')::BOOLEAN, true),
            COALESCE((NEW.settings_data->>'language')::VARCHAR(10), 'en')
        ) ON CONFLICT (user_id) DO UPDATE SET
            theme = EXCLUDED.theme,
            notifications_enabled = EXCLUDED.notifications_enabled,
            language = EXCLUDED.language,
            updated_at = NOW();
            
    ELSIF TG_OP = 'UPDATE' THEN
        -- Update profile if data changed
        IF OLD.profile_data IS DISTINCT FROM NEW.profile_data THEN
            UPDATE user_profiles SET
                bio = (NEW.profile_data->>'bio')::TEXT,
                avatar_url = (NEW.profile_data->>'avatar_url')::VARCHAR(500),
                updated_at = NOW()
            WHERE user_id = NEW.id;
        END IF;
        
        -- Update settings if data changed
        IF OLD.settings_data IS DISTINCT FROM NEW.settings_data THEN
            UPDATE user_settings SET
                theme = COALESCE((NEW.settings_data->>'theme')::VARCHAR(50), theme),
                notifications_enabled = COALESCE((NEW.settings_data->>'notifications_enabled')::BOOLEAN, notifications_enabled),
                language = COALESCE((NEW.settings_data->>'language')::VARCHAR(10), language),
                updated_at = NOW()
            WHERE user_id = NEW.id;
        END IF;
        
    ELSIF TG_OP = 'DELETE' THEN
        DELETE FROM user_profiles WHERE user_id = OLD.id;
        DELETE FROM user_settings WHERE user_id = OLD.id;
    END IF;
    
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER sync_user_tables_trigger
AFTER INSERT OR UPDATE OR DELETE ON users
FOR EACH ROW
EXECUTE FUNCTION sync_user_split_tables();

-- Phase 3: Backfill existing data
INSERT INTO user_profiles (user_id, bio, avatar_url)
SELECT 
    id,
    (profile_data->>'bio')::TEXT,
    (profile_data->>'avatar_url')::VARCHAR(500)
FROM users
ON CONFLICT (user_id) DO NOTHING;

INSERT INTO user_settings (user_id, theme, notifications_enabled, language)
SELECT 
    id,
    COALESCE((settings_data->>'theme')::VARCHAR(50), 'light'),
    COALESCE((settings_data->>'notifications_enabled')::BOOLEAN, true),
    COALESCE((settings_data->>'language')::VARCHAR(10), 'en')
FROM users
ON CONFLICT (user_id) DO NOTHING;

-- Phase 4: Create views for backward compatibility
CREATE OR REPLACE VIEW users_legacy AS
SELECT 
    u.id,
    u.email,
    jsonb_build_object(
        'bio', p.bio,
        'avatar_url', p.avatar_url
    ) as profile_data,
    jsonb_build_object(
        'theme', s.theme,
        'notifications_enabled', s.notifications_enabled,
        'language', s.language
    ) as settings_data,
    u.created_at,
    u.updated_at
FROM users u
LEFT JOIN user_profiles p ON u.id = p.user_id
LEFT JOIN user_settings s ON u.id = s.user_id;

-- Phase 5: After application migration, drop old columns
-- ALTER TABLE users DROP COLUMN profile_data;
-- ALTER TABLE users DROP COLUMN settings_data;
-- DROP VIEW users_legacy;
-- DROP TRIGGER sync_user_tables_trigger ON users;
-- DROP FUNCTION sync_user_split_tables();

// version-compatibility.go
package migration

import (
    "context"
    "fmt"
    "sync"
)

// VersionCompatibilityManager manages multi-version database support
type VersionCompatibilityManager struct {
    versions      map[string]VersionHandler
    activeVersion string
    router        QueryRouter
    mu            sync.RWMutex
}

// VersionHandler handles version-specific logic
type VersionHandler struct {
    Version        string
    SchemaAdapter  SchemaAdapter
    QueryRewriter  QueryRewriter
    DataTransformer DataTransformer
}

// QueryRouter routes queries to appropriate version handlers
type QueryRouter struct {
    rules    []RoutingRule
    fallback VersionHandler
}

// Execute query with version compatibility
func (vcm *VersionCompatibilityManager) ExecuteQuery(
    ctx context.Context,
    query Query,
    clientVersion string,
) (Result, error) {
    vcm.mu.RLock()
    handler, exists := vcm.versions[clientVersion]
    vcm.mu.RUnlock()
    
    if !exists {
        // Use compatibility layer for unknown versions
        handler = vcm.getCompatibleHandler(clientVersion)
    }
    
    // Rewrite query for target version
    rewrittenQuery, err := handler.QueryRewriter.Rewrite(query)
    if err != nil {
        return nil, fmt.Errorf("query rewrite failed: %w", err)
    }
    
