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# Channel Management
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The channel management system provides low-level networking channel implementations for both TCP transport and HTTP server operations. These channels handle connection lifecycle, data transmission, and resource management for all network communications.
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## Capabilities
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### TCP Server Channel Implementation
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Server-side TCP channel for accepting inbound connections from other Elasticsearch cluster nodes.
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```java { .api }
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/**
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 * Netty4 server channel implementation for TCP transport
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 * Binds to configured address and accepts connections from cluster peers
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 */
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public class Netty4TcpServerChannel implements TcpServerChannel {
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    /**
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     * Adds listener to be notified when server channel closes
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     * Useful for cleanup and monitoring of server lifecycle
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     * @param listener Callback invoked when channel closes
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     */
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    void addCloseListener(ActionListener<Void> listener);
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    /**
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     * Checks if server channel is currently open and accepting connections
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     * @return true if server is bound and accepting, false otherwise
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     */
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    boolean isOpen();
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    /**
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     * Gets local socket address this server channel is bound to
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     * Includes IP address and port number for client connections
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     * @return Local InetSocketAddress for bound server socket
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     */
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    InetSocketAddress getLocalAddress();
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    /**
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     * Closes server channel and stops accepting new connections
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     * Existing connections continue until naturally closed
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     */
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    void close();
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}
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```
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**Usage Example:**
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```java
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// Server channel created by transport layer during startup
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Netty4TcpServerChannel serverChannel = // from transport initialization
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// Monitor server lifecycle
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serverChannel.addCloseListener(ActionListener.wrap(
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    () -> {
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        logger.info("Transport server channel closed");
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        // Perform cleanup operations
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    },
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    exception -> {
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        logger.error("Transport server channel close failed", exception);
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        // Handle close failure
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    }
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));
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// Check server status
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if (serverChannel.isOpen()) {
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    InetSocketAddress address = serverChannel.getLocalAddress();
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    logger.info("Transport server listening on {}:{}", 
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                address.getHostString(), address.getPort());
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} else {
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    logger.warn("Transport server channel is not accepting connections");
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}
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```
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### TCP Client Channel Implementation
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Client-side TCP channel for outbound connections to other cluster nodes.
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```java { .api }
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/**
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 * Netty4 client channel implementation for TCP transport
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 * Handles individual connections to remote cluster nodes
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 */
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public class Netty4TcpChannel implements TcpChannel {
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    /**
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     * Adds listener to be notified when channel closes
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     * Called for both normal and exceptional channel closure
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     * @param listener Callback invoked on channel close
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     */
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    void addCloseListener(ActionListener<Void> listener);
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    /**
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     * Checks if channel is currently open and connected
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     * @return true if channel is active, false if closed or closing
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     */
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    boolean isOpen();
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    /**
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     * Gets local socket address for this connection
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     * @return Local InetSocketAddress of client socket
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     */
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    InetSocketAddress getLocalAddress();
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    /**
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     * Gets remote socket address for this connection
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     * @return Remote InetSocketAddress of connected peer node
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     */
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    InetSocketAddress getRemoteAddress();
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    /**
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     * Sends message bytes to remote peer asynchronously
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     * Messages are queued and sent in order
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     * @param reference Serialized message bytes to transmit
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     * @param listener Callback for send completion or failure
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     */
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    void sendMessage(BytesReference reference, ActionListener<Void> listener);
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    /**
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     * Closes channel connection to remote peer
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     * Pending operations complete before channel closure
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     */
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    void close();
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}
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```
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**Usage Example:**
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```java
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// Client channel obtained from transport connection
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Netty4TcpChannel clientChannel = // from transport.openConnection()
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// Send cluster message to remote node
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TransportMessage message = // serialized cluster message
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BytesReference messageBytes = // serialized message content
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clientChannel.sendMessage(messageBytes, ActionListener.wrap(
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    response -> {
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        logger.debug("Message sent successfully to {}", 
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                    clientChannel.getRemoteAddress());
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    },
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    exception -> {
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        logger.error("Failed to send message to {}: {}", 
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                    clientChannel.getRemoteAddress(), exception.getMessage());
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        // Handle send failure - may trigger connection retry
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    }
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));
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// Monitor connection health
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clientChannel.addCloseListener(ActionListener.wrap(
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    () -> {
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        logger.info("Connection to {} closed", clientChannel.getRemoteAddress());
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        // Update node connection status
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    },
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    exception -> {
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        logger.warn("Connection to {} closed with error: {}", 
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                   clientChannel.getRemoteAddress(), exception.getMessage());
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        // Trigger connection retry if appropriate
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    }
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));
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```
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### HTTP Server Channel Implementation
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Server-side HTTP channel for accepting HTTP client connections and serving REST API requests.
