Ensemble Providers

/**
 * Interface for providing ZooKeeper connection strings
 */
public interface EnsembleProvider extends Closeable {
    /**
     * Start the ensemble provider
     * @throws Exception if provider cannot be started
     */
    void start() throws Exception;
    
    /**
     * Get current ZooKeeper connection string
     * @return Connection string in format "host1:port1,host2:port2,..."
     */
    String getConnectionString();
    
    /**
     * Close the ensemble provider and release resources
     * @throws IOException if close operation fails
     */
    void close() throws IOException;
    
    /**
     * Set a new connection string (for providers that support updates)
     * @param connectionString New connection string to use
     */
    void setConnectionString(String connectionString);
    
    /**
     * Check if this provider supports server list updates
     * @return true if provider can update server list dynamically
     */
    boolean updateServerListEnabled();
}

/**
 * Ensemble provider with fixed connection string
 */
public class FixedEnsembleProvider implements EnsembleProvider {
    /**
     * Create fixed ensemble provider with connection string
     * @param connectionString ZooKeeper connection string
     */
    public FixedEnsembleProvider(String connectionString);
    
    /**
     * Create fixed ensemble provider with connection string and update flag
     * @param connectionString ZooKeeper connection string
     * @param updateServerListEnabled Whether server list updates are enabled
     */
    public FixedEnsembleProvider(String connectionString, boolean updateServerListEnabled);
}

import org.apache.curator.ensemble.fixed.FixedEnsembleProvider;
import org.apache.curator.CuratorZookeeperClient;
import org.apache.curator.retry.ExponentialBackoffRetry;

// Basic fixed ensemble
EnsembleProvider ensemble = new FixedEnsembleProvider("zk1:2181,zk2:2181,zk3:2181");

// Create client with ensemble provider
CuratorZookeeperClient client = new CuratorZookeeperClient(
    ensemble,
    5000,  // session timeout
    5000,  // connection timeout
    null,  // watcher
    new ExponentialBackoffRetry(1000, 3)
);

try {
    ensemble.start();
    client.start();
    // ... use client ...
} finally {
    client.close();
    ensemble.close();
}

/**
 * Ensemble provider that uses Netflix Exhibitor for dynamic ensemble discovery
 */
public class ExhibitorEnsembleProvider implements EnsembleProvider {
    /**
     * Create Exhibitor-based ensemble provider
     * @param exhibitors Collection of Exhibitor instances
     * @param restClient REST client for Exhibitor communication
     * @param pollingMs Polling interval in milliseconds for ensemble updates
     */
    public ExhibitorEnsembleProvider(Exhibitors exhibitors, ExhibitorRestClient restClient, int pollingMs);
    
    /**
     * Create Exhibitor ensemble provider with backup connection string
     * @param exhibitors Collection of Exhibitor instances  
     * @param restClient REST client for Exhibitor communication
     * @param backup Backup connection string provider
     * @param pollingMs Polling interval for ensemble updates
     */
    public ExhibitorEnsembleProvider(Exhibitors exhibitors, ExhibitorRestClient restClient, 
                                   BackupConnectionStringProvider backup, int pollingMs);
}

/**
 * Collection of Exhibitor instances for ensemble discovery
 */
public class Exhibitors {
    /**
     * Create Exhibitors collection
     * @param hostnames Collection of Exhibitor hostnames
     * @param restPort REST port for Exhibitor communication
     * @param backupConnectionStringProvider Backup connection string provider
     */
    public Exhibitors(Collection<String> hostnames, int restPort, 
                     BackupConnectionStringProvider backupConnectionStringProvider);
    
    /**
     * Get backup connection string provider
     * @return Backup connection string provider
     */
    public BackupConnectionStringProvider getBackupConnectionStringProvider();
    
    /**
     * Get Exhibitor hostnames
     * @return Collection of hostnames
     */
    public Collection<String> getHostnames();
    
    /**
     * Get REST port for Exhibitor communication
     * @return REST port number
     */
    public int getRestPort();
}