    // Execute query
    result, err := vcm.executeVersionedQuery(ctx, rewrittenQuery, handler)
    if err != nil {
        return nil, err
    }
    
    // Transform result for client version
    transformedResult, err := handler.DataTransformer.Transform(result, clientVersion)
    if err != nil {
        return nil, fmt.Errorf("result transformation failed: %w", err)
    }
    
    return transformedResult, nil
}

// SchemaAdapter provides version-specific schema adaptations
type SchemaAdapter interface {
    AdaptSchema(fromVersion, toVersion string) ([]Migration, error)
    ValidateCompatibility(version string) error
}

// PostgreSQL Schema Adapter
type PostgreSQLSchemaAdapter struct {
    db *sql.DB
}

func (psa *PostgreSQLSchemaAdapter) AdaptSchema(fromVersion, toVersion string) ([]Migration, error) {
    // Generate migrations based on version differences
    migrations := []Migration{}
    
    // Example: v1 to v2 adds new column
    if fromVersion == "v1" && toVersion == "v2" {
        migrations = append(migrations, Migration{
            ID: "add_feature_flags",
            Up: `
                ALTER TABLE users 
                ADD COLUMN IF NOT EXISTS feature_flags JSONB DEFAULT '{}';
                
                CREATE INDEX IF NOT EXISTS idx_users_feature_flags 
                ON users USING gin(feature_flags);
            `,
            Down: `
                ALTER TABLE users DROP COLUMN IF EXISTS feature_flags;
            `,
        })
    }
    
    // Example: v2 to v3 changes data types
    if fromVersion == "v2" && toVersion == "v3" {
        migrations = append(migrations, Migration{
            ID: "update_timestamp_precision",
            Up: `
                -- Add new column with microsecond precision
                ALTER TABLE events 
                ADD COLUMN created_at_precise TIMESTAMPTZ(6);
                
                -- Copy data with precision
                UPDATE events 
                SET created_at_precise = created_at;
                
                -- Swap columns
                ALTER TABLE events 
                DROP COLUMN created_at;
                
                ALTER TABLE events 
                RENAME COLUMN created_at_precise TO created_at;
            `,
            Down: `
                -- Revert to second precision
                ALTER TABLE events 
                ADD COLUMN created_at_standard TIMESTAMPTZ(0);
                
                UPDATE events 
                SET created_at_standard = created_at;
                
                ALTER TABLE events 
                DROP COLUMN created_at;
                
                ALTER TABLE events 
                RENAME COLUMN created_at_standard TO created_at;
            `,
        })
    }
    
    return migrations, nil
}

// Query rewriter for version compatibility
type QueryRewriter interface {
    Rewrite(query Query) (Query, error)
}

type SmartQueryRewriter struct {
    rules []RewriteRule
}

type RewriteRule struct {
    Pattern     string
    Replacement string
    Condition   func(Query) bool
}

func (sqr *SmartQueryRewriter) Rewrite(query Query) (Query, error) {
    rewritten := query
    
    for _, rule := range sqr.rules {
        if rule.Condition(query) {
            rewritten = applyRewriteRule(rewritten, rule)
        }
    }
    
    return rewritten, nil
}

// API Version Adapter for database changes
type APIVersionAdapter struct {
    versionMappings map[string]FieldMapping
}

type FieldMapping struct {
    OldField   string
    NewField   string
    Transform  func(interface{}) interface{}
}

// Transform API response based on client version
func (ava *APIVersionAdapter) TransformResponse(
    data interface{},
    clientVersion string,
) (interface{}, error) {
    mapping, exists := ava.versionMappings[clientVersion]
    if !exists {
        return data, nil // No transformation needed
    }
    
    // Apply field mappings
    transformed := make(map[string]interface{})
    
    switch d := data.(type) {
    case map[string]interface{}:
        for key, value := range d {
            if mapping.OldField == key {
                // Apply transformation
                if mapping.Transform != nil {
                    value = mapping.Transform(value)
                }
                transformed[mapping.NewField] = value
            } else {
                transformed[key] = value
            }
        }
    default:
        return data, nil
    }
    
    return transformed, nil
}

// Compatibility testing framework
type CompatibilityTester struct {
    versions []string
    testSuite TestSuite
}

func (ct *CompatibilityTester) TestCrossVersionCompatibility() error {
    results := make(map[string]map[string]TestResult)
    
    // Test all version combinations
    for _, fromVersion := range ct.versions {
        results[fromVersion] = make(map[string]TestResult)
        
        for _, toVersion := range ct.versions {
            result := ct.testVersionPair(fromVersion, toVersion)
            results[fromVersion][toVersion] = result
            
            if !result.Success {
                log.Printf("Compatibility test failed: %s -> %s: %v",
                    fromVersion, toVersion, result.Error)
            }
        }
    }
    
