Extension Points

package org.openqa.selenium.remote.server;

import org.openqa.selenium.Capabilities;
import org.openqa.selenium.WebDriver;

interface DriverProvider {
    /**
     * Get capabilities provided by this driver
     * @return Capabilities that this provider can handle
     */
    Capabilities getProvidedCapabilities();
    
    /**
     * Check if provider can create driver for given capabilities
     * @param capabilities Desired capabilities to check
     * @return true if provider supports these capabilities
     */
    boolean canCreateDriverInstanceFor(Capabilities capabilities);
    
    /**
     * Create new WebDriver instance
     * @param capabilities Capabilities for driver creation
     * @return New WebDriver instance
     */
    WebDriver newInstance(Capabilities capabilities);
}

public class CustomDriverProvider implements DriverProvider {
    @Override
    public Capabilities getProvidedCapabilities() {
        return new ImmutableCapabilities("browserName", "customBrowser");
    }
    
    @Override
    public boolean canCreateDriverInstanceFor(Capabilities capabilities) {
        return "customBrowser".equals(capabilities.getBrowserName());
    }
    
    @Override
    public WebDriver newInstance(Capabilities capabilities) {
        return new CustomWebDriver(capabilities);
    }
}

@FunctionalInterface
interface CommandHandler {
    /**
     * Execute HTTP request and generate response
     * @param req HTTP request to process
     * @param resp HTTP response to populate  
     * @throws IOException if request processing fails
     */
    void execute(HttpRequest req, HttpResponse resp) throws IOException;
}

public class CustomCommandHandler implements CommandHandler {
    @Override
    public void execute(HttpRequest req, HttpResponse resp) throws IOException {
        // Custom request processing logic
        resp.setStatus(200);
        resp.setContent(asJson(ImmutableMap.of("status", "custom response")));
    }
}

interface SessionFactory {
    /**
     * Check if this factory can create sessions for given capabilities
     * @param capabilities Desired browser capabilities
     * @return true if factory supports these capabilities
     */
    boolean isSupporting(Capabilities capabilities);
    
    /**
     * Create new active session instance
     * @param downstreamDialects WebDriver protocol dialects supported
     * @param capabilities Desired browser capabilities
     * @return ActiveSession if creation successful, empty otherwise
     */
    Optional<ActiveSession> apply(Set<Dialect> downstreamDialects, Capabilities capabilities);
}

public class CustomSessionFactory implements SessionFactory {
    @Override
    public boolean isSupporting(Capabilities capabilities) {
        return "customBrowser".equals(capabilities.getBrowserName());
    }
    
    @Override
    public Optional<ActiveSession> apply(Set<Dialect> downstreamDialects, Capabilities capabilities) {
        // Create custom session implementation
        return Optional.of(new CustomActiveSession(capabilities));
    }
}

interface ActiveSession extends CommandHandler, WrapsDriver {
    /**
     * Get unique session identifier
     * @return SessionId for this session
     */
    SessionId getId();
    
    /**
     * Get session capabilities
     * @return Capabilities used to create this session
     */
    Capabilities getCapabilities();
    
    /**
     * Get session creation timestamp
     * @return Instant when session was created
     */
    Instant getStartTime();
    
    /**
     * Get underlying WebDriver instance
     * @return WebDriver instance for this session
     */
    WebDriver getWrappedDriver();
}

interface CliCommand {
    /**
     * Get command name used in CLI
     * @return Command name string
     */
    String getName();
    
    /**
     * Get command description for help
     * @return Human-readable description
     */
    String getDescription();
    
    /**
     * Configure command with arguments and return executable
     * @param args Command line arguments
     * @return Executable that can be run
     */
    Executable configure(String... args);
    
    /**
     * Executable interface for running configured commands
     */
    interface Executable {
        void run();
    }
}

@AutoService(CliCommand.class)
public class CustomCommand implements CliCommand {
    @Override
    public String getName() {
        return "custom";
    }
    
    @Override
    public String getDescription() {
        return "Custom Grid command for specialized functionality";
    }
    
    @Override
    public Executable configure(String... args) {
        return () -> {
            // Custom command logic
            System.out.println("Executing custom command");
        };
    }
}

abstract class SessionMap implements Predicate<HttpRequest>, CommandHandler {
    /**
     * Add session to the session map
     * @param session Session to add
     * @return true if session was added successfully
     */
    public abstract boolean add(Session session);
    
    /**
     * Retrieve session by ID
     * @param id Session identifier
     * @return Session instance
     * @throws NoSuchSessionException if session not found
     */
    public abstract Session get(SessionId id) throws NoSuchSessionException;
    
    /**
     * Remove session from map
     * @param id Session identifier to remove
     */
    public abstract void remove(SessionId id);
    
