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Java implementation of Raft Consensus, this module contains only the core algorithm, and does not have any dependency except Java SE 8, for interface implementations that required by this module, please checkout jraft-ext module
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/**
 *
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  The ASF licenses 
 * this file to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package net.data.technology.jraft;

import java.nio.ByteBuffer;
import java.util.Calendar;
import java.util.HashMap;
import java.util.Map;
import java.util.Random;
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.CompletableFuture;

public class RaftClient {

    private Map rpcClients = new HashMap();
    private RpcClientFactory rpcClientFactory;
    private ClusterConfiguration configuration;
    private Logger logger;
    private Timer timer;
    private int leaderId;
    private boolean randomLeader;
    private Random random;

    public RaftClient(RpcClientFactory rpcClientFactory, ClusterConfiguration configuration, LoggerFactory loggerFactory){
        this.random = new Random(Calendar.getInstance().getTimeInMillis());
        this.rpcClientFactory = rpcClientFactory;
        this.configuration = configuration;
        this.leaderId = configuration.getServers().get(this.random.nextInt(configuration.getServers().size())).getId();
        this.randomLeader = true;
        this.logger = loggerFactory.getLogger(getClass());
        this.timer = new Timer();
    }

    public CompletableFuture appendEntries(byte[][] values){
        if(values == null || values.length == 0){
            throw new IllegalArgumentException("values cannot be null or empty");
        }

        LogEntry[] logEntries = new LogEntry[values.length];
        for(int i = 0; i < values.length; ++i){
            logEntries[i] = new LogEntry(0, values[i]);
        }

        RaftRequestMessage request = new RaftRequestMessage();
        request.setMessageType(RaftMessageType.ClientRequest);
        request.setLogEntries(logEntries);

        CompletableFuture result = new CompletableFuture();
        this.tryCurrentLeader(request, result, 0, 0);
        return result;
    }

    public CompletableFuture addServer(ClusterServer server){
        if(server == null){
            throw new IllegalArgumentException("server cannot be null");
        }

        LogEntry[] logEntries = new LogEntry[1];
        logEntries[0] = new LogEntry(0, server.toBytes(), LogValueType.ClusterServer);
        RaftRequestMessage request = new RaftRequestMessage();
        request.setMessageType(RaftMessageType.AddServerRequest);
        request.setLogEntries(logEntries);

        CompletableFuture result = new CompletableFuture();
        this.tryCurrentLeader(request, result, 0, 0);
        return result;
    }

    public CompletableFuture removeServer(int serverId){
        if(serverId < 0){
            throw new IllegalArgumentException("serverId must be equal or greater than zero");
        }

        ByteBuffer buffer = ByteBuffer.allocate(Integer.BYTES);
        buffer.putInt(serverId);
        LogEntry[] logEntries = new LogEntry[1];
        logEntries[0] = new LogEntry(0, buffer.array(), LogValueType.ClusterServer);
        RaftRequestMessage request = new RaftRequestMessage();
        request.setMessageType(RaftMessageType.RemoveServerRequest);
        request.setLogEntries(logEntries);

        CompletableFuture result = new CompletableFuture();
        this.tryCurrentLeader(request, result, 0, 0);
        return result;
    }

    private void tryCurrentLeader(RaftRequestMessage request, CompletableFuture future, int rpcBackoff, int retry){
        logger.debug("trying request to %d as current leader", this.leaderId);
        getOrCreateRpcClient().send(request).whenCompleteAsync((RaftResponseMessage response, Throwable error) -> {
            if(error == null){
                logger.debug("response from remote server, leader: %d, accepted: %s", response.getDestination(), String.valueOf(response.isAccepted()));
                if(response.isAccepted()){
                    future.complete(true);
                }else{
                    // set the leader return from the server
                    if(this.leaderId == response.getDestination() && !this.randomLeader){
                        future.complete(false);
                    }else{
                        this.randomLeader = false;
                        this.leaderId = response.getDestination();
                        tryCurrentLeader(request, future, rpcBackoff, retry);
                    }
                }
            }else{
                logger.info("rpc error, failed to send request to remote server (%s)", error.getMessage());
                if(retry > configuration.getServers().size()){
                    future.complete(false);
                    return;
                }

                // try a random server as leader
                this.leaderId = this.configuration.getServers().get(this.random.nextInt(this.configuration.getServers().size())).getId();
                this.randomLeader = true;
                refreshRpcClient();

                if(rpcBackoff > 0){
                    timer.schedule(new TimerTask(){

                        @Override
                        public void run() {
                            tryCurrentLeader(request, future, rpcBackoff + 50, retry + 1);

                        }}, rpcBackoff);
                }else{
                    tryCurrentLeader(request, future, rpcBackoff + 50, retry + 1);
                }
            }
        });
    }

    private RpcClient getOrCreateRpcClient(){
        synchronized(this.rpcClients){
            if(this.rpcClients.containsKey(this.leaderId)){
                return this.rpcClients.get(this.leaderId);
            }

            RpcClient client = this.rpcClientFactory.createRpcClient(getLeaderEndpoint());
            this.rpcClients.put(this.leaderId, client);
            return client;
        }
    }

    private RpcClient refreshRpcClient(){
        synchronized(this.rpcClients){
            RpcClient client = this.rpcClientFactory.createRpcClient(getLeaderEndpoint());
            this.rpcClients.put(this.leaderId, client);
            return client;
        }
    }

    private String getLeaderEndpoint(){
        for(ClusterServer server : this.configuration.getServers()){
            if(server.getId() == this.leaderId){
                return server.getEndpoint();
            }
        }

        logger.info("no endpoint could be found for leader %d, that usually means no leader is elected, retry the first one", this.leaderId);
        this.randomLeader = true;
        this.leaderId = this.configuration.getServers().get(0).getId();
        return this.configuration.getServers().get(0).getEndpoint();
    }
}