    // Generate compatibility matrix
    ct.generateCompatibilityReport(results)
    
    return nil
}

// rollback-handler.go
package migration

import (
    "context"
    "encoding/json"
    "fmt"
    "time"
)

// RollbackHandler manages migration rollbacks
type RollbackHandler struct {
    db               *sql.DB
    snapshotStore    SnapshotStore
    auditLog         AuditLogger
    validationEngine ValidationEngine
}

// RollbackPoint represents a point in time for rollback
type RollbackPoint struct {
    ID            string
    Timestamp     time.Time
    Version       string
    SchemaHash    string
    DataSnapshot  string
    Metadata      map[string]interface{}
}

// CreateRollbackPoint creates a new rollback point
func (rh *RollbackHandler) CreateRollbackPoint() (*RollbackPoint, error) {
    // Capture current schema
    schemaHash, err := rh.calculateSchemaHash()
    if err != nil {
        return nil, fmt.Errorf("failed to calculate schema hash: %w", err)
    }
    
    // Create data snapshot
    snapshotID, err := rh.snapshotStore.CreateSnapshot()
    if err != nil {
        return nil, fmt.Errorf("failed to create snapshot: %w", err)
    }
    
    rollbackPoint := &RollbackPoint{
        ID:           generateID(),
        Timestamp:    time.Now(),
        Version:      rh.getCurrentVersion(),
        SchemaHash:   schemaHash,
        DataSnapshot: snapshotID,
        Metadata: map[string]interface{}{
            "trigger": "migration",
            "user":    getCurrentUser(),
        },
    }
    
    // Store rollback point
    if err := rh.storeRollbackPoint(rollbackPoint); err != nil {
        return nil, err
    }
    
    return rollbackPoint, nil
}

// ExecuteRollback performs a rollback to a specific point
func (rh *RollbackHandler) ExecuteRollback(
    ctx context.Context,
    rollbackPoint *RollbackPoint,
) error {
    // Validate rollback is possible
    if err := rh.validateRollback(rollbackPoint); err != nil {
        return fmt.Errorf("rollback validation failed: %w", err)
    }
    
    // Create audit entry
    auditID := rh.auditLog.StartRollback(rollbackPoint)
    defer rh.auditLog.CompleteRollback(auditID)
    
    // Phase 1: Stop writes
    if err := rh.enableReadOnlyMode(); err != nil {
        return fmt.Errorf("failed to enable read-only mode: %w", err)
    }
    defer rh.disableReadOnlyMode()
    
    // Phase 2: Restore schema
    if err := rh.restoreSchema(ctx, rollbackPoint); err != nil {
        return fmt.Errorf("schema restoration failed: %w", err)
    }
    
    // Phase 3: Restore data
    if err := rh.restoreData(ctx, rollbackPoint); err != nil {
        return fmt.Errorf("data restoration failed: %w", err)
    }
    
    // Phase 4: Validate restoration
    if err := rh.validateRestoration(ctx, rollbackPoint); err != nil {
        return fmt.Errorf("restoration validation failed: %w", err)
    }
    
    return nil
}

// restoreSchema restores database schema
func (rh *RollbackHandler) restoreSchema(
    ctx context.Context,
    rollbackPoint *RollbackPoint,
) error {
    // Get schema differences
    currentSchema, err := rh.getCurrentSchema()
    if err != nil {
        return err
    }
    
    targetSchema, err := rh.getSchemaAtPoint(rollbackPoint)
    if err != nil {
        return err
    }
    
    // Generate reverse migrations
    reverseMigrations := rh.generateReverseMigrations(currentSchema, targetSchema)
    
    // Execute reverse migrations in order
    for _, migration := range reverseMigrations {
        log.Printf("Executing reverse migration: %s", migration.ID)
        
        if err := rh.executeMigration(ctx, migration); err != nil {
            return fmt.Errorf("reverse migration %s failed: %w", migration.ID, err)
        }
    }
    
    return nil
}

// Time-based rollback with point-in-time recovery
type PointInTimeRecovery struct {
    walArchive   WALArchive
    baseBackup   BaseBackup
    recoveryMgr  RecoveryManager
}

func (pitr *PointInTimeRecovery) RecoverToTimestamp(
    ctx context.Context,
    targetTime time.Time,
) error {
    // Find appropriate base backup
    backup, err := pitr.baseBackup.FindNearestBefore(targetTime)
    if err != nil {
        return fmt.Errorf("no suitable backup found: %w", err)
    }
    
    // Restore base backup
    if err := pitr.restoreBaseBackup(ctx, backup); err != nil {
        return fmt.Errorf("base backup restoration failed: %w", err)
    }
    