    /**
     * Check if request should be handled by this session map
     * @param req HTTP request
     * @return true if this session map should handle the request
     */
    @Override
    public abstract boolean test(HttpRequest req);
}

abstract class Distributor implements Predicate<HttpRequest>, CommandHandler {
    /**
     * Create new session using available nodes
     * @param payload New session request payload
     * @return Created session
     * @throws SessionNotCreatedException if session cannot be created
     */
    public abstract Session newSession(NewSessionPayload payload) throws SessionNotCreatedException;
    
    /**
     * Register node with distributor
     * @param node Node to add to available nodes
     */
    public abstract void add(Node node);
    
    /**
     * Remove node from distributor
     * @param nodeId Unique identifier of node to remove
     */
    public abstract void remove(UUID nodeId);
    
    /**
     * Get current distributor status
     * @return DistributorStatus with current state information
     */
    public abstract DistributorStatus getStatus();
}

abstract class Node implements Predicate<HttpRequest>, CommandHandler {
    /**
     * Create new session on this node
     * @param capabilities Desired browser capabilities
     * @return Session if created successfully, empty otherwise
     */
    public abstract Optional<Session> newSession(Capabilities capabilities);
    
    /**
     * Execute WebDriver command for existing session
     * @param req HTTP request containing WebDriver command
     * @param resp HTTP response to populate
     */
    public abstract void executeWebDriverCommand(HttpRequest req, HttpResponse resp);
    
    /**
     * Get existing session by ID
     * @param id Session identifier
     * @return Session instance
     * @throws NoSuchSessionException if session not found on this node
     */
    public abstract Session getSession(SessionId id) throws NoSuchSessionException;
    
    /**
     * Stop session and clean up resources
     * @param id Session identifier to stop
     * @throws NoSuchSessionException if session not found
     */
    public abstract void stop(SessionId id) throws NoSuchSessionException;
    
    /**
     * Check if this node owns the specified session
     * @param id Session identifier to check
     * @return true if this node owns the session
     */
    protected abstract boolean isSessionOwner(SessionId id);
    
    /**
     * Check if node supports given capabilities
     * @param capabilities Browser capabilities to check
     * @return true if node can handle these capabilities
     */
    public abstract boolean isSupporting(Capabilities capabilities);
    
    /**
     * Get current node status including capacity and active sessions
     * @return NodeStatus with current state
     */
    public abstract NodeStatus getStatus();
}

// Automatic service registration
@AutoService(CliCommand.class)
public class CustomCommand implements CliCommand { ... }

@AutoService(SessionFactory.class)  
public class CustomSessionFactory implements SessionFactory { ... }

// CLI command discovery (in Main.java)
Set<CliCommand> commands = new TreeSet<>(comparing(CliCommand::getName));
ServiceLoader.load(CliCommand.class).forEach(commands::add);

// Session factory discovery
ServiceLoader<SessionFactory> factories = ServiceLoader.load(SessionFactory.class);

@AutoService(SessionFactory.class)
public class CustomBrowserFactory implements SessionFactory {
    @Override
    public boolean isSupporting(Capabilities capabilities) {
        return "myCustomBrowser".equals(capabilities.getBrowserName());
    }
    
    @Override
    public Optional<ActiveSession> apply(Set<Dialect> downstreamDialects, Capabilities capabilities) {
        WebDriver driver = new CustomBrowserDriver(capabilities);
        return Optional.of(new CustomActiveSession(driver, capabilities));
    }
}

public class LoadBalancedDistributor extends Distributor {
    @Override
    public Session newSession(NewSessionPayload payload) throws SessionNotCreatedException {
        Node selectedNode = selectNodeByLoad(payload.getDesiredCapabilities());
        return selectedNode.newSession(payload.getDesiredCapabilities())
            .orElseThrow(() -> new SessionNotCreatedException("No capacity available"));
    }
    
    private Node selectNodeByLoad(Capabilities capabilities) {
        // Custom load balancing logic
        return nodes.stream()
            .filter(node -> node.isSupporting(capabilities))
            .min(Comparator.comparing(this::getCurrentLoad))
            .orElse(null);
    }
}

@AutoService(CliCommand.class)
public class HealthCheckCommand implements CliCommand {
    @Override
    public String getName() {
        return "healthcheck";
    }
    
    @Override
    public String getDescription() {
        return "Perform health check on Grid components";
    }
    
    @Override
    public Executable configure(String... args) {
        return () -> {
            // Health check implementation
            performHealthCheck();
        };
    }
}

// Access configuration in extensions
Config config = new CompoundConfig(
    new EnvConfig(),
    new SystemPropertyConfig(),
    new DefaultConfig()
);

String customValue = config.get("custom", "setting");
int customPort = config.getInt("custom", "port");
boolean customFlag = config.getBool("custom", "enabled");