A low-level design (LLD) for a content delivery network (CDN) might include the following components:

1. Edge servers: The edge servers are responsible for storing and serving the content to the users. They are distributed geographically to provide low latency and high availability.

2. Origin server: The origin server is the source of the content, and it is responsible for storing and serving the content to the edge servers.

3. DNS server: The domain name system (DNS) server is responsible for mapping domain names to IP addresses, allowing users to access the content using friendly URLs.

4. Load balancer: The load balancer is responsible for distributing the traffic among the edge servers to ensure that they are utilized efficiently and do not become overloaded.

5. Monitoring and management: The monitoring and management component is responsible for monitoring the performance and availability of the CDN, and for managing the configuration and deployment of the edge servers.

This is just a basic outline of the components that might be included in a low-level design of a CDN. The specific details and implementation of these components will depend on the specific requirements and use case of the CDN.

Code

import java.util.*;
import java.util.concurrent.*;
import java.security.*;

// Central CDN Class
public class ContentDeliveryNetwork {
    private List<EdgeServer> edgeServers;
    private OriginServer originServer;
    private DNSServer dnsServer;
    private LoadBalancer loadBalancer;
    private MonitoringAndManagement monitoringAndManagement;
    private CacheInvalidationService cacheInvalidationService;
    private SecurityService securityService;

    public ContentDeliveryNetwork() {
        this.edgeServers = new CopyOnWriteArrayList<>();
        this.originServer = new OriginServer();
        this.dnsServer = new DNSServer();
        this.loadBalancer = new RoundRobinLoadBalancer(edgeServers);
        this.monitoringAndManagement = new MonitoringAndManagement(edgeServers, loadBalancer);
        this.cacheInvalidationService = new CacheInvalidationService(edgeServers);
        this.securityService = new SecurityService();
    }

    // Main method to serve content with security validation
    public void serveContent(String url, String clientIP, String token) {
        if (!securityService.validateToken(token)) {
            throw new SecurityException("Unauthorized access.");
        }

        EdgeServer selectedEdgeServer = loadBalancer.selectServer(clientIP);
        if (!selectedEdgeServer.hasContent(url)) {
            String content = originServer.getContent(url);
            selectedEdgeServer.storeContent(url, content);
        }

        String ipAddress = dnsServer.getIPAddress(clientIP);
        selectedEdgeServer.serveContent(ipAddress, url);
    }

    // Invalidate content cache across all edge servers
    public void invalidateContent(String url) {
        cacheInvalidationService.invalidateContent(url);
    }

    // Register a new edge server
    public void registerEdgeServer(EdgeServer server) {
        edgeServers.add(server);
    }
}

// EdgeServer with multi-level caching and content replication
public class EdgeServer {
    private Map<String, CacheEntry> memoryCache;
    private FileCache fileCache;
    private ReplicationService replicationService;

    public EdgeServer() {
        this.memoryCache = new ConcurrentHashMap<>();
        this.fileCache = new FileCache();
        this.replicationService = new ReplicationService();
    }

    public void storeContent(String url, String content) {
        CacheEntry entry = new CacheEntry(content);
        memoryCache.put(url, entry);
        fileCache.saveToDisk(url, content);
        replicationService.replicateContent(url, content); // Replicate content across other edge servers
    }

    public void serveContent(String ipAddress, String url) {
        CacheEntry content = memoryCache.getOrDefault(url, fileCache.retrieveFromDisk(url));
        if (content != null) {
            serveWithHeaders(ipAddress, content.getContent());
        }
    }

    public boolean hasContent(String url) {
        return memoryCache.containsKey(url) || fileCache.existsOnDisk(url);
    }

    private void serveWithHeaders(String ipAddress, String content) {
        // Serve the content with headers, compression, and security
        System.out.println("Serving content to IP: " + ipAddress + ":\n" + content);
    }

    public void invalidateContent(String url) {
        memoryCache.remove(url);
        fileCache.removeFromDisk(url);
        replicationService.invalidate(url);  // Invalidate replicated content
    }
}