    // Apply WAL up to target time
    if err := pitr.applyWALToTime(ctx, backup.Timestamp, targetTime); err != nil {
        return fmt.Errorf("WAL replay failed: %w", err)
    }
    
    return nil
}

// Automated rollback decision engine
type RollbackDecisionEngine struct {
    metrics      MetricsCollector
    thresholds   RollbackThresholds
    aiPredictor  AnomalyDetector
}

type RollbackThresholds struct {
    ErrorRateThreshold    float64
    LatencyThreshold      time.Duration
    DataIntegrityFailures int
    CustomerImpact        float64
}

func (rde *RollbackDecisionEngine) ShouldRollback() (bool, string) {
    // Collect current metrics
    currentMetrics := rde.metrics.GetCurrentMetrics()
    
    // Check error rate
    if currentMetrics.ErrorRate > rde.thresholds.ErrorRateThreshold {
        return true, fmt.Sprintf("Error rate %.2f%% exceeds threshold %.2f%%",
            currentMetrics.ErrorRate*100, rde.thresholds.ErrorRateThreshold*100)
    }
    
    // Check latency
    if currentMetrics.P99Latency > rde.thresholds.LatencyThreshold {
        return true, fmt.Sprintf("P99 latency %v exceeds threshold %v",
            currentMetrics.P99Latency, rde.thresholds.LatencyThreshold)
    }
    
    // Check data integrity
    integrityErrors := rde.checkDataIntegrity()
    if integrityErrors > rde.thresholds.DataIntegrityFailures {
        return true, fmt.Sprintf("Data integrity failures: %d", integrityErrors)
    }
    
    // AI-based anomaly detection
    if anomaly := rde.aiPredictor.DetectAnomaly(currentMetrics); anomaly != nil {
        return true, fmt.Sprintf("Anomaly detected: %s", anomaly.Description)
    }
    
    return false, ""
}

// Rollback verification
func (rh *RollbackHandler) validateRestoration(
    ctx context.Context,
    rollbackPoint *RollbackPoint,
) error {
    validations := []ValidationCheck{
        {
            Name: "Schema Integrity",
            Check: func() error {
                return rh.validateSchemaIntegrity(rollbackPoint.SchemaHash)
            },
        },
        {
            Name: "Data Consistency",
            Check: func() error {
                return rh.validateDataConsistency(rollbackPoint)
            },
        },
        {
            Name: "Foreign Key Constraints",
            Check: func() error {
                return rh.validateForeignKeys()
            },
        },
        {
            Name: "Index Integrity",
            Check: func() error {
                return rh.validateIndexes()
            },
        },
    }
    
    for _, validation := range validations {
        if err := validation.Check(); err != nil {
            return fmt.Errorf("validation '%s' failed: %w", validation.Name, err)
        }
    }
    
    return nil
}

// flyway-enterprise-config.java
package com.enterprise.migration;

import org.flywaydb.core.Flyway;
import org.flywaydb.core.api.configuration.FluentConfiguration;
import org.flywaydb.core.api.callback.Callback;
import org.flywaydb.core.api.callback.Context;
import org.flywaydb.core.api.callback.Event;

public class FlywayEnterpriseConfig {
    
    public static Flyway configureEnterpriseFlyway() {
        FluentConfiguration config = Flyway.configure()
            .dataSource(getDataSource())
            .schemas("public", "audit", "archive")
            .table("schema_version")
            .baselineOnMigrate(true)
            .baselineVersion("1.0")
            .installedBy("migration-service")
            .mixed(true) // Allow mixing transactional and non-transactional
            .group(true) // Group multiple migrations in single transaction
            .outOfOrder(false) // Enforce order
            .validateOnMigrate(true)
            .cleanDisabled(true) // Prevent accidental clean in production
            .callbacks(
                new MigrationAuditCallback(),
                new PerformanceMonitorCallback(),
                new RollbackPrepareCallback()
            )
            .placeholders(Map.of(
                "environment", System.getenv("ENVIRONMENT"),
                "region", System.getenv("AWS_REGION")
            ))
            .locations(
                "classpath:db/migration",
                "filesystem:/opt/migrations/custom",
                "s3:my-bucket/migrations"
            )
            .resolvers(
                new ConditionalMigrationResolver(),
                new EncryptedMigrationResolver()
            )
            .target("latest")
            .cherryPick("2.1", "2.3", "3.0"); // Selective migrations
            
        return new Flyway(config);
    }
    
    // Custom callback for audit logging
    static class MigrationAuditCallback implements Callback {
        @Override
        public boolean supports(Event event, Context context) {
            return true;
        }
        
        @Override
        public boolean canHandleInTransaction(Event event, Context context) {
            return true;
        }
        
        @Override
        public void handle(Event event, Context context) {
            if (event == Event.BEFORE_MIGRATE) {
                logMigrationStart(context);
                createRollbackPoint(context);
            } else if (event == Event.AFTER_MIGRATE) {
                logMigrationComplete(context);
                validateMigration(context);
            } else if (event == Event.AFTER_MIGRATE_ERROR) {
                logMigrationFailure(context);
                if (shouldAutoRollback(context)) {
                    executeRollback(context);
                }
            }
        }
    }
    
