Event Handling

/**
 * Listen to transaction lifecycle events
 */
public interface TransactionEventListener<T> {
    
    /**
     * Called before a transaction is committed
     * @param data Transaction data containing changes
     * @param transaction The transaction being committed
     * @return State object passed to afterCommit/afterRollback
     * @throws Exception to prevent commit and trigger rollback
     */
    T beforeCommit(TransactionData data, Transaction transaction) throws Exception;
    
    /**
     * Called after a transaction is successfully committed
     * @param data Transaction data containing changes
     * @param state State object returned from beforeCommit
     * @param transaction The committed transaction
     */
    void afterCommit(TransactionData data, T state, Transaction transaction);
    
    /**
     * Called after a transaction is rolled back
     * @param data Transaction data containing changes
     * @param state State object returned from beforeCommit (may be null)
     * @param transaction The rolled back transaction
     */
    void afterRollback(TransactionData data, T state, Transaction transaction);
}

import org.neo4j.graphdb.event.TransactionEventListener;
import org.neo4j.graphdb.event.TransactionData;

public class AuditTransactionListener implements TransactionEventListener<Long> {
    
    @Override
    public Long beforeCommit(TransactionData data, Transaction transaction) throws Exception {
        long startTime = System.currentTimeMillis();
        
        // Validate changes before commit
        for (Node createdNode : data.createdNodes()) {
            if (createdNode.hasLabel(Label.label("User"))) {
                if (!createdNode.hasProperty("email")) {
                    throw new IllegalStateException("User nodes must have email property");
                }
            }
        }
        
        // Log significant changes
        if (data.createdNodes().iterator().hasNext() || 
            data.deletedNodes().iterator().hasNext()) {
            System.out.println("Transaction contains node changes");
        }
        
        return startTime;
    }
    
    @Override
    public void afterCommit(TransactionData data, Long startTime, Transaction transaction) {
        long duration = System.currentTimeMillis() - startTime;
        
        // Log successful commit
        System.out.println("Transaction committed in " + duration + "ms");
        System.out.println("  Nodes created: " + count(data.createdNodes()));
        System.out.println("  Nodes deleted: " + count(data.deletedNodes()));
        System.out.println("  Relationships created: " + count(data.createdRelationships()));
        System.out.println("  Relationships deleted: " + count(data.deletedRelationships()));
        
        // Trigger external systems
        notifyExternalSystems(data);
    }
    
    @Override
    public void afterRollback(TransactionData data, Long startTime, Transaction transaction) {
        if (startTime != null) {
            long duration = System.currentTimeMillis() - startTime;
            System.out.println("Transaction rolled back after " + duration + "ms");
        }
    }
    
    private int count(Iterable<?> iterable) {
        int count = 0;
        for (Object ignored : iterable) count++;
        return count;
    }
    
    private void notifyExternalSystems(TransactionData data) {
        // Send notifications to external systems
        // e.g., message queues, webhooks, etc.
    }
}

// Register the listener
DatabaseManagementService managementService = // ... create service
GraphDatabaseService db = managementService.database("neo4j");
db.registerTransactionEventListener(new AuditTransactionListener());

/**
 * Listen to database lifecycle events
 */
public interface DatabaseEventListener {
    
    /**
     * Called when a database is created
     * @param eventContext Context information about the event
     */
    void databaseCreate(DatabaseEventContext eventContext);
    
    /**
     * Called when a database is dropped
     * @param eventContext Context information about the event
     */
    void databaseDrop(DatabaseEventContext eventContext);
    
    /**
     * Called when a database is started
     * @param eventContext Context information about the event
     */
    void databaseStart(DatabaseEventContext eventContext);
    
    /**
     * Called when a database is shut down
     * @param eventContext Context information about the event
     */
    void databaseShutdown(DatabaseEventContext eventContext);
}

import org.neo4j.graphdb.event.DatabaseEventListener;
import org.neo4j.graphdb.event.DatabaseEventContext;

public class DatabaseLifecycleListener implements DatabaseEventListener {
    
    @Override
    public void databaseCreate(DatabaseEventContext eventContext) {
        String dbName = eventContext.getDatabaseName();
        System.out.println("Database created: " + dbName);
        
        // Initialize new database with default data
        initializeDatabase(dbName);
        
        // Log to audit system
        auditLog("DATABASE_CREATED", dbName, eventContext.getEventData());
    }
    
    @Override
    public void databaseDrop(DatabaseEventContext eventContext) {
        String dbName = eventContext.getDatabaseName();
        System.out.println("Database dropped: " + dbName);
        
        // Cleanup external resources
        cleanupExternalResources(dbName);
        
        // Log to audit system
        auditLog("DATABASE_DROPPED", dbName, eventContext.getEventData());
    }
    
    @Override
    public void databaseStart(DatabaseEventContext eventContext) {
        String dbName = eventContext.getDatabaseName();
        System.out.println("Database started: " + dbName);
        
        // Perform startup tasks
        performStartupTasks(dbName);
        
        // Register for monitoring
        registerForMonitoring(dbName);
    }
    
    @Override
    public void databaseShutdown(DatabaseEventContext eventContext) {
        String dbName = eventContext.getDatabaseName();
        System.out.println("Database shutting down: " + dbName);
        
        // Perform cleanup tasks
        performShutdownTasks(dbName);
        
        // Unregister from monitoring
        unregisterFromMonitoring(dbName);
    }
    
    private void initializeDatabase(String dbName) {
        // Add default constraints, indexes, or seed data
    }
    
    private void cleanupExternalResources(String dbName) {
        // Clean up external caches, connections, etc.
    }
    
    private void auditLog(String event, String dbName, Map<String, Object> data) {
        // Log to external audit system
    }
}