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```java { .api }
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/**
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 * Netty4 HTTP server channel implementation
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 * Binds to HTTP port and accepts client connections for REST API access
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 */
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public class Netty4HttpServerChannel implements HttpServerChannel {
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    /**
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     * Gets local socket address HTTP server is bound to
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     * Includes bind address and port for HTTP client connections
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     * @return Local InetSocketAddress for HTTP server
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     */
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    InetSocketAddress getLocalAddress();
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    /**
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     * Adds listener to be notified when HTTP server channel closes
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     * Useful for monitoring HTTP service availability
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     * @param listener Callback invoked on server close
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     */
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    void addCloseListener(ActionListener<Void> listener);
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    /**
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     * Checks if HTTP server is currently open and accepting connections
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     * @return true if server is bound and accepting HTTP requests
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     */
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    boolean isOpen();
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    /**
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     * Closes HTTP server channel and stops accepting connections
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     * Existing HTTP connections complete current requests before closing
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     */
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    void close();
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}
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```
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**Usage Example:**
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```java
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// HTTP server channel created during HTTP transport startup
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Netty4HttpServerChannel httpServer = // from HTTP transport initialization
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// Check HTTP server status
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if (httpServer.isOpen()) {
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    InetSocketAddress httpAddress = httpServer.getLocalAddress();
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    logger.info("HTTP server accepting connections on {}:{}", 
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                httpAddress.getHostString(), httpAddress.getPort());
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    // HTTP server is ready for client requests
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    // Clients can now send REST API requests to this address
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} else {
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    logger.error("HTTP server failed to bind - REST API unavailable");
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}
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// Monitor HTTP server lifecycle
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httpServer.addCloseListener(ActionListener.wrap(
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    () -> {
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        logger.info("HTTP server stopped - REST API no longer available");
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        // Update service registry or load balancer
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    },
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    exception -> {
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        logger.error("HTTP server close failed", exception);
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        // Handle server shutdown issues
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    }
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));
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```
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### HTTP Client Channel Implementation
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Client-side HTTP channel for individual HTTP client connections.
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```java { .api }
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/**
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 * Netty4 HTTP client channel implementation
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 * Handles individual HTTP connections from clients to process REST requests
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 */
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public class Netty4HttpChannel implements HttpChannel {
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    /**
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     * Sends HTTP response to client asynchronously
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     * Response is queued and transmitted to client
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     * @param response HTTP response object with status, headers, and body
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     * @param listener Callback for response transmission completion
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     */
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    void sendResponse(HttpResponse response, ActionListener<Void> listener);
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    /**
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     * Gets local address for this HTTP connection
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     * @return Local InetSocketAddress of server side
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     */
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    InetSocketAddress getLocalAddress();
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    /**
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     * Gets remote client address for this HTTP connection
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     * @return Remote InetSocketAddress of HTTP client
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     */
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    InetSocketAddress getRemoteAddress();
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    /**
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     * Closes HTTP connection to client
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     * Connection may be kept alive for reuse depending on HTTP headers
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     */
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    void close();
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}
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```
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**Usage Example:**
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```java
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// HTTP channel created for each client connection
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Netty4HttpChannel httpChannel = // from HTTP request pipeline
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// Process HTTP request and send response
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RestResponse restResponse = // from REST handler processing
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HttpResponse httpResponse = // converted to HTTP format
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httpChannel.sendResponse(httpResponse, ActionListener.wrap(
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    response -> {
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        logger.debug("HTTP response sent to client {}", 
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                    httpChannel.getRemoteAddress());
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        // Response successfully transmitted
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    },
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    exception -> {
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        logger.warn("Failed to send HTTP response to {}: {}", 
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                   httpChannel.getRemoteAddress(), exception.getMessage());
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        // Client may have disconnected or network error occurred
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    }
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));
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// Log client connection info
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InetSocketAddress clientAddr = httpChannel.getRemoteAddress();
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InetSocketAddress serverAddr = httpChannel.getLocalAddress();
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logger.debug("Processing HTTP request from {} to {}", clientAddr, serverAddr);
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```
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### Custom Socket Channel Implementations
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Specialized socket channel implementations providing enhanced functionality and performance optimizations.