    // Advanced migration with zero-downtime
    public static void executeZeroDowntimeMigration() {
        Flyway flyway = configureEnterpriseFlyway();
        
        // Phase 1: Validate pending migrations
        MigrationValidationResult validation = flyway.validateWithResult();
        if (!validation.validationSuccessful) {
            throw new RuntimeException("Validation failed: " + validation.errorDetails);
        }
        
        // Phase 2: Create pre-migration snapshot
        createDatabaseSnapshot();
        
        // Phase 3: Execute migrations with monitoring
        try {
            flyway.migrate();
        } catch (Exception e) {
            // Automatic rollback on failure
            rollbackToSnapshot();
            throw e;
        }
        
        // Phase 4: Post-migration validation
        performPostMigrationValidation();
    }
}

<!-- liquibase-enterprise.xml -->
<?xml version="1.0" encoding="UTF-8"?>
<databaseChangeLog
    xmlns="http://www.liquibase.org/xml/ns/dbchangelog"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:ext="http://www.liquibase.org/xml/ns/dbchangelog-ext"
    xsi:schemaLocation="http://www.liquibase.org/xml/ns/dbchangelog
        http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-4.17.xsd
        http://www.liquibase.org/xml/ns/dbchangelog-ext
        http://www.liquibase.org/xml/ns/dbchangelog/dbchangelog-ext.xsd">

    <!-- Properties for environment-specific configuration -->
    <property name="table.prefix" value="${env.TABLE_PREFIX}" global="true"/>
    <property name="index.tablespace" value="${env.INDEX_TABLESPACE}" global="true"/>
    
    <!-- Include modular changesets -->
    <include file="db/changelog/releases/v1.0/master.xml"/>
    <include file="db/changelog/releases/v2.0/master.xml"/>
    
    <!-- Conditional changeset based on database type -->
    <changeSet id="2024-01-001" author="migration-team">
        <preConditions onFail="MARK_RAN">
            <dbms type="postgresql"/>
            <sqlCheck expectedResult="0">
                SELECT COUNT(*) FROM pg_extension WHERE extname = 'uuid-ossp'
            </sqlCheck>
        </preConditions>
        <sql>CREATE EXTENSION IF NOT EXISTS "uuid-ossp";</sql>
    </changeSet>
    
    <!-- Zero-downtime column addition with backfill -->
    <changeSet id="2024-01-002" author="migration-team" runInTransaction="false">
        <comment>Add user_preferences column with zero downtime</comment>
        
        <!-- Step 1: Add nullable column -->
        <addColumn tableName="users">
            <column name="user_preferences" type="jsonb">
                <constraints nullable="true"/>
            </column>
        </addColumn>
        
        <!-- Step 2: Create backfill function -->
        <createProcedure>
            CREATE OR REPLACE FUNCTION backfill_user_preferences()
            RETURNS void AS $$
            DECLARE
                batch_size INTEGER := 1000;
                total_updated INTEGER := 0;
                batch_updated INTEGER;
            BEGIN
                LOOP
                    UPDATE users
                    SET user_preferences = jsonb_build_object(
                        'theme', 'light',
                        'language', 'en',
                        'notifications', true
                    )
                    WHERE user_preferences IS NULL
                    LIMIT batch_size;
                    
                    GET DIAGNOSTICS batch_updated = ROW_COUNT;
                    total_updated := total_updated + batch_updated;
                    
                    IF batch_updated = 0 THEN
                        EXIT;
                    END IF;
                    
                    -- Brief pause between batches
                    PERFORM pg_sleep(0.1);
                    
                    RAISE NOTICE 'Backfilled % rows', total_updated;
                END LOOP;
            END;
            $$ LANGUAGE plpgsql;
        </createProcedure>
        
        <!-- Step 3: Execute backfill -->
        <sql>SELECT backfill_user_preferences();</sql>
        
        <!-- Step 4: Add NOT NULL constraint -->
        <addNotNullConstraint 
            tableName="users" 
            columnName="user_preferences"
            constraintName="users_preferences_not_null"/>
            
        <rollback>
            <dropColumn tableName="users" columnName="user_preferences"/>
            <dropProcedure procedureName="backfill_user_preferences"/>
        </rollback>
    </changeSet>
    
    <!-- Complex migration with multiple steps -->
    <changeSet id="2024-01-003" author="migration-team" context="production">
        <validCheckSum>8:b3f2a4c5d6e7f8a9b0c1d2e3f4a5b6c7</validCheckSum>
        
        <preConditions onFail="HALT">
            <tableExists tableName="orders"/>
            <columnExists tableName="orders" columnName="status"/>
        </preConditions>
        