// Register the listener
DatabaseManagementService managementService = new DatabaseManagementServiceBuilder(dbPath)
    .addDatabaseListener(new DatabaseLifecycleListener())
    .build();

/**
 * Access transaction change data
 */
public interface TransactionData {
    
    /**
     * Get nodes created in this transaction
     * @return Iterable of created nodes
     */
    Iterable<Node> createdNodes();
    
    /**
     * Get nodes deleted in this transaction
     * @return Iterable of deleted nodes
     */
    Iterable<Node> deletedNodes();
    
    /**
     * Get relationships created in this transaction
     * @return Iterable of created relationships
     */
    Iterable<Relationship> createdRelationships();
    
    /**
     * Get relationships deleted in this transaction
     * @return Iterable of deleted relationships
     */
    Iterable<Relationship> deletedRelationships();
    
    /**
     * Check if a node was created in this transaction
     * @param node Node to check
     * @return true if node was created
     */
    boolean isCreated(Node node);
    
    /**
     * Check if a node was deleted in this transaction
     * @param node Node to check
     * @return true if node was deleted
     */
    boolean isDeleted(Node node);
    
    /**
     * Check if a relationship was created in this transaction
     * @param relationship Relationship to check
     * @return true if relationship was created
     */
    boolean isCreated(Relationship relationship);
    
    /**
     * Check if a relationship was deleted in this transaction
     * @param relationship Relationship to check
     * @return true if relationship was deleted
     */
    boolean isDeleted(Relationship relationship);
    
    /**
     * Get assigned properties for a node
     * @param node Node to get properties for
     * @return Property container with assigned properties
     */
    PropertyContainer assignedNodeProperties(Node node);
    
    /**
     * Get removed properties for a node
     * @param node Node to get properties for
     * @return Property container with removed properties
     */
    PropertyContainer removedNodeProperties(Node node);
    
    /**
     * Get assigned properties for a relationship
     * @param relationship Relationship to get properties for
     * @return Property container with assigned properties
     */
    PropertyContainer assignedRelationshipProperties(Relationship relationship);
    
    /**
     * Get removed properties for a relationship
     * @param relationship Relationship to get properties for
     * @return Property container with removed properties
     */
    PropertyContainer removedRelationshipProperties(Relationship relationship);
    
    /**
     * Get assigned labels for a node
     * @param node Node to get labels for
     * @return Iterable of assigned labels
     */
    Iterable<Label> assignedLabels(Node node);
    
    /**
     * Get removed labels for a node
     * @param node Node to get labels for
     * @return Iterable of removed labels
     */
    Iterable<Label> removedLabels(Node node);
}

public class ChangeTrackingListener implements TransactionEventListener<Map<String, Object>> {
    
    @Override
    public Map<String, Object> beforeCommit(TransactionData data, Transaction transaction) {
        Map<String, Object> changesSummary = new HashMap<>();
        
        // Track node changes
        Map<String, Integer> nodeChanges = new HashMap<>();
        
        // Count created nodes by label
        for (Node node : data.createdNodes()) {
            for (Label label : node.getLabels()) {
                nodeChanges.merge("created_" + label.name(), 1, Integer::sum);
            }
        }
        
        // Count deleted nodes by label
        for (Node node : data.deletedNodes()) {
            for (Label label : node.getLabels()) {
                nodeChanges.merge("deleted_" + label.name(), 1, Integer::sum);
            }
        }
        
        changesSummary.put("nodeChanges", nodeChanges);
        
        // Track relationship changes
        Map<String, Integer> relChanges = new HashMap<>();
        
        for (Relationship rel : data.createdRelationships()) {
            relChanges.merge("created_" + rel.getType().name(), 1, Integer::sum);
        }
        
        for (Relationship rel : data.deletedRelationships()) {
            relChanges.merge("deleted_" + rel.getType().name(), 1, Integer::sum);
        }
        
        changesSummary.put("relationshipChanges", relChanges);
        
        // Track property changes
        int propertyChanges = 0;
        
        // Check nodes for property changes
        for (Node node : data.createdNodes()) {
            PropertyContainer assigned = data.assignedNodeProperties(node);
            for (String key : assigned.getPropertyKeys()) {
                propertyChanges++;
            }
        }
        
        // Check for property updates on existing nodes
        // (This requires checking all nodes and comparing with removed properties)
        
        changesSummary.put("totalPropertyChanges", propertyChanges);
        
        return changesSummary;
    }
    
    @Override
    public void afterCommit(TransactionData data, Map<String, Object> changesSummary, 
                           Transaction transaction) {
        
        // Log detailed changes
        System.out.println("Transaction committed with changes:");
        System.out.println("  Node changes: " + changesSummary.get("nodeChanges"));
        System.out.println("  Relationship changes: " + changesSummary.get("relationshipChanges"));
        System.out.println("  Property changes: " + changesSummary.get("totalPropertyChanges"));
        
        // Send to monitoring system
        sendToMonitoringSystem(changesSummary);
        
        // Update search indexes or caches
        updateExternalSystems(data);
    }
    
    @Override
    public void afterRollback(TransactionData data, Map<String, Object> changesSummary, 
                             Transaction transaction) {
        System.out.println("Transaction rolled back - changes discarded");
        if (changesSummary != null) {
            System.out.println("  Would have applied: " + changesSummary);
        }
    }
    
    private void sendToMonitoringSystem(Map<String, Object> changes) {
        // Send metrics to monitoring system like Prometheus, DataDog, etc.
    }
    
    private void updateExternalSystems(TransactionData data) {
        // Update search indexes, caches, message queues, etc.
    }
}