Advanced Patterns and Techniques

public class ZabServer {
    enum ServerState {
        LOOKING, FOLLOWING, LEADING
    }
    
    private ServerState state;
    private long currentEpoch;
    private long lastZxid;
    private List<Transaction> history;
    private Map<Long, Transaction> pendingTransactions;
    private Set<ZabServer> quorum;
    
    // Leader election
    public void startLeaderElection() {
        state = ServerState.LOOKING;
        
        // Send out vote with last zxid
        Vote myVote = new Vote(getServerId(), currentEpoch, lastZxid);
        broadcastVote(myVote);
        
        // Collect votes and determine winner
        // (Simplified - actual implementation uses multiple rounds)
        Map<Integer, Vote> receivedVotes = collectVotes();
        Vote electedLeader = selectLeader(receivedVotes);
        
        if (electedLeader.getServerId() == getServerId()) {
            becomeLeader();
        } else {
            becomeFollower(electedLeader.getServerId());
        }
    }
    
    // Leader functionality
    private void becomeLeader() {
        state = ServerState.LEADING;
        currentEpoch++;
        
        // Discovery phase
        List<FollowerInfo> followers = collectFollowerInfo();
        long newEpochZxid = createNewEpochZxid();
        
        // Find highest zxid among followers
        long highestZxid = lastZxid;
        for (FollowerInfo info : followers) {
            if (info.getLastZxid() > highestZxid) {
                highestZxid = info.getLastZxid();
            }
        }
        
        // Synchronize followers
        for (FollowerInfo follower : followers) {
            synchronizeFollower(follower, highestZxid);
        }
        
        // Start processing client requests
        startProcessingRequests();
    }
    
    // Process a client request as leader
    public void processRequest(Request request) {
        // Create transaction
        Transaction txn = new Transaction(
            createZxid(),
            request.getOperation(),
            request.getData()
        );
        
        // Store in pending transactions
        pendingTransactions.put(txn.getZxid(), txn);
        
        // Broadcast to followers
        List<ZabServer> acks = broadcastTransaction(txn);
        
        // If majority acknowledge, commit
        if (acks.size() > quorum.size() / 2) {
            commit(txn);
            broadcastCommit(txn);
        }
    }
    
    // Follower functionality
    private void becomeFollower(int leaderId) {
        state = ServerState.FOLLOWING;
        
        // Connect to leader
        connectToLeader(leaderId);
        
        // Send follower info
        sendFollowerInfo(lastZxid);
        
        // Process messages from leader
        while (state == ServerState.FOLLOWING) {
            Message message = receiveFromLeader();
            
            if (message instanceof Transaction) {
                Transaction txn = (Transaction) message;
                
                // Validate transaction
                if (isValidTransaction(txn)) {
                    // Log transaction
                    logTransaction(txn);
                    
                    // Acknowledge
                    sendAck(txn.getZxid());
                }
            } else if (message instanceof Commit) {
                Commit commit = (Commit) message;
                
                // Apply committed transaction
                applyTransaction(commit.getZxid());
            }
        }
    }
    
    // Create a new zxid
    private long createZxid() {
        // zxid is a 64-bit number:
        // - high 32 bits: epoch
        // - low 32 bits: counter
        return (currentEpoch << 32) | (++lastZxid & 0xFFFFFFFFL);
    }
    
    // Apply a transaction to the state machine
    private void applyTransaction(long zxid) {
        Transaction txn = pendingTransactions.get(zxid);
        if (txn != null) {
            // Apply to state machine
            applyToStateMachine(txn);
            
            // Add to history
            history.add(txn);
            
            // Remove from pending
            pendingTransactions.remove(zxid);
        }
    }
}