        <!-- Create new enum type -->
        <sql splitStatements="false">
            DO $$
            BEGIN
                IF NOT EXISTS (SELECT 1 FROM pg_type WHERE typname = 'order_status_v2') THEN
                    CREATE TYPE order_status_v2 AS ENUM (
                        'pending', 'processing', 'shipped', 'delivered', 'cancelled', 'refunded'
                    );
                END IF;
            END $$;
        </sql>
        
        <!-- Add new column with new type -->
        <addColumn tableName="orders">
            <column name="status_v2" type="order_status_v2"/>
        </addColumn>
        
        <!-- Migrate data with mapping -->
        <sql>
            UPDATE orders SET status_v2 = 
                CASE status
                    WHEN 'new' THEN 'pending'::order_status_v2
                    WHEN 'in_progress' THEN 'processing'::order_status_v2
                    WHEN 'completed' THEN 'delivered'::order_status_v2
                    ELSE status::order_status_v2
                END;
        </sql>
        
        <rollback>
            <dropColumn tableName="orders" columnName="status_v2"/>
            <sql>DROP TYPE IF EXISTS order_status_v2;</sql>
        </rollback>
    </changeSet>

</databaseChangeLog>

// custom-migration-framework.go
package migration

import (
    "context"
    "crypto/sha256"
    "database/sql"
    "encoding/hex"
    "fmt"
    "io/fs"
    "path/filepath"
    "regexp"
    "sort"
    "time"
)

// CustomMigrationEngine provides advanced migration capabilities
type CustomMigrationEngine struct {
    db              *sql.DB
    config          MigrationConfig
    parser          MigrationParser
    executor        MigrationExecutor
    validator       MigrationValidator
    hooks           MigrationHooks
}

// MigrationConfig holds configuration for migrations
type MigrationConfig struct {
    MigrationsPath   string
    MigrationsTable  string
    SchemaName       string
    LockTimeout      time.Duration
    StatementTimeout time.Duration
    DryRun          bool
    ValidateChecksums bool
    AllowOutOfOrder  bool
    BaselineVersion  string
}

// Migration represents a single migration
type Migration struct {
    Version     string
    Description string
    Type        MigrationType
    Checksum    string
    Script      string
    UpSQL       []string
    DownSQL     []string
    Metadata    map[string]string
}

// MigrationType indicates the type of migration
type MigrationType int

const (
    MigrationTypeSQL MigrationType = iota
    MigrationTypeGo
    MigrationTypeProcedural
)

// Execute runs all pending migrations
func (cme *CustomMigrationEngine) Execute(ctx context.Context) error {
    // Acquire migration lock
    lock, err := cme.acquireLock(ctx)
    if err != nil {
        return fmt.Errorf("failed to acquire migration lock: %w", err)
    }
    defer lock.Release()

    // Get migration history
    history, err := cme.getMigrationHistory()
    if err != nil {
        return err
    }

    // Discover migrations
    migrations, err := cme.discoverMigrations()
    if err != nil {
        return err
    }

    // Determine pending migrations
    pending := cme.determinePendingMigrations(migrations, history)
    if len(pending) == 0 {
        log.Println("No pending migrations")
        return nil
    }

    log.Printf("Found %d pending migrations", len(pending))

    // Execute migrations
    for _, migration := range pending {
        if err := cme.executeMigration(ctx, migration); err != nil {
            return fmt.Errorf("migration %s failed: %w", migration.Version, err)
        }
    }

    return nil
}

// executeMigration executes a single migration with full lifecycle
func (cme *CustomMigrationEngine) executeMigration(ctx context.Context, migration Migration) error {
    log.Printf("Executing migration %s: %s", migration.Version, migration.Description)

    // Pre-execution hooks
    if err := cme.hooks.PreMigration(ctx, migration); err != nil {
        return fmt.Errorf("pre-migration hook failed: %w", err)
    }

    // Start transaction
    tx, err := cme.db.BeginTx(ctx, &sql.TxOptions{
        Isolation: sql.LevelSerializable,
    })
    if err != nil {
        return err
    }
    defer tx.Rollback()

    // Set timeouts
    if _, err := tx.ExecContext(ctx, fmt.Sprintf("SET LOCAL lock_timeout = '%s'", cme.config.LockTimeout)); err != nil {
        return err
    }
    if _, err := tx.ExecContext(ctx, fmt.Sprintf("SET LOCAL statement_timeout = '%s'", cme.config.StatementTimeout)); err != nil {
        return err
    }

    // Execute migration based on type
    switch migration.Type {
    case MigrationTypeSQL:
        err = cme.executeSQLMigration(ctx, tx, migration)
    case MigrationTypeGo:
        err = cme.executeGoMigration(ctx, tx, migration)
    case MigrationTypeProcedural:
        err = cme.executeProceduralMigration(ctx, tx, migration)
    default:
        err = fmt.Errorf("unknown migration type: %v", migration.Type)
    }

    if err != nil {
        return err
    }

    // Record migration
    if err := cme.recordMigration(ctx, tx, migration); err != nil {
        return fmt.Errorf("failed to record migration: %w", err)
    }