// Cache entry class with TTL support
class CacheEntry {
    private String content;
    private long timestamp;
    private static final long TTL = 3600 * 1000; // 1-hour TTL

    public CacheEntry(String content) {
        this.content = content;
        this.timestamp = System.currentTimeMillis();
    }

    public String getContent() {
        if (isExpired()) {
            return null;
        }
        return content;
    }

    public boolean isExpired() {
        return System.currentTimeMillis() - timestamp > TTL;
    }
}

// Disk-based file cache
class FileCache {
    private Map<String, String> diskStorage;

    public FileCache() {
        this.diskStorage = new ConcurrentHashMap<>();
    }

    public void saveToDisk(String url, String content) {
        diskStorage.put(url, content);
    }

    public String retrieveFromDisk(String url) {
        return diskStorage.get(url);
    }

    public boolean existsOnDisk(String url) {
        return diskStorage.containsKey(url);
    }

    public void removeFromDisk(String url) {
        diskStorage.remove(url);
    }
}

// Replication service to replicate content across servers
class ReplicationService {
    public void replicateContent(String url, String content) {
        // Replicate content to other edge servers
        System.out.println("Replicating content: " + url);
    }

    public void invalidate(String url) {
        // Invalidate replicated content
        System.out.println("Invalidating replicated content: " + url);
    }
}

// LoadBalancer with intelligent selection of servers based on location and load
public class RoundRobinLoadBalancer extends LoadBalancer {
    private int currentIndex = 0;

    public RoundRobinLoadBalancer(List<EdgeServer> edgeServers) {
        super(edgeServers);
    }

    @Override
    public EdgeServer selectServer(String clientIP) {
        EdgeServer server = edgeServers.get(currentIndex);
        currentIndex = (currentIndex + 1) % edgeServers.size();
        return server;
    }
}

// Security service for validating tokens and encryption
class SecurityService {
    private static final String SECRET_KEY = "SuperSecretKey";

    public boolean validateToken(String token) {
        return token != null && token.equals(SECRET_KEY);  // Simplified token validation
    }
}

// DNS Server class to resolve domain names to IP addresses
public class DNSServer {
    private Map<String, String> domainMappings;

    public DNSServer() {
        this.domainMappings = new HashMap<>();
        domainMappings.put("cdn.example.com", "192.168.1.1");
    }

    public String getIPAddress(String clientIP) {
        return domainMappings.getOrDefault("cdn.example.com", "0.0.0.0");
    }
}

// Monitoring and management service for real-time monitoring and scaling
public class MonitoringAndManagement {
    private List<EdgeServer> edgeServers;
    private LoadBalancer loadBalancer;

    public MonitoringAndManagement(List<EdgeServer> edgeServers, LoadBalancer loadBalancer) {
        this.edgeServers = edgeServers;
        this.loadBalancer = loadBalancer;
    }

    public void monitorPerformance() {
        // Real-time monitoring of edge servers
        System.out.println("Monitoring edge servers' performance...");
    }

    public void autoScaleEdgeServers() {
        // Automatically scale edge servers based on traffic
        System.out.println("Auto-scaling edge servers based on traffic...");
    }
}

// Cache invalidation service
class CacheInvalidationService {
    private List<EdgeServer> edgeServers;

    public CacheInvalidationService(List<EdgeServer> edgeServers) {
        this.edgeServers = edgeServers;
    }

    public void invalidateContent(String url) {
        for (EdgeServer server : edgeServers) {
            server.invalidateContent(url);
        }
    }
}

// Abstract LoadBalancer class
abstract class LoadBalancer {
    protected List<EdgeServer> edgeServers;

    public LoadBalancer(List<EdgeServer> edgeServers) {
        this.edgeServers = edgeServers;
    }

    public abstract EdgeServer selectServer(String clientIP);
}