/**
 * Interface for REST client communication with Exhibitor
 */
public interface ExhibitorRestClient {
    /**
     * Make raw REST request to Exhibitor
     * @param hostname Exhibitor hostname
     * @param port Exhibitor port
     * @param uriPath URI path for the request
     * @param mimeType Expected MIME type of response
     * @return Raw response string
     * @throws Exception if request fails
     */
    String getRaw(String hostname, int port, String uriPath, String mimeType) throws Exception;
}

/**
 * Default implementation of ExhibitorRestClient
 */
public class DefaultExhibitorRestClient implements ExhibitorRestClient {
    /**
     * Create default REST client with connection timeout
     * @param connectionTimeoutMs Connection timeout in milliseconds
     */
    public DefaultExhibitorRestClient(int connectionTimeoutMs);
    
    /**
     * Create default REST client with custom socket timeout
     * @param connectionTimeoutMs Connection timeout in milliseconds
     * @param socketTimeoutMs Socket timeout in milliseconds
     */
    public DefaultExhibitorRestClient(int connectionTimeoutMs, int socketTimeoutMs);
}

import org.apache.curator.ensemble.exhibitor.*;
import java.util.Arrays;

// Configure Exhibitor instances
Exhibitors exhibitors = new Exhibitors(
    Arrays.asList("exhibitor1.example.com", "exhibitor2.example.com"), 
    8080, 
    () -> "fallback1:2181,fallback2:2181"  // Backup connection string
);

// Create REST client
ExhibitorRestClient restClient = new DefaultExhibitorRestClient(5000);

// Create dynamic ensemble provider
ExhibitorEnsembleProvider ensembleProvider = new ExhibitorEnsembleProvider(
    exhibitors, 
    restClient, 
    10000  // Poll every 10 seconds
);

// Use with Curator client
CuratorZookeeperClient client = new CuratorZookeeperClient(
    ensembleProvider,
    5000, 5000, null, 
    new ExponentialBackoffRetry(1000, 3)
);

try {
    ensembleProvider.start();
    client.start();
    
    // Ensemble provider will automatically update connection string
    // as ZooKeeper cluster membership changes
    
} finally {
    client.close();
    ensembleProvider.close();
}

/**
 * Functional interface for backup connection string provision
 */
@FunctionalInterface
public interface BackupConnectionStringProvider {
    /**
     * Get backup connection string when primary discovery fails
     * @return Backup ZooKeeper connection string
     * @throws Exception if backup cannot be provided
     */
    String getBackupConnectionString() throws Exception;
}

// Lambda expression for backup connection
BackupConnectionStringProvider backup = () -> "backup1:2181,backup2:2181,backup3:2181";

// Method reference
BackupConnectionStringProvider configBackup = this::getConfiguredBackupString;

// Anonymous class implementation
BackupConnectionStringProvider dynamicBackup = new BackupConnectionStringProvider() {
    @Override
    public String getBackupConnectionString() throws Exception {
        // Query configuration service or database for backup servers
        return configService.getZookeeperBackupServers();
    }
};

// Pattern 1: Fixed ensemble for stable clusters
EnsembleProvider fixedEnsemble = new FixedEnsembleProvider("zk1:2181,zk2:2181,zk3:2181");

// Pattern 2: Dynamic ensemble with Exhibitor
Exhibitors exhibitors = new Exhibitors(
    Arrays.asList("exhibitor1.com", "exhibitor2.com"), 8080, 
    () -> "backup1:2181,backup2:2181"
);
EnsembleProvider dynamicEnsemble = new ExhibitorEnsembleProvider(
    exhibitors, new DefaultExhibitorRestClient(5000), 30000
);

// Both patterns work identically with CuratorZookeeperClient
CuratorZookeeperClient client = new CuratorZookeeperClient(
    ensembleProvider, 5000, 5000, null, retryPolicy
);

// Always ensure proper resource cleanup
try (EnsembleProvider ensemble = new FixedEnsembleProvider("localhost:2181");
     CuratorZookeeperClient client = new CuratorZookeeperClient(ensemble, 5000, 5000, null, retryPolicy)) {
    
    ensemble.start();
    client.start();
    
    // Use client...
    
} // Resources automatically closed