    // Validate migration
    if err := cme.validator.Validate(ctx, tx, migration); err != nil {
        return fmt.Errorf("migration validation failed: %w", err)
    }

    // Commit transaction
    if err := tx.Commit(); err != nil {
        return fmt.Errorf("failed to commit migration: %w", err)
    }

    // Post-execution hooks
    if err := cme.hooks.PostMigration(ctx, migration); err != nil {
        log.Printf("Warning: post-migration hook failed: %v", err)
    }

    log.Printf("Successfully completed migration %s", migration.Version)
    return nil
}

// Advanced migration discovery with multiple sources
func (cme *CustomMigrationEngine) discoverMigrations() ([]Migration, error) {
    var migrations []Migration

    // Discover from filesystem
    fsMigrations, err := cme.discoverFileSystemMigrations()
    if err != nil {
        return nil, err
    }
    migrations = append(migrations, fsMigrations...)

    // Discover from embedded resources
    embeddedMigrations, err := cme.discoverEmbeddedMigrations()
    if err != nil {
        return nil, err
    }
    migrations = append(migrations, embeddedMigrations...)

    // Discover from remote sources (S3, Git, etc.)
    remoteMigrations, err := cme.discoverRemoteMigrations()
    if err != nil {
        return nil, err
    }
    migrations = append(migrations, remoteMigrations...)

    // Sort migrations by version
    sort.Slice(migrations, func(i, j int) bool {
        return migrations[i].Version < migrations[j].Version
    })

    // Validate migration sequence
    if err := cme.validateMigrationSequence(migrations); err != nil {
        return nil, err
    }

    return migrations, nil
}

// Parallel migration execution for independent migrations
type ParallelMigrationExecutor struct {
    engine      *CustomMigrationEngine
    maxWorkers  int
}

func (pme *ParallelMigrationExecutor) ExecuteParallel(ctx context.Context, migrations []Migration) error {
    // Build dependency graph
    graph := pme.buildDependencyGraph(migrations)
    
    // Topological sort
    executionOrder := graph.TopologicalSort()
    
    // Execute in parallel respecting dependencies
    sem := make(chan struct{}, pme.maxWorkers)
    errChan := make(chan error, len(migrations))
    completed := make(map[string]bool)
    var mu sync.Mutex

    for _, batch := range executionOrder {
        var wg sync.WaitGroup
        
        for _, migration := range batch {
            wg.Add(1)
            go func(m Migration) {
                defer wg.Done()
                
                sem <- struct{}{}
                defer func() { <-sem }()
                
                // Wait for dependencies
                pme.waitForDependencies(m, completed, &mu)
                
                // Execute migration
                if err := pme.engine.executeMigration(ctx, m); err != nil {
                    errChan <- fmt.Errorf("migration %s failed: %w", m.Version, err)
                    return
                }
                
                // Mark as completed
                mu.Lock()
                completed[m.Version] = true
                mu.Unlock()
            }(migration)
        }
        
        wg.Wait()
    }

    close(errChan)
    
    // Check for errors
    for err := range errChan {
        if err != nil {
            return err
        }
    }

    return nil
}

// Migration validation framework
type MigrationValidator struct {
    rules []ValidationRule
}

type ValidationRule struct {
    Name     string
    Validate func(context.Context, *sql.Tx, Migration) error
}

func (mv *MigrationValidator) Validate(ctx context.Context, tx *sql.Tx, migration Migration) error {
    for _, rule := range mv.rules {
        if err := rule.Validate(ctx, tx, migration); err != nil {
            return fmt.Errorf("validation rule '%s' failed: %w", rule.Name, err)
        }
    }
    return nil
}

// Default validation rules
func DefaultValidationRules() []ValidationRule {
    return []ValidationRule{
        {
            Name: "table_count",
            Validate: func(ctx context.Context, tx *sql.Tx, m Migration) error {
                var count int
                err := tx.QueryRowContext(ctx, `
                    SELECT COUNT(*) 
                    FROM information_schema.tables 
                    WHERE table_schema = 'public'
                `).Scan(&count)
                
                if err != nil {
                    return err
                }
                
                if count == 0 {
                    return fmt.Errorf("no tables found after migration")
                }
                
                return nil
            },
        },
        {
            Name: "foreign_key_integrity",
            Validate: func(ctx context.Context, tx *sql.Tx, m Migration) error {
                var violations int
                err := tx.QueryRowContext(ctx, `
                    SELECT COUNT(*)
                    FROM pg_constraint
                    WHERE contype = 'f'
                    AND NOT convalidated
                `).Scan(&violations)
                