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```java { .api }
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/**
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 * Custom NIO socket channel with Elasticsearch-specific optimizations
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 * Extends Netty's NioSocketChannel with additional features
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public class Netty4NioSocketChannel extends NioSocketChannel {
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    // Enhanced socket channel with ES-specific networking optimizations
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    // Provides better integration with Elasticsearch's networking stack
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}
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/**
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 * Socket channel implementation that copies bytes for certain operations
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 * Provides additional safety for buffer management in specific scenarios
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 */
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public class CopyBytesSocketChannel extends Netty4NioSocketChannel {
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    // Specialized channel that copies bytes when needed
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    // Used for scenarios requiring buffer isolation or special handling
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}
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/**
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 * Server socket channel that copies bytes for connection handling
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 * Provides enhanced buffer management for server-side operations
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public class CopyBytesServerSocketChannel extends NioServerSocketChannel {
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    // Server-side equivalent of CopyBytesSocketChannel
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    // Handles incoming connections with specialized byte copying behavior
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}
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```
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### Channel Lifecycle Management
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Channel implementations provide comprehensive lifecycle management:
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**Connection Establishment**: Proper handshake and negotiation for both TCP and HTTP protocols
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**Resource Allocation**: Efficient allocation of buffers, threads, and system resources per connection
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**Error Handling**: Graceful handling of network errors, timeouts, and connection failures
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**Connection Pooling**: Reuse of connections where appropriate to minimize overhead
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**Graceful Shutdown**: Proper cleanup and resource release during channel closure
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**Monitoring Integration**: Detailed metrics and logging for connection health and performance
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## Channel Configuration and Tuning
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### TCP Channel Configuration
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```java
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// TCP transport channel settings
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Settings tcpSettings = Settings.builder()
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    .put("transport.tcp.port", "9300-9400") // Port range
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    .put("transport.bind_host", "0.0.0.0") // Bind address
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    .put("transport.publish_host", "192.168.1.100") // Advertised address
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    .put("transport.tcp.connect_timeout", "30s") // Connection timeout
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    .put("transport.tcp.compress", true) // Message compression
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    .build();
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```
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### HTTP Channel Configuration
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```java
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// HTTP server channel settings
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Settings httpSettings = Settings.builder()
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    .put("http.port", "9200-9300") // HTTP port range
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    .put("http.bind_host", "0.0.0.0") // HTTP bind address
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    .put("http.publish_host", "192.168.1.100") // HTTP advertised address
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    .put("http.max_content_length", "100mb") // Request size limit
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    .put("http.compression", true) // Response compression
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    .put("http.cors.enabled", true) // CORS support
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    .build();
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```
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## Integration with Elasticsearch Networking
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The channel management system integrates deeply with Elasticsearch's networking infrastructure:
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1. **Transport Integration**: Channels are created and managed by transport implementations
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2. **Thread Pool Integration**: Channel operations use appropriate Elasticsearch thread pools
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3. **Circuit Breaker Integration**: Channels respect resource limits and circuit breakers
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4. **Security Integration**: Channels support TLS encryption and authentication
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5. **Monitoring Integration**: Channel statistics are exposed via Elasticsearch monitoring APIs
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6. **Plugin Integration**: Other plugins can extend channel functionality through Elasticsearch's plugin system
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The channel management system provides the foundation for all network communication in Elasticsearch, ensuring reliable, performant, and secure data transmission between cluster nodes and client applications.