                if err != nil {
                    return err
                }
                
                if violations > 0 {
                    return fmt.Errorf("found %d invalid foreign key constraints", violations)
                }
                
                return nil
            },
        },
        {
            Name: "index_validity",
            Validate: func(ctx context.Context, tx *sql.Tx, m Migration) error {
                var invalid int
                err := tx.QueryRowContext(ctx, `
                    SELECT COUNT(*)
                    FROM pg_index
                    WHERE NOT indisvalid
                `).Scan(&invalid)
                
                if err != nil {
                    return err
                }
                
                if invalid > 0 {
                    return fmt.Errorf("found %d invalid indexes", invalid)
                }
                
                return nil
            },
        },
    }
}

// canary-deployment.go
package deployment

import (
    "context"
    "fmt"
    "sync"
    "time"
)

// CanaryDeploymentController manages canary deployments for database changes
type CanaryDeploymentController struct {
    primaryDB    *sql.DB
    canaryDB     *sql.DB
    router       TrafficRouter
    monitor      HealthMonitor
    rollback     RollbackController
}

// DeploymentStrategy defines how to roll out changes
type DeploymentStrategy struct {
    InitialCanaryPercent float64
    IncrementPercent     float64
    IncrementInterval    time.Duration
    SuccessThreshold     MetricThreshold
    RollbackThreshold    MetricThreshold
    MaxDuration          time.Duration
}

// ExecuteCanaryDeployment performs a canary deployment
func (cdc *CanaryDeploymentController) ExecuteCanaryDeployment(
    ctx context.Context,
    migration Migration,
    strategy DeploymentStrategy,
) error {
    // Apply migration to canary
    if err := cdc.applyToCanary(ctx, migration); err != nil {
        return fmt.Errorf("failed to apply migration to canary: %w", err)
    }

    // Start with initial canary traffic
    if err := cdc.router.SetCanaryTraffic(strategy.InitialCanaryPercent); err != nil {
        return fmt.Errorf("failed to set initial canary traffic: %w", err)
    }

    // Monitor and gradually increase traffic
    ticker := time.NewTicker(strategy.IncrementInterval)
    defer ticker.Stop()

    timeout := time.After(strategy.MaxDuration)
    currentPercent := strategy.InitialCanaryPercent

    for {
        select {
        case <-ctx.Done():
            return ctx.Err()
        
        case <-timeout:
            return fmt.Errorf("deployment timeout exceeded")
        
        case <-ticker.C:
            // Check metrics
            metrics := cdc.monitor.GetCurrentMetrics()
            
            // Check for rollback conditions
            if cdc.shouldRollback(metrics, strategy.RollbackThreshold) {
                log.Printf("Rollback triggered: %v", metrics)
                return cdc.executeRollback(ctx)
            }
            
            // Check for success conditions
            if cdc.meetsSuccessThreshold(metrics, strategy.SuccessThreshold) {
                if currentPercent >= 100 {
                    // Deployment complete
                    return cdc.finalizeDeployment(ctx)
                }
                
                // Increase canary traffic
                currentPercent = min(currentPercent + strategy.IncrementPercent, 100)
                if err := cdc.router.SetCanaryTraffic(currentPercent); err != nil {
                    return fmt.Errorf("failed to update canary traffic: %w", err)
                }
                
                log.Printf("Increased canary traffic to %.1f%%", currentPercent)
            }
        }
    }
}

// TrafficRouter manages traffic distribution
type TrafficRouter struct {
    config      RouterConfig
    connections sync.Map
}

func (tr *TrafficRouter) RouteQuery(query Query) (*sql.DB, error) {
    // Get user/session hash for consistent routing
    hash := tr.hashUserSession(query.UserID, query.SessionID)
    
    // Check if this user is in canary group
    canaryPercent := tr.getCanaryPercent()
    threshold := uint32(float64(^uint32(0)) * canaryPercent / 100)
    
    if hash <= threshold {
        return tr.getCanaryConnection()
    }
    
    return tr.getPrimaryConnection()
}

// Feature flag integration for gradual rollout
type FeatureFlagController struct {
    flags sync.Map
}

func (ffc *FeatureFlagController) IsEnabled(feature string, context map[string]interface{}) bool {
    flag, exists := ffc.flags.Load(feature)
    if !exists {
        return false
    }
    
    ff := flag.(*FeatureFlag)
    
    // Check if globally enabled
    if ff.Enabled && ff.Percentage >= 100 {
        return true
    }
    
    // Check percentage rollout
    if ff.Enabled && ff.Percentage > 0 {
        hash := ffc.hashContext(context)
        threshold := uint32(float64(^uint32(0)) * ff.Percentage / 100)
        return hash <= threshold
    }
    
    // Check specific rules
    for _, rule := range ff.Rules {
        if rule.Matches(context) {
            return rule.Enabled
        }
    }
    
    return false
}